fix: cache resolvers via OnceLock, blocklist takes precedence, use TEST-NET IPs in tests

Signed-off-by: danbugs <danilochiarlone@gmail.com>

Author: danbugs

host/src/lib.rs

@@ -304,52 +304,47 @@ impl BlockList {
     }
 }
 
-/// Parse `/etc/resolv.conf` and return the set of nameserver IPs.
-///
-/// On non-Unix platforms this returns an empty set (DNS goes through the
-/// system resolver and never needs a raw port-53 exemption).
-fn parse_resolv_conf() -> HashSet<IpAddr> {
-    #[cfg(unix)]
-    {
-        let mut resolvers = HashSet::new();
-        if let Ok(contents) = std::fs::read_to_string("/etc/resolv.conf") {
-            for line in contents.lines() {
-                let line = line.trim();
-                if let Some(rest) = line.strip_prefix("nameserver") {
-                    if let Some(ip_str) = rest.split_whitespace().next() {
-                        if let Ok(ip) = ip_str.parse::<IpAddr>() {
-                            resolvers.insert(ip);
+fn dns_resolvers() -> &'static HashSet<IpAddr> {
+    static RESOLVERS: std::sync::OnceLock<HashSet<IpAddr>> = std::sync::OnceLock::new();
+    RESOLVERS.get_or_init(|| {
+        #[cfg(unix)]
+        {
+            let mut set = HashSet::new();
+            if let Ok(contents) = std::fs::read_to_string("/etc/resolv.conf") {
+                for line in contents.lines() {
+                    let line = line.trim();
+                    if let Some(rest) = line.strip_prefix("nameserver") {
+                        if let Some(ip_str) = rest.split_whitespace().next() {
+                            if let Ok(ip) = ip_str.parse::<IpAddr>() {
+                                set.insert(ip);
+                            }
                         }
                     }
                 }
             }
+            set
         }
-        resolvers
-    }
-    #[cfg(not(unix))]
-    {
-        HashSet::new()
-    }
+        #[cfg(not(unix))]
+        {
+            HashSet::new()
+        }
+    })
 }
 
 impl NetworkPolicy {
     fn check(&self, addr: &std::net::SocketAddr) -> Result<()> {
         match self {
             NetworkPolicy::AllowAll => Ok(()),
             NetworkPolicy::AllowList(al) => {
-                if (addr.port() == 53 && parse_resolv_conf().contains(&addr.ip()))
-                    || al.is_allowed(&addr.ip())
+                if al.is_allowed(&addr.ip())
+                    || (addr.port() == 53 && dns_resolvers().contains(&addr.ip()))
                 {
                     Ok(())
                 } else {
                     Err(anyhow!("network policy denies connection to {}", addr))
                 }
             }
             NetworkPolicy::BlockList(bl) => {
-                // Allow DNS (port 53) only to configured resolver IPs.
-                if addr.port() == 53 && parse_resolv_conf().contains(&addr.ip()) {
-                    return Ok(());
-                }
                 if bl.is_blocked(&addr.ip()) {
                     Err(anyhow!("network policy denies connection to {}", addr))
                 } else {
@@ -2656,11 +2651,10 @@ mod tests {
 
     #[test]
     fn test_port53_arbitrary_ip_blocked() {
-        // Only 1.1.1.1 is in the allowlist — 8.8.8.8:53 must be denied
-        // because 8.8.8.8 is (almost certainly) not in /etc/resolv.conf.
-        let al = AllowList::from_hosts(&["1.1.1.1"]).unwrap();
+        // RFC5737 TEST-NET-2 address — guaranteed not in /etc/resolv.conf.
+        let al = AllowList::from_hosts(&["198.51.100.1"]).unwrap();
         let policy = NetworkPolicy::AllowList(al);
-        let addr: std::net::SocketAddr = "8.8.8.8:53".parse().unwrap();
+        let addr: std::net::SocketAddr = "198.51.100.99:53".parse().unwrap();
         assert!(
             policy.check(&addr).is_err(),
             "port 53 to a non-resolver IP must be denied"
@@ -2670,16 +2664,14 @@ mod tests {
     #[cfg(unix)]
     #[test]
     fn test_port53_real_resolver_allowed() {
-        // Grab the first resolver from /etc/resolv.conf and verify port
-        // 53 to that IP is allowed even when the IP is not in the
-        // AllowList itself.
-        let resolvers = parse_resolv_conf();
+        let resolvers = dns_resolvers();
         if resolvers.is_empty() {
             eprintln!("skipping: no resolvers found in /etc/resolv.conf");
             return;
         }
         let resolver_ip = *resolvers.iter().next().unwrap();
-        let al = AllowList::from_hosts(&["1.1.1.1"]).unwrap();
+        // Allowlist uses a TEST-NET IP that won't match the resolver.
+        let al = AllowList::from_hosts(&["198.51.100.1"]).unwrap();
         let policy = NetworkPolicy::AllowList(al);
         let addr = std::net::SocketAddr::new(resolver_ip, 53);
         assert!(
@@ -2691,8 +2683,7 @@ mod tests {
 
     #[test]
     fn test_port53_blocklist_enforced() {
-        // Even on port 53, a blocklisted IP must be denied — unless it
-        // happens to be a real resolver. Use a bogus IP to be safe.
+        // RFC5737 TEST-NET-1 address — blocklist always wins.
         let bl = BlockList::from_hosts(&["192.0.2.1"]).unwrap();
         let policy = NetworkPolicy::BlockList(bl);
         let addr: std::net::SocketAddr = "192.0.2.1:53".parse().unwrap();
@@ -2735,13 +2726,13 @@ mod tests {
 
     #[test]
     fn network_policy_blocklist_denies_blocked_ip_on_port53() {
-        // A blocked IP must not be exempted just because the port is 53.
-        let bl = BlockList::from_hosts(&["1.2.3.4"]).unwrap();
+        // RFC5737 TEST-NET-3 — blocklist always wins over DNS exemption.
+        let bl = BlockList::from_hosts(&["203.0.113.1"]).unwrap();
         let policy = NetworkPolicy::BlockList(bl);
-        let addr: std::net::SocketAddr = "1.2.3.4:53".parse().unwrap();
+        let addr: std::net::SocketAddr = "203.0.113.1:53".parse().unwrap();
         assert!(
             policy.check(&addr).is_err(),
-            "blocked IP 1.2.3.4 must be denied even on port 53"
+            "blocked IP must be denied even on port 53"
         );
     }