Outgoing ping required in container environment (even with PersistentKeepalive)

[Nico Schottelius]

Good morning,

another day news from the container land. When running wireguard in
kubernetes, deleting the existing pod and replacing it with a new one, I
see the following behaviour:

- The assigned IPv4 address stops being reachable (good so far)
- The assigned IPv4 address is then shortly reachable for about 5 seconds
- The assigned IPv4 address stops being reachable (not good)
- The assigned IPv4 address is again reachable, if I trigger a ping
  through the tunnel inside the container (ok, but why?)

I am using the following configuration:

--------------------------------------------------------------------------------
[Interface]
PrivateKey = ...
ListenPort = 51828
Address = 185.155.29.81/32
PostUp = iptables -t nat -I POSTROUTING -o ipv4 -j MASQUERADE

# upstream
[Peer]
Endpoint = vpn-...:51820
PublicKey = 6BRnQ+dmeFzVCH9RbM1pbJ7u3y3qrl+zUzzYCmC88kE=
AllowedIPs = 0.0.0.0/1, 128.0.0.0/1
PersistentKeepalive = 25
--------------------------------------------------------------------------------

And the following container specification:

--------------------------------------------------------------------------------
  spec:
      containers:
        - name: wireguard
          image: ungleich/ungleich-wireguard:{{ $.Chart.AppVersion }}
          # We only support 1 listener at the moment
          # Outgoing connections are not affected
          ports:
            - containerPort: 51820
          securityContext:
            capabilities:
              # NET_ADMIN for wg
              # NET_RAW for iptables
              add: ["NET_ADMIN", "NET_RAW" ]
          volumeMounts:
            - name: wireguard-config
              mountPath: "/etc/wireguard"
          resources:
            requests:
              memory: {{ $v.memory | default "1Gi" }}
              cpu: {{ $v.cpu | default "1000m" }}
            limits:
              memory: {{ $v.memory | default "1Gi" }}
              cpu: {{ $v.cpu | default "1000m" }}
--------------------------------------------------------------------------------

The strange thing is that after issuing the ping once inside the
container:

--------------------------------------------------------------------------------
[8:41] nb2:~% kubectl -n wireguard exec -ti wireguard-vpn-server-7db664db6f-zl4fz -- ping -c2 -4 google.com
PING google.com (172.217.168.78): 56 data bytes
64 bytes from 172.217.168.78: seq=0 ttl=116 time=9.110 ms
64 bytes from 172.217.168.78: seq=1 ttl=116 time=6.664 ms

--- google.com ping statistics ---
2 packets transmitted, 2 packets received, 0% packet loss
round-trip min/avg/max = 6.664/7.887/9.110 ms
--------------------------------------------------------------------------------

The connection stays correctly established.

If anyone has a pointer on what might be going on, any help is
appreciated.

Best regards,

Nico


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