Socket.io Client Not Connecting to AKS-Deployed Socket.io Server (Only Receiving 40 Without sid)

Question

I’m facing an issue where my Socket.io client fails to fully connect to a Socket.io server deployed on AKS.

• When running Socket.io locally, the client correctly receives the 40{"sid":"<socket_id>"} message, meaning the connection is successful
• When connecting to the AKS-deployed server, the client only receives 40 without the sid, and the connection does not complete
• The issue happens over wss://, but the same client works fine when connecting to a local server.

Server: Node.js with Socket.io (socket.io@4.7.5)

io.on("connection", (socket) => {

        console.log("Socket connected:", socket.id);

      });

Client: Angularapp (socket.io-client@4.7.5)

 this.socket = io("https://myapplication.com",

      {

      transports: ["websocket"],

       path: "/nodeapp/socket.io/"

      });

      this.socket.on("connect", (socketdata:any) => {

      console.log("Connected to server");

      });

Deployment: AKS with NGINX Ingress Ingress Annotations:

ingress:

  enabled: true

  className: "nginx"

  annotations:

    nginx.ingress.kubernetes.io/use-regex: "true"

    nginx.ingress.kubernetes.io/rewrite-target: /$2

    nginx.ingress.kubernetes.io/error-log-level: "debug"

    nginx.ingress.kubernetes.io/enable-websocket: "true"

    nginx.ingress.kubernetes.io/proxy-read-timeout: "3600"

    nginx.ingress.kubernetes.io/proxy-send-timeout: "3600" 

    nginx.ingress.kubernetes.io/proxy-buffering: "off"

    nginx.ingress.kubernetes.io/proxy-body-size: "100m"

  hosts:

   - host: 

     paths:

       - path: /nodeapp(/|$)(.*)

         pathType: ImplementationSpecific

Why does the Socket.io client only receive 40 (without sid) when connecting via Ingress?

Could this be an issue with Ingress proxy headers stripping the sid? Any known Azure or NGINX settings that affect WebSocket handshake completion? Would appreciate any insights or troubleshooting tips! Someone confirms a known issue with Socket.io, NGINX Ingress and suggests a fix.

Answer 1

Yes, this is a common issue when using Socket.io behind an NGINX Ingress Controller in AKS. The problem is likely caused by NGINX's handling of WebSocket upgrades and path rewriting. The connection works fine and you receive 40{"sid":"<socket_id>"}, confirming a successful WebSocket handshake but on AKS, the client only gets 40 without the sid, meaning the connection does not fully establish. Even though you've added nginx.ingress.kubernetes.io/enable-websocket: "true"

To fix this, you can try to add these headers explicitly in your Ingress annotations:

nginx.ingress.kubernetes.io/configuration-snippet: |
  proxy_set_header Upgrade $http_upgrade;
  proxy_set_header Connection "Upgrade";
  proxy_http_version 1.1;
  proxy_set_header Host $host;

This should ensure that WebSocket upgrade requests are properly forwarded.

Also kindly share the output of

kubectl logs -n ingress-nginx -l app.kubernetes.io/name=ingress-nginx

Look for any errors related to WebSockets.

In general, to set up a Socket.io server on AKS with NGINX Ingress Controller

Deploy NGINX Ingress Controller

Below is a sample Node.js Express Server with Socket.io and it's being deployed on AKS.

socket-server.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: socket-server
spec:
  replicas: 2
  selector:
    matchLabels:
      app: socket-server
  template:
    metadata:
      labels:
        app: socket-server
    spec:
      containers:
        - name: socket-server
          image:arkoacr.azurecr.io/socket-server:latest
          ports:
            - containerPort: 3000
---
apiVersion: v1
kind: Service
metadata:
  name: socket-service
spec:
  selector:
    app: socket-server
  ports:
    - protocol: TCP
      port: 80
      targetPort: 3000
  type: ClusterIP

Dockerfile for the same-

FROM node:18
WORKDIR /app
COPY package.json package-lock.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD ["node", "server.js"]

server.js file-

const express = require("express");
const http = require("http");
const { Server } = require("socket.io");

const app = express();
const server = http.createServer(app);

const io = new Server(server, {
  cors: {
    origin: "*",
    methods: ["GET", "POST"]
  }
});

io.on("connection", (socket) => {
  console.log("New client connected:", socket.id);
  
  socket.on("disconnect", () => {
    console.log("Client disconnected:", socket.id);
  });
});

server.listen(3000, () => {
  console.log("Socket.io server running on port 3000");
});

package.json

{
  "name": "socket-server",
  "version": "1.0.0",
  "description": "Socket.io server for Kubernetes",
  "main": "server.js",
  "dependencies": {
    "express": "^4.17.1",
    "socket.io": "^4.7.5"
  }
}

containerise it and push it to your acr

Create Ingress to Expose the Server

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: socket-ingress
  annotations:
    nginx.ingress.kubernetes.io/use-regex: "true"
    nginx.ingress.kubernetes.io/rewrite-target: /$2
    nginx.ingress.kubernetes.io/error-log-level: "debug"
    nginx.ingress.kubernetes.io/enable-websocket: "true"
    nginx.ingress.kubernetes.io/proxy-read-timeout: "3600"
    nginx.ingress.kubernetes.io/proxy-send-timeout: "3600"
    nginx.ingress.kubernetes.io/proxy-buffering: "off"
spec:
  ingressClassName: nginx
  rules:
    - host: myapp.example.com
      http:
        paths:
          - path: /socket(/|$)(.*)
            pathType: ImplementationSpecific
            backend:
              service:
                name: socket-service
                port:
                  number: 80

finally deploy the Angular Client. If issue persists. Kindly share the outputs for

kubectl logs -l app=socket-server and kubectl logs -n ingress-nginx -l app.kubernetes.io/name=ingress-nginx

Adding to your follow up comment-

Since the WebSocket connection initially succeeds but then immediately disconnects, the issue is likely not just NGINX, but also how Socket.io is handling WebSocket upgrades behind a proxy. Could you kindly try to update your server.js as below and let me know?

const express = require("express");
const http = require("http");
const { Server } = require("socket.io");
const app = express();
const server = http.createServer(app);
const io = new Server(server, {
  cors: {
    origin: "*",
    methods: ["GET", "POST"]
  }
});

io.engine.set("trust proxy", 1);
io.on("connection", (socket) => {
  console.log("New client connected:", socket.id);
  
  socket.on("disconnect", () => {
    console.log("Client disconnected:", socket.id);
  });
});
server.listen(8083, () => {
  console.log("Socket.io server running on port 8083");
});

and your ingress yaml as below-

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: socket-ingress
  annotations:
    nginx.ingress.kubernetes.io/use-regex: "true"
    nginx.ingress.kubernetes.io/rewrite-target: /$2
    nginx.ingress.kubernetes.io/error-log-level: "debug"
    nginx.ingress.kubernetes.io/enable-websocket: "true"
    nginx.ingress.kubernetes.io/proxy-read-timeout: "3600"
    nginx.ingress.kubernetes.io/proxy-send-timeout: "3600"
    nginx.ingress.kubernetes.io/proxy-buffering: "off"
    nginx.ingress.kubernetes.io/proxy-body-size: "100m"
    nginx.ingress.kubernetes.io/configuration-snippet: |
      proxy_set_header Upgrade $http_upgrade;
      proxy_set_header Connection "Upgrade";
      proxy_http_version 1.1;
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header Host $host;
spec:
  ingressClassName: nginx
  rules:
    - host: myapp.example.com
      http:
        paths:
          - path: /socket(/|$)(.*)
            pathType: ImplementationSpecific
            backend:
              service:
                name: socket-service
                port:
                  number: 80

and modify your angular client to force Web Sockets

this.socket = io("wss://myapplication.com", {
  transports: ["websocket"],
  path: "/nodeapp/socket.io/",
  reconnectionAttempts: 10,   
  reconnectionDelay: 1000
});

I requested you for the kubectl logs -n ingress-nginx -l app.kubernetes.io/name=ingress-nginx. Could you kindly share that with me so that I can have a better insight. I have pinged you in Private message. Let's take it forward from there. Thanks