Limit allowed private address space to avoid long CIDR strings (#215 )

Allocate a /29 network instead of /30 to work around bootpd bug (#214 )
Bump the all-updates group with 6 updates (#210 )
2026-10-17 12:15:38 +02:00 · 2026-10-09 16:15:40 +01:00 · 2026-10-09 16:15:19 +01:00 · 2026-10-06 19:11:12 +01:00 · 2026-09-28 22:51:16 +01:00 · 2026-09-28 22:50:26 +01:00
27 changed files with 3892 additions and 1073 deletions
@@ -1,2 +1,5 @@
+[env]
+MACOSX_DEPLOYMENT_TARGET = "26.0"
+
 [target.aarch64-apple-darwin]
 runner = 'sudo -E'
@@ -17,7 +17,7 @@ jobs:
    runs-on: macos-26
    timeout-minutes: 30
    steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6.1.0
      - name: Install Rust
        run: |
          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile minimal
@@ -34,7 +34,7 @@ jobs:
    runs-on: macos-26
    timeout-minutes: 30
    steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6.1.0
      - name: Install Rust
        run: |
          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile minimal
@@ -17,7 +17,7 @@ jobs:
    environment: publish
    timeout-minutes: 60
    steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6.1.0
        with:
          fetch-depth: 0
          persist-credentials: false
@@ -45,7 +45,7 @@ jobs:
          permission-contents: write
          permission-pull-requests: write
      - name: Release
-        uses: goreleaser/goreleaser-action@v7
+        uses: goreleaser/goreleaser-action@f06c13b6b1a9625abc9e6e439d9c05a8f2190e94 # v7.2.3
        with:
          distribution: goreleaser-pro
          version: "~> v2"
@@ -61,7 +61,7 @@ jobs:
    runs-on: macos-26
    timeout-minutes: 60
    steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6.1.0
        with:
          fetch-depth: 0
          persist-credentials: false
@@ -72,13 +72,13 @@ jobs:
      - name: Install release targets
        run: rustup target add aarch64-apple-darwin x86_64-apple-darwin
      - name: Release dry run
-        uses: goreleaser/goreleaser-action@v7
+        uses: goreleaser/goreleaser-action@f06c13b6b1a9625abc9e6e439d9c05a8f2190e94 # v7.2.3
        with:
          distribution: goreleaser-pro
          version: "~> v2"
          args: release --skip=publish --snapshot --clean
      - name: Upload dry-run artifacts
-        uses: actions/upload-artifact@v6
+        uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6.0.0
        with:
          name: softnet-snapshot
          path: dist/**
@@ -5,6 +5,8 @@ project_name: softnet
 builds:
  - builder: rust
    command: build
+    env:
+      - SOFTNET_VERSION={{ .Version }}-{{ .ShortCommit }}
    targets:
      - aarch64-apple-darwin
      - x86_64-apple-darwin
@@ -36,5 +38,5 @@ brews:
    skip_upload: auto
    custom_block: |
      on_macos do
-        depends_on :macos => :sequoia
+        depends_on :macos => :tahoe
      end
@@ -16,7 +16,7 @@ smoltcp = "0"
 libc = "0"
 polling = "3"
 dhcproto = { git = "https://github.com/bluecatengineering/dhcproto.git", branch = "master" }
-vmnet = "0.5.1"
+vmnet = "0.7.0"
 clap = { version = "4", features = ["derive"] }
 mac_address = "1"
 privdrop = "0"
@@ -28,12 +28,17 @@ num_enum = "0"
 sentry = { version = "0", features = ["debug-images"] }
 sentry-anyhow = { version = "0", features = ["backtrace"] }
 nix = { version = "0", features = ["signal", "socket"] }
+network-interface = "2"
 prefix-trie = "0"
 ipnet = "2"
+rand = "0.10"
 oslog = "0.2.0"
 log = "0.4.29"
-serial_test = "3"
+serial_test = "4"
 coarsetime = "0.1.37"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+jsonrpsee-types = "0.26"

 [profile.release]
 debug = true
@@ -23,15 +23,39 @@ Softnet solves two problems:
 2. DHCP exhaustion
    * macOS built-in DHCP-server allocates a `/24` subnet with 86400 seconds lease time by default, which only allows for ~253 VMs a day (or 1 VM every ~6 minutes) to be spawned without causing a denial-of-service, which is pretty limiting for CI services like Cirrus CI

-And assumes that:
+Each VM gets a separate [`vmnet` network](https://developer.apple.com/documentation/vmnet/vmnet_network_create(_:_:)) with a private `/29` subnet selected to avoid overlap with existing host interface subnets. Softnet reserves an IP-address for the VM's MAC-address, delivers it through its own DHCP-server, and uses the reserved address for packet filtering.

-1. Tart gives it's VMs unique MAC-addresses
-2. macOS built-in DHCP-server won't re-use the IP-addresses from it's pool until their lease expire
+### Stateful flow authorization

-...otherwise it's possible for two VMs to receive an identical IP-address from the macOS built-in DHCP-server (even in the presence of Softnet's packet filtering) and thus bypass the protections offered by Softnet.
+Stateful `in`/`out` rules use a bounded authorization cache that records the
+direction and exact transport tuple of policy-approved flows, allowing matching
+return traffic without treating it as a new flow and thus requiring a separate
+policy entry. It is not a complete TCP connection tracker: endpoint transport
+stacks remain responsible for validating sequence numbers, receive windows,
+resets, and application-level traffic.
+
+This cache deliberately favors security, bounded resource use, and a simple
+implementation over availability. Only policy-authorized initiator traffic
+renews an entry; return traffic does not. Softnet does not maintain fairness
+quotas, eviction heuristics, or complete TCP lifecycle state.
+
+A packet admitted by a stateful rule is denied when Softnet cannot represent its
+flow, including when the cache is full. High flow churn, long idle connections,
+or ambiguous retransmissions may therefore interrupt networking and require the
+affected VM to reconnect.
+
+For TCP, a bare TCP SYN on an existing tuple is deliberately returned to policy
+because Softnet cannot distinguish a retransmission from tuple reuse without
+tracking TCP sequence state. If authorized, it may replace the tuple's previous
+cache lifetime; this can reduce availability but cannot grant traffic that
+policy did not permit.
+
+For ICMP, stateful flow authorization supports only echo requests and replies.

 ## Installing

+Softnet requires macOS 26 or newer; building also requires the macOS 26 SDK or newer.
+
 For proper functioning, Softnet binary requires two things:

 * a [SUID-bit](https://en.wikipedia.org/wiki/Setuid#SUID) to be set on the binary or a [passwordless sudo](https://serverfault.com/questions/160581/how-to-setup-passwordless-sudo-on-linux) to be configured, which effectively gives the binary `root` privileges
@@ -43,3 +67,20 @@ For proper functioning, Softnet binary requires two things:
 ## Running

 Softnet is started and managed automatically by Tart if `--net-softnet` flag is provided when calling `tart run`.
+
+### Dynamic network policy
+
+Softnet can update the running VM's IPv4 policy without restarting the VM. Pass a connected Unix stream socket as `--control-fd` to enable a newline-delimited [JSON-RPC 2.0](https://www.jsonrpc.org/specification) control channel. The socket is duplex and must be separate from `--vm-fd`, which carries VM packets.
+
+The supported methods are `softnet.policy.get` and `softnet.policy.set`. A complete policy update looks like this (each request and response occupies one line):
+
+```json
+{"jsonrpc":"2.0","id":"42","method":"softnet.policy.set","params":{"allow":["@host","10.0.0.0/8"],"block":["0.0.0.0/0"]}}
+{"jsonrpc":"2.0","id":"42","result":{"allow":["10.0.0.0/8","@host"],"block":["0.0.0.0/0"],"ruleCount":3}}
+```
+
+Every request must include a non-null string (at most 256 bytes) or non-negative integer `id`; notifications are rejected so policy changes always have an acknowledgment. Policy updates are atomic: all rules are parsed and a new prefix map is built before the active policy changes. Longest-prefix matching and block precedence for identical rules are preserved. Rules are normalized and deduplicated. A policy update may contain at most 4096 combined allow/block rules, and a request frame may not exceed 1 MiB.
+
+When the normalized allow or block policy changes, Softnet clears the flow table so the new policy applies to established flows immediately. This may interrupt active connections. Repeating the same normalized policy is a no-op and preserves the flow table.
+
+Use `block=["0.0.0.0/0"]` with specific allow rules for a default-deny egress policy. Closing the control socket leaves the last accepted policy active.
@@ -0,0 +1,389 @@
+use anyhow::Result;
+use dhcproto::v4::{
+    CLIENT_PORT, DhcpOption, Flags, HType, Message, MessageType, Opcode, OptionCode, SERVER_PORT,
+};
+use dhcproto::{Decodable, Encodable};
+use smoltcp::phy::ChecksumCapabilities;
+use smoltcp::wire::{
+    ETHERNET_HEADER_LEN, EthernetAddress, EthernetFrame, EthernetProtocol, IpAddress, IpProtocol,
+    Ipv4Address, Ipv4Packet, Ipv4Repr, UDP_HEADER_LEN, UdpPacket, UdpRepr,
+};
+
+pub(crate) const DHCP_SERVER_MAC: EthernetAddress = EthernetAddress([0x02, 0, 0, 0, 0, 1]);
+
+pub(crate) struct DhcpServer {
+    vm_mac: EthernetAddress,
+    vm_ip: Ipv4Address,
+    subnet_mask: Ipv4Address,
+    gateway_ip: Ipv4Address,
+}
+
+impl DhcpServer {
+    /// Creates a DHCP server for the VM's reserved IPv4 configuration.
+    pub fn new(
+        vm_mac: EthernetAddress,
+        vm_ip: Ipv4Address,
+        subnet_mask: Ipv4Address,
+        gateway_ip: Ipv4Address,
+    ) -> Self {
+        Self {
+            vm_mac,
+            vm_ip,
+            subnet_mask,
+            gateway_ip,
+        }
+    }
+
+    /// Returns a reply for a request sent to the DHCP server port.
+    pub fn receive_vm(
+        &self,
+        ip: &Ipv4Packet<&[u8]>,
+        udp: &UdpPacket<&[u8]>,
+    ) -> Result<Option<Vec<u8>>> {
+        // Accept only intact DHCP client datagrams with valid checksums
+        if udp.src_port() != CLIENT_PORT
+            || !ip.verify_checksum()
+            || ip.more_frags()
+            || ip.frag_offset() != 0
+            || UdpRepr::parse(
+                udp,
+                &IpAddress::Ipv4(ip.src_addr()),
+                &IpAddress::Ipv4(ip.dst_addr()),
+                &ChecksumCapabilities::default(),
+            )
+            .is_err()
+        {
+            return Ok(None);
+        }
+
+        // Only accept broadcasts or requests addressed to our gateway
+        if !(ip.dst_addr().is_broadcast() || ip.dst_addr() == self.gateway_ip) {
+            return Ok(None);
+        }
+
+        // Decode the DHCP request
+        let Ok(request) = Message::from_bytes(udp.payload()) else {
+            return Ok(None);
+        };
+
+        // Allow REQUESTs from an obsolete address so the VM can receive a NAK
+        if request.opts().msg_type() != Some(MessageType::Request)
+            && !ip.src_addr().is_unspecified()
+            && self.vm_ip != ip.src_addr()
+        {
+            return Ok(None);
+        }
+
+        // Build a reply if the request is supported
+        if let Some(reply) = self.receive(&request) {
+            // Broadcast NAKs and requested broadcasts; otherwise unicast to the VM
+            let (destination_mac, destination_ip) =
+                if reply.opts().msg_type() == Some(MessageType::Nak) || request.flags().broadcast()
+                {
+                    (EthernetAddress::BROADCAST, Ipv4Address::BROADCAST)
+                } else if !request.ciaddr().is_unspecified() {
+                    (self.vm_mac, request.ciaddr())
+                } else {
+                    (self.vm_mac, reply.yiaddr())
+                };
+
+            // Encode the reply for delivery to the VM
+            let packet = encode_packet(
+                &reply,
+                DHCP_SERVER_MAC,
+                destination_mac,
+                self.gateway_ip,
+                destination_ip,
+                SERVER_PORT,
+                CLIENT_PORT,
+            )?;
+            return Ok(Some(packet));
+        }
+
+        Ok(None)
+    }
+
+    /// Handles a DHCP request from the attached VM.
+    fn receive(&self, request: &Message) -> Option<Message> {
+        // Only accept direct Ethernet requests from the attached VM
+        if request.opcode() != Opcode::BootRequest
+            || request.htype() != HType::Eth
+            || request.hlen() != self.vm_mac.0.len() as u8
+            || request.chaddr() != self.vm_mac.0
+            || !request.giaddr().is_unspecified()
+        {
+            return None;
+        }
+
+        // Ignore requests that select another DHCP server
+        let server_id = match request.opts().get(OptionCode::ServerIdentifier) {
+            Some(DhcpOption::ServerIdentifier(address)) => Some(*address),
+            _ => None,
+        };
+        if server_id.is_some_and(|address| address != self.gateway_ip) {
+            return None;
+        }
+
+        // Read the requested address, if supplied
+        let requested_ip = match request.opts().get(OptionCode::RequestedIpAddress) {
+            Some(DhcpOption::RequestedIpAddress(address)) => Some(*address),
+            _ => None,
+        };
+
+        // Offer the reserved address or validate a request for it
+        match request.opts().msg_type()? {
+            MessageType::Discover if request.ciaddr().is_unspecified() && server_id.is_none() => {
+                Some(self.address_reply(request, MessageType::Offer))
+            }
+            MessageType::Request => {
+                // Use option 50 for SELECTING/INIT-REBOOT and ciaddr for RENEWING/REBINDING
+                let address = match (server_id, requested_ip, request.ciaddr().is_unspecified()) {
+                    (_, Some(address), true) => address,
+                    (None, None, false) => request.ciaddr(),
+                    _ => return None,
+                };
+
+                // Reject requests for any address other than the VM's reservation
+                if address != self.vm_ip {
+                    return Some(self.reply(request, MessageType::Nak));
+                }
+
+                // Acknowledge the reserved address
+                Some(self.address_reply(request, MessageType::Ack))
+            }
+            _ => None,
+        }
+    }
+
+    /// Builds an OFFER or ACK with the VM's reserved address and network configuration.
+    fn address_reply(&self, request: &Message, message_type: MessageType) -> Message {
+        // Include the reserved address and network configuration with an infinite lease
+        let mut reply = self.reply(request, message_type);
+        reply.set_yiaddr(self.vm_ip);
+        let options = reply.opts_mut();
+        options.insert(DhcpOption::SubnetMask(self.subnet_mask));
+        options.insert(DhcpOption::Router(vec![self.gateway_ip]));
+        options.insert(DhcpOption::DomainNameServer(vec![self.gateway_ip]));
+        options.insert(DhcpOption::AddressLeaseTime(u32::MAX));
+
+        reply
+    }
+
+    /// Builds a DHCP reply with the request's transaction and client identifiers.
+    fn reply(&self, request: &Message, message_type: MessageType) -> Message {
+        // Create a reply for the same client and transaction
+        let nak = message_type == MessageType::Nak;
+        let mut reply = Message::default();
+        reply
+            .set_xid(request.xid())
+            .set_chaddr(request.chaddr())
+            .set_opcode(Opcode::BootReply);
+
+        // Broadcast NAKs; otherwise retain the client's address and flags
+        if nak {
+            reply.set_flags(Flags::default().set_broadcast());
+        } else {
+            reply
+                .set_ciaddr(request.ciaddr())
+                .set_flags(request.flags());
+        }
+
+        // Identify the reply type and this DHCP server
+        let options = reply.opts_mut();
+        options.insert(DhcpOption::MessageType(message_type));
+        options.insert(DhcpOption::ServerIdentifier(self.gateway_ip));
+
+        // RFC 6842 requires echoing the client's identifier in every reply
+        if let Some(identifier) = request.opts().get(OptionCode::ClientIdentifier) {
+            options.insert(identifier.clone());
+        }
+
+        reply
+    }
+}
+
+/// Encodes a DHCP message in an Ethernet frame with IPv4 and UDP headers.
+fn encode_packet(
+    message: &Message,
+    source_mac: EthernetAddress,
+    destination_mac: EthernetAddress,
+    source_ip: Ipv4Address,
+    destination_ip: Ipv4Address,
+    source_port: u16,
+    destination_port: u16,
+) -> Result<Vec<u8>> {
+    // Pad the encoded message to the minimum BOOTP size, including short NAKs
+    let mut payload = message.to_vec()?;
+    payload.resize(payload.len().max(dhcproto::v4::MIN_PACKET_SIZE), 0);
+
+    // Allocate space for Ethernet, IPv4, UDP, and the DHCP payload
+    let ip_repr = Ipv4Repr {
+        src_addr: source_ip,
+        dst_addr: destination_ip,
+        next_header: IpProtocol::Udp,
+        payload_len: UDP_HEADER_LEN + payload.len(),
+        hop_limit: 64,
+    };
+    let mut bytes = vec![0; ETHERNET_HEADER_LEN + ip_repr.buffer_len() + ip_repr.payload_len];
+
+    // Write the Ethernet header
+    let mut ethernet = EthernetFrame::new_unchecked(bytes.as_mut_slice());
+    ethernet.set_src_addr(source_mac);
+    ethernet.set_dst_addr(destination_mac);
+    ethernet.set_ethertype(EthernetProtocol::Ipv4);
+
+    // Write the IPv4 header and checksum
+    let mut ip = Ipv4Packet::new_unchecked(ethernet.payload_mut());
+    ip_repr.emit(&mut ip, &ChecksumCapabilities::default());
+
+    // Write the UDP header, DHCP payload, and checksum
+    let mut udp = UdpPacket::new_unchecked(ip.payload_mut());
+    UdpRepr {
+        src_port: source_port,
+        dst_port: destination_port,
+    }
+    .emit(
+        &mut udp,
+        &IpAddress::Ipv4(source_ip),
+        &IpAddress::Ipv4(destination_ip),
+        payload.len(),
+        |buffer| buffer.copy_from_slice(&payload),
+        &ChecksumCapabilities::default(),
+    );
+
+    Ok(bytes)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{DHCP_SERVER_MAC, DhcpServer, encode_packet};
+    use dhcproto::Decodable;
+    use dhcproto::v4::{
+        CLIENT_PORT, DhcpOption, Message, MessageType, Opcode, OptionCode, SERVER_PORT,
+    };
+    use smoltcp::phy::ChecksumCapabilities;
+    use smoltcp::wire::{
+        EthernetAddress, EthernetFrame, Ipv4Address, Ipv4Packet, Ipv4Repr, UdpPacket, UdpRepr,
+    };
+
+    const VM: EthernetAddress = EthernetAddress([2, 0, 0, 0, 0, 2]);
+    const ADDRESS: Ipv4Address = Ipv4Address::new(192, 168, 1, 2);
+    const GATEWAY: Ipv4Address = Ipv4Address::new(192, 168, 1, 1);
+    const MASK: Ipv4Address = Ipv4Address::new(255, 255, 255, 252);
+    const CLIENT_ID: &[u8] = &[1, 2, 0, 0, 0, 0, 2];
+
+    #[test]
+    fn assigns_reserved_address() {
+        // Create a server and a client request
+        let dhcp = DhcpServer::new(VM, ADDRESS, MASK, GATEWAY);
+        let mut message = request(MessageType::Discover);
+
+        // Discover the reserved address
+        let (offer, destination) = exchange(&dhcp, &message, Ipv4Address::BROADCAST);
+        assert_eq!(offer.opts().msg_type(), Some(MessageType::Offer));
+        assert_eq!(offer.yiaddr(), ADDRESS);
+        assert_eq!(destination, ADDRESS);
+
+        // Request the offered address
+        message
+            .opts_mut()
+            .insert(DhcpOption::MessageType(MessageType::Request));
+        message
+            .opts_mut()
+            .insert(DhcpOption::RequestedIpAddress(offer.yiaddr()));
+        message
+            .opts_mut()
+            .insert(DhcpOption::ServerIdentifier(GATEWAY));
+        let (ack, destination) = exchange(&dhcp, &message, Ipv4Address::BROADCAST);
+
+        // Check the address and configuration in the acknowledgement
+        assert_eq!(ack.opcode(), Opcode::BootReply);
+        assert_eq!(ack.opts().msg_type(), Some(MessageType::Ack));
+        assert_eq!(ack.xid(), message.xid());
+        assert_eq!(ack.chaddr(), VM.0);
+        assert_eq!(ack.yiaddr(), ADDRESS);
+        assert_eq!(destination, ADDRESS);
+        for option in [
+            DhcpOption::ServerIdentifier(GATEWAY),
+            DhcpOption::SubnetMask(MASK),
+            DhcpOption::Router(vec![GATEWAY]),
+            DhcpOption::DomainNameServer(vec![GATEWAY]),
+            DhcpOption::AddressLeaseTime(u32::MAX),
+            DhcpOption::ClientIdentifier(CLIENT_ID.to_vec()),
+        ] {
+            assert_eq!(ack.opts().get(OptionCode::from(&option)), Some(&option));
+        }
+    }
+
+    #[test]
+    fn renews_reserved_address_and_rejects_stale_address() {
+        // Renew the reserved address
+        let dhcp = DhcpServer::new(VM, ADDRESS, MASK, GATEWAY);
+        let mut message = request(MessageType::Request);
+        message.set_ciaddr(ADDRESS);
+        let (ack, destination) = exchange(&dhcp, &message, GATEWAY);
+        assert_eq!(ack.opts().msg_type(), Some(MessageType::Ack));
+        assert_eq!(ack.yiaddr(), ADDRESS);
+        assert_eq!(destination, ADDRESS);
+
+        // Broadcast a NAK when the client renews an old address
+        message.set_ciaddr(Ipv4Address::new(192, 168, 2, 2));
+        let (nak, destination) = exchange(&dhcp, &message, GATEWAY);
+        assert_eq!(nak.opts().msg_type(), Some(MessageType::Nak));
+        assert!(nak.yiaddr().is_unspecified());
+        assert_eq!(destination, Ipv4Address::BROADCAST);
+    }
+
+    fn request(kind: MessageType) -> Message {
+        let mut message = Message::default();
+        message.set_xid(42).set_chaddr(&VM.0);
+        message.opts_mut().insert(DhcpOption::MessageType(kind));
+        message
+            .opts_mut()
+            .insert(DhcpOption::ClientIdentifier(CLIENT_ID.to_vec()));
+        message
+    }
+
+    fn exchange(
+        dhcp: &DhcpServer,
+        request: &Message,
+        destination: Ipv4Address,
+    ) -> (Message, Ipv4Address) {
+        // Encode the client request
+        let frame = encode_packet(
+            request,
+            VM,
+            EthernetAddress::BROADCAST,
+            request.ciaddr(),
+            destination,
+            CLIENT_PORT,
+            SERVER_PORT,
+        )
+        .unwrap();
+
+        // Pass the packet to the DHCP server
+        let ethernet = EthernetFrame::new_checked(frame.as_slice()).unwrap();
+        let ip = Ipv4Packet::new_checked(ethernet.payload()).unwrap();
+        let udp = UdpPacket::new_unchecked(ip.payload());
+        let frame = dhcp
+            .receive_vm(&ip, &udp)
+            .unwrap()
+            .expect("expected a DHCP reply");
+
+        // Decode and validate the reply
+        let ethernet = EthernetFrame::new_checked(frame.as_slice()).unwrap();
+        let ip = Ipv4Packet::new_unchecked(ethernet.payload());
+        Ipv4Repr::parse(&ip, &ChecksumCapabilities::default()).unwrap();
+        let udp = UdpPacket::new_unchecked(ip.payload());
+        UdpRepr::parse(
+            &udp,
+            &ip.src_addr().into(),
+            &ip.dst_addr().into(),
+            &ChecksumCapabilities::default(),
+        )
+        .unwrap();
+        assert_eq!(ethernet.src_addr(), DHCP_SERVER_MAC);
+        assert_eq!(ip.src_addr(), GATEWAY);
+        (Message::from_bytes(udp.payload()).unwrap(), ip.dst_addr())
+    }
+}
@@ -1,102 +0,0 @@
-use dhcproto::Decodable;
-use dhcproto::v4::{DhcpOption, MessageType, OptionCode};
-use smoltcp::wire::Ipv4Address;
-use std::collections::HashSet;
-use std::time::Duration;
-
-#[derive(Default)]
-pub struct DhcpSnooper {
-    vm_lease: Option<Lease>,
-    uncertainty_duration: Duration,
-}
-
-impl DhcpSnooper {
-    pub fn new(uncertainty_duration: Duration) -> Self {
-        DhcpSnooper {
-            uncertainty_duration,
-            ..Default::default()
-        }
-    }
-
-    pub fn register_dhcp_reply(&mut self, dhcp_packet: &[u8]) {
-        let mut decoder = dhcproto::v4::Decoder::new(dhcp_packet);
-
-        let message = match dhcproto::v4::Message::decode(&mut decoder) {
-            Ok(message) => message,
-            Err(_) => return,
-        };
-
-        match message.opts().msg_type() {
-            Some(MessageType::Ack) => {
-                let lease_time = match message.opts().get(OptionCode::AddressLeaseTime) {
-                    Some(DhcpOption::AddressLeaseTime(lease_time)) => lease_time,
-                    _ => return,
-                };
-
-                let dns_ips = match message.opts().get(OptionCode::DomainNameServer) {
-                    Some(DhcpOption::DomainNameServer(dns_ips)) => {
-                        HashSet::from_iter(dns_ips.iter().cloned())
-                    }
-                    _ => HashSet::new(),
-                };
-
-                let mut lease_duration = Duration::from_secs(*lease_time as u64);
-
-                // Adjust for uncertainty caused by using a coarse clock
-                lease_duration = lease_duration.saturating_sub(self.uncertainty_duration);
-
-                self.vm_lease = Some(Lease::new(message.yiaddr(), lease_duration, dns_ips))
-            }
-            Some(MessageType::Nak) => {
-                self.vm_lease = None;
-            }
-            _ => {}
-        };
-    }
-
-    #[cfg(test)]
-    pub(crate) fn set_lease(&mut self, vm_lease: Option<Lease>) {
-        self.vm_lease = vm_lease
-    }
-
-    pub fn lease(&self) -> &Option<Lease> {
-        &self.vm_lease
-    }
-
-    pub fn valid_dns_target(&self, addr: &Ipv4Address) -> bool {
-        if let Some(lease) = &self.vm_lease {
-            return lease.dns_ips.contains(addr);
-        }
-
-        false
-    }
-}
-
-#[derive(Debug)]
-pub struct Lease {
-    address: Ipv4Address,
-    valid_until: coarsetime::Instant,
-    dns_ips: HashSet<Ipv4Address>,
-}
-
-impl Lease {
-    pub fn new(address: Ipv4Address, lease_time: Duration, dns_ips: HashSet<Ipv4Address>) -> Lease {
-        Lease {
-            address,
-            valid_until: coarsetime::Instant::recent() + lease_time.into(),
-            dns_ips,
-        }
-    }
-
-    pub fn address(&self) -> Ipv4Address {
-        self.address
-    }
-
-    pub fn valid(&self) -> bool {
-        coarsetime::Instant::recent() < self.valid_until
-    }
-
-    pub fn valid_ip_source(&self, address: Ipv4Address) -> bool {
-        self.address == address && self.valid()
-    }
-}
@@ -1,15 +1,16 @@
+use crate::subnet_finder;
 use anyhow::{Context, Result, anyhow};
 use clap::ValueEnum;
 use log::info;
+use smoltcp::wire::EthernetAddress;
 use std::net::Ipv4Addr;
 use std::os::unix::io::{AsRawFd, RawFd};
 use std::os::unix::net::UnixDatagram;
-use std::str::FromStr;
 use std::sync::mpsc::{SyncSender, sync_channel};
-use vmnet::mode::Mode;
+use vmnet::network::Mode;
 use vmnet::parameters::{Parameter, ParameterKind};
 use vmnet::port_forwarding::{AddressFamily, Protocol};
-use vmnet::{Batch, Events, Options};
+use vmnet::{Batch, Events, Network, NetworkConfiguration, Options};

 #[derive(ValueEnum, Clone, Debug)]
 pub enum NetType {
@@ -27,6 +28,8 @@ pub struct Host {
    interface: vmnet::Interface,
    new_packets_rx: UnixDatagram,
    callback_can_continue_tx: SyncSender<()>,
+    pub vm_ip: Ipv4Addr,
+    pub subnet_mask: Ipv4Addr,
    pub gateway_ip: smoltcp::wire::Ipv4Address,
    pub max_packet_size: u64,
    pub read_max_packets: u64,
@@ -34,31 +37,49 @@ pub struct Host {
 }

 impl Host {
-    pub fn new(vm_net_type: NetType, enable_isolation: bool) -> Result<Host> {
-        // Initialize a vmnet.framework NAT or Host interface with isolation enabled
-        let mut interface = vmnet::Interface::new(
-            match vm_net_type {
-                NetType::Nat => Mode::Shared(Default::default()),
-                NetType::Host => Mode::Host(Default::default()),
-            },
+    pub fn new(
+        vm_net_type: NetType,
+        vm_mac: EthernetAddress,
+        enable_isolation: bool,
+    ) -> Result<Host> {
+        // Find an unused /29 subnet for the gateway and VM
+        let (gateway_ip, vm_ip, subnet) = subnet_finder::find_available_subnet(29)?;
+
+        // Create vmnet's network configuration
+        let mut configuration = NetworkConfiguration::new(match vm_net_type {
+            NetType::Nat => Mode::Shared,
+            NetType::Host => Mode::Host,
+        })
+        .context("failed to create vmnet network configuration")?;
+
+        // Configure the gateway address and subnet mask
+        let subnet_mask = subnet.netmask();
+        configuration
+            .set_ipv4_subnet(gateway_ip, subnet_mask)
+            .context("failed to configure IPv4 subnet")?;
+
+        // Reserve the VM's address for its MAC
+        configuration
+            .add_dhcp_reservation(vm_mac.0, vm_ip)
+            .context("failed to add DHCP address reservation")?;
+
+        // Create vmnet's network
+        let network = Network::new(&configuration)
+            .with_context(|| format!("failed to create vmnet network {subnet}"))?;
+
+        // Instantiate vmnet's interface
+        //
+        // The interface retains the network reservation until it is stopped
+        let mut interface = vmnet::Interface::with_network(
+            &network,
            Options {
+                allocate_mac_address: Some(false),
                enable_isolation: Some(enable_isolation),
                ..Default::default()
            },
        )
        .context("failed to initialize vmnet interface")?;

-        // Retrieve first IP (gateway) used for this interface
-        let Some(Parameter::StartAddress(gateway_ip)) =
-            interface.parameters().get(ParameterKind::StartAddress)
-        else {
-            return Err(anyhow!(
-                "failed to retrieve vmnet's interface start address"
-            ));
-        };
-        let gateway_ip = Ipv4Addr::from_str(&gateway_ip)
-            .context("failed to parse vmnet's interface start address")?;
-
        // Retrieve max packet size for this interface
        let Some(Parameter::MaxPacketSize(max_packet_size)) =
            interface.parameters().get(ParameterKind::MaxPacketSize)
@@ -104,6 +125,8 @@ impl Host {
            interface,
            new_packets_rx,
            callback_can_continue_tx,
+            vm_ip,
+            subnet_mask,
            gateway_ip,
            max_packet_size,
            read_max_packets,
@@ -135,15 +158,6 @@ impl Host {
            .map_err(|err| anyhow!("failed to add port forwarding rule {details}: {err}"))
    }

-    pub fn port_forwarding_remove_rule(&mut self, external_port: u16) -> Result<()> {
-        let details = format!("external_port={external_port}");
-
-        self.interface
-            .port_forwarding_rule_remove(AddressFamily::Ipv4, Protocol::Tcp, external_port)
-            .map(|_| info!("removed port forwarding rule {details}"))
-            .map_err(|err| anyhow!("failed to remove port forwarding rule {details}: {err}"))
-    }
-
    pub fn read(&mut self, batch: &mut Batch, bufs: &mut [Vec<u8>]) -> vmnet::Result<usize> {
        // Dequeue dummy datagram from the socket (if any)
        // to free up buffer space and reduce false-positives
@@ -1,6 +1,7 @@
-mod dhcp_snooper;
+mod dhcp_server;
 mod host;
 pub use host::NetType;
 mod poller;
 pub mod proxy;
+mod subnet_finder;
 mod vm;
@@ -10,8 +10,10 @@ pub struct Poller<'poller> {
    poller: polling::Poller,
    events: polling::Events,
    timeout: Duration,
+    last_periodic_tick: coarsetime::Instant,
    vm_fd: BorrowedFd<'poller>,
    host_fd: BorrowedFd<'poller>,
+    control_fd: Option<BorrowedFd<'poller>>,
 }

 #[derive(IntoPrimitive)]
@@ -19,6 +21,7 @@ pub struct Poller<'poller> {
 enum EventKey {
    VM,
    Host,
+    Control,
    Interrupt,
 }

@@ -26,6 +29,7 @@ impl Poller<'_> {
    pub fn new<'poller>(
        vm_fd: RawFd,
        host_fd: RawFd,
+        control_fd: Option<RawFd>,
        timeout: Duration,
    ) -> Result<Poller<'poller>> {
        let poller = polling::Poller::new()?;
@@ -34,8 +38,10 @@ impl Poller<'_> {
            poller,
            events: polling::Events::new(),
            timeout,
+            last_periodic_tick: coarsetime::Instant::recent(),
            vm_fd: unsafe { BorrowedFd::borrow_raw(vm_fd) },
            host_fd: unsafe { BorrowedFd::borrow_raw(host_fd) },
+            control_fd: control_fd.map(|fd| unsafe { BorrowedFd::borrow_raw(fd) }),
        })
    }

@@ -46,6 +52,14 @@ impl Poller<'_> {
                self.vm_interest(),
                PollMode::Edge,
            )?;
+
+            if let Some(control_fd) = self.control_fd {
+                self.poller.add_with_mode(
+                    control_fd.as_raw_fd(),
+                    polling::Event::all(EventKey::Control.into()),
+                    PollMode::Edge,
+                )?;
+            }
            self.poller.add_with_mode(
                self.host_fd.as_raw_fd(),
                self.host_interest(),
@@ -79,10 +93,27 @@ impl Poller<'_> {
            .events
            .iter()
            .any(|ev| ev.key == Into::<usize>::into(EventKey::Interrupt));
-
        Ok((vm_readable, host_readable, interrupt))
    }

+    pub fn periodic_tick_due(&mut self) -> bool {
+        let due = self.last_periodic_tick.elapsed_since_recent() >= self.timeout.into();
+
+        if due {
+            self.last_periodic_tick = coarsetime::Instant::recent();
+        }
+
+        due
+    }
+
+    pub fn remove_control(&mut self) -> Result<()> {
+        if let Some(control_fd) = self.control_fd.take() {
+            self.poller.delete(control_fd)?;
+        }
+
+        Ok(())
+    }
+
    fn vm_interest(&self) -> polling::Event {
        polling::Event::readable(EventKey::VM.into())
    }
@@ -0,0 +1,173 @@
+use super::{FlowDirection, FlowKey, FlowMatch, FlowTable};
+use coarsetime::{Duration, Instant};
+use smoltcp::wire::{Icmpv4Message, Icmpv4Packet, Ipv4Address, Ipv4Packet};
+
+const ECHO_TIMEOUT: Duration = Duration::from_secs(30);
+
+impl FlowTable {
+    pub(super) fn inspect_icmp(
+        &mut self,
+        ipv4_pkt: &Ipv4Packet<&[u8]>,
+        direction: FlowDirection,
+        now: Instant,
+    ) -> FlowMatch {
+        let Ok(icmp) = Icmpv4Packet::new_checked(ipv4_pkt.payload()) else {
+            return FlowMatch::Denied;
+        };
+        if !icmp.verify_checksum() {
+            return FlowMatch::Denied;
+        }
+
+        match (icmp.msg_type(), icmp.msg_code()) {
+            (Icmpv4Message::EchoRequest, 0) => {
+                self.inspect_echo(ipv4_pkt, direction, icmp.echo_ident(), true, now)
+            }
+            (Icmpv4Message::EchoReply, 0) => {
+                self.inspect_echo(ipv4_pkt, direction, icmp.echo_ident(), false, now)
+            }
+            // Other ICMP messages follow normal source policy: treating errors that quote
+            // tracked tuples as RELATED requires validation beyond this exact-tuple table
+            //
+            // Potential degradation of PMTU discovery and traceroute is an accepted tradeoff.
+            _ => FlowMatch::Untracked,
+        }
+    }
+
+    fn inspect_echo(
+        &mut self,
+        ipv4_pkt: &Ipv4Packet<&[u8]>,
+        direction: FlowDirection,
+        ident: u16,
+        is_request: bool,
+        now: Instant,
+    ) -> FlowMatch {
+        // We need to preserve the request direction so opposite-direction echo flows remain distinct
+        let initiating_direction = if is_request {
+            direction
+        } else {
+            direction.opposite()
+        };
+
+        let key = FlowKey::icmp_echo(
+            ipv4_pkt.src_addr(),
+            ipv4_pkt.dst_addr(),
+            direction,
+            initiating_direction,
+            ident,
+        );
+
+        if let Some(matched) = self.match_existing_flow(key, direction, now, ECHO_TIMEOUT) {
+            return matched;
+        }
+
+        if is_request {
+            FlowMatch::candidate(key, initiating_direction, now, ECHO_TIMEOUT)
+        } else {
+            // Only replies matching an admitted request are tracked
+            FlowMatch::Untracked
+        }
+    }
+}
+
+impl FlowKey {
+    fn icmp_echo(
+        src_addr: Ipv4Address,
+        dst_addr: Ipv4Address,
+        direction: FlowDirection,
+        initiating_direction: FlowDirection,
+        ident: u16,
+    ) -> Self {
+        let (host_addr, vm_addr) = direction.host_vm_pair(src_addr, dst_addr);
+        Self::IcmpEcho {
+            host_addr,
+            vm_addr,
+            ident,
+            initiating_direction,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::super::test_support::{HOST, VM, inspect_from_host, inspect_from_vm, ipv4_packet};
+    use super::super::{FlowMatch, FlowTable};
+    use smoltcp::wire::{Icmpv4Message, Icmpv4Packet, IpProtocol, Ipv4Address};
+
+    #[test]
+    fn echo_request_and_reply_are_tracked_in_both_directions() {
+        let mut tracker = FlowTable::new();
+        let vm_request = echo_packet(VM, HOST, Icmpv4Message::EchoRequest, 7, 1);
+
+        let FlowMatch::Candidate(pending) = inspect_from_vm(&mut tracker, &vm_request) else {
+            panic!("expected a new flow");
+        };
+        assert!(tracker.commit(pending));
+
+        let host_reply = echo_packet(HOST, VM, Icmpv4Message::EchoReply, 7, 1);
+        assert!(matches!(
+            inspect_from_host(&mut tracker, &host_reply),
+            FlowMatch::Allowed
+        ));
+
+        let wrong_ident = echo_packet(HOST, VM, Icmpv4Message::EchoReply, 8, 1);
+        assert!(matches!(
+            inspect_from_host(&mut tracker, &wrong_ident),
+            FlowMatch::Untracked
+        ));
+
+        // The opposite direction is a distinct flow even with the same identifier.
+        let host_request = echo_packet(HOST, VM, Icmpv4Message::EchoRequest, 7, 1);
+        let FlowMatch::Candidate(pending) = inspect_from_host(&mut tracker, &host_request) else {
+            panic!("expected a new flow");
+        };
+        assert!(tracker.commit(pending));
+
+        let vm_reply = echo_packet(VM, HOST, Icmpv4Message::EchoReply, 7, 1);
+        assert!(matches!(
+            inspect_from_vm(&mut tracker, &vm_reply),
+            FlowMatch::Allowed
+        ));
+    }
+
+    #[test]
+    fn invalid_echo_is_denied_and_unsolicited_echo_is_untracked() {
+        let mut tracker = FlowTable::new();
+        let unsolicited = echo_packet(HOST, VM, Icmpv4Message::EchoReply, 7, 1);
+        assert!(matches!(
+            inspect_from_host(&mut tracker, &unsolicited),
+            FlowMatch::Untracked
+        ));
+
+        let mut wrong_code = echo_packet(HOST, VM, Icmpv4Message::EchoRequest, 7, 1);
+        wrong_code[21] = 1;
+        Icmpv4Packet::new_unchecked(&mut wrong_code[20..]).fill_checksum();
+        assert!(matches!(
+            inspect_from_host(&mut tracker, &wrong_code),
+            FlowMatch::Untracked
+        ));
+
+        let mut bad_checksum = echo_packet(HOST, VM, Icmpv4Message::EchoRequest, 7, 1);
+        bad_checksum[27] ^= 1;
+        assert!(matches!(
+            inspect_from_host(&mut tracker, &bad_checksum),
+            FlowMatch::Denied
+        ));
+    }
+
+    fn echo_packet(
+        src_addr: Ipv4Address,
+        dst_addr: Ipv4Address,
+        message: Icmpv4Message,
+        ident: u16,
+        sequence: u16,
+    ) -> Vec<u8> {
+        let mut bytes = ipv4_packet(src_addr, dst_addr, IpProtocol::Icmp, 8);
+        let mut icmp = Icmpv4Packet::new_unchecked(&mut bytes[20..]);
+        icmp.set_msg_type(message);
+        icmp.set_msg_code(0);
+        icmp.set_echo_ident(ident);
+        icmp.set_echo_seq_no(sequence);
+        icmp.fill_checksum();
+        bytes
+    }
+}
@@ -0,0 +1,390 @@
+mod icmp;
+mod tcp;
+mod udp;
+
+use coarsetime::{Duration, Instant};
+use smoltcp::wire::{IpProtocol, Ipv4Address, Ipv4Packet};
+use std::collections::HashMap;
+
+const MAX_FLOWS: usize = 32_768;
+const SWEEP_INTERVAL: Duration = Duration::from_secs(1);
+
+/// A bounded cache of exact-tuple permissions for return traffic.
+///
+/// The table establishes only the direction in which a flow was authorized.
+/// This is deliberately not a TCP state machine. Endpoint transport stacks remain
+/// responsible for TCP handshakes, teardown, sequence numbers, and receive windows,
+/// as well as ICMP echo sequences.
+///
+/// Capacity is intentionally enforced with one fail-closed limit per VM.
+/// Exhaustion may deny further networking for that VM; this availability
+/// tradeoff is accepted to avoid fairness quotas, admission scans, and eviction.
+#[derive(Debug)]
+pub(crate) struct FlowTable {
+    flows: HashMap<FlowKey, Flow>,
+    next_sweep: Instant,
+}
+
+/// A proposed flow-table entry awaiting policy authorization.
+pub(crate) struct PendingFlow {
+    key: FlowKey,
+    flow: Flow,
+}
+
+/// Metadata stored for an admitted flow.
+#[derive(Clone, Copy, Debug)]
+struct Flow {
+    initiating_direction: FlowDirection,
+    expires_at: Instant,
+}
+
+/// The result of classifying a packet against the flow table.
+pub(crate) enum FlowMatch {
+    /// The packet is exact-tuple return traffic for an admitted flow.
+    Allowed,
+
+    /// Policy must authorize this initiator-side packet before committing it.
+    Candidate(PendingFlow),
+
+    /// The packet is malformed or violates a transport invariant that we deliberately enforce.
+    Denied,
+
+    /// The flow table has no stateful interpretation for this packet.
+    Untracked,
+}
+
+impl FlowMatch {
+    fn candidate(
+        key: FlowKey,
+        initiating_direction: FlowDirection,
+        now: Instant,
+        timeout: Duration,
+    ) -> Self {
+        Self::Candidate(PendingFlow {
+            key,
+            flow: Flow {
+                initiating_direction,
+                expires_at: now + timeout,
+            },
+        })
+    }
+}
+
+/// The canonical identity used to index a flow-table entry.
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+enum FlowKey {
+    IcmpEcho {
+        host_addr: Ipv4Address,
+        vm_addr: Ipv4Address,
+        ident: u16,
+        initiating_direction: FlowDirection,
+    },
+    Tcp {
+        host_addr: Ipv4Address,
+        host_port: u16,
+        vm_addr: Ipv4Address,
+        vm_port: u16,
+    },
+    Udp {
+        host_addr: Ipv4Address,
+        host_port: u16,
+        vm_addr: Ipv4Address,
+        vm_port: u16,
+    },
+}
+
+/// A packet direction across the host–VM boundary.
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+pub(super) enum FlowDirection {
+    FromHost,
+    FromVm,
+}
+
+impl FlowTable {
+    pub(crate) fn new() -> Self {
+        Self {
+            flows: HashMap::new(),
+            next_sweep: Instant::recent() + SWEEP_INTERVAL,
+        }
+    }
+
+    pub(super) fn inspect(
+        &mut self,
+        ipv4_pkt: &Ipv4Packet<&[u8]>,
+        direction: FlowDirection,
+    ) -> FlowMatch {
+        self.inspect_at(ipv4_pkt, direction, Instant::recent())
+    }
+
+    fn inspect_at(
+        &mut self,
+        ipv4_pkt: &Ipv4Packet<&[u8]>,
+        direction: FlowDirection,
+        now: Instant,
+    ) -> FlowMatch {
+        // Perform lazy garbage collection of expired flow entries
+        self.sweep_if_due(now);
+
+        // Later fragments do not contain enough transport information to bind
+        // them to an exact flow, so packet policy must decide
+        if ipv4_pkt.more_frags() || ipv4_pkt.frag_offset() != 0 {
+            return FlowMatch::Untracked;
+        }
+
+        match ipv4_pkt.next_header() {
+            IpProtocol::Icmp => self.inspect_icmp(ipv4_pkt, direction, now),
+            IpProtocol::Tcp => self.inspect_tcp(ipv4_pkt, direction, now),
+            IpProtocol::Udp => self.inspect_udp(ipv4_pkt, direction, now),
+            _ => FlowMatch::Untracked,
+        }
+    }
+
+    pub(crate) fn commit(&mut self, pending: PendingFlow) -> bool {
+        let PendingFlow { key, flow } = pending;
+
+        // Reject new entries when full while allowing existing entries to be updated
+        if !self.flows.contains_key(&key) && self.flows.len() >= MAX_FLOWS {
+            return false;
+        }
+
+        self.flows.insert(key, flow);
+
+        true
+    }
+
+    fn sweep_if_due(&mut self, now: Instant) {
+        if now < self.next_sweep {
+            return;
+        }
+
+        self.flows.retain(|_, flow| now < flow.expires_at);
+
+        self.next_sweep = now + SWEEP_INTERVAL;
+    }
+
+    fn match_existing_flow(
+        &self,
+        key: FlowKey,
+        direction: FlowDirection,
+        now: Instant,
+        timeout: Duration,
+    ) -> Option<FlowMatch> {
+        let existing = self.flows.get(&key).copied()?;
+
+        // Treat expired entries as missing even before the next sweep
+        if now >= existing.expires_at {
+            return None;
+        }
+
+        if direction == existing.initiating_direction {
+            Some(FlowMatch::candidate(
+                key,
+                existing.initiating_direction,
+                now,
+                timeout,
+            ))
+        } else {
+            Some(FlowMatch::Allowed)
+        }
+    }
+}
+
+impl FlowDirection {
+    fn host_vm_pair<T>(self, src: T, dst: T) -> (T, T) {
+        match self {
+            Self::FromHost => (src, dst),
+            Self::FromVm => (dst, src),
+        }
+    }
+
+    fn opposite(self) -> Self {
+        match self {
+            Self::FromHost => Self::FromVm,
+            Self::FromVm => Self::FromHost,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::test_support::{HOST, VM, inspect_from_host, ipv4_packet, udp_packet};
+    use super::{
+        Duration, FlowDirection, FlowMatch, FlowTable, Instant, MAX_FLOWS, SWEEP_INTERVAL,
+    };
+    use smoltcp::wire::{IpProtocol, Ipv4Packet};
+
+    #[test]
+    fn all_ipv4_fragments_are_untracked() {
+        for (more_fragments, offset) in [(true, 0), (false, 8)] {
+            let mut bytes = udp_packet(HOST, 50_000, VM, 53);
+            let mut packet = Ipv4Packet::new_unchecked(bytes.as_mut_slice());
+            packet.set_frag_offset(offset);
+            packet.set_more_frags(more_fragments);
+
+            let mut tracker = FlowTable::new();
+            assert!(matches!(
+                inspect_from_host(&mut tracker, &bytes),
+                FlowMatch::Untracked
+            ));
+        }
+    }
+
+    #[test]
+    fn expired_tuple_is_not_matched_before_the_next_sweep() {
+        let mut tracker = FlowTable::new();
+        let datagram = udp_packet(HOST, 50_000, VM, 53);
+        let FlowMatch::Candidate(pending) = inspect_from_host(&mut tracker, &datagram) else {
+            panic!("expected a candidate");
+        };
+        assert!(tracker.commit(pending));
+
+        let now = Instant::recent();
+        let flow = tracker.flows.values_mut().next().unwrap();
+        flow.expires_at = now - Duration::from_secs(1);
+        tracker.next_sweep = now + SWEEP_INTERVAL;
+
+        let reply = udp_packet(VM, 53, HOST, 50_000);
+        assert!(matches!(
+            tracker.inspect_at(
+                &Ipv4Packet::new_checked(reply.as_slice()).unwrap(),
+                FlowDirection::FromVm,
+                now,
+            ),
+            FlowMatch::Candidate(_)
+        ));
+    }
+
+    #[test]
+    fn global_limit_rejects_new_tuple_but_allows_replacement() {
+        let mut tracker = FlowTable::new();
+        for port in 10_000..10_000 + MAX_FLOWS as u16 {
+            let datagram = udp_packet(HOST, port, VM, 53);
+            let FlowMatch::Candidate(pending) = inspect_from_host(&mut tracker, &datagram) else {
+                panic!("expected a candidate");
+            };
+            assert!(tracker.commit(pending));
+        }
+
+        let over_limit = udp_packet(HOST, 50_000, VM, 53);
+        let FlowMatch::Candidate(pending) = inspect_from_host(&mut tracker, &over_limit) else {
+            panic!("expected a candidate");
+        };
+        assert!(!tracker.commit(pending));
+
+        let replacement = udp_packet(HOST, 10_000, VM, 53);
+        let FlowMatch::Candidate(pending) = inspect_from_host(&mut tracker, &replacement) else {
+            panic!("expected a candidate");
+        };
+        assert!(tracker.commit(pending));
+    }
+
+    #[test]
+    fn unsupported_protocol_is_untracked() {
+        let bytes = ipv4_packet(HOST, VM, IpProtocol::Unknown(253), 0);
+        let mut tracker = FlowTable::new();
+        assert!(matches!(
+            inspect_from_host(&mut tracker, &bytes),
+            FlowMatch::Untracked
+        ));
+    }
+}
+
+#[cfg(test)]
+mod test_support {
+    use super::{FlowDirection, FlowMatch, FlowTable};
+    use smoltcp::wire::{IpProtocol, Ipv4Address, Ipv4Packet, TcpPacket, UdpPacket};
+
+    pub(super) const HOST: Ipv4Address = Ipv4Address::new(192, 168, 64, 1);
+    pub(super) const VM: Ipv4Address = Ipv4Address::new(192, 168, 64, 2);
+
+    pub(super) fn inspect_from_host(tracker: &mut FlowTable, bytes: &[u8]) -> FlowMatch {
+        let packet = Ipv4Packet::new_checked(bytes).unwrap();
+        tracker.inspect(&packet, FlowDirection::FromHost)
+    }
+
+    pub(super) fn inspect_from_vm(tracker: &mut FlowTable, bytes: &[u8]) -> FlowMatch {
+        let packet = Ipv4Packet::new_checked(bytes).unwrap();
+        tracker.inspect(&packet, FlowDirection::FromVm)
+    }
+
+    #[derive(Clone, Copy)]
+    pub(super) struct TcpFlags {
+        syn: bool,
+        ack: bool,
+        rst: bool,
+        fin: bool,
+    }
+
+    impl TcpFlags {
+        pub(super) const SYN: Self = Self {
+            syn: true,
+            ack: false,
+            rst: false,
+            fin: false,
+        };
+        pub(super) const SYN_ACK: Self = Self {
+            syn: true,
+            ack: true,
+            rst: false,
+            fin: false,
+        };
+        pub(super) const ACK: Self = Self {
+            syn: false,
+            ack: true,
+            rst: false,
+            fin: false,
+        };
+    }
+
+    pub(super) fn tcp_packet(
+        src_addr: Ipv4Address,
+        src_port: u16,
+        dst_addr: Ipv4Address,
+        dst_port: u16,
+        flags: TcpFlags,
+    ) -> Vec<u8> {
+        let mut bytes = ipv4_packet(src_addr, dst_addr, IpProtocol::Tcp, 20);
+        let mut tcp = TcpPacket::new_unchecked(&mut bytes[20..]);
+        tcp.set_src_port(src_port);
+        tcp.set_dst_port(dst_port);
+        tcp.set_header_len(20);
+        tcp.set_syn(flags.syn);
+        tcp.set_ack(flags.ack);
+        tcp.set_rst(flags.rst);
+        tcp.set_fin(flags.fin);
+        tcp.set_window_len(u16::MAX);
+        bytes
+    }
+
+    pub(super) fn udp_packet(
+        src_addr: Ipv4Address,
+        src_port: u16,
+        dst_addr: Ipv4Address,
+        dst_port: u16,
+    ) -> Vec<u8> {
+        let mut bytes = ipv4_packet(src_addr, dst_addr, IpProtocol::Udp, 8);
+        let mut udp = UdpPacket::new_unchecked(&mut bytes[20..]);
+        udp.set_src_port(src_port);
+        udp.set_dst_port(dst_port);
+        udp.set_len(8);
+        bytes
+    }
+
+    pub(super) fn ipv4_packet(
+        src_addr: Ipv4Address,
+        dst_addr: Ipv4Address,
+        protocol: IpProtocol,
+        payload_len: usize,
+    ) -> Vec<u8> {
+        let mut bytes = vec![0; 20 + payload_len];
+        let total_len = bytes.len() as u16;
+        let mut ipv4 = Ipv4Packet::new_unchecked(bytes.as_mut_slice());
+        ipv4.set_version(4);
+        ipv4.set_header_len(20);
+        ipv4.set_total_len(total_len);
+        ipv4.set_next_header(protocol);
+        ipv4.set_src_addr(src_addr);
+        ipv4.set_dst_addr(dst_addr);
+        bytes
+    }
+}
@@ -0,0 +1,156 @@
+use super::{FlowDirection, FlowKey, FlowMatch, FlowTable};
+use coarsetime::{Duration, Instant};
+use smoltcp::wire::{Ipv4Address, Ipv4Packet, TcpPacket};
+
+const SYN_TIMEOUT: Duration = Duration::from_secs(60);
+const TCP_TIMEOUT: Duration = Duration::from_secs(5 * 60);
+
+impl FlowTable {
+    pub(super) fn inspect_tcp(
+        &mut self,
+        ipv4_pkt: &Ipv4Packet<&[u8]>,
+        direction: FlowDirection,
+        now: Instant,
+    ) -> FlowMatch {
+        let Ok(tcp) = TcpPacket::new_checked(ipv4_pkt.payload()) else {
+            return FlowMatch::Denied;
+        };
+        if tcp.src_port() == 0 || tcp.dst_port() == 0 {
+            return FlowMatch::Denied;
+        }
+
+        let key = FlowKey::tcp(
+            direction,
+            (ipv4_pkt.src_addr(), tcp.src_port()),
+            (ipv4_pkt.dst_addr(), tcp.dst_port()),
+        );
+
+        // A bare SYN may be either a retransmission or a new connection reusing
+        // the tuple in either direction. Always return it to policy, and do not
+        // replace an existing permission until the candidate is committed.
+        let is_initial_syn = tcp.syn() && !tcp.ack() && !tcp.fin() && !tcp.rst();
+
+        if is_initial_syn {
+            return FlowMatch::candidate(key, direction, now, SYN_TIMEOUT);
+        }
+
+        self.match_existing_flow(key, direction, now, TCP_TIMEOUT)
+            .unwrap_or(FlowMatch::Untracked)
+    }
+}
+
+impl FlowKey {
+    fn tcp(direction: FlowDirection, src: (Ipv4Address, u16), dst: (Ipv4Address, u16)) -> Self {
+        let ((host_addr, host_port), (vm_addr, vm_port)) = direction.host_vm_pair(src, dst);
+        Self::Tcp {
+            host_addr,
+            host_port,
+            vm_addr,
+            vm_port,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::super::test_support::{
+        HOST, TcpFlags, VM, inspect_from_host, inspect_from_vm, tcp_packet,
+    };
+    use super::super::{FlowMatch, FlowTable};
+
+    fn admit_host_syn(tracker: &mut FlowTable) {
+        let syn = tcp_packet(HOST, 49_152, VM, 22, TcpFlags::SYN);
+        let FlowMatch::Candidate(pending) = inspect_from_host(tracker, &syn) else {
+            panic!("expected a candidate");
+        };
+        assert!(tracker.commit(pending));
+    }
+
+    #[test]
+    fn admitted_syn_creates_an_exact_reverse_permission() {
+        let mut tracker = FlowTable::new();
+        admit_host_syn(&mut tracker);
+        let original_expiry = tracker.flows.values().next().unwrap().expires_at;
+
+        let reply = tcp_packet(VM, 22, HOST, 49_152, TcpFlags::ACK);
+        assert!(matches!(
+            inspect_from_vm(&mut tracker, &reply),
+            FlowMatch::Allowed
+        ));
+        assert_eq!(
+            tracker.flows.values().next().unwrap().expires_at,
+            original_expiry,
+            "return traffic must not refresh a tuple"
+        );
+
+        let initiator_data = tcp_packet(HOST, 49_152, VM, 22, TcpFlags::ACK);
+        let FlowMatch::Candidate(pending) = inspect_from_host(&mut tracker, &initiator_data) else {
+            panic!("expected an initiator candidate");
+        };
+        assert_eq!(
+            tracker.flows.values().next().unwrap().expires_at,
+            original_expiry,
+            "inspection must not refresh a tuple before policy accepts it"
+        );
+        assert!(tracker.commit(pending));
+        assert!(tracker.flows.values().next().unwrap().expires_at > original_expiry);
+    }
+
+    #[test]
+    fn new_tuple_requires_a_clean_syn() {
+        let mut tracker = FlowTable::new();
+
+        for flags in [TcpFlags::ACK, TcpFlags::SYN_ACK] {
+            let packet = tcp_packet(HOST, 49_152, VM, 22, flags);
+            assert!(matches!(
+                inspect_from_host(&mut tracker, &packet),
+                FlowMatch::Untracked
+            ));
+        }
+
+        let syn = tcp_packet(HOST, 49_152, VM, 22, TcpFlags::SYN);
+        assert!(matches!(
+            inspect_from_host(&mut tracker, &syn),
+            FlowMatch::Candidate(_)
+        ));
+    }
+
+    #[test]
+    fn reply_requires_the_exact_tuple() {
+        let mut tracker = FlowTable::new();
+        admit_host_syn(&mut tracker);
+
+        let wrong_port = tcp_packet(VM, 22, HOST, 49_153, TcpFlags::ACK);
+        assert!(matches!(
+            inspect_from_vm(&mut tracker, &wrong_port),
+            FlowMatch::Untracked
+        ));
+    }
+
+    #[test]
+    fn reverse_bare_syn_is_a_new_policy_candidate() {
+        let mut tracker = FlowTable::new();
+        admit_host_syn(&mut tracker);
+
+        let reverse_syn = tcp_packet(VM, 22, HOST, 49_152, TcpFlags::SYN);
+        let FlowMatch::Candidate(pending) = inspect_from_vm(&mut tracker, &reverse_syn) else {
+            panic!("reverse SYN must return to policy");
+        };
+
+        // Inspection is provisional: a policy rejection leaves the admitted
+        // flow untouched.
+        let old_flow_reply = tcp_packet(VM, 22, HOST, 49_152, TcpFlags::ACK);
+        assert!(matches!(
+            inspect_from_vm(&mut tracker, &old_flow_reply),
+            FlowMatch::Allowed
+        ));
+
+        assert!(tracker.commit(pending));
+
+        let reverse_reply = tcp_packet(HOST, 49_152, VM, 22, TcpFlags::SYN_ACK);
+        assert!(matches!(
+            inspect_from_host(&mut tracker, &reverse_reply),
+            FlowMatch::Allowed
+        ));
+    }
+}
@@ -0,0 +1,92 @@
+use super::{FlowDirection, FlowKey, FlowMatch, FlowTable};
+use coarsetime::{Duration, Instant};
+use smoltcp::wire::{Ipv4Address, Ipv4Packet, UdpPacket};
+
+const UDP_TIMEOUT: Duration = Duration::from_secs(30);
+
+impl FlowTable {
+    pub(super) fn inspect_udp(
+        &mut self,
+        ipv4_pkt: &Ipv4Packet<&[u8]>,
+        direction: FlowDirection,
+        now: Instant,
+    ) -> FlowMatch {
+        let Ok(udp) = UdpPacket::new_checked(ipv4_pkt.payload()) else {
+            return FlowMatch::Denied;
+        };
+        if udp.dst_port() == 0 {
+            return FlowMatch::Denied;
+        }
+
+        // Deliberate policy, not merely a packet-format validation:
+        // RFC 8085, §5.1 says UDP senders SHOULD NOT use source port zero.
+        //
+        // We enforce this recommendation to retain source-port entropy
+        // and protection against off-path packet injection.
+        if udp.src_port() == 0 {
+            return FlowMatch::Denied;
+        }
+
+        let key = FlowKey::udp(
+            direction,
+            (ipv4_pkt.src_addr(), udp.src_port()),
+            (ipv4_pkt.dst_addr(), udp.dst_port()),
+        );
+
+        if let Some(matched) = self.match_existing_flow(key, direction, now, UDP_TIMEOUT) {
+            return matched;
+        }
+
+        FlowMatch::candidate(key, direction, now, UDP_TIMEOUT)
+    }
+}
+
+impl FlowKey {
+    fn udp(direction: FlowDirection, src: (Ipv4Address, u16), dst: (Ipv4Address, u16)) -> Self {
+        let ((host_addr, host_port), (vm_addr, vm_port)) = direction.host_vm_pair(src, dst);
+        Self::Udp {
+            host_addr,
+            host_port,
+            vm_addr,
+            vm_port,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::super::test_support::{HOST, VM, inspect_from_host, inspect_from_vm, udp_packet};
+    use super::super::{FlowMatch, FlowTable};
+
+    #[test]
+    fn rejects_zero_source_port_per_rfc_8085() {
+        let mut tracker = FlowTable::new();
+        let datagram = udp_packet(HOST, 0, VM, 5353);
+
+        assert!(matches!(
+            inspect_from_host(&mut tracker, &datagram),
+            FlowMatch::Denied
+        ));
+    }
+
+    #[test]
+    fn udp_reply_requires_an_exact_admitted_request() {
+        let mut tracker = FlowTable::new();
+        let request = udp_packet(HOST, 50_000, VM, 5353);
+        let reply = udp_packet(VM, 5353, HOST, 50_000);
+
+        assert!(matches!(
+            inspect_from_vm(&mut tracker, &reply),
+            FlowMatch::Candidate(_)
+        ));
+
+        let FlowMatch::Candidate(pending) = inspect_from_host(&mut tracker, &request) else {
+            panic!("expected a candidate");
+        };
+        assert!(tracker.commit(pending));
+        assert!(matches!(
+            inspect_from_vm(&mut tracker, &reply),
+            FlowMatch::Allowed
+        ));
+    }
+}
@@ -1,7 +1,9 @@
-use crate::proxy::Proxy;
-use crate::proxy::udp_packet_helper::UdpPacketHelper;
+use crate::proxy::flows::{FlowDirection, FlowMatch};
+use crate::proxy::{Direction, PolicyDecision, Proxy};
 use anyhow::{Context, Result};
-use smoltcp::wire::{EthernetFrame, EthernetProtocol, Ipv4Packet, UdpPacket};
+use dhcproto::v4::{CLIENT_PORT, SERVER_PORT};
+use smoltcp::phy::ChecksumCapabilities;
+use smoltcp::wire::{EthernetFrame, EthernetProtocol, IpProtocol, Ipv4Packet, Ipv4Repr, UdpPacket};

 impl Proxy<'_> {
    pub(crate) fn process_frame_from_host(&mut self, frame: &EthernetFrame<&[u8]>) -> Result<()> {
@@ -10,13 +12,11 @@ impl Proxy<'_> {
            return Ok(());
        }

-        // Snoop bootpd(8) replies from the host to
-        // figure out the IP assigned to the VM
-        if frame.dst_addr() == self.vm_mac_address {
-            self.snoop(frame);
-        }
+        self.write_to_vm(frame.as_ref())
+    }

-        match self.vm.write(frame.as_ref()) {
+    pub(super) fn write_to_vm(&mut self, packet: &[u8]) -> Result<()> {
+        match self.vm.write(packet) {
            Ok(_) => Ok(()),
            Err(err) => {
                if let Some(libc::ENOBUFS) = err.raw_os_error() {
@@ -39,38 +39,69 @@ impl Proxy<'_> {
    fn allowed_from_host(&mut self, frame: &EthernetFrame<&[u8]>) -> Option<()> {
        match frame.ethertype() {
            EthernetProtocol::Arp => Some(()),
-            EthernetProtocol::Ipv4 => Some(()),
+            EthernetProtocol::Ipv4 => {
+                let ipv4_pkt = Ipv4Packet::new_unchecked(frame.payload());
+                Ipv4Repr::parse(&ipv4_pkt, &ChecksumCapabilities::ignored()).ok()?;
+
+                self.allowed_from_host_ipv4(&ipv4_pkt)
+            }
            _ => None,
        }
    }

-    fn snoop(&mut self, frame: &EthernetFrame<&[u8]>) {
-        if frame.ethertype() != EthernetProtocol::Ipv4 {
-            return;
+    pub(super) fn allowed_from_host_ipv4(&mut self, ipv4_pkt: &Ipv4Packet<&[u8]>) -> Option<()> {
+        // Drop external DHCP replies, including malformed and fragmented replies
+        if ipv4_pkt.next_header() == IpProtocol::Udp
+            && ipv4_pkt.frag_offset() == 0
+            && ipv4_pkt.payload().len() >= 4
+        {
+            let udp = UdpPacket::new_unchecked(ipv4_pkt.payload());
+            if udp.src_port() == SERVER_PORT && udp.dst_port() == CLIENT_PORT {
+                return None;
+            }
        }

-        let ipv4_pkt = match Ipv4Packet::new_checked(frame.payload()) {
-            Ok(ipv4_pkt) => ipv4_pkt,
-            _ => return,
+        // Backwards compatibility with Softnet consumers that only use stateless rules
+        if self.flows.is_none() {
+            return Some(());
+        }
+
+        // Consult the flow table before evaluating inbound policy
+        // so established flows are not treated as new traffic
+        let pending = if self.host.vm_ip == ipv4_pkt.dst_addr() {
+            match self
+                .flows
+                .as_mut()?
+                .inspect(ipv4_pkt, FlowDirection::FromHost)
+            {
+                FlowMatch::Allowed => return Some(()),
+                FlowMatch::Denied => return None,
+                FlowMatch::Candidate(pending) => Some(pending),
+                FlowMatch::Untracked => None,
+            }
+        } else {
+            None
        };

-        if ipv4_pkt.src_addr() != self.host.gateway_ip {
-            return;
+        // The flow is either pending or untracked, evaluate it against inbound policy
+        match self
+            .rules
+            .policy_decision(ipv4_pkt.src_addr(), Direction::In)
+        {
+            // Return traffic was handled above; enforce explicit inbound blocks here
+            Some(PolicyDecision::Block) => None,
+
+            // Stateless policy is outbound-only; fail closed if this invariant is violated
+            Some(PolicyDecision::AllowStateless) => None,
+
+            // Untracked packets cannot satisfy stateful policy
+            Some(PolicyDecision::AllowStateful) => self.admit_with_tracking(pending?),
+
+            // No inbound rule matched, so allow by default. Track the flow when needed
+            // so its reply is not treated as a new outbound flow
+            None => {
+                self.admit_with_tracking_if_stateful(pending, ipv4_pkt.src_addr(), Direction::Out)
+            }
        }
-
-        if ipv4_pkt.next_header() != smoltcp::wire::IpProtocol::Udp {
-            return;
-        }
-
-        let udp_pkt = match UdpPacket::new_checked(ipv4_pkt.payload()) {
-            Ok(udp_pkt) => udp_pkt,
-            Err(_) => return,
-        };
-
-        if !udp_pkt.is_dhcp_response() {
-            return;
-        }
-
-        self.dhcp_snooper.register_dhcp_reply(udp_pkt.payload());
    }
 }
@@ -1,24 +1,27 @@
+mod control;
 mod exposed_port;
+mod flows;
 mod host;
-mod port_forwarder;
-mod udp_packet_helper;
+mod rule;
+mod rules;
 mod vm;

-use crate::dhcp_snooper::DhcpSnooper;
+use crate::dhcp_server::{DHCP_SERVER_MAC, DhcpServer};
 use crate::host::Host;
 use crate::host::NetType;
 use crate::poller::Poller;
 use crate::vm::VM;
-use anyhow::Result;
+use anyhow::{Context, Result, bail};
+use control::{Control, normalize_rules};
 pub use exposed_port::ExposedPort;
+use flows::{FlowTable, PendingFlow};
 use ipnet::Ipv4Net;
 use mac_address::MacAddress;
-use port_forwarder::PortForwarder;
-use prefix_trie::{Prefix, PrefixMap};
-use smoltcp::wire::EthernetFrame;
+pub use rule::{Direction, Rule, Target};
+pub(crate) use rules::{PolicyDecision, Rules};
+use smoltcp::wire::{EthernetAddress, EthernetFrame, Ipv4Address};
 use std::io::ErrorKind;
 use std::os::unix::io::{AsRawFd, RawFd};
-use std::str::FromStr;
 use std::time::Duration;
 use vmnet::Batch;

@@ -27,34 +30,11 @@ pub struct Proxy<'proxy> {
    host: Host,
    poller: Poller<'proxy>,
    vm_mac_address: smoltcp::wire::EthernetAddress,
-    dhcp_snooper: DhcpSnooper,
-    rules: PrefixMap<Ipv4Net, Action>,
+    dhcp_server: DhcpServer,
+    rules: Rules,
+    control: Option<Control>,
+    flows: Option<FlowTable>,
    enobufs_encountered: bool,
-    port_forwarder: PortForwarder,
-}
-
-#[derive(Debug, Clone, PartialEq, Eq)]
-pub enum Target {
-    Prefix(Ipv4Net),
-    Host,
-}
-
-impl FromStr for Target {
-    type Err = ipnet::AddrParseError;
-
-    fn from_str(s: &str) -> std::result::Result<Self, Self::Err> {
-        if s == "@host" {
-            return Ok(Target::Host);
-        }
-
-        Ipv4Net::from_str(s).map(Target::Prefix)
-    }
-}
-
-#[derive(Debug, Clone, PartialEq)]
-pub(crate) enum Action {
-    Block,
-    Allow,
 }

 impl Proxy<'_> {
@@ -62,51 +42,70 @@ impl Proxy<'_> {
        vm_fd: RawFd,
        vm_mac_address: MacAddress,
        vm_net_type: NetType,
-        allow: Vec<Target>,
-        block: Vec<Target>,
+        allow: Vec<Rule>,
+        block: Vec<Rule>,
        exposed_ports: Vec<ExposedPort>,
+        control_fd: Option<RawFd>,
    ) -> Result<Proxy<'proxy>> {
-        let vm = VM::new(vm_fd)?;
-        let host = Host::new(
-            vm_net_type,
-            !allow.contains(&Target::Prefix(Ipv4Net::zero())),
-        )?;
-        let poller_timeout = Duration::from_millis(100);
-        let poller = Poller::new(vm.as_raw_fd(), host.as_raw_fd(), poller_timeout)?;
-
-        // Craft packet filter rules
-        //
-        // SECURITY: blocking rules must always take precedence
-        // over allowing rules when prefixes are identical.
-        let mut rules = PrefixMap::new();
-
-        for allow_target in allow {
-            let allow_prefix = match allow_target {
-                Target::Prefix(prefix) => prefix,
-                Target::Host => host.gateway_ip.into(),
-            };
-
-            rules.insert(allow_prefix, Action::Allow);
+        // Ensure that VM's MAC address won't conflict with our DHCP server's MAC address
+        let vm_mac_address = EthernetAddress(vm_mac_address.bytes());
+        if vm_mac_address == DHCP_SERVER_MAC {
+            bail!("VM MAC address {vm_mac_address} is reserved for the DHCP server");
        }

-        for block_target in block {
-            let block_prefix = match block_target {
-                Target::Prefix(prefix) => prefix,
-                Target::Host => host.gateway_ip.into(),
-            };
+        let allow = normalize_rules(allow);
+        let block = normalize_rules(block);

-            rules.insert(block_prefix, Action::Block);
+        let vm = VM::new(vm_fd)?;
+        let mut host = Host::new(
+            vm_net_type,
+            vm_mac_address,
+            !allow.contains(&Rule::Stateless(Target::Prefix(Ipv4Net::default()))),
+        )?;
+        let poller_timeout = Duration::from_millis(100);
+        let control = control_fd
+            .map(|control_fd| {
+                Control::new(control_fd, host.gateway_ip, allow.clone(), block.clone())
+            })
+            .transpose()?;
+        let poller = Poller::new(
+            vm.as_raw_fd(),
+            host.as_raw_fd(),
+            control.as_ref().map(AsRawFd::as_raw_fd),
+            poller_timeout,
+        )?;
+
+        let rules = Rules::new(host.gateway_ip, &allow, &block);
+
+        // Any stateful rule enables flow inspection for the whole VM, including
+        // traffic admitted through implicit global and gateway fallbacks
+        let flows = rules.has_stateful().then(FlowTable::new);
+
+        // vmnet owns the gateway (including ARP and DNS); the VM address is
+        // reserved for this MAC for the lifetime of the attached interface.
+        let dhcp_server = DhcpServer::new(
+            vm_mac_address,
+            host.vm_ip,
+            host.subnet_mask,
+            host.gateway_ip,
+        );
+
+        // Install port forwardings for the reserved VM's IP address
+        for port in exposed_ports {
+            host.port_forwarding_add_rule(port.external_port, host.vm_ip, port.internal_port)
+                .context("failed to configure port forwarding")?;
        }

        Ok(Proxy {
            vm,
            host,
            poller,
-            vm_mac_address: smoltcp::wire::EthernetAddress(vm_mac_address.bytes()),
-            dhcp_snooper: DhcpSnooper::new(poller_timeout),
+            vm_mac_address,
+            dhcp_server,
            rules,
+            control,
+            flows,
            enobufs_encountered: false,
-            port_forwarder: PortForwarder::new(exposed_ports),
        })
    }

@@ -126,9 +125,13 @@ impl Proxy<'_> {
        loop {
            let (vm_readable, host_readable, interrupt) = self.poller.wait()?;

-            // Update coarse time for the DHCP snooper
+            // Update coarse time for flows
            coarsetime::Instant::update();

+            // Service control on every wake (including timeouts) so a bounded read or a pending
+            // response continues making progress even when no new edge is generated.
+            self.service_control();
+
            if vm_readable {
                self.read_from_vm(buf.as_mut_slice())?;
            }
@@ -142,10 +145,12 @@ impl Proxy<'_> {
                return Ok(());
            }

-            // Timeout
-            if !vm_readable && !host_readable && !interrupt {
-                self.port_forwarder
-                    .tick(&mut self.host, self.dhcp_snooper.lease());
+            // Periodic maintenance
+            if self.poller.periodic_tick_due() {
+                // kqueue(2) does not report peer disconnects for Unix datagram sockets
+                if !self.vm.is_connected()? {
+                    return Ok(());
+                }
            }

            self.poller.rearm();
@@ -153,15 +158,23 @@ impl Proxy<'_> {
    }

    fn read_from_vm(&mut self, buf: &mut [u8]) -> Result<()> {
+        let mut packets_read = 0;
+
        loop {
            match self.vm.read(buf) {
                Ok(n) => {
-                    // Update coarse time for the DHCP snooper
+                    // Update coarse time for flows
                    coarsetime::Instant::update();

                    if let Ok(frame) = EthernetFrame::new_checked(&buf[..n]) {
                        self.process_frame_from_vm(frame)?;
                    }
+
+                    packets_read += 1;
+                    if packets_read == 128 {
+                        self.service_control();
+                        packets_read = 0;
+                    }
                }
                Err(err) => {
                    if err.kind() == ErrorKind::WouldBlock {
@@ -178,7 +191,7 @@ impl Proxy<'_> {
        loop {
            match self.host.read(batch, bufs) {
                Ok(pktcnt) => {
-                    // Update coarse time for the DHCP snooper
+                    // Update coarse time for flows
                    coarsetime::Instant::update();

                    for buf in batch.packet_sized_bufs(bufs).take(pktcnt) {
@@ -186,6 +199,8 @@ impl Proxy<'_> {
                            self.process_frame_from_host(&pkt)?;
                        }
                    }
+
+                    self.service_control();
                }
                Err(err) => {
                    if let vmnet::Error::VmnetReadNothing = err {
@@ -197,82 +212,177 @@ impl Proxy<'_> {
            }
        }
    }
+
+    fn service_control(&mut self) {
+        let Some(control) = self.control.as_mut() else {
+            return;
+        };
+
+        let keep_open = match control.service(&mut self.rules) {
+            Ok(keep_open) => keep_open,
+            Err(err) => {
+                log::warn!("disabling Softnet control socket: {err:#}");
+                false
+            }
+        };
+
+        // Invalidate tracked flows whenever the policy changes
+        if control.policy_changed() {
+            self.flows = self.rules.has_stateful().then(FlowTable::new);
+        }
+
+        if keep_open {
+            return;
+        }
+
+        if let Err(err) = self.poller.remove_control() {
+            log::warn!("failed to remove Softnet control socket from the poller: {err:#}");
+        }
+
+        if let Some(control) = self.control.take()
+            && let Err(err) = control.shutdown()
+        {
+            log::warn!("failed to shut down Softnet control socket: {err:#}");
+        }
+    }
+
+    /// Commits the pending flow, rejecting the packet if the table cannot store it.
+    fn admit_with_tracking(&mut self, pending: PendingFlow) -> Option<()> {
+        self.flows.as_mut()?.commit(pending).then_some(())
+    }
+
+    /// Commits a pending flow for trackable packets; untracked packets proceed without one.
+    fn admit_with_tracking_if_trackable(&mut self, pending: Option<PendingFlow>) -> Option<()> {
+        match pending {
+            Some(pending) => self.admit_with_tracking(pending),
+            None => Some(()),
+        }
+    }
+
+    /// Commits a pending flow when the return-direction rule is stateful.
+    fn admit_with_tracking_if_stateful(
+        &mut self,
+        pending: Option<PendingFlow>,
+        peer_addr: Ipv4Address,
+        return_direction: Direction,
+    ) -> Option<()> {
+        if self.rules.is_stateful(peer_addr, return_direction) {
+            self.admit_with_tracking_if_trackable(pending)
+        } else {
+            Some(())
+        }
+    }
 }

 #[cfg(test)]
 mod tests {
    use crate::NetType;
-    use crate::dhcp_snooper::Lease;
-    use crate::proxy::{Action, Proxy};
-    use ipnet::Ipv4Net;
+    use crate::proxy::Proxy;
    use mac_address::MacAddress;
    use nix::sys::socket::{AddressFamily, SockFlag, SockType, socketpair};
-    use prefix_trie::PrefixMap;
    use serial_test::serial;
-    use smoltcp::wire::{Ipv4Address, Ipv4Packet};
-    use std::collections::HashSet;
+    use smoltcp::wire::{IpProtocol, Ipv4Address, Ipv4Packet, UdpPacket};
    use std::os::fd::AsRawFd;
    use std::str::FromStr;
-    use std::time::Duration;

    #[test]
    #[serial]
    fn test_blocking_takes_precedence() {
-        let vm_ip = Ipv4Address::from_str("192.168.0.2").unwrap();
-        let proxy = create_proxy(vm_ip, vec!["66.66.0.0/16"], vec!["66.66.0.0/16"]);
+        let mut proxy = create_proxy(vec!["66.66.0.0/16"], vec!["66.66.0.0/16"]);
+        let vm_ip = proxy.host.vm_ip;

-        assert_eq!(
-            proxy.rules,
-            PrefixMap::<Ipv4Net, Action>::from_iter(vec![(
-                Ipv4Net::from_str("66.66.0.0/16").unwrap(),
-                Action::Block
-            ),])
-        );
+        assert_eq!(proxy.rules.len(), 1);

-        assert!(allowed_from_vm_ipv4(&proxy, vm_ip, "66.66.66.66").is_none());
+        assert!(allowed_from_vm_ipv4(&mut proxy, vm_ip, "66.66.66.66").is_none());
    }

    #[test]
    #[serial]
    fn test_longest_prefix_match_wins() {
-        let vm_ip = Ipv4Address::from_str("192.168.0.2").unwrap();
-        let proxy = create_proxy(vm_ip, vec!["33.33.33.33/32"], vec!["33.33.33.0/24"]);
+        let mut proxy = create_proxy(vec!["33.33.33.33/32"], vec!["33.33.33.0/24"]);
+        let vm_ip = proxy.host.vm_ip;

-        assert_eq!(
-            proxy.rules,
-            PrefixMap::<Ipv4Net, Action>::from_iter(vec![
-                (Ipv4Net::from_str("33.33.33.33/32").unwrap(), Action::Allow),
-                (Ipv4Net::from_str("33.33.33.0/24").unwrap(), Action::Block),
-            ])
-        );
+        assert_eq!(proxy.rules.len(), 2);

-        assert!(allowed_from_vm_ipv4(&proxy, vm_ip, "33.33.33.32").is_none());
-        assert!(allowed_from_vm_ipv4(&proxy, vm_ip, "33.33.33.33").is_some());
-        assert!(allowed_from_vm_ipv4(&proxy, vm_ip, "33.33.33.34").is_none());
+        assert!(allowed_from_vm_ipv4(&mut proxy, vm_ip, "33.33.33.32").is_none());
+        assert!(allowed_from_vm_ipv4(&mut proxy, vm_ip, "33.33.33.33").is_some());
+        assert!(allowed_from_vm_ipv4(&mut proxy, vm_ip, "33.33.33.34").is_none());
    }

    #[test]
    #[serial]
    fn test_allow_host() {
-        let vm_ip = Ipv4Address::from_str("192.168.0.2").unwrap();
-        let proxy = create_proxy(vm_ip, vec!["@host"], vec!["0.0.0.0/0"]);
+        let mut proxy = create_proxy(vec!["@host"], vec!["0.0.0.0/0"]);
+        let vm_ip = proxy.host.vm_ip;

-        assert_eq!(
-            proxy.rules,
-            PrefixMap::from_iter(vec![
-                (proxy.host.gateway_ip.into(), Action::Allow),
-                (Ipv4Net::from_str("0.0.0.0/0").unwrap(), Action::Block),
-            ])
-        );
+        assert_eq!(proxy.rules.len(), 2);

        // Access to global IPs should be disallowed because of --block=0.0.0.0/0
-        assert!(allowed_from_vm_ipv4(&proxy, vm_ip, "8.8.8.8").is_none());
+        assert!(allowed_from_vm_ipv4(&mut proxy, vm_ip, "8.8.8.8").is_none());

        // Despite the above, access to host IP address should be possible because of --allow=@host
-        assert!(allowed_from_vm_ipv4(&proxy, vm_ip, &proxy.host.gateway_ip.to_string()).is_some());
+        let gateway_ip = proxy.host.gateway_ip.to_string();
+        assert!(allowed_from_vm_ipv4(&mut proxy, vm_ip, &gateway_ip).is_some());
    }

-    fn create_proxy<'test>(vm_ip: Ipv4Address, allow: Vec<&str>, block: Vec<&str>) -> Proxy<'test> {
+    #[test]
+    #[serial]
+    fn test_bare_default_block_applies_in_both_directions_in_stateful_mode() {
+        let mut proxy = create_proxy(vec!["in 192.0.2.0/24"], vec!["0.0.0.0/0"]);
+        let vm_ip = proxy.host.vm_ip;
+        let fallback_peer = Ipv4Address::new(203, 0, 113, 1);
+        let explicitly_allowed_peer = Ipv4Address::new(192, 0, 2, 1);
+
+        let fallback_request = udp_packet(fallback_peer, 40_000, vm_ip, 1_234);
+        let fallback_request = Ipv4Packet::new_checked(fallback_request.as_slice()).unwrap();
+        assert!(proxy.allowed_from_host_ipv4(&fallback_request).is_none());
+
+        let fallback_reply = udp_packet(vm_ip, 1_234, fallback_peer, 40_000);
+        let fallback_reply = Ipv4Packet::new_checked(fallback_reply.as_slice()).unwrap();
+        assert!(proxy.allowed_from_vm_ipv4(fallback_reply).is_none());
+
+        let allowed_request = udp_packet(explicitly_allowed_peer, 40_000, vm_ip, 1_234);
+        let allowed_request = Ipv4Packet::new_checked(allowed_request.as_slice()).unwrap();
+        assert!(proxy.allowed_from_host_ipv4(&allowed_request).is_some());
+
+        let allowed_reply = udp_packet(vm_ip, 1_234, explicitly_allowed_peer, 40_000);
+        let allowed_reply = Ipv4Packet::new_checked(allowed_reply.as_slice()).unwrap();
+        assert!(proxy.allowed_from_vm_ipv4(allowed_reply).is_some());
+    }
+
+    #[test]
+    #[serial]
+    fn test_directional_egress_block_does_not_block_reply_to_unmatched_inbound_flow() {
+        let mut proxy = create_proxy(vec![], vec!["out 203.0.113.0/24"]);
+        let vm_ip = proxy.host.vm_ip;
+        let peer = Ipv4Address::new(203, 0, 113, 1);
+
+        let request = udp_packet(peer, 40_000, vm_ip, 1_234);
+        let request = Ipv4Packet::new_checked(request.as_slice()).unwrap();
+        assert!(proxy.allowed_from_host_ipv4(&request).is_some());
+
+        let reply = udp_packet(vm_ip, 1_234, peer, 40_000);
+        let reply = Ipv4Packet::new_checked(reply.as_slice()).unwrap();
+        assert!(proxy.allowed_from_vm_ipv4(reply).is_some());
+    }
+
+    #[test]
+    #[serial]
+    fn test_directional_ingress_block_does_not_block_reply_to_bare_outbound_allow() {
+        let mut proxy = create_proxy(vec!["203.0.113.0/24"], vec!["in 203.0.113.0/24"]);
+        let vm_ip = proxy.host.vm_ip;
+        let peer = Ipv4Address::new(203, 0, 113, 1);
+
+        let request = udp_packet(vm_ip, 1_234, peer, 40_000);
+        let request = Ipv4Packet::new_checked(request.as_slice()).unwrap();
+        assert!(proxy.allowed_from_vm_ipv4(request).is_some());
+
+        let reply = udp_packet(peer, 40_000, vm_ip, 1_234);
+        let reply = Ipv4Packet::new_checked(reply.as_slice()).unwrap();
+        assert!(proxy.allowed_from_host_ipv4(&reply).is_some());
+    }
+
+    fn create_proxy<'test>(allow: Vec<&str>, block: Vec<&str>) -> Proxy<'test> {
        let (vm_fd, _) = socketpair(
            AddressFamily::Unix,
            SockType::Datagram,
@@ -282,32 +392,25 @@ mod tests {
        .unwrap();
        let vm_fd = Box::leak(Box::new(vm_fd));

-        let mut proxy = Proxy::new(
+        Proxy::new(
            vm_fd.as_raw_fd(),
-            MacAddress::from_str("02:00:00:00:00:01").unwrap(),
+            MacAddress::from_str("02:00:00:00:00:02").unwrap(),
            NetType::Nat,
            allow
                .into_iter()
-                .map(|cidr| cidr.parse().unwrap())
+                .map(|value| value.parse().unwrap())
                .collect(),
            block
                .into_iter()
-                .map(|cidr| cidr.parse().unwrap())
+                .map(|value| value.parse().unwrap())
                .collect(),
            Vec::default(),
+            None,
        )
-        .unwrap();
-
-        proxy.dhcp_snooper.set_lease(Some(Lease::new(
-            vm_ip,
-            Duration::from_secs(600),
-            HashSet::new(),
-        )));
-
-        proxy
+        .unwrap()
    }

-    fn allowed_from_vm_ipv4(proxy: &Proxy, src: Ipv4Address, dst: &str) -> Option<()> {
+    fn allowed_from_vm_ipv4(proxy: &mut Proxy, src: Ipv4Address, dst: &str) -> Option<()> {
        let mut buf = vec![0; 1500];

        let mut ipv4_pkt_mut = Ipv4Packet::new_unchecked(&mut buf[..]);
@@ -318,4 +421,26 @@ mod tests {

        proxy.allowed_from_vm_ipv4(ipv4_pkt)
    }
+
+    fn udp_packet(
+        src_addr: Ipv4Address,
+        src_port: u16,
+        dst_addr: Ipv4Address,
+        dst_port: u16,
+    ) -> Vec<u8> {
+        let mut bytes = vec![0; 28];
+        let mut ipv4 = Ipv4Packet::new_unchecked(bytes.as_mut_slice());
+        ipv4.set_version(4);
+        ipv4.set_header_len(20);
+        ipv4.set_total_len(28);
+        ipv4.set_next_header(IpProtocol::Udp);
+        ipv4.set_src_addr(src_addr);
+        ipv4.set_dst_addr(dst_addr);
+
+        let mut udp = UdpPacket::new_unchecked(ipv4.payload_mut());
+        udp.set_src_port(src_port);
+        udp.set_dst_port(dst_port);
+        udp.set_len(8);
+        bytes
+    }
 }
@@ -1,99 +0,0 @@
-use crate::dhcp_snooper::Lease;
-use crate::host::Host;
-use crate::proxy::exposed_port::ExposedPort;
-use anyhow::Result;
-use log::error;
-use std::net::Ipv4Addr;
-
-#[derive(Default)]
-pub struct PortForwarder {
-    port_forwardings: Vec<PortForwarding>,
-    failed: bool,
-}
-
-#[derive(Debug, Clone, Copy, Default)]
-struct PortForwarding {
-    exposed_port: ExposedPort,
-    forwarding_to_addr: Option<Ipv4Addr>,
-}
-
-impl PortForwarder {
-    pub fn new(exposed_ports: Vec<ExposedPort>) -> PortForwarder {
-        let port_forwardings = exposed_ports
-            .into_iter()
-            .map(|exposed_port| PortForwarding {
-                exposed_port,
-                ..Default::default()
-            })
-            .collect();
-
-        PortForwarder {
-            port_forwardings,
-            ..Default::default()
-        }
-    }
-
-    pub fn tick(&mut self, host: &mut Host, lease: &Option<Lease>) {
-        if self.failed {
-            return;
-        }
-
-        if let Err(err) = self.tick_inner(host, lease) {
-            error!("port-forwarding failed: {}", err);
-
-            self.failed = true;
-        }
-    }
-
-    fn tick_inner(&mut self, host: &mut Host, lease: &Option<Lease>) -> Result<()> {
-        if let Some(lease) = lease {
-            // Lease exists, but is not valid, remove all port forwardings
-            if !lease.valid() {
-                self.remove_all_port_forwardings(host)?;
-
-                return Ok(());
-            }
-
-            // Lease exists and is valid, install/re-install port forwardings
-            for port_forwarding in &mut self.port_forwardings {
-                if let Some(installed_addr) = port_forwarding.forwarding_to_addr {
-                    // Port forwarding already installed, perhaps it's outdated?
-                    if installed_addr == lease.address() {
-                        // Nope, the port forwarding is up to date
-                        continue;
-                    }
-
-                    // Remove port forwarding since the lease address had changed
-                    host.port_forwarding_remove_rule(port_forwarding.exposed_port.external_port)?;
-                    port_forwarding.forwarding_to_addr = None;
-                }
-
-                // Install new port forwarding
-                host.port_forwarding_add_rule(
-                    port_forwarding.exposed_port.external_port,
-                    lease.address(),
-                    port_forwarding.exposed_port.internal_port,
-                )?;
-                port_forwarding.forwarding_to_addr = Some(lease.address());
-            }
-        } else {
-            // Lease does not exist, remove all port forwardings
-            self.remove_all_port_forwardings(host)?;
-        }
-
-        Ok(())
-    }
-
-    fn remove_all_port_forwardings(&mut self, host: &mut Host) -> Result<()> {
-        for port_forwarding in &mut self.port_forwardings {
-            if port_forwarding.forwarding_to_addr.is_none() {
-                continue;
-            }
-
-            host.port_forwarding_remove_rule(port_forwarding.exposed_port.external_port)?;
-            port_forwarding.forwarding_to_addr = None;
-        }
-
-        Ok(())
-    }
-}
@@ -0,0 +1,158 @@
+use ipnet::Ipv4Net;
+use std::fmt::{self, Display, Formatter};
+use std::str::FromStr;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Rule {
+    Stateless(Target),
+    Stateful {
+        direction: Direction,
+        target: Target,
+    },
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Target {
+    Prefix(Ipv4Net),
+    Host,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Direction {
+    In,
+    Out,
+}
+
+impl Rule {
+    pub(super) fn normalized(self) -> Self {
+        match self {
+            Rule::Stateless(target) => Rule::Stateless(target.normalized()),
+            Rule::Stateful { direction, target } => Rule::Stateful {
+                direction,
+                target: target.normalized(),
+            },
+        }
+    }
+}
+
+impl FromStr for Rule {
+    type Err = ipnet::AddrParseError;
+
+    fn from_str(input: &str) -> Result<Self, Self::Err> {
+        let (direction, target) = if let Some(target) = input.strip_prefix("in ") {
+            (Direction::In, target)
+        } else if let Some(target) = input.strip_prefix("out ") {
+            (Direction::Out, target)
+        } else {
+            return input.parse().map(Rule::Stateless);
+        };
+
+        let target = target.trim_start_matches(' ').parse()?;
+        Ok(Rule::Stateful { direction, target })
+    }
+}
+
+impl Display for Rule {
+    fn fmt(&self, formatter: &mut Formatter<'_>) -> fmt::Result {
+        match self {
+            Rule::Stateless(target) => Display::fmt(target, formatter),
+            Rule::Stateful { direction, target } => match direction {
+                Direction::In => write!(formatter, "in {target}"),
+                Direction::Out => write!(formatter, "out {target}"),
+            },
+        }
+    }
+}
+
+impl Target {
+    fn normalized(self) -> Self {
+        match self {
+            Target::Prefix(prefix) => Target::Prefix(prefix.trunc()),
+            Target::Host => Target::Host,
+        }
+    }
+}
+
+impl FromStr for Target {
+    type Err = ipnet::AddrParseError;
+
+    fn from_str(input: &str) -> Result<Self, Self::Err> {
+        if input == "@host" {
+            Ok(Target::Host)
+        } else {
+            input.parse().map(Target::Prefix)
+        }
+    }
+}
+
+impl Display for Target {
+    fn fmt(&self, formatter: &mut Formatter<'_>) -> fmt::Result {
+        match self {
+            Target::Prefix(prefix) => Display::fmt(prefix, formatter),
+            Target::Host => formatter.write_str("@host"),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{Direction, Rule, Target};
+    use ipnet::Ipv4Net;
+    use std::str::FromStr;
+
+    #[test]
+    fn parses_stateless_target() {
+        assert_eq!(
+            "@host".parse::<Rule>().unwrap(),
+            Rule::Stateless(Target::Host)
+        );
+    }
+
+    #[test]
+    fn parses_stateful_directions() {
+        let private_network = Target::Prefix(Ipv4Net::from_str("10.0.0.0/8").unwrap());
+
+        assert_eq!(
+            "in   @host".parse::<Rule>().unwrap(),
+            Rule::Stateful {
+                direction: Direction::In,
+                target: Target::Host,
+            }
+        );
+        assert_eq!(
+            "out 10.0.0.0/8".parse::<Rule>().unwrap(),
+            Rule::Stateful {
+                direction: Direction::Out,
+                target: private_network,
+            }
+        );
+    }
+
+    #[test]
+    fn displays_normalized_rules() {
+        for (input, expected) in [("in 10.1.2.3/8", "in 10.0.0.0/8"), ("@host", "@host")] {
+            assert_eq!(
+                input.parse::<Rule>().unwrap().normalized().to_string(),
+                expected
+            );
+        }
+    }
+
+    #[test]
+    fn rejects_invalid_rules() {
+        for input in [
+            "",
+            "from @host",
+            "in",
+            "out",
+            "in from @host",
+            "out to @host",
+            "infrom @host",
+            " in @host",
+            "in @host ",
+            "in\t@host",
+        ] {
+            assert!(input.parse::<Rule>().is_err(), "{input:?} should fail");
+        }
+    }
+}
@@ -0,0 +1,279 @@
+use super::{Direction, Rule, Target};
+use ipnet::Ipv4Net;
+use prefix_trie::PrefixMap;
+use smoltcp::wire::Ipv4Address;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) enum Action {
+    Block,
+    Allow,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) enum PolicyDecision {
+    Block,
+    AllowStateless,
+    AllowStateful,
+}
+
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
+enum Mode {
+    #[default]
+    Legacy,
+    Stateful,
+}
+
+#[derive(Default)]
+pub(crate) struct Rules {
+    mode: Mode,
+    inbound: PrefixMap<Ipv4Net, Action>,
+    outbound: PrefixMap<Ipv4Net, Action>,
+}
+
+impl Rules {
+    pub(crate) fn new(host_address: Ipv4Address, allow: &[Rule], block: &[Rule]) -> Self {
+        // Preserve legacy behavior for bare-only policies. Once a directional rule
+        // is present, compile the whole policy using directional semantics.
+        let mode = if allow
+            .iter()
+            .chain(block)
+            .any(|rule| matches!(rule, Rule::Stateful { .. }))
+        {
+            Mode::Stateful
+        } else {
+            Mode::Legacy
+        };
+
+        let mut rules = Self {
+            mode,
+            ..Self::default()
+        };
+
+        for &rule in allow {
+            rules.insert(rule, Action::Allow, host_address);
+        }
+
+        // SECURITY: blocking rules must always take precedence
+        // over allowing rules when the rules are identical.
+        for &rule in block {
+            rules.insert(rule, Action::Block, host_address);
+        }
+
+        rules
+    }
+
+    pub(crate) fn policy_decision(
+        &self,
+        address: Ipv4Address,
+        direction: Direction,
+    ) -> Option<PolicyDecision> {
+        match (self.select(address, direction)?, self.mode) {
+            (Action::Block, _) => Some(PolicyDecision::Block),
+            (Action::Allow, Mode::Legacy) => Some(PolicyDecision::AllowStateless),
+            (Action::Allow, Mode::Stateful) => Some(PolicyDecision::AllowStateful),
+        }
+    }
+
+    pub(crate) fn is_stateful(&self, address: Ipv4Address, direction: Direction) -> bool {
+        self.mode == Mode::Stateful && self.select(address, direction).is_some()
+    }
+
+    pub(crate) fn len(&self) -> usize {
+        self.inbound.len() + self.outbound.len()
+    }
+
+    pub(crate) fn has_stateful(&self) -> bool {
+        self.mode == Mode::Stateful
+    }
+
+    fn select(&self, address: Ipv4Address, direction: Direction) -> Option<Action> {
+        let entries = match direction {
+            Direction::In => &self.inbound,
+            Direction::Out => &self.outbound,
+        };
+
+        entries
+            .get_lpm(&Ipv4Net::from(address))
+            .map(|(_, action)| *action)
+    }
+
+    fn insert(&mut self, rule: Rule, action: Action, host_address: Ipv4Address) {
+        match rule {
+            Rule::Stateless(target) => {
+                // Bare rules apply in both directions in stateful mode
+                if self.mode == Mode::Stateful {
+                    self.insert_direction(Direction::In, target, action, host_address);
+                }
+
+                self.insert_direction(Direction::Out, target, action, host_address);
+            }
+            Rule::Stateful { direction, target } => {
+                self.insert_direction(direction, target, action, host_address);
+            }
+        }
+    }
+
+    fn insert_direction(
+        &mut self,
+        direction: Direction,
+        target: Target,
+        action: Action,
+        host_address: Ipv4Address,
+    ) {
+        let prefix = match target {
+            Target::Prefix(prefix) => prefix,
+            Target::Host => host_address.into(),
+        };
+        let entries = match direction {
+            Direction::In => &mut self.inbound,
+            Direction::Out => &mut self.outbound,
+        };
+
+        entries.insert(prefix, action);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{Action, Mode, PolicyDecision, Rules};
+    use crate::proxy::Direction;
+    use smoltcp::wire::Ipv4Address;
+
+    const HOST: Ipv4Address = Ipv4Address::new(192, 168, 64, 1);
+
+    fn stateful_rules() -> Rules {
+        Rules {
+            mode: Mode::Stateful,
+            ..Rules::default()
+        }
+    }
+
+    #[test]
+    fn test_policy_precedence() {
+        let target = Ipv4Address::new(10, 0, 0, 1);
+        let mut rules = stateful_rules();
+
+        rules.insert("0.0.0.0/0".parse().unwrap(), Action::Block, HOST);
+        rules.insert("in 10.0.0.0/8".parse().unwrap(), Action::Allow, HOST);
+
+        assert_eq!(
+            rules.policy_decision(target, Direction::In),
+            Some(PolicyDecision::AllowStateful)
+        );
+        assert_eq!(
+            rules.policy_decision(target, Direction::Out),
+            Some(PolicyDecision::Block)
+        );
+
+        rules.insert("10.0.0.1/32".parse().unwrap(), Action::Allow, HOST);
+        assert_eq!(
+            rules.policy_decision(target, Direction::Out),
+            Some(PolicyDecision::AllowStateful)
+        );
+    }
+
+    #[test]
+    fn test_directional_rules_at_same_prefix_are_independent() {
+        let mut rules = stateful_rules();
+
+        for (target, action) in [
+            ("in @host", Action::Allow),
+            ("out @host", Action::Allow),
+            ("in @host", Action::Block),
+        ] {
+            rules.insert(target.parse().unwrap(), action, HOST);
+        }
+
+        assert_eq!(
+            rules.policy_decision(HOST, Direction::In),
+            Some(PolicyDecision::Block)
+        );
+        assert_eq!(
+            rules.policy_decision(HOST, Direction::Out),
+            Some(PolicyDecision::AllowStateful)
+        );
+        assert_eq!(rules.len(), 2);
+    }
+
+    #[test]
+    fn test_stateless_rules_are_outbound_only() {
+        let target = Ipv4Address::new(10, 1, 2, 3);
+        let mut rules = Rules::default();
+
+        rules.insert("10.0.0.0/8".parse().unwrap(), Action::Block, HOST);
+
+        assert!(rules.policy_decision(target, Direction::In).is_none());
+        assert_eq!(
+            rules.policy_decision(target, Direction::Out),
+            Some(PolicyDecision::Block)
+        );
+    }
+
+    #[test]
+    fn test_inbound_selection_uses_more_specific_bare_rule() {
+        let target = Ipv4Address::new(10, 1, 2, 3);
+        let mut rules = stateful_rules();
+
+        rules.insert("10.1.0.0/16".parse().unwrap(), Action::Allow, HOST);
+        rules.insert("in 10.0.0.0/8".parse().unwrap(), Action::Block, HOST);
+
+        assert_eq!(
+            rules.policy_decision(target, Direction::In),
+            Some(PolicyDecision::AllowStateful)
+        );
+    }
+
+    #[test]
+    fn test_block_wins_over_allow_at_same_outbound_prefix() {
+        let target = Ipv4Address::new(10, 1, 2, 3);
+
+        for (allow, block) in [
+            ("10.0.0.0/8", "out 10.0.0.0/8"),
+            ("out 10.0.0.0/8", "10.0.0.0/8"),
+        ] {
+            let mut rules = stateful_rules();
+            rules.insert(allow.parse().unwrap(), Action::Allow, HOST);
+            rules.insert(block.parse().unwrap(), Action::Block, HOST);
+
+            assert_eq!(
+                rules.policy_decision(target, Direction::Out),
+                Some(PolicyDecision::Block)
+            );
+        }
+    }
+
+    #[test]
+    fn test_directional_rule_makes_bare_rules_stateful() {
+        let allow = "out @host".parse().unwrap();
+        let block = "0.0.0.0/0".parse().unwrap();
+        let rules = Rules::new(HOST, &[allow], &[block]);
+
+        assert_eq!(rules.len(), 3);
+        assert!(rules.has_stateful());
+        assert_eq!(
+            rules.policy_decision(HOST, Direction::In),
+            Some(PolicyDecision::Block)
+        );
+        assert_eq!(
+            rules.policy_decision(HOST, Direction::Out),
+            Some(PolicyDecision::AllowStateful)
+        );
+    }
+
+    #[test]
+    fn test_return_tracking_applies_to_all_rules_in_stateful_mode() {
+        let stateful_target = Ipv4Address::new(10, 1, 2, 3);
+        let stateless_target = Ipv4Address::new(192, 0, 2, 1);
+        let mut rules = stateful_rules();
+
+        rules.insert("0.0.0.0/0".parse().unwrap(), Action::Block, HOST);
+        rules.insert("out 10.0.0.0/8".parse().unwrap(), Action::Block, HOST);
+
+        assert!(rules.is_stateful(stateful_target, Direction::Out));
+        assert!(rules.is_stateful(stateless_target, Direction::Out));
+        assert!(rules.is_stateful(stateful_target, Direction::In));
+
+        rules.insert("10.0.0.0/8".parse().unwrap(), Action::Allow, HOST);
+        assert!(rules.is_stateful(stateful_target, Direction::Out));
+    }
+}
@@ -1,66 +0,0 @@
-use smoltcp::wire::UdpPacket;
-
-pub(crate) trait UdpPacketHelper {
-    const DNS_PORT: u16 = 53;
-    const BOOTPS_PORT: u16 = 67;
-    const BOOTPC_PORT: u16 = 68;
-
-    fn is_dns_request(&self) -> bool;
-
-    fn is_dhcp_request(&self) -> bool;
-    fn is_dhcp_response(&self) -> bool;
-}
-
-impl UdpPacketHelper for UdpPacket<&[u8]> {
-    fn is_dns_request(&self) -> bool {
-        self.dst_port() == Self::DNS_PORT
-    }
-
-    fn is_dhcp_request(&self) -> bool {
-        self.src_port() == Self::BOOTPC_PORT && self.dst_port() == Self::BOOTPS_PORT
-    }
-
-    fn is_dhcp_response(&self) -> bool {
-        self.src_port() == Self::BOOTPS_PORT && self.dst_port() == Self::BOOTPC_PORT
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use super::UdpPacketHelper;
-    use smoltcp::wire::UdpPacket;
-
-    #[test]
-    fn test_is_dhcp_request_requires_both_standard_ports() {
-        assert!(is_dhcp_request(68, 67));
-        assert!(!is_dhcp_request(68, 9999));
-        assert!(!is_dhcp_request(9999, 67));
-    }
-
-    #[test]
-    fn test_is_dhcp_response_requires_both_standard_ports() {
-        assert!(is_dhcp_response(67, 68));
-        assert!(!is_dhcp_response(67, 9999));
-        assert!(!is_dhcp_response(9999, 68));
-    }
-
-    fn is_dhcp_request(src_port: u16, dst_port: u16) -> bool {
-        let buffer = udp_packet_buffer(src_port, dst_port);
-        let udp_pkt = UdpPacket::new_unchecked(&buffer[..]);
-        udp_pkt.is_dhcp_request()
-    }
-
-    fn is_dhcp_response(src_port: u16, dst_port: u16) -> bool {
-        let buffer = udp_packet_buffer(src_port, dst_port);
-        let udp_pkt = UdpPacket::new_unchecked(&buffer[..]);
-        udp_pkt.is_dhcp_response()
-    }
-
-    fn udp_packet_buffer(src_port: u16, dst_port: u16) -> [u8; 8] {
-        let mut buffer = [0; 8];
-        let mut udp_pkt = UdpPacket::new_unchecked(&mut buffer[..]);
-        udp_pkt.set_src_port(src_port);
-        udp_pkt.set_dst_port(dst_port);
-        buffer
-    }
-}
@@ -1,14 +1,16 @@
-use crate::dhcp_snooper::Lease;
-use crate::proxy::udp_packet_helper::UdpPacketHelper;
-use crate::proxy::{Action, Proxy};
+use crate::proxy::flows::{FlowDirection, FlowMatch};
+use crate::proxy::{Direction, PolicyDecision, Proxy};
 use anyhow::Context;
 use anyhow::Result;
-use ipnet::Ipv4Net;
+use dhcproto::v4::SERVER_PORT;
+use smoltcp::phy::ChecksumCapabilities;
 use smoltcp::wire::{
-    ArpOperation, ArpPacket, ArpRepr, EthernetFrame, EthernetProtocol, IpProtocol, Ipv4Packet,
-    UdpPacket,
+    ArpOperation, ArpPacket, ArpRepr, EthernetFrame, EthernetProtocol, IpProtocol, Ipv4Address,
+    Ipv4Packet, Ipv4Repr, UdpPacket,
 };

+const IPV4_HEADER_LEN_WITHOUT_OPTIONS: u8 = 20;
+
 impl Proxy<'_> {
    pub(crate) fn process_frame_from_vm(&mut self, frame: EthernetFrame<&[u8]>) -> Result<()> {
        if self.allowed_from_vm(&frame).is_none() {
@@ -22,7 +24,7 @@ impl Proxy<'_> {
            .context("failed to write to the host")
    }

-    fn allowed_from_vm(&self, frame: &EthernetFrame<&[u8]>) -> Option<()> {
+    fn allowed_from_vm(&mut self, frame: &EthernetFrame<&[u8]>) -> Option<()> {
        if frame.src_addr() != self.vm_mac_address {
            return None;
        }
@@ -33,7 +35,14 @@ impl Proxy<'_> {
                self.allowed_from_vm_arp(arp_pkt)
            }
            EthernetProtocol::Ipv4 => {
-                let ipv4_pkt = Ipv4Packet::new_checked(frame.payload()).ok()?;
+                let ipv4_pkt = Ipv4Packet::new_unchecked(frame.payload());
+                Ipv4Repr::parse(&ipv4_pkt, &ChecksumCapabilities::ignored()).ok()?;
+
+                // Reject IPv4 options because source routing could bypass destination-based policy
+                if ipv4_pkt.header_len() != IPV4_HEADER_LEN_WITHOUT_OPTIONS {
+                    return None;
+                }
+
                self.allowed_from_vm_ipv4(ipv4_pkt)
            }
            _ => None,
@@ -41,71 +50,105 @@ impl Proxy<'_> {
    }

    fn allowed_from_vm_arp(&self, arp_pkt: ArpPacket<&[u8]>) -> Option<()> {
-        vm_arp_allowed(arp_pkt, self.vm_mac_address, self.dhcp_snooper.lease())
+        vm_arp_allowed(arp_pkt, self.vm_mac_address, self.host.vm_ip)
    }

-    pub(crate) fn allowed_from_vm_ipv4(&self, ipv4_pkt: Ipv4Packet<&[u8]>) -> Option<()> {
-        // Is this packet coming from VM's IP address that we've learned from DHCP snooping?
-        if let Some(lease) = &self.dhcp_snooper.lease()
-            && lease.valid_ip_source(ipv4_pkt.src_addr())
-        {
+    pub(crate) fn allowed_from_vm_ipv4(&mut self, ipv4_pkt: Ipv4Packet<&[u8]>) -> Option<()> {
+        // Consume DHCP before enforcing the reserved source address
+        if self.consume_dhcp(&ipv4_pkt) {
+            return None;
+        }
+
+        // Is this packet coming from the VM's reserved IP address?
+        if self.host.vm_ip == ipv4_pkt.src_addr() {
+            // Consult the flow table before evaluating outbound policy
+            // so established flows are not treated as new traffic
+            let pending = match self
+                .flows
+                .as_mut()
+                .map(|flows| flows.inspect(&ipv4_pkt, FlowDirection::FromVm))
+                .unwrap_or(FlowMatch::Untracked)
+            {
+                FlowMatch::Allowed => return Some(()),
+                FlowMatch::Denied => return None,
+                FlowMatch::Candidate(pending) => Some(pending),
+                FlowMatch::Untracked => None,
+            };
+
+            // The flow is either pending or untracked, evaluate it against outbound policy
            let dst_addr = ipv4_pkt.dst_addr();

-            // Filter traffic based on user-specified rules first
-            if !self.rules.is_empty() {
-                let dst_net = Ipv4Net::from(dst_addr);
+            match self.rules.policy_decision(dst_addr, Direction::Out) {
+                // Return traffic was handled above; enforce explicit outbound blocks here
+                Some(PolicyDecision::Block) => return None,

-                if let Some((_, action)) = self.rules.get_lpm(&dst_net) {
-                    return match action {
-                        Action::Allow => Some(()),
-                        Action::Block => None,
-                    };
+                // Track statelessly allowed traffic only when needed so its reply is not
+                // treated as a new inbound flow
+                Some(PolicyDecision::AllowStateless) => {
+                    return self.admit_with_tracking_if_stateful(pending, dst_addr, Direction::In);
                }
+
+                // Untracked packets cannot satisfy stateful policy
+                Some(PolicyDecision::AllowStateful) => return self.admit_with_tracking(pending?),
+
+                // No outbound rule matched; apply the built-in fallbacks below
+                None => {}
            }

            // When no user-specified rules matched, simply allow all global traffic
            if ip_network::IpNetwork::from(dst_addr).is_global() {
-                return Some(());
+                return self.admit_with_tracking_if_trackable(pending);
            }

            // Additionally, allow communication with the host,
            // otherwise things like SSH to a VM won't work
-            if ipv4_pkt.dst_addr() == self.host.gateway_ip {
-                return Some(());
-            }
-
-            // Additionally, allow DNS requests to DNS-servers
-            // provided to a VM by the host's DHCP server
-            if ipv4_pkt.next_header() == IpProtocol::Udp {
-                let udp_pkt = UdpPacket::new_checked(ipv4_pkt.payload()).ok()?;
-
-                if udp_pkt.is_dns_request()
-                    && self.dhcp_snooper.valid_dns_target(&ipv4_pkt.dst_addr())
-                {
-                    return Some(());
-                }
-            }
-        }
-
-        // Allow outgoing DHCP requests to broadcast addresses,
-        // otherwise DHCP snooper will never be populated
-        if ipv4_pkt.next_header() == IpProtocol::Udp {
-            let udp_pkt = UdpPacket::new_checked(ipv4_pkt.payload()).ok()?;
-
-            // Allow DHCP communication with the bootpd(8) on host via broadcast address
-            if udp_pkt.is_dhcp_request() && ipv4_pkt.dst_addr().is_broadcast() {
-                return Some(());
+            if dst_addr == self.host.gateway_ip {
+                return self.admit_with_tracking_if_trackable(pending);
            }
        }

        None
    }
+
+    fn consume_dhcp(&mut self, ipv4_pkt: &Ipv4Packet<&[u8]>) -> bool {
+        // Only inspect UDP packets that contain the source and destination ports
+        if ipv4_pkt.next_header() != IpProtocol::Udp
+            || ipv4_pkt.frag_offset() != 0
+            || ipv4_pkt.payload().len() < 4
+        {
+            return false;
+        }
+
+        // Only consume packets addressed to the DHCP server port
+        let udp = UdpPacket::new_unchecked(ipv4_pkt.payload());
+        if udp.dst_port() != SERVER_PORT {
+            return false;
+        }
+
+        // Pass the request to the DHCP server and send any reply
+        let result = match self.dhcp_server.receive_vm(ipv4_pkt, &udp) {
+            Ok(Some(reply)) => self.write_to_vm(&reply),
+            Ok(None) => Ok(()),
+            Err(err) => Err(err),
+        };
+
+        // Report failures to generate or send the reply
+        if let Err(err) = result {
+            sentry::capture_message(
+                &format!("Failed to reply to DHCP request: {err:#}"),
+                sentry::Level::Warning,
+            );
+        }
+
+        // Consume the packet even if the request was rejected or the reply failed
+        true
+    }
 }

 fn vm_arp_allowed(
    arp_pkt: ArpPacket<&[u8]>,
    vm_mac_address: smoltcp::wire::EthernetAddress,
-    lease: &Option<Lease>,
+    address: Ipv4Address,
 ) -> Option<()> {
    let (operation, source_hardware_addr, source_protocol_addr) =
        match ArpRepr::parse(&arp_pkt).ok()? {
@@ -126,54 +169,34 @@ fn vm_arp_allowed(
        return None;
    }

-    if let Some(lease) = lease {
-        if lease.valid_ip_source(source_protocol_addr) {
-            return Some(());
-        }
-    } else if source_protocol_addr.is_unspecified() {
-        return Some(());
-    }
-
-    None
+    (source_protocol_addr == address || source_protocol_addr.is_unspecified()).then_some(())
 }

 #[cfg(test)]
 mod tests {
-    use crate::dhcp_snooper::Lease;
    use smoltcp::wire::{
        ArpHardware, ArpOperation, ArpPacket, EthernetAddress, EthernetProtocol, Ipv4Address,
    };
-    use std::collections::HashSet;
-    use std::time::Duration;

    #[test]
-    fn test_allowed_from_vm_arp_allows_unspecified_request_without_lease() {
+    fn test_allowed_from_vm_arp_allows_unspecified_request() {
        let vm_mac_address = EthernetAddress([0x02, 0x00, 0x00, 0x00, 0x00, 0x01]);
        let buf = arp_packet(vm_mac_address.0, [0, 0, 0, 0], ArpOperation::Request, 6, 4);
        let arp_pkt = ArpPacket::new_checked(buf.as_slice()).unwrap();

-        assert!(super::vm_arp_allowed(arp_pkt, vm_mac_address, &None).is_some());
+        assert!(
+            super::vm_arp_allowed(arp_pkt, vm_mac_address, Ipv4Address::new(1, 2, 3, 4)).is_some()
+        );
    }

    #[test]
    fn test_allowed_from_vm_arp_allows_reply_for_leased_ip() {
        let vm_mac_address = EthernetAddress([0x02, 0x00, 0x00, 0x00, 0x00, 0x01]);
-        let lease_ip = Ipv4Address::new(192, 168, 0, 2);
-        let lease = Some(Lease::new(
-            lease_ip,
-            Duration::from_secs(600),
-            HashSet::new(),
-        ));
-        let buf = arp_packet(
-            vm_mac_address.0,
-            lease_ip.octets(),
-            ArpOperation::Reply,
-            6,
-            4,
-        );
+        let vm_ip = Ipv4Address::new(192, 168, 0, 2);
+        let buf = arp_packet(vm_mac_address.0, vm_ip.octets(), ArpOperation::Reply, 6, 4);
        let arp_pkt = ArpPacket::new_checked(buf.as_slice()).unwrap();

-        assert!(super::vm_arp_allowed(arp_pkt, vm_mac_address, &lease).is_some());
+        assert!(super::vm_arp_allowed(arp_pkt, vm_mac_address, vm_ip).is_some());
    }

    #[test]
@@ -188,7 +211,9 @@ mod tests {
        );
        let arp_pkt = ArpPacket::new_checked(buf.as_slice()).unwrap();

-        assert!(super::vm_arp_allowed(arp_pkt, vm_mac_address, &None).is_none());
+        assert!(
+            super::vm_arp_allowed(arp_pkt, vm_mac_address, Ipv4Address::new(1, 2, 3, 4)).is_none()
+        );
    }

    #[test]
@@ -199,7 +224,9 @@ mod tests {
        arp_pkt.set_hardware_type(ArpHardware::Unknown(2));
        let arp_pkt = ArpPacket::new_checked(buf.as_slice()).unwrap();

-        assert!(super::vm_arp_allowed(arp_pkt, vm_mac_address, &None).is_none());
+        assert!(
+            super::vm_arp_allowed(arp_pkt, vm_mac_address, Ipv4Address::new(1, 2, 3, 4)).is_none()
+        );
    }

    #[test]
@@ -210,7 +237,9 @@ mod tests {
        arp_pkt.set_protocol_type(EthernetProtocol::Ipv6);
        let arp_pkt = ArpPacket::new_checked(buf.as_slice()).unwrap();

-        assert!(super::vm_arp_allowed(arp_pkt, vm_mac_address, &None).is_none());
+        assert!(
+            super::vm_arp_allowed(arp_pkt, vm_mac_address, Ipv4Address::new(1, 2, 3, 4)).is_none()
+        );
    }

    #[test]
@@ -219,7 +248,9 @@ mod tests {
        let buf = arp_packet(vm_mac_address.0, [0, 0, 0], ArpOperation::Request, 6, 3);
        let arp_pkt = ArpPacket::new_checked(buf.as_slice()).unwrap();

-        assert!(super::vm_arp_allowed(arp_pkt, vm_mac_address, &None).is_none());
+        assert!(
+            super::vm_arp_allowed(arp_pkt, vm_mac_address, Ipv4Address::new(1, 2, 3, 4)).is_none()
+        );
    }

    fn arp_packet(
@@ -0,0 +1,92 @@
+use anyhow::{Context, Result};
+use ipnet::{Ipv4AddrRange, Ipv4Net};
+use network_interface::{Addr, NetworkInterface, NetworkInterfaceConfig};
+use rand::seq::{IndexedRandom, IteratorRandom};
+use std::net::Ipv4Addr;
+
+const MAX_ATTEMPTS: usize = 128;
+
+/// Find an available private subnet and its first two usable host addresses
+pub(crate) fn find_available_subnet(prefix_len: u8) -> Result<(Ipv4Addr, Ipv4Addr, Ipv4Net)> {
+    // Add private address space (as defined in RFC 1918[1]) with one exception
+    //
+    // We narrow the 192.168.0.0/16 allocation pool to work around a macOS bug
+    // where long subnet address strings lose their CIDR prefix in the generated
+    // firewall rules, causing connection issues.
+    //
+    // [1]: https://datatracker.ietf.org/doc/html/rfc1918#section-3
+    let private_subnets: Vec<Ipv4Net> = vec![
+        "10.0.0.0/8".parse()?,
+        "172.16.0.0/12".parse()?,
+        "192.168.0.0/18".parse()?,  // third octet 0–63
+        "192.168.64.0/19".parse()?, // third octet 64–95
+        "192.168.96.0/22".parse()?, // third octet 96–99
+    ];
+
+    // Figure out which address space is already utilized on the host
+    let mut used_subnets = Vec::new();
+
+    for interface in NetworkInterface::show()? {
+        for addr in interface.addr {
+            // We only support IPv4 for now
+            if let Addr::V4(addr) = addr {
+                let mask = addr.netmask.context("IPv4 interface has no netmask")?;
+                used_subnets.push(Ipv4Net::with_netmask(addr.ip, mask)?);
+            }
+        }
+    }
+
+    // Try up to MAX_ATTEMPTS random subnets until we're able to
+    // get an available subnet of the desired length
+    for _ in 0..MAX_ATTEMPTS {
+        // Give each private range an equal chance, even if a larger one is occupied
+        let private_subnet = private_subnets.choose(&mut rand::rng()).unwrap();
+
+        // Skip private ranges too small to contain the requested subnet
+        if prefix_len < private_subnet.prefix_len() {
+            continue;
+        }
+
+        // Pick a subnet of the desired length within this private range
+        let address = Ipv4AddrRange::new(private_subnet.network(), private_subnet.broadcast())
+            .choose(&mut rand::rng())
+            .unwrap();
+        let candidate = Ipv4Net::new(address, prefix_len)?.trunc();
+
+        // Only use the subnet if it doesn't overlap address space
+        // that is already utilized on the host
+        let overlaps = used_subnets
+            .iter()
+            .any(|used_subnet| candidate.contains(used_subnet) || used_subnet.contains(&candidate));
+        if overlaps {
+            continue;
+        }
+
+        // Take the first two hosts from the subnet
+        let mut hosts = candidate.hosts();
+        if let (Some(first_host), Some(second_host)) = (hosts.next(), hosts.next()) {
+            return Ok((first_host, second_host, candidate));
+        }
+    }
+
+    anyhow::bail!(
+        "failed to find an unused private IPv4 /{prefix_len} subnet after {MAX_ATTEMPTS} attempts"
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    #[serial_test::serial]
+    fn finds_two_usable_addresses() {
+        // Find a subnet on the current host
+        let (gateway_ip, vm_ip, subnet) = find_available_subnet(29).unwrap();
+
+        // Check the subnet and its first two usable addresses
+        assert_eq!(subnet.prefix_len(), 29);
+        assert!(subnet.network().is_private());
+        assert_eq!(subnet.hosts().collect::<Vec<_>>()[..2], [gateway_ip, vm_ip]);
+    }
+}
@@ -26,6 +26,14 @@ impl VM {
    pub fn read(&self, buf: &mut [u8]) -> std::io::Result<usize> {
        self.sock.recv(buf)
    }
+
+    pub fn is_connected(&self) -> io::Result<bool> {
+        match self.sock.peer_addr() {
+            Ok(_) => Ok(true),
+            Err(error) if error.kind() == io::ErrorKind::NotConnected => Ok(false),
+            Err(error) => Err(error),
+        }
+    }
 }

 fn duplicate_vm_fd(vm_fd: RawFd) -> Result<RawFd> {
@@ -115,10 +123,12 @@ impl AsRawFd for VM {
 #[cfg(test)]
 mod tests {
    use super::VM;
+    use polling::{Event, Events, PollMode, Poller};
    use std::fs::File;
    use std::net::UdpSocket;
    use std::os::fd::AsRawFd;
    use std::os::unix::net::{UnixDatagram, UnixStream};
+    use std::time::Duration;

    #[test]
    fn test_new_rejects_negative_fd() {
@@ -187,4 +197,62 @@ mod tests {

        assert!(socket_fd_is_open);
    }
+
+    #[test]
+    fn test_connected_socket_has_peer() {
+        let (socket, _peer) = UnixDatagram::pair().unwrap();
+        let vm = VM::new(socket.as_raw_fd()).unwrap();
+
+        assert!(vm.is_connected().unwrap());
+    }
+
+    #[test]
+    fn test_disconnected_peer_is_detected_without_kqueue_event() {
+        let (socket, peer) = UnixDatagram::pair().unwrap();
+        let vm = VM::new(socket.as_raw_fd()).unwrap();
+        let poller = Poller::new().unwrap();
+        let mut events = Events::new();
+
+        unsafe {
+            poller
+                .add_with_mode(vm.as_raw_fd(), Event::readable(0), PollMode::Edge)
+                .unwrap();
+        }
+        drop(peer);
+
+        poller
+            .wait(&mut events, Some(Duration::from_millis(20)))
+            .unwrap();
+
+        assert!(events.is_empty());
+        assert!(!vm.is_connected().unwrap());
+    }
+
+    #[test]
+    fn test_disconnected_peer_is_detected_when_another_socket_wakes_kqueue() {
+        let (socket, peer) = UnixDatagram::pair().unwrap();
+        let (host, host_peer) = UnixDatagram::pair().unwrap();
+        let vm = VM::new(socket.as_raw_fd()).unwrap();
+        let poller = Poller::new().unwrap();
+        let mut events = Events::new();
+
+        unsafe {
+            poller
+                .add_with_mode(vm.as_raw_fd(), Event::readable(0), PollMode::Edge)
+                .unwrap();
+            poller
+                .add_with_mode(host.as_raw_fd(), Event::readable(1), PollMode::Edge)
+                .unwrap();
+        }
+        drop(peer);
+        host_peer.send(&[1]).unwrap();
+
+        poller
+            .wait(&mut events, Some(Duration::from_millis(20)))
+            .unwrap();
+
+        assert!(events.iter().any(|event| event.key == 1));
+        assert!(!events.iter().any(|event| event.key == 0));
+        assert!(!vm.is_connected().unwrap());
+    }
 }
@@ -7,22 +7,31 @@ use privdrop::PrivDrop;
 use softnet::NetType;
 use softnet::proxy::ExposedPort;
 use softnet::proxy::Proxy;
-use softnet::proxy::Target;
-use std::borrow::Cow;
+use softnet::proxy::Rule;
 use std::env;
 use std::os::raw::c_int;
 use std::os::unix::io::RawFd;
 use std::os::unix::process::CommandExt;
 use std::process::{Command, ExitCode};
 use system_configuration::core_foundation::base::TCFType;
+use system_configuration::core_foundation::boolean::CFBoolean;
 use system_configuration::core_foundation::dictionary::CFDictionary;
 use system_configuration::core_foundation::number::CFNumber;
 use system_configuration::core_foundation::string::CFString;
 use system_configuration::preferences::SCPreferences;
-use system_configuration::sys::preferences::{SCPreferencesCommitChanges, SCPreferencesSetValue};
+use system_configuration::sys::preferences::{
+    SCPreferencesApplyChanges, SCPreferencesCommitChanges, SCPreferencesLock,
+    SCPreferencesSetValue, SCPreferencesUnlock,
+};
 use uzers::{get_current_groupname, get_current_username, get_effective_uid};

+const VERSION: &str = match option_env!("SOFTNET_VERSION") {
+    Some(version) => version,
+    None => "unknown-unknown",
+};
+
 #[derive(Parser, Debug)]
+#[command(version = VERSION)]
 struct Args {
    #[clap(
        long,
@@ -31,6 +40,13 @@ struct Args {
    )]
    vm_fd: c_int,

+    #[clap(
+        long,
+        value_parser = parse_vm_fd,
+        help = "connected Unix stream FD for newline-delimited JSON-RPC policy control"
+    )]
+    control_fd: Option<c_int>,
+
    #[clap(long, help = "MAC address to enforce for the VM")]
    vm_mac_address: mac_address::MacAddress,

@@ -52,34 +68,51 @@ struct Args {

    #[clap(
        long,
-        help = "Comma-separated list of CIDRs to allow the traffic to \
-        (e.g. --allow=192.168.0.0/24 may be used to allow a LAN access for a VM), \
-        plus supported @-aliases. Currently the only supported @-alias is @host, \
-        which matches the vmnet bridge gateway IP. \
-        When used with --block, the longest prefix match always wins. \
-        In case an identical prefix is both --allow'ed and --block'ed, \
-        blocking will take precedence. --allow=0.0.0.0/0 is a special case, \
-        it additionally disables bridge isolation (even when --block=0.0.0.0/0 is specified).",
-        value_name = "comma-separated CIDRs or @-alias",
+        help = "Comma-separated rules for allowing traffic, in the following forms:\n\n\
+        * TARGET: traffic sent from the VM to TARGET; reverse traffic is not filtered by this rule\n\
+        * in TARGET: flows initiated from TARGET to the VM\n\
+        * out TARGET: flows initiated from the VM to TARGET\n\n\
+        Targets are:\n\n\
+        * IPv4 CIDRs\n\
+        * @host, which matches the vmnet bridge gateway IP\n\n\
+        Directional rules make bare TARGET rules stateful in both directions.\n\n\
+        When used with --block, the longest prefix match wins. If an identical rule is both \
+        allowed and blocked, blocking takes precedence.\n\n\
+        --allow=0.0.0.0/0 additionally disables bridge isolation, even when \
+        --block=0.0.0.0/0 is specified.\n\n\
+        Examples:\n\n\
+        * --allow=192.168.0.0/24 — allow stateless traffic with this LAN\n\
+        * --allow=\"in @host\" — allow stateful flows initiated from @host\n\
+        * --allow=\"out 192.168.0.0/24\" — allow stateful flows initiated toward this LAN\n\
+        * --allow=\"in @host,out 192.168.0.0/24\" — multiple rules may be comma-separated",
+        value_name = "comma-separated rules",
        use_value_delimiter = true,
        action = clap::ArgAction::Set
    )]
-    allow: Vec<Target>,
+    allow: Vec<Rule>,

    #[clap(
        long,
-        help = "Comma-separated list of CIDRs to block the traffic to \
-        (e.g. --block=0.0.0.0/0 may be used to establish a default deny policy \
-        that is further relaxed with --allow), plus supported @-aliases. \
-        Currently the only supported @-alias is @host, which matches the vmnet bridge gateway IP. \
-        When used with --allow, the longest prefix match always wins. \
-        In case an identical prefix is both --allow'ed and --block'ed, \
-        blocking will take precedence.",
-        value_name = "comma-separated CIDRs or @-alias",
+        help = "Comma-separated rules for blocking traffic, in the following forms:\n\n\
+        * TARGET: traffic sent from the VM to TARGET; reverse traffic is not filtered by this rule\n\
+        * in TARGET: flows initiated from TARGET to the VM\n\
+        * out TARGET: flows initiated from the VM to TARGET\n\n\
+        Targets are:\n\n\
+        * IPv4 CIDRs\n\
+        * @host, which matches the vmnet bridge gateway IP\n\n\
+        Directional rules make bare TARGET rules stateful in both directions.\n\n\
+        When used with --allow, the longest prefix match wins. If an identical rule is both \
+        allowed and blocked, blocking takes precedence.\n\n\
+        Examples:\n\n\
+        * --block=0.0.0.0/0 — establish a stateless default-deny egress policy\n\
+        * --block=\"out @host\" — block stateful flows initiated toward @host\n\
+        * --block=\"out 66.66.66.0/24\" — block stateful flows initiated toward this CIDR\n\
+        * --block=\"out @host,out 66.66.66.0/24\" — multiple rules may be comma-separated",
+        value_name = "comma-separated rules",
        use_value_delimiter = true,
        action = clap::ArgAction::Set
    )]
-    block: Vec<Target>,
+    block: Vec<Rule>,

    #[clap(
        long,
@@ -106,10 +139,10 @@ fn main() -> ExitCode {
    }

    // Initialize Sentry
-    let _sentry = sentry::init(sentry::ClientOptions {
-        release: option_env!("CIRRUS_TAG").map(|tag| Cow::from(format!("softnet@{tag}"))),
-        ..Default::default()
-    });
+    let _sentry = sentry::init(
+        sentry::ClientOptions::default()
+            .maybe_release(option_env!("CIRRUS_TAG").map(|tag| format!("softnet@{tag}"))),
+    );

    // Enrich future events with Cirrus CI-specific tags
    if let Ok(tags) = env::var("CIRRUS_SENTRY_TAGS") {
@@ -192,8 +225,8 @@ fn try_main() -> anyhow::Result<()> {
        ));
    }

-    // Set bootpd(8) min/max lease time while still having the root privileges
-    set_bootpd_lease_time(args.bootpd_lease_time);
+    // Configure bootpd(8) while still having the root privileges
+    configure_bootpd(args.bootpd_lease_time)?;

    // Initialize the proxy while still having the root privileges
    let mut proxy = Proxy::new(
@@ -203,6 +236,7 @@ fn try_main() -> anyhow::Result<()> {
        args.allow,
        args.block,
        args.expose,
+        args.control_fd.map(|fd| fd as RawFd),
    )
    .context("failed to initialize proxy")?;

@@ -244,26 +278,59 @@ fn sudo_escalation_works() -> bool {
        .unwrap_or(false)
 }

-fn set_bootpd_lease_time(lease_time: u32) {
+fn configure_bootpd(lease_time: u32) -> anyhow::Result<()> {
    let prefs = SCPreferences::group(
        &CFString::new("softnet"),
        &CFString::new("com.apple.InternetSharing.default.plist"),
    );

-    let bootpd_dict = CFDictionary::from_CFType_pairs(&[(
-        CFString::new("DHCPLeaseTimeSecs"),
-        CFNumber::from(lease_time as i32),
-    )]);
+    let bootpd_dict = CFDictionary::from_CFType_pairs(&[
+        (
+            CFString::new("DHCPLeaseTimeSecs"),
+            CFNumber::from(lease_time as i32).as_CFType(),
+        ),
+        (
+            CFString::new("dhcp_ignore_client_identifier"),
+            CFBoolean::true_value().as_CFType(),
+        ),
+    ]);

    unsafe {
-        SCPreferencesSetValue(
-            prefs.as_concrete_TypeRef(),
-            CFString::new("bootpd").as_concrete_TypeRef(),
-            bootpd_dict.as_concrete_TypeRef().cast(),
+        let prefs = prefs.as_concrete_TypeRef();
+        anyhow::ensure!(
+            SCPreferencesLock(prefs, 1) != 0,
+            "failed to lock bootpd preferences"
        );

-        SCPreferencesCommitChanges(prefs.as_concrete_TypeRef());
+        let result = (|| -> anyhow::Result<()> {
+            anyhow::ensure!(
+                SCPreferencesSetValue(
+                    prefs,
+                    CFString::new("bootpd").as_concrete_TypeRef(),
+                    bootpd_dict.as_concrete_TypeRef().cast(),
+                ) != 0,
+                "failed to set bootpd preferences"
+            );
+
+            anyhow::ensure!(
+                SCPreferencesCommitChanges(prefs) != 0,
+                "failed to commit bootpd preferences"
+            );
+
+            anyhow::ensure!(
+                SCPreferencesApplyChanges(prefs) != 0,
+                "failed to apply bootpd preferences"
+            );
+
+            Ok(())
+        })();
+
+        let unlocked = SCPreferencesUnlock(prefs) != 0;
+        result?;
+        anyhow::ensure!(unlocked, "failed to unlock bootpd preferences");
    }
+
+    Ok(())
 }

 #[cfg(test)]
@@ -286,4 +353,21 @@ mod tests {
                .contains("file descriptor must be non-negative")
        );
    }
+
+    #[test]
+    fn test_cli_rejects_negative_control_fd_before_startup() {
+        let error = Args::try_parse_from([
+            "softnet",
+            "--vm-fd=0",
+            "--control-fd=-1",
+            "--vm-mac-address=02:00:00:00:00:01",
+        ])
+        .unwrap_err();
+
+        assert!(
+            error
+                .to_string()
+                .contains("file descriptor must be non-negative")
+        );
+    }
 }