Implement DNS notifications from resolved via varlink

* The new varlink interface exposes a method to subscribe to DNS
resolutions on the system. The socket permissions are open for owner and
group only.
* Notifications are sent to subscriber(s), if any, after successful
resolution of A and AAAA records.

This feature could be used by applications for auditing/logging services
downstream of the resolver. It could also be used to asynchronously
update the firewall. For example, a system that has a tightly configured
firewall could open up connections selectively to known good hosts based
on a known allow-list of hostnames. Of course, updating the firewall
asynchronously will require other design considerations (such as
queueing packets in the user space while a verdict is made).

See also:
https://lists.freedesktop.org/archives/systemd-devel/2022-August/048202.html
https://lists.freedesktop.org/archives/systemd-devel/2022-February/047441.html

test/units/testsuite-75.sh

@@ -8,6 +8,8 @@ set -o pipefail
 : >/failed
 
 RUN_OUT="$(mktemp)"
+NOTIFICATION_SUBSCRIPTION_SCRIPT="/tmp/subscribe.sh"
+NOTIFICATION_LOGS="/tmp/notifications.txt"
 
 run() {
     "$@" |& tee "$RUN_OUT"
@@ -34,10 +36,22 @@ DNSSEC=allow-downgrade
 DNS=10.0.0.1
 EOF
 
+# Script to dump DNS notifications to a txt file
+cat >$NOTIFICATION_SUBSCRIPTION_SCRIPT <<EOF
+#!/bin/sh
+printf '
+{
+  "method": "io.systemd.Resolve.Monitor.SubscribeDnsResolves",
+  "more": true
+}\0' | nc -U /run/systemd/resolve/io.systemd.Resolve.Monitor > $NOTIFICATION_LOGS
+EOF
+chmod a+x $NOTIFICATION_SUBSCRIPTION_SCRIPT
+
 {
     echo "FallbackDNS="
     echo "DNSSEC=allow-downgrade"
     echo "DNSOverTLS=opportunistic"
+    echo "Monitor=yes"
 } >>/etc/systemd/resolved.conf
 ln -svf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf
 # Override the default NTA list, which turns off DNSSEC validation for (among
@@ -78,6 +92,13 @@ networkctl status
 resolvectl status
 resolvectl log-level debug
 
+# Verify that DNS notifications are enabled (Monitor=yes)
+run busctl get-property org.freedesktop.resolve1 /org/freedesktop/resolve1 org.freedesktop.resolve1.Manager Monitor
+grep -qF 'b true' "$RUN_OUT"
+
+# Start monitoring DNS notifications
+systemd-run $NOTIFICATION_SUBSCRIPTION_SCRIPT
+
 # We need to manually propagate the DS records of onlinesign.test. to the parent
 # zone, since they're generated online
 knotc zone-begin test.
@@ -99,6 +120,7 @@ knotc reload
 # Sanity check
 run getent -s resolve hosts ns1.unsigned.test
 grep -qE "^10\.0\.0\.1\s+ns1\.unsigned\.test" "$RUN_OUT"
+grep -aF "ns1.unsigned.test" $NOTIFICATION_LOGS | grep -qF "[10,0,0,1]"
 
 # Issue: https://github.com/systemd/systemd/issues/18812
 # PR: https://github.com/systemd/systemd/pull/18896
@@ -191,6 +213,7 @@ grep -qF "; fully validated" "$RUN_OUT"
 run resolvectl query -t A cname-chain.signed.test
 grep -qF "follow14.final.signed.test IN A 10.0.0.14" "$RUN_OUT"
 grep -qF "authenticated: yes" "$RUN_OUT"
+grep -aF "cname-chain.signed.test" $NOTIFICATION_LOGS | grep -qF "[10,0,0,14]"
 # Non-existing RR + CNAME chain
 run dig +dnssec AAAA cname-chain.signed.test
 grep -qF "status: NOERROR" "$RUN_OUT"
@@ -226,6 +249,10 @@ run resolvectl query -t TXT this.should.be.authenticated.wild.onlinesign.test
 grep -qF 'this.should.be.authenticated.wild.onlinesign.test IN TXT "this is an onlinesign wildcard"' "$RUN_OUT"
 grep -qF "authenticated: yes" "$RUN_OUT"
 
+# Resolve via dbus method
+run busctl call org.freedesktop.resolve1 /org/freedesktop/resolve1 org.freedesktop.resolve1.Manager ResolveHostname 'isit' 0 secondsub.onlinesign.test 0 0
+grep -qF '10 0 0 134 "secondsub.onlinesign.test"' "$RUN_OUT"
+grep -aF "secondsub.onlinesign.test" $NOTIFICATION_LOGS | grep -qF "[10,0,0,134]"
 
 : "--- ZONE: untrusted.test (DNSSEC without propagated DS records) ---"
 run dig +short untrusted.test
@@ -244,6 +271,7 @@ grep -qF "authenticated: no" "$RUN_OUT"
 #run dig +dnssec this.does.not.exist.untrusted.test
 #grep -qF "status: NXDOMAIN" "$RUN_OUT"
 
+cat $NOTIFICATION_LOGS
 
 touch /testok
 rm /failed