On Ubuntu 22 (systemd 249), systemd-resolved applies interface-scoped
routing to per-link DNS servers. Configuring `resolvectl dns fips0
127.0.0.1:5354` caused resolved to attempt reaching 127.0.0.1 through
fips0 (a TUN with only fd00::/8 routes), silently failing. The DNS
responder never received queries. Newer systemd versions (250+) have
explicit handling for loopback servers on non-loopback interfaces.
Changes:
- DNS responder default bind_addr changed from "127.0.0.1" to "::"
so it listens on all interfaces, including fips0. Bind logic in
lifecycle.rs now parses bind_addr as IpAddr and constructs a
SocketAddr, handling IPv6 literal formatting. Factored into
Node::bind_dns_socket with explicit IPV6_V6ONLY=0 via socket2, so
IPv4 clients on 127.0.0.1:5354 still reach the responder
regardless of the kernel's net.ipv6.bindv6only sysctl.
- fips-dns-setup resolvectl backend now waits for fips0 to have a
global IPv6 address, then configures resolved with
[<fips0-addr>]:5354. That address is locally delivered by the
kernel regardless of which interface resolved tries to route
through. The dnsmasq and NetworkManager backends still use
127.0.0.1 (they don't have the interface-scoping issue).
- Dropped hardcoded `bind_addr: "127.0.0.1"` from the packaged
fips.yaml (Debian + OpenWrt). The shipped config was overriding
the new default.
- DNS queries are only accepted from the localhost.
Verified end-to-end in a privileged Ubuntu 22.04 systemd container:
dig @127.0.0.53 AAAA <npub>.fips resolves cleanly through
systemd-resolved.
The dns-delegate backend (systemd 258+) still uses 127.0.0.1; it
has not been verified whether that backend has the same routing
issue.
Replace the resolvectl-only fips-dns.service with a detection script
that configures whichever DNS resolver is available:
1. systemd dns-delegate (systemd >= 258, declarative drop-in)
2. systemd-resolved via resolvectl (most systemd distros)
3. dnsmasq (standalone)
4. NetworkManager with dnsmasq plugin
5. Warning with manual instructions if none found
Service reloads are non-fatal — config is written and the backend
is recorded even if the reload fails, preventing state file cleanup
issues under set -e.
Teardown reads the recorded backend from /run/fips/dns-backend and
reverses the configuration, or cleans up all possible backends if
the state file is missing.
Includes a Docker-based test harness (testing/dns-resolver/test.sh)
covering all five backends across Debian 12, Debian 13, Fedora,
and bare systems.
Fixes#52.
Johnathan CorganandGitHub