wlb: T7966: Restore default route when interface disconnects/reconnects

1 files changed, 41 insertions, 0 deletions

diff --git a/src/helpers/vyos-load-balancer.py b/src/helpers/vyos-load-balancer.py
index 5df83b31e..249433f0b 100755
--- a/src/helpers/vyos-load-balancer.py
+++ b/src/helpers/vyos-load-balancer.py
@@ -24,6 +24,7 @@ import time
 from vyos.config import Config
 from vyos.template import render
 from vyos.utils.commit import commit_in_progress
+from vyos.utils.dict import dict_search_args
 from vyos.utils.network import get_interface_address
 from vyos.utils.process import rc_cmd
 from vyos.utils.process import run
@@ -125,6 +126,43 @@ def dynamic_nexthop_update(lb, ifname):
 
     return False
 
+def restore_default_route(lb: dict, ifname: str) -> None:
+    """
+    Restores a missing default route for a WAN interface in its policy routing table.
+
+    When a link flap or DHCP/PPP renegotiation removes the per-interface default route,
+    this function checks the interface’s assigned table for an existing default entry.
+    If none is found, it determines the proper next-hop (from DHCP, PPP, or static config)
+    and reinstalls the route using:
+      ip route replace table <table_num> default dev <ifname> via <nexthop>
+
+    @param lb       Load-balancer state/config dictionary.
+    @param ifname   Interface name whose default route should be verified and restored.
+    @returns        None — exits quietly if the table number or next-hop cannot be found.
+    """
+    table_num = dict_search_args(lb, 'health_state', ifname, 'table_number')
+    if not table_num:
+        return
+
+    rc, out = rc_cmd(f'ip -j route show default table {table_num}')
+    if rc == 0:
+        rt_table = json.loads(out)
+        if len(rt_table) > 0:
+            return
+        else:
+            if 'dhcp_nexthop' in lb['health_state'][ifname]:
+                if ifname[:5] == 'pppoe':
+                    nexthop_addr = parse_ppp_nexthop(ifname)
+                else:
+                    nexthop_addr = parse_dhcp_nexthop(ifname)
+            else:
+                nexthop_addr = dict_search_args(lb, 'interface_health', ifname, 'nexthop')
+
+            if nexthop_addr:
+                run(f'ip route replace table {table_num} default dev {ifname} via {nexthop_addr}')
+    else:
+        return
+
 def nftables_update(lb):
     # Atomically reload nftables table from template
     if not os.path.exists(nftables_wlb_conf):
@@ -295,6 +333,9 @@ if __name__ == '__main__':
                     if dynamic_nexthop_update(lb, ifname):
                         ip_change = True
 
+                    restore_default_route(lb, ifname)
+
+
             if any(state['state_changed'] for ifname, state in lb['health_state'].items()):
                 if not nftables_update(lb):
                     break