Handling CORS

Configuration

As of Socket.IO v2, the server will automatically add the necessary headers in order to support Cross-Origin Resource Sharing (CORS)

The origins option should be used to provide a list of authorized domains:

const io = require("socket.io")(httpServer, {
  origins: ["https://example.com"]
});

Please note that by default, ALL domains are authorized. You should explicitly allow/disallow cross-origin requests in order to keep your application secure:

• without CORS (server and client are served from the same domain):

const io = require("socket.io")(httpServer, {
  allowRequest: (req, callback) => {
    callback(null, req.headers.origin === undefined); // cross-origin requests will not be allowed
  }
});

• with CORS (server and client are served from distinct domains):

io.origins(["http://localhost:3000"]); // for local development
io.origins(["https://example.com"]);

The handlePreflightRequest option can be used to customize the Access-Control-Allow-xxx headers sent in response to the preflight request.

Example with cookies (withCredentials) and additional headers:

// server-side
const io = require("socket.io")(httpServer, {
  origins: ["https://example.com"],

  handlePreflightRequest: (req, res) => {
    res.writeHead(200, {
      "Access-Control-Allow-Origin": "https://example.com",
      "Access-Control-Allow-Methods": "GET,POST",
      "Access-Control-Allow-Headers": "my-custom-header",
      "Access-Control-Allow-Credentials": true
    });
    res.end();
  }
});

// client-side
const io = require("socket.io-client");
const socket = io("https://api.example.com", {
  withCredentials: true,
  transportOptions: {
    polling: {
      extraHeaders: {
        "my-custom-header": "abcd"
      }
    }
  }
});

Troubleshooting

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at xxx/socket.io/?EIO=3&transport=polling&t=NMnp2WI. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing).

If you have properly configured your server (see above), this could mean that your browser wasn't able to reach the Socket.IO server.

The following command:

curl "https://api.example.com/socket.io/?EIO=3&transport=polling"

should return something like:

96:0{"sid":"Lbo5JLzTotvW3g2LAAAA","upgrades":["websocket"],"pingInterval":25000,"pingTimeout":5000}

If that's not the case, please check that your server is listening and is actually reachable on the given port.