networkd: add support for resolved hook for DHCP server

Let's synthesize DNS RRs for leases handed out by our DHCP server. This
way local VMs can have resolvable hostnames locally.

This does not implement reverse look ups for now. We can add this
later in similar fashion.

test/test-network/conf/25-dhcp-client-resolve-hook.network

@@ -0,0 +1,10 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Match]
+Name=veth99
+
+[Network]
+DHCP=yes
+
+[DHCPv4]
+SendHostname=yes
+Hostname=flummy

test/test-network/conf/25-dhcp-server-resolve-hook.network

@@ -0,0 +1,13 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Match]
+Name=veth-peer
+
+[Network]
+IPv6AcceptRA=false
+DHCPServer=yes
+
+[DHCPServer]
+ServerAddress=192.168.5.1/24
+PoolOffset=20
+PoolSize=10
+LocalLeaseDomain=_networkdtest

test/test-network/systemd-networkd-tests.py

@@ -426,6 +426,7 @@ def save_active_units():
     for u in [
             'systemd-networkd.socket',
             'systemd-networkd-varlink.socket',
+            'systemd-networkd-resolve-hook.socket',
             'systemd-networkd.service',
             'systemd-resolved-monitor.socket',
             'systemd-resolved-varlink.socket',
@@ -449,6 +450,10 @@ def restore_active_units():
         call('systemctl stop systemd-networkd-varlink.socket')
         has_network_socket = True
 
+    if 'systemd-networkd-resolve-hook.socket' in active_units:
+        call('systemctl stop systemd-networkd-resolve-hook.socket')
+        has_network_socket = True
+
     if 'systemd-resolved-monitor.socket' in active_units:
         call('systemctl stop systemd-resolved-monitor.socket')
         has_resolve_socket = True
@@ -525,6 +530,7 @@ def setup_system_units():
                 'systemd-networkd.service',
                 'systemd-networkd.socket',
                 'systemd-networkd-varlink.socket',
+                'systemd-networkd-resolve-hook.socket',
                 'systemd-networkd-persistent-storage.service',
                 'systemd-resolved.service',
                 'systemd-timesyncd.service',
@@ -572,6 +578,13 @@ def setup_system_units():
             'StartLimitIntervalSec=0',
         ]
     )
+    create_unit_dropin(
+        'systemd-networkd-resolve-hook.socket',
+        [
+            '[Unit]',
+            'StartLimitIntervalSec=0',
+        ]
+    )
     create_unit_dropin(
         'systemd-networkd-persistent-storage.service',
         [
@@ -604,6 +617,7 @@ def clear_system_units():
     rm_unit('systemd-networkd.service')
     rm_unit('systemd-networkd.socket')
     rm_unit('systemd-networkd-varlink.socket')
+    rm_unit('systemd-networkd-resolve-hook.socket')
     rm_unit('systemd-networkd-persistent-storage.service')
     rm_unit('systemd-resolved.service')
     rm_unit('systemd-timesyncd.service')
@@ -990,10 +1004,12 @@ def stop_networkd(show_logs=True, check_failed=True):
     if check_failed:
         check_output('systemctl stop systemd-networkd.socket')
         check_output('systemctl stop systemd-networkd-varlink.socket')
+        check_output('systemctl stop systemd-networkd-resolve-hook.socket')
         check_output('systemctl stop systemd-networkd.service')
     else:
         call('systemctl stop systemd-networkd.socket')
         call('systemctl stop systemd-networkd-varlink.socket')
+        call('systemctl stop systemd-networkd-resolve-hook.socket')
         call('systemctl stop systemd-networkd.service')
 
     if show_logs:
@@ -7366,6 +7382,16 @@ class NetworkdDHCPServerTests(unittest.TestCase, Utilities):
         data = json.loads(output)
         self.assertEqual(data['DHCPv4Client']['Lease']['Hostname'], 'fqdn.example.com')
 
+    def test_dhcp_server_resolve_hook(self):
+        copy_network_unit('25-veth.netdev', '25-dhcp-client-resolve-hook.network', '25-dhcp-server-resolve-hook.network')
+        start_networkd()
+        self.wait_online('veth99:routable', 'veth-peer:routable')
+
+        output = check_output('resolvectl query flummy._networkdtest')
+        print(output)
+        self.assertIn('192.168.5.2', output)
+
+
 class NetworkdDHCPServerRelayAgentTests(unittest.TestCase, Utilities):
 
     def setUp(self):