dhcp: 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.

This is similar to the capability exposed via kea hook
`libdhcp_run_script.so` which is invoked internally
by kea when a single lease changes state.

Since the kea hook is currently implemented for DHCPv4
only, this helper script is implemented for DHCPv4
only as well.

1 files changed, 112 insertions, 0 deletions

diff --git a/src/system/sync-dhcp-lease-to-hosts.py b/src/system/sync-dhcp-lease-to-hosts.py
new file mode 100755
index 000000000..5c8b18faf
--- /dev/null
+++ b/src/system/sync-dhcp-lease-to-hosts.py
@@ -0,0 +1,112 @@
+#!/usr/bin/env python3
+#
+# Copyright (C) 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
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import argparse
+import logging
+
+import vyos.opmode
+import vyos.hostsd_client
+
+from vyos.configquery import ConfigTreeQuery
+
+from vyos.kea import kea_get_active_config
+from vyos.kea import kea_get_dhcp_pools
+from vyos.kea import kea_get_server_leases
+
+# Configure logging
+logger = logging.getLogger(__name__)
+# set stream as output
+logs_handler = logging.StreamHandler()
+logger.addHandler(logs_handler)
+
+
+def _get_all_server_leases(inet_suffix='4') -> list:
+    mappings = []
+    try:
+        active_config = kea_get_active_config(inet_suffix)
+    except Exception:
+        raise vyos.opmode.DataUnavailable('Cannot fetch DHCP server configuration')
+
+    try:
+        pools = kea_get_dhcp_pools(active_config, inet_suffix)
+        mappings = kea_get_server_leases(
+            active_config, inet_suffix, pools, state=[], origin=None
+        )
+    except Exception:
+        raise vyos.opmode.DataUnavailable('Cannot fetch DHCP server leases')
+
+    return mappings
+
+
+if __name__ == '__main__':
+    # Parse command arguments
+    parser = argparse.ArgumentParser()
+    group = parser.add_mutually_exclusive_group(required=True)
+    group.add_argument('--inet', action='store_true', help='Use IPv4 DHCP leases')
+    group.add_argument('--inet6', action='store_true', help='Use IPv6 DHCP leases')
+    args = parser.parse_args()
+
+    inet_suffix = '4' if args.inet else '6'
+    service_suffix = '' if args.inet else 'v6'
+
+    if inet_suffix == '6':
+        raise vyos.opmode.UnsupportedOperation(
+            'Syncing IPv6 DHCP leases are not supported yet'
+        )
+
+    # Load configuration
+    config = ConfigTreeQuery()
+
+    # Check if DHCP server is configured
+    # Using warning instead of error since this check may fail during first-time
+    # DHCP server setup when the service is not yet configured in the config tree.
+    # This happens when called from systemd's ExecStartPost the first time.
+    if not config.exists(f'service dhcp{service_suffix}-server'):
+        logger.warning(f'DHCP{service_suffix} server is not configured')
+
+    # Check if hostfile-update is enabled
+    if not config.exists(f'service dhcp{service_suffix}-server hostfile-update'):
+        logger.debug(
+            f'Hostfile update is disabled for DHCP{service_suffix} server, skipping hosts update'
+        )
+        exit(0)
+
+    lease_data = _get_all_server_leases(inet_suffix)
+
+    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()
+
+        logger.debug('Hosts store updated successfully')
+
+    except vyos.hostsd_client.VyOSHostsdError as e:
+        raise vyos.opmode.InternalError(str(e))