Socket.io gives CORS error even if I allowed cors it on server

Question

My node server and client are running on different ports(3001,5347 respectively). On client I had used,

var socket = io('http://127.0.0.1:3001');

On server I tried all of following one by one

    1) io.set('origins', '*:*');
    2) io.set('origins', 'http://127.0.0.1:5347');
    3) io.set('origins', '*');
    4) io.set('origins', '127.0.0.1:5347');

But I'm getting following error:

XMLHttpRequest cannot load http://127.0.0.1:3001/socket.io/?EIO=3&transport=polling&t=1456799439856-4590. A wildcard '*' cannot be used in the 'Access-Control-Allow-Origin' header when the credentials flag is true. Origin 'null' is therefore not allowed access.

Note: I'm using express on server and I'm already using cors middleware in following way:

app.use(cors());

Where app and cors are instances of express and cors respectively.

Answer 1

If running Express as follows:

var app = express();
var server = app.listen(3000);

So to start socket.io, we can do the following:

var io = require('socket.io').listen(server);

The problem was because of following code:

var http = require('http').Server(app);
var io = require('socket.io')(http);

The server object passed to socket.io was not the same as the server object I'm listening on.

Answer 2

The accepted answer is outdated

According to the official docs, you can add it to an existing http Server https://socket.io/docs/server-initialization/#Attached-to-an-existing-HTTP-server

const server = require('http').createServer();
const options = { /* ... */ };
const io = require('socket.io')(server, options);
io.on('connection', socket => { /* ... */ });
server.listen(3000);

to enable cors using this options object seemed to do the trick

options={
 cors:true,
 origins:["http://127.0.0.1:5347"],
}

Answer 3

Back in 2014 they added support for method as part of the origin parameter:

let socket = io(web_server, {
    cors: { 
        origin: function(origin, fn) {
            if(origin == 'http://example.com')
                return fn(null, origin);
            return fn('Error Invalid domain');
        } 
    },
});

You can validate the domain with an external call, a database or an array...

The function fn execute the following logic:

function fn(err2, origin) {
    if (err2 || !origin) {
        next(err2);
    } else {
        corsOptions.origin = origin;
        cors(corsOptions, req, res, next);
    }
}

Ability to dynamically define CORS headers https://github.com/socketio/socket.io/issues/1772

While migrating to socket.io v3.0+, there is a migration change on how to setup CORS. https://socket.io/docs/v3/migrating-from-2-x-to-3-0/index.html#Configuration

Before socket.io v3.0:

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

  // optional, useful for custom headers
  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();
  }
});

After socket.io v3.0:

const io = require("socket.io")(httpServer, {
  cors: {
    origin: "https://example.com",
    methods: ["GET", "POST"],
    allowedHeaders: ["my-custom-header"],
    credentials: true
  }
});

The function is supported on the origin parameter in v3.0 of socket.io

Answer 4

Note that you can use cors() with app before you pass app into require('http'), which is then passed into socket.io. In this order. This should work too.

const app = express();

app.use(cors());
const http = require('http').Server(app);
const io = require('socket.io')(http);

Hope it helps.

Answer 5

If you get socket.io app working on Chrome, Safari and other browsers but you still encounter CORS issues in Firefox, and you are using a self-signed certificate, then the problem is that Firefox does not accept self-signed certificates by default, and you have to add an exception by going to Firefox's Preferences > Certificates > View Certificates > Add Exception.

If you don't do this, then Firefox shows the error you posted which is misleading, but deep within its Developer Tools, you will find this error: MOZILLA_PKIX_ERROR_SELF_SIGNED_CERT. This indicates that Firefox is not accepting the certificate at all because it is self-signed.

Answer 6

Following code helped:

const PORT = process.env.PORT || 4000;
const app = express();
const server = app.listen(PORT, () => {
    console.log(`Server running on port ${PORT}`);
});
const io = socket(server);

Answer 7

Acording with the doc (you can ignore the typescript stuff if you're using JS):

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

Answer 8

This worked for me in python- Flask-SocketIO

socketio =  SocketIO(app,cors_allowed_origins="*")

Answer 9

Installing socket.io version 2 solved my problem

npm install socket.io@2

Answer 10

If you are using Express then define a middleware like this before your serve start code...

app.use(function(request, response, next) {
  response.header("Access-Control-Allow-Origin", "*");
  response.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
  next();
});