Support TCP port exposure via "--expose" command-line argument (#70)

Bumps [anyhow](https://github.com/dtolnay/anyhow) from 1.0.94 to 1.0.95.
- [Release notes](https://github.com/dtolnay/anyhow/releases)
- [Commits](https://github.com/dtolnay/anyhow/compare/1.0.94...1.0.95)

---
updated-dependencies:
- dependency-name: anyhow
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "cargo"
+    directory: "/"
+    schedule:
+      interval: "weekly"

.gitignore

@@ -1 +1,3 @@
+/.idea
+/dist
 /target

.goreleaser.yml

@@ -1,31 +1,19 @@
 project_name: softnet
 
 builds:
-  - id: softnet-arm64
+  - id: softnet
     builder: prebuilt
+    goamd64: [v1]
     goos:
       - darwin
     goarch:
       - arm64
-    prebuilt:
-      path: "target/aarch64-apple-darwin/release-with-debug/softnet"
-  - id: softnet-x86
-    builder: prebuilt
-    goos:
-      - darwin
-    goarch:
       - amd64
     prebuilt:
-      path: "target/x86_64-apple-darwin/release-with-debug/softnet"
+      path: 'target/{{- if eq .Arch "arm64" }}aarch64{{- else }}x86_64{{ end }}-apple-darwin/release-with-debug/softnet'
 
 archives:
-  - id: regular-arm64
-    builds:
-      - softnet-arm64
-    name_template: "{{ .ProjectName }}-{{ .Arch }}"
-  - id: regular-x86
-    builds:
-      - softnet-x86
+  - id: regular
     name_template: "{{ .ProjectName }}-{{ .Arch }}"
 
 release:
@@ -33,10 +21,7 @@ release:
 
 brews:
   - name: softnet
-    ids:
-      - regular-arm64
-      - regular-x86
-    tap:
+    repository:
       owner: cirruslabs
       name: homebrew-cli
     caveats: See the Github repository for more information

Cargo.toml

@@ -12,19 +12,23 @@ inherits = "release"
 debug = true
 
 [dependencies]
-smoltcp = "0.8.1"
-libc = "0.2.126"
-polling = { git = "https://github.com/smol-rs/polling.git" }
-dhcproto = "0.7.0"
-vmnet = "0.1.1"
-clap = { version = "3.1.18", features = ["derive"] }
-mac_address = "1.1.3"
-privdrop = "0.5.2"
-anyhow = { version = "1.0.66", features = ["backtrace"] }
-ip_network = "0.4.1"
-uzers = "0.11.3"
-system-configuration = "0.5.0"
-num_enum = "0.5.7"
-sentry = { version = "0.29.1", features = ["debug-images"] }
-sentry-anyhow = { version = "0.29.1", features = ["backtrace"] }
-nix = "0.26.2"
+smoltcp = "0"
+libc = "0"
+polling = "3"
+dhcproto = { git = "https://github.com/bluecatengineering/dhcproto.git", branch = "master" }
+vmnet = "0"
+clap = { version = "4", features = ["derive"] }
+mac_address = "1"
+privdrop = "0"
+anyhow = { version = "1", features = ["backtrace"] }
+ip_network = "0"
+uzers = "0"
+system-configuration = "0"
+num_enum = "0"
+sentry = { version = "0", features = ["debug-images"] }
+sentry-anyhow = { version = "0", features = ["backtrace"] }
+nix = { version = "0", features = ["signal"] }
+prefix-trie = "0"
+ipnet = "2"
+oslog = "0.2.0"
+log = "0.4.22"

README.md

@@ -8,9 +8,9 @@ Please check out [this blog post](https://cirrus-ci.org/blog/2022/07/07/isolatin
 Softnet solves two problems:
 
 1. VM network isolation
-  * [`VZNATNetworkDeviceAttachment`](https://developer.apple.com/documentation/virtualization/vznatnetworkdeviceattachment) (the default networking in Tart) enables [vmnet's bridge isolation](https://developer.apple.com/documentation/vmnet/vmnet_enable_isolation_key) by default and prevents cross-VM traffic, however it's still possible for any VM to spoof the host's ARP-table and capture other VMs traffic
+    * [`VZNATNetworkDeviceAttachment`](https://developer.apple.com/documentation/virtualization/vznatnetworkdeviceattachment) (the default networking in Tart) enables [vmnet's bridge isolation](https://developer.apple.com/documentation/vmnet/vmnet_enable_isolation_key) by default and prevents cross-VM traffic, however it's still possible for any VM to spoof the host's ARP-table and capture other VMs traffic by using tools that enable conducting the [ARP spoofing attacks](https://en.wikipedia.org/wiki/ARP_spoofing) (e.g. [arpspoof](https://www.monkey.org/~dugsong/dsniff/), [arpoison](http://www.arpoison.net/) and so on)
 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
+    * 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:
 

lib/dhcp_snooper.rs

@@ -26,14 +26,14 @@ impl DhcpSnooper {
                 };
 
                 let dns_ips = match message.opts().get(OptionCode::DomainNameServer) {
-                    Some(DhcpOption::DomainNameServer(dns_ips)) => HashSet::from_iter(
-                        dns_ips.iter().map(|dns_ip| Ipv4Address(dns_ip.octets())),
-                    ),
+                    Some(DhcpOption::DomainNameServer(dns_ips)) => {
+                        HashSet::from_iter(dns_ips.iter().cloned())
+                    }
                     _ => HashSet::new(),
                 };
 
                 self.vm_lease = Some(Lease::new(
-                    message.yiaddr().into(),
+                    message.yiaddr(),
                     Duration::from_secs(*lease_time as u64),
                     dns_ips,
                 ))
@@ -73,7 +73,15 @@ impl Lease {
         }
     }
 
+    pub fn address(&self) -> Ipv4Address {
+        self.address
+    }
+
+    pub fn valid(&self) -> bool {
+        Instant::now() < self.valid_until
+    }
+
     pub fn valid_ip_source(&self, address: Ipv4Address) -> bool {
-        self.address == address && Instant::now() < self.valid_until
+        self.address == address && self.valid()
     }
 }

lib/host.rs

@@ -1,4 +1,6 @@
 use anyhow::{anyhow, Context, Result};
+use clap::ValueEnum;
+use log::info;
 use std::net::Ipv4Addr;
 use std::os::unix::io::{AsRawFd, RawFd};
 use std::os::unix::net::UnixDatagram;
@@ -6,8 +8,21 @@ use std::str::FromStr;
 use std::sync::mpsc::{sync_channel, SyncSender};
 use vmnet::mode::Mode;
 use vmnet::parameters::{Parameter, ParameterKind};
+use vmnet::port_forwarding::{AddressFamily, Protocol};
 use vmnet::{Events, Options};
 
+#[derive(ValueEnum, Clone, Debug)]
+pub enum NetType {
+    /// Shared network
+    ///
+    /// Uses NAT-translation to give guests access to the global network
+    Nat,
+    /// Host network
+    ///
+    /// Guests will be able to talk only to the host without access to global network
+    Host,
+}
+
 pub struct Host {
     interface: vmnet::Interface,
     new_packets_rx: UnixDatagram,
@@ -18,27 +33,38 @@ pub struct Host {
 }
 
 impl Host {
-    pub fn new() -> Result<Host> {
-        // Initialize a vmnet.framework NAT interface with isolation enabled
+    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(
-            Mode::Shared(Default::default()),
+            match vm_net_type {
+                NetType::Nat => Mode::Shared(Default::default()),
+                NetType::Host => Mode::Host(Default::default()),
+            },
             Options {
-                enable_isolation: Some(true),
+                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 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) else {
-            return Err(anyhow!("failed to retrieve vmnet's interface max packet size"));
+        let Some(Parameter::MaxPacketSize(max_packet_size)) =
+            interface.parameters().get(ParameterKind::MaxPacketSize)
+        else {
+            return Err(anyhow!(
+                "failed to retrieve vmnet's interface max packet size"
+            ));
         };
 
         // Set up a socketpair() to emulate polling of the vmnet interface
@@ -50,7 +76,9 @@ impl Host {
         interface
             .set_event_callback(Events::PACKETS_AVAILABLE, move |_mask, _params| {
                 // Send a dummy datagram to make the other end of socketpair() readable
-                new_packets_tx.send(&[0; 1]).unwrap();
+                // and ignore the error as this merely a signalling channel to wake up
+                // the poller
+                new_packets_tx.send(&[0; 1]).ok();
 
                 // Wait for the permission to continue to avoid
                 // wasting CPU cycles or in case of termination,
@@ -66,7 +94,7 @@ impl Host {
             interface,
             new_packets_rx,
             callback_can_continue_tx,
-            gateway_ip: gateway_ip.into(),
+            gateway_ip,
             max_packet_size,
             finalized: false,
         })
@@ -74,6 +102,35 @@ impl Host {
 }
 
 impl Host {
+    pub fn port_forwarding_add_rule(
+        &mut self,
+        external_port: u16,
+        internal_addr: Ipv4Addr,
+        internal_port: u16,
+    ) -> Result<()> {
+        let details = format!("external_port={external_port}, internal_addr={internal_addr}, internal_port={internal_port}");
+
+        self.interface
+            .port_forwarding_rule_add(
+                AddressFamily::Ipv4,
+                Protocol::Tcp,
+                external_port,
+                internal_addr.into(),
+                internal_port,
+            )
+            .map(|_| info!("added port forwarding rule {details}"))
+            .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, buf: &mut [u8]) -> vmnet::Result<usize> {
         // Dequeue dummy datagram from the socket (if any)
         // to free up buffer space and reduce false-positives

lib/mod.rs

@@ -1,5 +1,6 @@
 mod dhcp_snooper;
 mod host;
+pub use host::NetType;
 mod poller;
 pub mod proxy;
 mod vm;

lib/poller.rs

@@ -2,14 +2,15 @@ use anyhow::Result;
 use num_enum::IntoPrimitive;
 use polling::os::kqueue::PollerKqueueExt;
 use polling::PollMode;
+use std::os::fd::{AsRawFd, BorrowedFd};
 use std::os::unix::io::RawFd;
 use std::time::Duration;
 
-pub struct Poller {
+pub struct Poller<'poller> {
     poller: polling::Poller,
-    events: Vec<polling::Event>,
-    vm_fd: RawFd,
-    host_fd: RawFd,
+    events: polling::Events,
+    vm_fd: BorrowedFd<'poller>,
+    host_fd: BorrowedFd<'poller>,
 }
 
 #[derive(IntoPrimitive)]
@@ -20,22 +21,25 @@ enum EventKey {
     Interrupt,
 }
 
-impl Poller {
-    pub fn new(vm_fd: RawFd, host_fd: RawFd) -> Result<Poller> {
+impl Poller<'_> {
+    pub fn new<'poller>(vm_fd: RawFd, host_fd: RawFd) -> Result<Poller<'poller>> {
         let poller = polling::Poller::new()?;
 
         Ok(Poller {
             poller,
-            events: Vec::new(),
-            vm_fd,
-            host_fd,
+            events: polling::Events::new(),
+            vm_fd: unsafe { BorrowedFd::borrow_raw(vm_fd) },
+            host_fd: unsafe { BorrowedFd::borrow_raw(host_fd) },
         })
     }
 
     pub fn arm(&self) -> Result<()> {
-        self.poller.add(self.vm_fd as RawFd, self.vm_interest())?;
-        self.poller
-            .add(self.host_fd as RawFd, self.host_interest())?;
+        unsafe {
+            self.poller
+                .add(self.vm_fd.as_raw_fd(), self.vm_interest())?;
+            self.poller
+                .add(self.host_fd.as_raw_fd(), self.host_interest())?;
+        }
 
         let interrupt_signal = polling::os::kqueue::Signal(libc::SIGINT);
         self.poller
@@ -52,10 +56,8 @@ impl Poller {
     pub fn rearm(&mut self) -> Result<()> {
         self.events.clear();
 
-        self.poller
-            .modify(self.vm_fd as RawFd, self.vm_interest())?;
-        self.poller
-            .modify(self.host_fd as RawFd, self.host_interest())?;
+        self.poller.modify(self.vm_fd, self.vm_interest())?;
+        self.poller.modify(self.host_fd, self.host_interest())?;
 
         let interrupt_signal = polling::os::kqueue::Signal(libc::SIGINT);
         self.poller.modify_filter(

lib/proxy/exposed_port.rs

@@ -0,0 +1,47 @@
+use anyhow::{anyhow, Context, Error};
+use std::str::FromStr;
+
+#[derive(Debug, Clone, Copy, Default)]
+pub struct ExposedPort {
+    pub external_port: u16,
+    pub internal_port: u16,
+}
+
+impl FromStr for ExposedPort {
+    type Err = Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let splits: Vec<&str> = s.split(':').collect();
+
+        match splits.len() {
+            2 => Ok(ExposedPort {
+                external_port: splits[0]
+                    .parse()
+                    .context(format!("invalid external port {:?}", splits[0]))?,
+                internal_port: splits[1]
+                    .parse()
+                    .context(format!("invalid internal port {:?}", splits[1]))?,
+            }),
+            _ => Err(anyhow!(
+                "invalid exposed port specification {:?}, the format should be EXTERNAL:INTERNAL",
+                s
+            )),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::proxy::exposed_port::ExposedPort;
+
+    #[test]
+    fn exposed_port() {
+        assert_eq!(
+            ExposedPort {
+                external_port: 2222,
+                internal_port: 22
+            },
+            "2222:22".parse().unwrap()
+        );
+    }
+}

lib/proxy/host.rs

@@ -3,7 +3,7 @@ use crate::proxy::Proxy;
 use anyhow::{Context, Result};
 use smoltcp::wire::{EthernetFrame, EthernetProtocol, Ipv4Packet, UdpPacket};
 
-impl Proxy {
+impl Proxy<'_> {
     pub(crate) fn process_frame_from_host(&mut self, frame: &EthernetFrame<&[u8]>) -> Result<()> {
         if self.allowed_from_host(frame).is_none() {
             // Block packet by not forwarding it to the VM
@@ -58,7 +58,7 @@ impl Proxy {
             return;
         }
 
-        if ipv4_pkt.protocol() != smoltcp::wire::IpProtocol::Udp {
+        if ipv4_pkt.next_header() != smoltcp::wire::IpProtocol::Udp {
             return;
         }
 

lib/proxy/mod.rs

@@ -1,30 +1,45 @@
+mod exposed_port;
 mod host;
+mod port_forwarder;
 mod udp_packet_helper;
 mod vm;
 
 use crate::dhcp_snooper::DhcpSnooper;
 use crate::host::Host;
+use crate::host::NetType;
 use crate::poller::Poller;
 use crate::vm::VM;
 use anyhow::Result;
+pub use exposed_port::ExposedPort;
+use ipnet::Ipv4Net;
 use mac_address::MacAddress;
+use port_forwarder::PortForwarder;
+use prefix_trie::{Prefix, PrefixSet};
 use smoltcp::wire::EthernetFrame;
 use std::io::ErrorKind;
 use std::os::unix::io::{AsRawFd, RawFd};
 
-pub struct Proxy {
+pub struct Proxy<'proxy> {
     vm: VM,
     host: Host,
-    poller: Poller,
+    poller: Poller<'proxy>,
     vm_mac_address: smoltcp::wire::EthernetAddress,
     dhcp_snooper: DhcpSnooper,
+    allow: PrefixSet<Ipv4Net>,
     enobufs_encountered: bool,
+    port_forwarder: PortForwarder,
 }
 
-impl Proxy {
-    pub fn new(vm_fd: RawFd, vm_mac_address: MacAddress) -> Result<Proxy> {
+impl Proxy<'_> {
+    pub fn new<'proxy>(
+        vm_fd: RawFd,
+        vm_mac_address: MacAddress,
+        vm_net_type: NetType,
+        allow: PrefixSet<Ipv4Net>,
+        exposed_ports: Vec<ExposedPort>,
+    ) -> Result<Proxy<'proxy>> {
         let vm = VM::new(vm_fd)?;
-        let host = Host::new()?;
+        let host = Host::new(vm_net_type, !allow.contains(&Ipv4Net::zero()))?;
         let poller = Poller::new(vm.as_raw_fd(), host.as_raw_fd())?;
 
         Ok(Proxy {
@@ -33,7 +48,9 @@ impl Proxy {
             poller,
             vm_mac_address: smoltcp::wire::EthernetAddress(vm_mac_address.bytes()),
             dhcp_snooper: Default::default(),
+            allow,
             enobufs_encountered: false,
+            port_forwarder: PortForwarder::new(exposed_ports),
         })
     }
 
@@ -58,6 +75,12 @@ impl Proxy {
                 return Ok(());
             }
 
+            // Timeout
+            if !vm_readable && !host_readable && !interrupt {
+                self.port_forwarder
+                    .tick(&mut self.host, self.dhcp_snooper.lease());
+            }
+
             self.poller.rearm()?;
         }
     }

lib/proxy/port_forwarder.rs

@@ -0,0 +1,99 @@
+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(())
+    }
+}

lib/proxy/vm.rs

@@ -2,12 +2,13 @@ use crate::proxy::udp_packet_helper::UdpPacketHelper;
 use crate::proxy::Proxy;
 use anyhow::Context;
 use anyhow::Result;
+use ipnet::Ipv4Net;
 use smoltcp::wire::{
     ArpPacket, EthernetFrame, EthernetProtocol, IpProtocol, Ipv4Packet, UdpPacket,
 };
 use std::net::Ipv4Addr;
 
-impl Proxy {
+impl Proxy<'_> {
     pub(crate) fn process_frame_from_vm(&mut self, frame: EthernetFrame<&[u8]>) -> Result<()> {
         if self.allowed_from_vm(&frame).is_none() {
             // Block packet by not forwarding it to the host
@@ -47,7 +48,7 @@ impl Proxy {
         let source_protocol_addr = Ipv4Addr::from(source_protocol_addr);
 
         if let Some(lease) = self.dhcp_snooper.lease() {
-            if lease.valid_ip_source(source_protocol_addr.into()) {
+            if lease.valid_ip_source(source_protocol_addr) {
                 return Some(());
             }
         } else if source_protocol_addr.is_unspecified() {
@@ -58,15 +59,26 @@ impl Proxy {
     }
 
     fn allowed_from_vm_ipv4(&self, ipv4_pkt: Ipv4Packet<&[u8]>) -> Option<()> {
-        // Once we've learned the VM's IP from the DHCP snooping,
-        // allow all global traffic for that VM's IP
+        // Have we learned the VM's IP from the DHCP snooping?
         if let Some(lease) = &self.dhcp_snooper.lease() {
-            let dst_is_global =
-                ip_network::IpNetwork::from(Ipv4Addr::from(ipv4_pkt.dst_addr().0)).is_global();
+            // If so, allow all global traffic
+            let dst_addr = ipv4_pkt.dst_addr();
+            let dst_is_global = ip_network::IpNetwork::from(dst_addr).is_global();
 
             if lease.valid_ip_source(ipv4_pkt.src_addr()) && dst_is_global {
                 return Some(());
             }
+
+            // Also allow all traffic to the user-specified CIDRs
+            let dst_net = Ipv4Net::from(dst_addr);
+
+            // Use get_lpm() instead of get_spm() to work around prefix-trie
+            // not handling prefixes like 0.0.0.0/0 correctly[1]
+            //
+            // [1]: https://github.com/tiborschneider/prefix-trie/issues/8
+            if self.allow.get_lpm(&dst_net).is_some() {
+                return Some(());
+            }
         }
 
         // Allow communication with host
@@ -74,7 +86,7 @@ impl Proxy {
             return Some(());
         }
 
-        if ipv4_pkt.protocol() == IpProtocol::Udp {
+        if ipv4_pkt.next_header() == IpProtocol::Udp {
             let udp_pkt = UdpPacket::new_checked(ipv4_pkt.payload()).ok()?;
 
             // Allow DNS communication with the DNS-servers provided by DHCP

rust-toolchain

@@ -1 +0,0 @@
-nightly

rust-toolchain.toml

@@ -0,0 +1,2 @@
+[toolchain]
+channel = "nightly"

src/main.rs

@@ -1,8 +1,14 @@
 use anyhow::{anyhow, Context};
 use clap::Parser;
+use ipnet::Ipv4Net;
+use log::LevelFilter;
 use nix::sys::signal::{signal, SigHandler, Signal};
+use oslog::OsLogger;
+use prefix_trie::PrefixSet;
 use privdrop::PrivDrop;
+use softnet::proxy::ExposedPort;
 use softnet::proxy::Proxy;
+use softnet::NetType;
 use std::borrow::Cow;
 use std::env;
 use std::os::raw::c_int;
@@ -28,6 +34,9 @@ struct Args {
     #[clap(long, help = "MAC address to enforce for the VM")]
     vm_mac_address: mac_address::MacAddress,
 
+    #[clap(long, value_enum, help = "type of network to use for the VM", default_value_t=NetType::Nat)]
+    vm_net_type: NetType,
+
     #[clap(
         long,
         help = "set bootpd(8) lease time to this value (in seconds) before starting the VM",
@@ -41,6 +50,24 @@ struct Args {
     #[clap(long, help = "group name to drop privileges to")]
     group: Option<String>,
 
+    #[clap(
+        long,
+        help = "comma-separated list of CIDRs to allow the traffic to (e.g. --allow=192.168.0.0/24)",
+        value_name = "comma-separated CIDRs",
+        use_value_delimiter = true,
+        action = clap::ArgAction::Set
+    )]
+    allow: Vec<Ipv4Net>,
+
+    #[clap(
+        long,
+        help = "comma-separated list of TCP ports to expose (e.g. --expose 2222:22,8080:80)",
+        value_name = "comma-separated port specifications",
+        use_value_delimiter = true,
+        action = clap::ArgAction::Set
+    )]
+    expose: Vec<ExposedPort>,
+
     #[clap(long, hide = true)]
     sudo_escalation_probing: bool,
 
@@ -85,6 +112,11 @@ fn main() -> ExitCode {
 }
 
 fn try_main() -> anyhow::Result<()> {
+    // Initialize logger
+    OsLogger::new("org.cirruslabs.softnet")
+        .level_filter(LevelFilter::Info)
+        .init()?;
+
     // The default signal(3)[1] action for SIGINT is to interrupt program,
     // but we want to handle SIGINT ourselves, so we ignore it. The kqueue(2)'s[2]
     // EVFILT_SIGNAL will receive it anyways, because it has lower precedence.
@@ -140,8 +172,14 @@ fn try_main() -> anyhow::Result<()> {
     set_bootpd_lease_time(args.bootpd_lease_time);
 
     // Initialize the proxy while still having the root privileges
-    let mut proxy = Proxy::new(args.vm_fd as RawFd, args.vm_mac_address)
-        .context("failed to initialize proxy")?;
+    let mut proxy = Proxy::new(
+        args.vm_fd as RawFd,
+        args.vm_mac_address,
+        args.vm_net_type,
+        PrefixSet::from_iter(args.allow),
+        args.expose,
+    )
+    .context("failed to initialize proxy")?;
 
     // Drop effective privileges to the user
     // and group which have had invoked us