GKE NLB Simple UDP Echo Server

01-udp-echo.yaml

# from https://cloud.google.com/load-balancing/docs/network/udp-with-network-load-balancing
apiVersion: v1
kind: Namespace
metadata:
  name: udp-echo
---
apiVersion: v1
kind: ConfigMap
metadata:
  name: echo
  namespace: udp-echo
data:
  server.py: |
    #!/usr/bin/python3
    import socket,struct
    def loop_on_socket(s):
      while True:
        d, ctl, flg, addr = s.recvmsg(1500, 1024)
        # ctl contains the destination address information
        s.sendmsg(["ECHO: ".encode("utf8"),d], ctl, 0, addr)

    if __name__ == "__main__":
      HOST, PORT = "0.0.0.0", 60002
      s = socket.socket(type=socket.SocketKind.SOCK_DGRAM)
      s.setsockopt(0,   # level is 0 (IPPROTO_IP)
                    8,   # optname is 8 (IP_PKTINFO)
                    1)

      s.bind((HOST, PORT))
      loop_on_socket(s)
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: udp-echo
  namespace: udp-echo
  labels:
    app: udp-echo
spec:
  selector:
    matchLabels:
      app: udp-echo
  template:
    metadata:
      labels:
        app: udp-echo
    spec:
      containers:
      - name: udp-echo
        image: cgr.dev/chainguard/python:latest-dev
        args: ["/scripts/server.py"]
        # or for just sleep forever use:
        #command: ["/bin/sleep"]
        #args: ["infinity"]
        ports:
        - containerPort: 60002
        volumeMounts:
        - name: src
          mountPath: /scripts
      volumes:
      - name: src
        configMap:
          name: echo
          defaultMode: 0744
---
apiVersion: v1
kind: Service
metadata:
  name: udp-echo
  namespace: udp-echo
spec:
  #ipFamilyPolicy: RequireDualStack
  ports:
  - port: 8000
    targetPort: 60002
    protocol: UDP
    name: echo
  selector:
    app: udp-echo
  type: LoadBalancer

02-tshark-node.txt

# Test using udp client like:
echo hello | nc -vu -m1 35.192.82.200 8000

# apt install tshark inside toolbox on node, and then watch for packets with port 8000
gregbray@gke-gke-iowa-default-pool-fa46c430-p74m ~ $ toolbox
Spawning container gregbray-us.gcr.iocos-cloudtoolbox-v20230615 on /var/lib/toolbox/gregbray-us.gcr.io_cos-cloud_toolbox-v20230615.
Press ^] three times within 1s to kill container.
root@gke-gke-iowa-default-pool-fa46c430-p74m:~# tshark -f "udp port 8000" -T text
Running as user "root" and group "root". This could be dangerous.
Capturing on 'eth0'
    1 0.000000000 174.162.248.150 → 35.192.82.200 UDP 48 46872 → 8000 Len=6
    2 0.000365026 35.192.82.200 → 174.162.248.150 UDP 54 8000 → 46872 Len=12

# That also shows the return packet src is using NLB ip
# which doesn't seem to match https://cloud.google.com/load-balancing/docs/network/udp-with-network-load-balancing#an_issue_with_udp_return_packets

This is follow-up test from STUN example at https://gist.github.com/gbrayut/ee14dbbb7de21c67cdd25065fa67bcc9
after finding https://cloud.google.com/load-balancing/docs/network/udp-with-network-load-balancing which outlines common issue with UDP return packets for NLB on GCE, but I cannot reproduce the results on that GCE documentation