Firewall kaspersky/avast blocked messages when I'm using a websocket server (socket.io on node.

What can be done?

  • Do you mean it is blocking incoming connections to port 8080 and 8000? – kanaka Mar 14 '11 at 23:17
  • 1
    1. with avast don't working websockets and flashsockets protocol, 2 KIS blocking outcoming messages even by AJAX multipart streaming – Bart Mar 15 '11 at 6:41
  • sorry I didn't understand your comment. Can you clarify your setup? Which system is running the browser, which system is running socket.io/node.js, and where the "firewall" is running? Also, please try temporarily disabling your firewall software (make sure you are behind a router with basic firewalling first) and see if that gets things working. Then post you config and results of that test. – kanaka Mar 15 '11 at 14:34
  • Why do you have 2 anti virus programs? – Dave Jul 20 '17 at 8:11

I can tell you 100% that Avast for example blocks not only websockets, but from what I see from my Socket.IO app also xhr-polling and other stuff.

This is unacceptable, I've even tested on port 80 and it doesn't work. What an awful piece of software.

Edit: this is a list of blocked ports by antiviruses (Dead 2017-07-20)


First rule of Firewall: it can't block what it can't sense.

Does the firewall allow TLS? If so, serve content over WSS instead of WS.

  • A usual WSS client will open a TLS connection to you (source) and send you this:

    GET /demo HTTP/1.1
    Upgrade: WebSocket
    Connection: Upgrade
    Host: example.com
    Origin: http://example.com
    WebSocket-Protocol: sample 

    The firewall can't detect it because it is over TLS.

  • A HTTP-CONNECT-WSS client will send you this:

    CONNECT example.com:443 HTTP/1.1
    Host: example.com 

    The firewall sees it as yet another HTTP-CONNECT-TLS connection. (source)

Sidenote: websockets can fail due to websocket-unaware intermediary proxies buffering connections even if the firewall doesn't block it. The same is true of HTTP "comet" streaming.

  • some companies do packet inspection by installing custom root certs on their computers – wutzebaer Oct 8 '19 at 18:27
  • Changing the protocol from "ws://" to "wss://" solved for me a similar problem with UFW (server-side firewall). – Francesco Galgani Jan 25 '20 at 16:13

Many ISP companies block the port 80 to prevent their customers of hosting a web servers on their home computers.

It might beyond your power to unblock the port 80 from your ISP.

Also, as far as I am concerned, Avast is not a firewall.


You can try to use port 443 (which is used for https) if it's possible in your case.

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