T6948: Keep DHCP server leases in sync with hostd records

Keep DHCP server leases in sync with vyos-hostd records
via helper script invoked with `ExecStartPost` directive
in kea-dhcp4-server.service.

The helper script updates VyOS hostd records from
DHCP server leases. This ensures that hostd records
with the DHCP server leases are kept in sync with
VyOS hostd records right after DHCP server is started.

Currently, this is implemented for DHCPv4 only.

src/op_mode/dhcp.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 #
-# Copyright (C) 2022-2024 VyOS maintainers and contributors
+# Copyright (C) 2022-2025 VyOS maintainers and contributors
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 2 or later as
@@ -25,16 +25,19 @@
 from tabulate import tabulate
 
 import vyos.opmode
+import vyos.hostsd_client
 
 from vyos.base import Warning
 from vyos.configquery import ConfigTreeQuery
 
 from vyos.kea import kea_get_active_config
 from vyos.kea import kea_get_leases
 from vyos.kea import kea_get_pool_from_subnet_id
+from vyos.kea import kea_get_domain_from_subnet_id
 from vyos.kea import kea_delete_lease
-from vyos.utils.process import is_systemd_service_running
 from vyos.utils.process import call
+from vyos.utils.process import is_systemd_service_running
+from vyos.utils.process import process_named_running
 
 time_string = "%a %b %d %H:%M:%S %Z %Y"
 
@@ -109,6 +112,7 @@ def _get_raw_server_leases(family='inet', pool=None, sorted=None, state=[], orig
         lease_state_long = {0: 'active', 1: 'rejected', 2: 'expired'}
         data_lease['state'] = lease_state_long[lease['state']]
         data_lease['pool'] = kea_get_pool_from_subnet_id(active_config, inet_suffix, lease['subnet-id']) if active_config else '-'
+        data_lease['domain'] = kea_get_domain_from_subnet_id(active_config, inet_suffix, lease['subnet-id']) if active_config else ''
         data_lease['end'] = lease['expire_timestamp'].timestamp() if lease['expire_timestamp'] else None
         data_lease['origin'] = 'local' # TODO: Determine remote in HA
         data_lease['hostname'] = lease.get('hostname', '-')
@@ -406,6 +410,39 @@ def show_server_static_mappings(raw: bool, family: ArgFamily, pool: typing.Optio
     else:
         return _get_formatted_server_static_mappings(static_mappings, family=family)
 
+@_verify
+def update_dhcp_server_lease_to_hostd_state(family: ArgFamily, pool: typing.Optional[str]):
+    """
+    Update DHCP server leases to VyOS hostsd via vyos-hostsd-client.
+    """
+    # if dhcp server is down, inactive leases may still be shown as active, so warn the user.
+    v = '6' if family == 'inet6' else '4'
+    if not process_named_running(f'kea-dhcp{v}'):
+        Warning('DHCP server is configured but not started. Data may be stale.')
+
+    v = 'v6' if family == 'inet6' else ''
+    if pool and pool not in _get_dhcp_pools(family=family):
+        raise vyos.opmode.IncorrectValue(f'DHCP{v} pool "{pool}" does not exist!')
+
+    lease_data = _get_raw_server_leases(family=family, pool=pool, sorted=None, state=['active'], origin=None)
+
+    try:
+        hc = vyos.hostsd_client.Client()
+
+        for mapping in lease_data:
+            ip_addr = mapping.get('ip')
+            mac_addr = mapping.get('mac')
+            name = mapping.get('hostname')
+            name = name if name else f"host-{mac_addr.replace(':', '-')}"
+            domain = mapping.get('domain')
+            fqdn = f"{name}.{domain}" if domain else name
+            hc.add_hosts({f'dhcp-server-{ip_addr}': {fqdn: {'address': [ip_addr], 'aliases': []}}})
+
+        hc.apply()
+
+    except vyos.hostsd_client.VyOSHostsdError as e:
+        raise vyos.opmode.InternalError(str(e))
+
 def _lease_valid(inet, address):
     leases = kea_get_leases(inet)
     for lease in leases: