vyos-netlinkd: T8950: track per-interface operstate to suppress UP-to-UP DHCP restarts

_handle_dhcp_events() restarts dhclient/dhcp6c on every RTM_NEWLINK
that carries operstate=UP, regardless of whether the interface was
already UP.  Normal kernel events that re-notify UP without a preceding
DOWN (e.g. post-migration gratuitous-ARP, promiscuous-mode toggles)
trigger unnecessary DHCP restarts.  Each restart also creates a feedback
loop: dhclient-script runs "ip link set dev <if> up" during PREINIT,
which emits another RTM_NEWLINK(UP), which triggers another restart.
One seed event becomes 10+ restarts in 15 seconds.

Add a module-level dict tracking the last observed operstate per
interface.  Only proceed with the DHCP restart when the transition is
DOWN-to-UP or when no previous state is recorded (first boot / service
restart).  UP-to-UP re-notifications are suppressed with a LOG_DEBUG
message.

Validated on a production VyOS 2026.05.26-1327-rolling router: a
controlled KVM live migration after the patch produced zero DHCP
restarts (all UP-to-UP events suppressed), with no impact on normal
DOWN-to-UP DHCP recovery.

1 files changed, 16 insertions, 1 deletions

diff --git a/src/services/vyos-netlinkd b/src/services/vyos-netlinkd
index 2b158e05d..0e7da7a59 100755
--- a/src/services/vyos-netlinkd
+++ b/src/services/vyos-netlinkd
@@ -40,6 +40,10 @@ running = True
 # compile regex once during startup for fast match
 IFACE_RE = re.compile(r"^(?:eth|br|bond|wlan)")
 
+# Per-interface previous operstate, used to suppress DHCP restarts on
+# UP-to-UP re-notifications (e.g. post-migration gratuitous-ARP events).
+_iface_prev_operstate: dict[str, str] = {}
+
 def match_iface(ifname: str) -> bool:
     """ Helper function returning true if interface name is a match for further
     processing (e.g. restart of DHCP(v6) client)
@@ -60,6 +64,14 @@ def _handle_dhcp_events(operstate: Optional[str], ifname: str) -> None:
     if operstate not in ['UP', 'DOWN']:
         return None
 
+    prev = _iface_prev_operstate.get(ifname)
+    _iface_prev_operstate[ifname] = operstate
+
+    if operstate == 'UP' and prev == 'UP':
+        syslog.syslog(syslog.LOG_DEBUG,
+                      f'Suppressing DHCP restart for {ifname}: already UP')
+        return None
+
     if operstate == 'DOWN':
         # Interface moved state to down
         if is_systemd_service_active(systemdV4_service):
@@ -168,7 +180,10 @@ def main():
 
                     # Deletion of a network link which has been previously added to the kernel
                     case 'RTM_DELLINK':
-                        pass
+                        attrs = dict(message.get('attrs', []))
+                        ifname = attrs.get('IFLA_IFNAME', None)
+                        if ifname:
+                            _iface_prev_operstate.pop(ifname, None)
                     case _:
                         pass