Question 85483 in Docker Question posted in
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Pod-to-pod communication doens't work with Python socket in k8s – Docker

Problem

In our project, we want two pods working as server-client that communicate via the Python socket library. Both containers are built locally with docker build, pulled locally via imagePullPolicy: IfNotPresent on the yaml files and run on the same node of the k8s cluster (I’m running kubernetes vanilla, if that’s important).

The communication works well when we

  • run both python scripts in the command line
  • run both scripts as containers using docker build and docker run
  • the server app container is deployed in the K8s cluster and the client app is run either on the command line or as a docker container.

The communication fails when both server and client are deployed in K8s. kubectl logs client -f returns :

Traceback (most recent call last):
  File "client.py", line 7, in <module>
    client_socket.connect((IP_Server,PORT_Server))
TimeoutError: [Errno 110] Connection timed out

I suspect there’s a problem with the outgoing request from the client script when it’s deployed on the cluster, but I can’t find where the problem lies.

Codes

server.py

import socket

IP = "0.0.0.0"
PORT = 1234

server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server_socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
server_socket.bind((IP, PORT))
server_socket.listen()

...

server.yaml

apiVersion: v1
kind: Service
metadata:
  name: server
  labels:
    app: server
spec:
  ports:
    - port: 1234
      targetPort: 1234
      protocol: TCP
  selector:
    app: server
---
apiVersion: v1
kind: Pod
metadata:
  name: server
  labels:
    app: server
spec:
  containers:
  - name: server
    image: server:latest
    imagePullPolicy: IfNotPresent
    ports:
    - containerPort: 1234

client.py

import socket

IP_Server = # the IP of the server service, obtained from "kubectl get svc" 
PORT_Server = 1234

client_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
client_socket.connect((IP_Server,PORT_Server)) # fails here

...

client.yaml

apiVersion: v1
kind: Pod
metadata:
  name: client
  labels:
    app: client
spec:
  containers:
  - name: client
    image: client:latest
    imagePullPolicy: IfNotPresent
Tags:

3

Answers


  1. Chosen as BEST ANSWER
    0

    In case anyone gets here in the future, I found a solution that worked for me by executing

    iptables -P INPUT ACCEPT
iptables -P FORWARD ACCEPT
iptables -P OUTPUT ACCEPT
iptables -F

    and then deleting all coredns pods.

    Reference : https://github.com/kubernetes/kubernetes/issues/86762#issuecomment-836338017


  2. 0

    In a default setup – there shouldn’t be anything preventing you to connect between 2 pods. However, you shouldn’t be relying on the IP addresses for communication inside your cluster. Try using the service name:

    $ kubectl get svc
NAME         TYPE        CLUSTER-IP    EXTERNAL-IP   PORT(S)   AGE
kubernetes   ClusterIP   10.96.0.1     <none>        443/TCP   10m
server       ClusterIP   10.98.48.92   <none>        80/TCP    9m39s

    server should be typically available to all pods running inside the same namespace.

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  3. 0

    There may be something wrong with the way you resolve the IP address of the server, but the "service" created by the server should implicitly be accessible over DNS (e.g. as server:1234), as explained here, so maybe you can use that instead?

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