veth: T7990: fix stale DHCP clients when removing virtual Ethernet pairs

When removing a veth interface, the kernel automatically deletes its peer
interface, since veth devices always exist in pairs. However, this automatic
removal does not trigger the remove() helper in vyos.ifconfig.interfaces, which
can leave associated DHCP(v6) clients running indefinitely.

For example:
  ip link add veth0 type veth peer name veth1
  ip link del dev veth1

Removing veth1 will also delete veth0 in the kernel, but the VyOS cleanup
routines are never called for veth0. This patch ensures that peer removal
correctly purges both interfaces and their associated state.

This drops the workarounds in veth smoketest and falls back to the common
interface smoketests from the inherited base class.

2 files changed, 27 insertions, 3 deletions

diff --git a/python/vyos/ifconfig/interface.py b/python/vyos/ifconfig/interface.py
index 761ba1799..491651aac 100644
--- a/python/vyos/ifconfig/interface.py
+++ b/python/vyos/ifconfig/interface.py
@@ -371,7 +371,7 @@ class Interface(Control):
         if 'netns' in self.config: cmd = f'ip netns exec {netns} {cmd}'
         self._cmd(cmd)
 
-    def remove(self):
+    def remove(self, skip_delete=False):
         """
         Remove interface from operating system. Removing the interface
         deconfigures all assigned IP addresses and clear possible DHCP(v6)
@@ -388,12 +388,20 @@ class Interface(Control):
 
         # remove all assigned IP addresses from interface - this is a bit redundant
         # as the kernel will remove all addresses on interface deletion, but we
-        # can not delete ALL interfaces, see below
+        # can not delete ALL interfaces, see below.
+        #
+        # This will internally stop DHCP(v6) if running
         self.flush_addrs()
 
         # remove interface from conntrack VRF interface map
         self._del_interface_from_ct_iface_map()
 
+        # Some interfaces - mainly veth pairs - should be properly de-configured
+        # but not deleted. Deleting one veth pair member will delete the other,
+        # we need need a way to skip the deletion.
+        if skip_delete:
+            return
+
         # ---------------------------------------------------------------------
         # Any class can define an eternal regex in its definition
         # interface matching the regex will not be deleted
diff --git a/python/vyos/ifconfig/veth.py b/python/vyos/ifconfig/veth.py
index f4075fa02..9868ea526 100644
--- a/python/vyos/ifconfig/veth.py
+++ b/python/vyos/ifconfig/veth.py
@@ -14,7 +14,8 @@
 # License along with this library.  If not, see <http://www.gnu.org/licenses/>.
 
 from vyos.ifconfig.interface import Interface
-
+from vyos.utils.dict import dict_search
+from vyos.utils.network import get_interface_config
 
 @Interface.register
 class VethIf(Interface):
@@ -51,3 +52,18 @@ class VethIf(Interface):
 
         # interface is always A/D down. It needs to be enabled explicitly
         self.set_admin_state('down')
+
+    def remove(self):
+        # The oddity with virtual-ethernet pairs is, if you delete one - the OS
+        # Kernel will immediately delete the other interface, leaving no
+        # possibility to properly shutdown the peer-interface.
+        #
+        # We deconfigure the second veth pair interface during deletion, but
+        # skip it's actual call to "ip link del dev ..."
+        tmp = get_interface_config(self.ifname)
+        peer = tmp.get('link', dict_search('linkinfo.info_kind', tmp))
+        if peer != None:
+            Interface(peer).remove(skip_delete=True)
+
+        # always forward to the base class
+        super().remove()