fix: scope DNS port-53 exemption to configured resolver IPs - #32
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Pull request overview
This PR tightens outbound network policy enforcement by removing the blanket “port 53 is always allowed” exemption and scoping DNS allowance to the host’s configured resolver IPs (from /etc/resolv.conf), preventing a guest from bypassing allow/block lists by choosing destination port 53.
Changes:
- Added
parse_resolv_conf()to parse nameserver IPs from/etc/resolv.conf(Unix; empty set otherwise). - Updated
NetworkPolicy::check()to exempt port 53 only when the destination IP is a configured resolver. - Updated/added tests to reflect the new port-53 behavior.
Comments suppressed due to low confidence (1)
host/src/lib.rs:349
- In
NetworkPolicy::BlockList, the port-53 resolver exemption is checked beforebl.is_blocked(), which means a blocklisted resolver IP will still be allowed on port 53. If blocklist entries are intended to be enforced regardless of port (as the tests/documentation suggest), checkbl.is_blocked(&addr.ip())first and only apply the DNS exemption for non-blocked IPs (or remove the exemption for blocklist entirely).
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))
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| if (addr.port() == 53 && parse_resolv_conf().contains(&addr.ip())) | ||
| || al.is_allowed(&addr.ip()) |
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| 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); | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| resolvers |
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| 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(); | ||
| let policy = NetworkPolicy::AllowList(al); | ||
| let addr: std::net::SocketAddr = "8.8.8.8:53".parse().unwrap(); | ||
| assert!( | ||
| policy.check(&addr).is_err(), | ||
| "port 53 to a non-resolver IP must be denied" | ||
| ); |
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| #[test] | ||
| fn network_policy_blocklist_allows_dns() { | ||
| 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(); | ||
| let policy = NetworkPolicy::BlockList(bl); | ||
| let addr: std::net::SocketAddr = "1.2.3.4:53".parse().unwrap(); | ||
| assert!(policy.check(&addr).is_ok()); | ||
| assert!( | ||
| policy.check(&addr).is_err(), | ||
| "blocked IP 1.2.3.4 must be denied even on port 53" | ||
| ); |
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| // Allow DNS (port 53) only to configured resolver IPs. | ||
| if addr.port() == 53 && parse_resolv_conf().contains(&addr.ip()) { | ||
| return Ok(()); | ||
| } |
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Linux Benchmarks
Details
| Benchmark suite | Current: 82fdb86 | Previous: f5eb506 | Ratio |
|---|---|---|---|
hello_world (median) |
20 ms |
20 ms |
1 |
pandas (median) |
100 ms |
110 ms |
0.91 |
density (per VM) |
7 MB |
7 MB |
1 |
snapshot (disk) |
385 MiB |
385 MiB |
1 |
This comment was automatically generated by workflow using github-action-benchmark.
Signed-off-by: danbugs <danilochiarlone@gmail.com>
…ST-NET IPs in tests Signed-off-by: danbugs <danilochiarlone@gmail.com>
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Windows Benchmarks
Details
| Benchmark suite | Current: 82fdb86 | Previous: f5eb506 | Ratio |
|---|---|---|---|
hello_world (median) |
260 ms |
221 ms |
1.18 |
pandas (median) |
827 ms |
697 ms |
1.19 |
density (per VM) |
6 MB |
6 MB |
1 |
snapshot (disk) |
393 MiB |
393 MiB |
1 |
This comment was automatically generated by workflow using github-action-benchmark.
This was referenced May 15, 2026
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Summary
NetworkPolicy::check()unconditionally allowed any connection whereaddr.port() == 53, regardless of destination IP. A guest could bypassAllowList or BlockList restrictions entirely by setting the destination port
to 53 — the guest controls this field via
parse_sockaddr.parse_resolv_conf()that reads/etc/resolv.conffor nameserver IPs(returns empty set on non-Unix)
resolver, for both AllowList and BlockList policies
Test plan
test_port53_arbitrary_ip_blocked— non-resolver IP on port 53 deniedtest_port53_real_resolver_allowed— actual resolv.conf IP on port 53 allowedtest_port53_blocklist_enforced— blocklisted IP denied even on port 53