src/main.rs

@@ -231,8 +231,23 @@ async fn main() -> numa::Result<()> {
         None
     };
 
+    let socket = match UdpSocket::bind(&config.server.bind_addr).await {
+        Ok(s) => s,
+        Err(e)
+            if e.kind() == std::io::ErrorKind::AddrInUse
+                && numa::system_dns::is_port_53(&config.server.bind_addr) =>
+        {
+            eprint!(
+                "{}",
+                numa::system_dns::port53_conflict_advisory(&config.server.bind_addr)
+            );
+            std::process::exit(1);
+        }
+        Err(e) => return Err(e.into()),
+    };
+
     let ctx = Arc::new(ServerCtx {
-        socket: UdpSocket::bind(&config.server.bind_addr).await?,
+        socket,
         zone_map: build_zone_map(&config.zones)?,
         cache: RwLock::new(DnsCache::new(
             config.cache.max_entries,

src/system_dns.rs

@@ -46,6 +46,49 @@ pub fn discover_system_dns() -> SystemDnsInfo {
     }
 }
 
+/// True if `bind_addr` targets DNS port 53. Used to scope the port-53
+/// conflict advisory — we only want to print the systemd-resolved /
+/// Dnscache hint when the user is actually trying to bind the DNS port.
+pub fn is_port_53(bind_addr: &str) -> bool {
+    bind_addr
+        .parse::<SocketAddr>()
+        .map(|s| s.port() == 53)
+        .unwrap_or(false)
+}
+
+/// Human-readable diagnostic for port-53 bind conflicts. Offers two
+/// concrete fixes: install Numa as the system resolver, or bind to a
+/// non-privileged port.
+pub fn port53_conflict_advisory(bind_addr: &str) -> String {
+    let o = "\x1b[1;38;2;192;98;58m"; // bold orange
+    let r = "\x1b[0m";
+    format!(
+        "
+{o}Numa{r} — cannot bind to {bind_addr}: port 53 is already in use.
+
+  Another process is already bound to port 53. On Linux this is
+  typically systemd-resolved; on Windows, the DNS Client service.
+
+  Fix — pick one:
+
+    1. Install Numa as the system resolver (frees port 53):
+
+         sudo numa install       (on Windows, run as Administrator)
+
+    2. Run on a non-privileged port for testing.
+       Create ~/.config/numa/numa.toml with:
+
+         [server]
+         bind_addr = \"127.0.0.1:5354\"
+         api_port  = 5380
+
+       Then run:  numa
+       Test with: dig @127.0.0.1 -p 5354 example.com
+
+"
+    )
+}
+
 #[cfg(target_os = "macos")]
 fn discover_macos() -> SystemDnsInfo {
     use log::{debug, warn};

tests/docker/smoke-port53.sh

@@ -0,0 +1,138 @@
+#!/usr/bin/env bash
+#
+# Port-53 conflict advisory integration test.
+#
+# Builds numa from source inside a debian:bookworm container, pre-binds
+# port 53 with a UDP socket, then runs numa bare (default bind_addr
+# 0.0.0.0:53). Verifies:
+#   - process exits with code 1
+#   - stderr contains the advisory ("cannot bind to")
+#   - stderr contains both fix suggestions ("numa install", "bind_addr")
+#
+# This is the end-to-end test for the fix in:
+#   src/main.rs — AddrInUse match arm → eprint advisory + process::exit(1)
+#
+# No systemd-resolved needed — the conflict is simulated by a Python
+# UDP socket held open before numa starts.
+#
+# Requirements: docker
+# Usage:        ./tests/docker/smoke-port53.sh
+
+set -euo pipefail
+
+cd "$(dirname "$0")/../.."
+
+GREEN="\033[32m"; RED="\033[31m"; RESET="\033[0m"
+
+pass() { printf "  ${GREEN}✓${RESET} %s\n" "$1"; }
+fail() { printf "  ${RED}✗${RESET} %s\n" "$1"; printf "    %s\n" "$2"; FAILED=$((FAILED+1)); }
+FAILED=0
+
+echo "── smoke-port53: building + testing numa on debian:bookworm ──"
+echo "  (first run is slow: image pull + cold cargo build, ~5-8 min)"
+echo
+
+OUTPUT=$(docker run --rm \
+    --platform linux/amd64 \
+    -v "$PWD:/src:ro" \
+    -v numa-port53-cargo:/root/.cargo \
+    -v numa-port53-target:/work/target \
+    debian:bookworm bash -c '
+set -e
+
+apt-get update -qq && apt-get install -y -qq curl build-essential python3 2>&1 | tail -3
+
+# Install rustup if not already in the cargo cache volume
+if ! command -v cargo &>/dev/null; then
+    curl -sSf https://sh.rustup.rs | sh -s -- -y --profile minimal --quiet
+fi
+. "$HOME/.cargo/env"
+
+# Copy source to a writable workdir
+mkdir -p /work
+tar -C /src --exclude=./target --exclude=./.git -cf - . | tar -C /work -xf -
+cd /work
+
+echo "── cargo build --release --locked ──"
+cargo build --release --locked 2>&1 | tail -5
+echo
+
+# Write the holder script to a file to avoid quoting hell.
+# Holds port 53 until killed — no sleep race.
+cat > /tmp/hold53.py << '"'"'PYEOF'"'"'
+import socket, signal
+s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 0)
+s.bind(("", 53))
+signal.pause()
+PYEOF
+
+python3 /tmp/hold53.py &
+HOLDER_PID=$!
+
+# Verify the holder is actually up before proceeding
+sleep 0.3
+if ! kill -0 $HOLDER_PID 2>/dev/null; then
+    echo "holder_failed=1"
+    exit 1
+fi
+
+echo "── running numa with port 53 already bound ──"
+# timeout 5: guards against numa not exiting (advisory not fired, bug present)
+# Capture stderr to a file so the exit code is not clobbered by || or $()
+set +e
+timeout 5 ./target/release/numa > /tmp/numa-stderr.txt 2>&1
+EXIT_CODE=$?
+set -e
+STDERR=$(cat /tmp/numa-stderr.txt)
+
+kill $HOLDER_PID 2>/dev/null || true
+
+echo "exit_code=$EXIT_CODE"
+printf "%s" "$STDERR" | sed "s/^/  numa: /"
+' 2>&1)
+
+echo "$OUTPUT"
+
+echo
+echo "── assertions ──"
+
+if echo "$OUTPUT" | grep -q "holder_failed=1"; then
+    echo "  SETUP FAILED: could not pre-bind port 53 inside container"
+    exit 1
+fi
+
+EXIT_CODE=$(echo "$OUTPUT" | grep '^exit_code=' | cut -d= -f2)
+
+if [ "${EXIT_CODE:-}" = "1" ]; then
+    pass "exits with code 1"
+else
+    fail "exits with code 1" "got: exit_code=${EXIT_CODE:-<missing>}"
+fi
+
+if echo "$OUTPUT" | grep -q "cannot bind to"; then
+    pass "advisory printed to stderr"
+else
+    fail "advisory printed to stderr" "stderr did not contain 'cannot bind to'"
+fi
+
+if echo "$OUTPUT" | grep -q "numa install"; then
+    pass "advisory offers 'sudo numa install'"
+else
+    fail "advisory offers 'sudo numa install'" "not found in output"
+fi
+
+if echo "$OUTPUT" | grep -q "bind_addr"; then
+    pass "advisory offers non-privileged port alternative"
+else
+    fail "advisory offers non-privileged port alternative" "'bind_addr' not found in output"
+fi
+
+echo
+if [ "$FAILED" -eq 0 ]; then
+    printf "${GREEN}── smoke-port53 passed ──${RESET}\n"
+    exit 0
+else
+    printf "${RED}── smoke-port53 failed ($FAILED assertion(s)) ──${RESET}\n"
+    exit 1
+fi