--allow: support "@host" syntax (#160)

* --allow: support "@host" syntax

To make communication with the guest VM possible
when using --block=0.0.0.0/0.

* Use "@-alias" wording instead of "@host"

Cargo.toml

@@ -16,7 +16,7 @@ smoltcp = "0"
 libc = "0"
 polling = "3"
 dhcproto = { git = "https://github.com/bluecatengineering/dhcproto.git", branch = "master" }
-vmnet = "0.5.0"
+vmnet = "0.5.1"
 clap = { version = "4", features = ["derive"] }
 mac_address = "1"
 privdrop = "0"
@@ -33,6 +33,7 @@ ipnet = "2"
 oslog = "0.2.0"
 log = "0.4.29"
 serial_test = "3"
+coarsetime = "0.1.37"
 
 [profile.release]
 debug = true

lib/dhcp_snooper.rs

@@ -2,14 +2,22 @@ use dhcproto::Decodable;
 use dhcproto::v4::{DhcpOption, MessageType, OptionCode};
 use smoltcp::wire::Ipv4Address;
 use std::collections::HashSet;
-use std::time::{Duration, Instant};
+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);
 
@@ -32,11 +40,12 @@ impl DhcpSnooper {
                     _ => HashSet::new(),
                 };
 
-                self.vm_lease = Some(Lease::new(
-                    message.yiaddr(),
-                    Duration::from_secs(*lease_time as u64),
-                    dns_ips,
-                ))
+                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;
@@ -66,7 +75,7 @@ impl DhcpSnooper {
 #[derive(Debug)]
 pub struct Lease {
     address: Ipv4Address,
-    valid_until: Instant,
+    valid_until: coarsetime::Instant,
     dns_ips: HashSet<Ipv4Address>,
 }
 
@@ -74,7 +83,7 @@ impl Lease {
     pub fn new(address: Ipv4Address, lease_time: Duration, dns_ips: HashSet<Ipv4Address>) -> Lease {
         Lease {
             address,
-            valid_until: Instant::now() + lease_time,
+            valid_until: coarsetime::Instant::recent() + lease_time.into(),
             dns_ips,
         }
     }
@@ -84,7 +93,7 @@ impl Lease {
     }
 
     pub fn valid(&self) -> bool {
-        Instant::now() < self.valid_until
+        coarsetime::Instant::recent() < self.valid_until
     }
 
     pub fn valid_ip_source(&self, address: Ipv4Address) -> bool {

lib/poller.rs

@@ -9,6 +9,7 @@ use std::time::Duration;
 pub struct Poller<'poller> {
     poller: polling::Poller,
     events: polling::Events,
+    timeout: Duration,
     vm_fd: BorrowedFd<'poller>,
     host_fd: BorrowedFd<'poller>,
 }
@@ -22,12 +23,17 @@ enum EventKey {
 }
 
 impl Poller<'_> {
-    pub fn new<'poller>(vm_fd: RawFd, host_fd: RawFd) -> Result<Poller<'poller>> {
+    pub fn new<'poller>(
+        vm_fd: RawFd,
+        host_fd: RawFd,
+        timeout: Duration,
+    ) -> Result<Poller<'poller>> {
         let poller = polling::Poller::new()?;
 
         Ok(Poller {
             poller,
             events: polling::Events::new(),
+            timeout,
             vm_fd: unsafe { BorrowedFd::borrow_raw(vm_fd) },
             host_fd: unsafe { BorrowedFd::borrow_raw(host_fd) },
         })
@@ -59,8 +65,7 @@ impl Poller<'_> {
     }
 
     pub fn wait(&mut self) -> Result<(bool, bool, bool)> {
-        self.poller
-            .wait(&mut self.events, Some(Duration::from_millis(100)))?;
+        self.poller.wait(&mut self.events, Some(self.timeout))?;
 
         let vm_readable = self
             .events

lib/proxy/mod.rs

@@ -14,10 +14,12 @@ pub use exposed_port::ExposedPort;
 use ipnet::Ipv4Net;
 use mac_address::MacAddress;
 use port_forwarder::PortForwarder;
-use prefix_trie::{Prefix, PrefixMap, PrefixSet};
+use prefix_trie::{Prefix, PrefixMap};
 use smoltcp::wire::EthernetFrame;
 use std::io::ErrorKind;
 use std::os::unix::io::{AsRawFd, RawFd};
+use std::str::FromStr;
+use std::time::Duration;
 use vmnet::Batch;
 
 pub struct Proxy<'proxy> {
@@ -31,6 +33,24 @@ pub struct Proxy<'proxy> {
     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,
@@ -42,13 +62,17 @@ impl Proxy<'_> {
         vm_fd: RawFd,
         vm_mac_address: MacAddress,
         vm_net_type: NetType,
-        allow: PrefixSet<Ipv4Net>,
-        block: PrefixSet<Ipv4Net>,
+        allow: Vec<Target>,
+        block: Vec<Target>,
         exposed_ports: Vec<ExposedPort>,
     ) -> Result<Proxy<'proxy>> {
         let vm = VM::new(vm_fd)?;
-        let host = Host::new(vm_net_type, !allow.contains(&Ipv4Net::zero()))?;
-        let poller = Poller::new(vm.as_raw_fd(), host.as_raw_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
         //
@@ -56,12 +80,22 @@ impl Proxy<'_> {
         // over allowing rules when prefixes are identical.
         let mut rules = PrefixMap::new();
 
-        for allow_net in allow {
-            rules.insert(allow_net, Action::Allow);
+        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);
         }
 
-        for block_net in block {
-            rules.insert(block_net, Action::Block);
+        for block_target in block {
+            let block_prefix = match block_target {
+                Target::Prefix(prefix) => prefix,
+                Target::Host => host.gateway_ip.into(),
+            };
+
+            rules.insert(block_prefix, Action::Block);
         }
 
         Ok(Proxy {
@@ -69,7 +103,7 @@ impl Proxy<'_> {
             host,
             poller,
             vm_mac_address: smoltcp::wire::EthernetAddress(vm_mac_address.bytes()),
-            dhcp_snooper: Default::default(),
+            dhcp_snooper: DhcpSnooper::new(poller_timeout),
             rules,
             enobufs_encountered: false,
             port_forwarder: PortForwarder::new(exposed_ports),
@@ -92,6 +126,9 @@ impl Proxy<'_> {
         loop {
             let (vm_readable, host_readable, interrupt) = self.poller.wait()?;
 
+            // Update coarse time for the DHCP snooper
+            coarsetime::Instant::update();
+
             if vm_readable {
                 self.read_from_vm(buf.as_mut_slice())?;
             }
@@ -119,6 +156,9 @@ impl Proxy<'_> {
         loop {
             match self.vm.read(buf) {
                 Ok(n) => {
+                    // Update coarse time for the DHCP snooper
+                    coarsetime::Instant::update();
+
                     if let Ok(frame) = EthernetFrame::new_checked(&buf[..n]) {
                         self.process_frame_from_vm(frame)?;
                     }
@@ -138,6 +178,9 @@ impl Proxy<'_> {
         loop {
             match self.host.read(batch, bufs) {
                 Ok(pktcnt) => {
+                    // Update coarse time for the DHCP snooper
+                    coarsetime::Instant::update();
+
                     for buf in batch.packet_sized_bufs(bufs).take(pktcnt) {
                         if let Ok(pkt) = EthernetFrame::new_checked(buf) {
                             self.process_frame_from_host(&pkt)?;
@@ -164,7 +207,7 @@ mod tests {
     use ipnet::Ipv4Net;
     use mac_address::MacAddress;
     use nix::sys::socket::{AddressFamily, SockFlag, SockType, socketpair};
-    use prefix_trie::{PrefixMap, PrefixSet};
+    use prefix_trie::PrefixMap;
     use serial_test::serial;
     use smoltcp::wire::{Ipv4Address, Ipv4Packet};
     use std::collections::HashSet;
@@ -208,6 +251,27 @@ mod tests {
         assert!(allowed_from_vm_ipv4(&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"]);
+
+        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),
+            ])
+        );
+
+        // 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());
+
+        // 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());
+    }
+
     fn create_proxy<'test>(vm_ip: Ipv4Address, allow: Vec<&str>, block: Vec<&str>) -> Proxy<'test> {
         let (vm_fd, _) = socketpair(
             AddressFamily::Unix,
@@ -222,16 +286,14 @@ mod tests {
             vm_fd.as_raw_fd(),
             MacAddress::from_str("02:00:00:00:00:01").unwrap(),
             NetType::Nat,
-            PrefixSet::from_iter(
-                allow
-                    .into_iter()
-                    .map(|cidr| Ipv4Net::from_str(cidr).unwrap()),
-            ),
-            PrefixSet::from_iter(
-                block
-                    .into_iter()
-                    .map(|cidr| Ipv4Net::from_str(cidr).unwrap()),
-            ),
+            allow
+                .into_iter()
+                .map(|cidr| cidr.parse().unwrap())
+                .collect(),
+            block
+                .into_iter()
+                .map(|cidr| cidr.parse().unwrap())
+                .collect(),
             Vec::default(),
         )
         .unwrap();

src/main.rs

@@ -1,22 +1,19 @@
-use anyhow::{Context, Error, anyhow};
+use anyhow::{Context, anyhow};
 use clap::Parser;
-use ipnet::Ipv4Net;
 use log::LevelFilter;
 use nix::sys::signal::{SigHandler, Signal, signal};
 use oslog::OsLogger;
-use prefix_trie::PrefixSet;
 use privdrop::PrivDrop;
 use softnet::NetType;
 use softnet::proxy::ExposedPort;
 use softnet::proxy::Proxy;
+use softnet::proxy::Target;
 use std::borrow::Cow;
 use std::env;
-use std::net::{Ipv4Addr, SocketAddr, ToSocketAddrs};
 use std::os::raw::c_int;
 use std::os::unix::io::RawFd;
 use std::os::unix::process::CommandExt;
 use std::process::{Command, ExitCode};
-use std::str::FromStr;
 use system_configuration::core_foundation::base::TCFType;
 use system_configuration::core_foundation::dictionary::CFDictionary;
 use system_configuration::core_foundation::number::CFNumber;
@@ -25,67 +22,6 @@ use system_configuration::preferences::SCPreferences;
 use system_configuration::sys::preferences::{SCPreferencesCommitChanges, SCPreferencesSetValue};
 use uzers::{get_current_groupname, get_current_username, get_effective_uid};
 
-#[derive(Debug, Clone)]
-enum AllowBlockEntry {
-    Net(Ipv4Net),
-    Domain(String),
-}
-
-impl FromStr for AllowBlockEntry {
-    type Err = Error;
-
-    fn from_str(s: &str) -> Result<Self, Self::Err> {
-        let trimmed = s.trim();
-        if trimmed.is_empty() {
-            return Err(anyhow!("empty allow/block entry"));
-        }
-
-        if let Ok(net) = trimmed.parse::<Ipv4Net>() {
-            return Ok(AllowBlockEntry::Net(net));
-        }
-
-        if let Ok(addr) = trimmed.parse::<Ipv4Addr>() {
-            return Ok(AllowBlockEntry::Net(Ipv4Net::from(addr)));
-        }
-
-        Ok(AllowBlockEntry::Domain(trimmed.to_string()))
-    }
-}
-
-fn resolve_allow_block_entries(
-    kind: &str,
-    entries: Vec<AllowBlockEntry>,
-) -> anyhow::Result<Vec<Ipv4Net>> {
-    let mut nets = Vec::new();
-
-    for entry in entries {
-        match entry {
-            AllowBlockEntry::Net(net) => nets.push(net),
-            AllowBlockEntry::Domain(domain) => {
-                // A-record resolution happens here via ToSocketAddrs, then we keep only IPv4s.
-                let resolved: Vec<Ipv4Net> = (domain.as_str(), 0)
-                    .to_socket_addrs()
-                    .with_context(|| format!("failed to resolve {kind} entry {domain}"))?
-                    .filter_map(|addr| match addr {
-                        SocketAddr::V4(v4) => Some(Ipv4Net::from(*v4.ip())),
-                        SocketAddr::V6(_) => None,
-                    })
-                    .collect();
-
-                if resolved.is_empty() {
-                    return Err(anyhow!(
-                        "no IPv4 addresses found for {kind} entry {domain}"
-                    ));
-                }
-
-                nets.extend(resolved);
-            }
-        }
-    }
-
-    Ok(nets)
-}
-
 #[derive(Parser, Debug)]
 struct Args {
     #[clap(
@@ -115,32 +51,34 @@ struct Args {
 
     #[clap(
         long,
-        help = "Comma-separated list of CIDRs, IPs, or domains to allow the traffic to \
-        (e.g. --allow=192.168.0.0/24 or --allow=example.com). Domains are resolved to A records \
-        at startup. \
+        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/IPs/domains",
+        value_name = "comma-separated CIDRs or @-alias",
         use_value_delimiter = true,
         action = clap::ArgAction::Set
     )]
-    allow: Vec<AllowBlockEntry>,
+    allow: Vec<Target>,
 
     #[clap(
         long,
-        help = "Comma-separated list of CIDRs, IPs, or domains to block the traffic to \
+        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). Domains are resolved to A records at startup. \
-        When used with --allow, \
-        the longest prefix match always wins. In case the same prefix is both \
-        --allow'ed and --block'ed, blocking takes precedence.",
-        value_name = "comma-separated CIDRs/IPs/domains",
+        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",
         use_value_delimiter = true,
         action = clap::ArgAction::Set
     )]
-    block: Vec<AllowBlockEntry>,
+    block: Vec<Target>,
 
     #[clap(
         long,
@@ -210,7 +148,7 @@ fn try_main() -> anyhow::Result<()> {
     // [2]: https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/kqueue.2.html
     unsafe { signal(Signal::SIGINT, SigHandler::SigIgn) }?;
 
-    let mut args: Args = Args::parse();
+    let args: Args = Args::parse();
 
     // No need to run anything, just return
     // so that the invoker process knows we
@@ -253,9 +191,6 @@ fn try_main() -> anyhow::Result<()> {
         ));
     }
 
-    let allow = resolve_allow_block_entries("allow", std::mem::take(&mut args.allow))?;
-    let block = resolve_allow_block_entries("block", std::mem::take(&mut args.block))?;
-
     // Set bootpd(8) min/max lease time while still having the root privileges
     set_bootpd_lease_time(args.bootpd_lease_time);
 
@@ -264,8 +199,8 @@ fn try_main() -> anyhow::Result<()> {
         args.vm_fd as RawFd,
         args.vm_mac_address,
         args.vm_net_type,
-        PrefixSet::from_iter(allow),
-        PrefixSet::from_iter(block),
+        args.allow,
+        args.block,
         args.expose,
     )
     .context("failed to initialize proxy")?;
@@ -317,24 +252,3 @@ fn set_bootpd_lease_time(lease_time: u32) {
         SCPreferencesCommitChanges(prefs.as_concrete_TypeRef());
     }
 }
-
-#[cfg(test)]
-mod tests {
-    use super::{AllowBlockEntry, resolve_allow_block_entries};
-    use ipnet::Ipv4Net;
-    use std::net::Ipv4Addr;
-
-    #[test]
-    fn resolve_domain_to_ipv4_nets_dns_google() {
-        let nets = resolve_allow_block_entries(
-            "allow",
-            vec![AllowBlockEntry::Domain("dns.google".to_string())],
-        )
-        .unwrap();
-
-        assert!(
-            nets.contains(&Ipv4Net::from(Ipv4Addr::new(8, 8, 8, 8)))
-                || nets.contains(&Ipv4Net::from(Ipv4Addr::new(8, 8, 4, 4)))
-        );
-    }
-}