Karma + Node.js 17: can't connect to karma server. There is no server listening on port 9876

When running under Node 17, running karma reports:

 can't connect to karma server. There is no server listening on port 9876 

Switching back to Node 16 seems to “fix” the issue.

See also: https://youtrack.jetbrains.com/issue/WEB-53632

[devoto13]

https://github.com/nodejs/node/issues/40702 seems to be the culprit. Node 17 changed the DNS resolution, so now it resolves localhost according to the OS settings instead of IPv4 first. The Karma server only listens on IPv4 address (0.0.0.0) by default, but the requests are sent to localhost and localhost is resolved into IPv6 address (::) in Node 17+. So the run request is unable to reach the Karma server.

I’ll think what’s the best way to make it work out of the box. In the meantime you can address the issue by setting listenAddress to :: either in config or using LISTEN_ADDR environment variable.

[cherniavskii]

Hey @devoto13 Thanks for looking into this.

Indeed, I just realised that the root cause of the issue is that I use VPN (NordVPN to be specific). Without it, the server starts just fine, so I guess VPN is messing with my network configuration.

Anyway, I’ve changed karma net-utils.js code locally to omit host - and it works both with VPN and without it. For now I can use listenAddress: 'localhost' as a workaround.