--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"

lib/proxy/mod.rs

@@ -14,10 +14,11 @@ 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;
 
@@ -32,6 +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,
@@ -43,12 +62,15 @@ impl Proxy<'_> {
         vm_fd: RawFd,
         vm_mac_address: MacAddress,
         vm_net_type: NetType,
-        allow: PrefixSet<Ipv4Net>,
+        allow: Vec<Target>,
-        block: PrefixSet<Ipv4Net>,
+        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 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)?;
 
@@ -58,12 +80,22 @@ impl Proxy<'_> {
         // over allowing rules when prefixes are identical.
         let mut rules = PrefixMap::new();
 
-        for allow_net in allow {
+        for allow_target in allow {
-            rules.insert(allow_net, Action::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 {
+        for block_target in block {
-            rules.insert(block_net, Action::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 {
@@ -175,7 +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;
@@ -219,6 +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,
@@ -233,16 +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()),
+                .map(|cidr| cidr.parse().unwrap())
-            ),
+                .collect(),
-            PrefixSet::from_iter(
             block
                 .into_iter()
-                    .map(|cidr| Ipv4Net::from_str(cidr).unwrap()),
+                .map(|cidr| cidr.parse().unwrap())
-            ),
+                .collect(),
             Vec::default(),
         )
         .unwrap();

src/main.rs

@@ -1,14 +1,13 @@
 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::os::raw::c_int;
@@ -53,29 +52,33 @@ 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). \
+        (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",
+        value_name = "comma-separated CIDRs or @-alias",
         use_value_delimiter = true,
         action = clap::ArgAction::Set
     )]
-    allow: Vec<Ipv4Net>,
+    allow: Vec<Target>,
 
     #[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). When used with --allow, \
+        that is further relaxed with --allow), plus supported @-aliases. \
-        the longest prefix match always wins. In case the same prefix is both \
+        Currently the only supported @-alias is @host, which matches the vmnet bridge gateway IP. \
-        --allow'ed and --block'ed, blocking takes precedence.",
+        When used with --allow, the longest prefix match always wins. \
-        value_name = "comma-separated CIDRs",
+        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<Ipv4Net>,
+    block: Vec<Target>,
 
     #[clap(
         long,
@@ -196,8 +199,8 @@ fn try_main() -> anyhow::Result<()> {
         args.vm_fd as RawFd,
         args.vm_mac_address,
         args.vm_net_type,
-        PrefixSet::from_iter(args.allow),
+        args.allow,
-        PrefixSet::from_iter(args.block),
+        args.block,
         args.expose,
     )
     .context("failed to initialize proxy")?;