Cannot get websocket connection working with ec2 + application load balancer

Question

I have an aws application load balancer with an https listener on port 9999, forwarding to a group on port 9999 with an ec2-instance being the target.

If I run my websocket server with the host name configured to my domain api.example.com, then when the client tries to open a websocket connection it gets:

Error during WebSocket handshake: Unexpected response code: 502

However, if I configure my websocket server with an empty string instead of the domain, then it connects just fine!

This is problematic because the server I am intending to run on this instance automatically launches a websocket server ONLY IF there is a websocket host name configuration provided, so a blank string means the websocket server will never launch! I would prefer to not have to hack the library to get around that condition. So I am wondering, why in the world a blank string works, but the domain name does not?

I tried localhost, the ip of the box, etc, everything results in a 502 except a blank string!

Solution 1

My issue was solved by setting my websocket server host to 0.0.0.0

Solution 2

The ALB does not use a host name for the health check. Thus, if your server does not support requests without host names, the health check will fail, resulting in 5xx errors when accessing the ALB.

• Check the target group the ALB is using for health check errors
• Check your application server logs for health check related errors

If your server does not support this you have two options:

• Use a proxy (e.g. nginx) on your server that can handle requests without host name, and handle the problem there
• Override the ALB health check port, redirecting it to something that returns HTTP 200 all the time (e.g. an Apache running on the same host, but on another port).

Comments

[patrick]

sorry, this has nothing to do with the health check. I actually had no idea how to do a "health check" for a websocket server, so I just made the health check go to my webserver (https 443) just to get it to pass. So the target on my port 9999 group is in fact "healthy." As I said, I can connect via websockets just fine only if I run the websocket server without a host name, and that is a problem as I need to specify a host name.

[Ben Butterworth]

To explain why this works, localhost or 127.0.0.1 is only accessible through other local devices. 0.0.0.0 means 'no particular address' source, and in this context, this means let the application bind to all addresses. What does it mean to bind() a socket to any address other than localhost?