Proxy assumes server can bind to IPv6 when specifying localhost

[tomjal]

I updated from 0.9.5 to 1.0.5 and it says that:

"Proxy error: Could not proxy request /ui_host/api/v1/account/context from localhost:3000 to http://[::1]:5000/."

Any ideas?

[Timer]

Can you downgrade to 1.0.4 and try?

[Timer]

Normally, this error would mean that your server is not running on http://[::1]:5000/. Are you sure your server is running?

[tomjal]

Im 100% sure that server works and is running properly. Im not sure what this [::1] means? My package.json has "proxy": "http://localhost:5000"

[gaearon]

What does your proxy config look like?

[Timer]

[::1] is the IPv6 equivalent of localhost.

@tomjal can you try changing your proxy property from localhost to 127.0.0.1?
What host are you binding to with your application running on 5000?

If your application on 5000 is binding to 127.0.0.1 and you're specifying localhost in your proxy, this is expected to not work.

[tomjal]

@Timer you were right, it works with 127.0.0.1. The server is .NET Core Kestrel and for some reason it does not work with IPv6. Thanks

[Timer]

I'm going to reopen this because I think we can make this smarter.
We resolve localhost to IPv6 because normally that's what it is, but that is obviously painful for servers that don't support it.
I think we should act more in line with the underlying network and try both, or only perform this resolution when you're offline.

[Timer]

What version of .NET core are you on? This announcement seems relevant: aspnet/Announcements#185.

Configuring Kestrel to bind to localhost used to only bind to the IPv4 loopback interface. This caused confusion among users, especially those putting Kestrel behind reverse proxies that would see failures or delays because the proxies where trying to establish an IPv6 connection to Kestrel on localhost.

Seems like in >=1.0.0 they fixed their bug and bind to [::1] as well. If they don't, this may be a regression on their part.

[uvtzxpm]

For what it's worth, I had exactly the same problem with a Google App Engine (dev_appserver.py) backend. I fixed it by changing the proxy from localhost to 127.0.0.1, as already suggested.

There might be other strange and wonderful servers that people are using that can be affected by this.

[Timer]

While inspecting these individual cases, it's amazing how deep into the rabbit hole you can go.

In @uvtzxpm's case, dev_appserver.py connects to localhost but Python 2 by default doesn't enable IPv6 (you have to configure it with --enable-ipv6; Python 3 has it on by default afaik).

Most of this testing was done with Node servers on windows which do support IPv6.