http3: prefer IPv6 over IPv4 when dialing

[marten-seemann]

Fixes #3755.

I'm not sure this is the correct fix. It seems like we should be able to handle multiple DNS responses, and fall back from IPv6 to IPv4 if IPv6 is not available (for example). That would be a bigger change, and would probably need to be done in the quic and not the http3 package.

[codecov]

Codecov Report

Merging #3770 (53b9550) into master (af517bd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3770   +/-   ##
=======================================
  Coverage   84.49%   84.49%           
=======================================
  Files         142      142           
  Lines       14114    14118    +4     
=======================================
+ Hits        11925    11929    +4     
  Misses       1769     1769           
  Partials      420      420           

Impacted Files	Coverage Δ
http3/roundtrip.go	89.44% <100.00%> (+0.31%)	⬆️

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[kgersen]

this doesn't fix the issue in our app because we use this kind of code:

	rtTransport = &http3.RoundTripper{
		DisableCompression: true,
		TLSClientConfig:    netTransport.TLSClientConfig,
		QuicConfig:         quicConf,
		Dial: func(ctx context.Context, addr string, tlsCfg *tls.Config, cfg *quic.Config) (quic.EarlyConnection, error) {
			j.Logger().Debug().Msgf("HTTP3 dialing to: %s", addr)
			return quic.DialAddrEarlyContext(ctx, addr, tlsCfg, cfg)
		},
	}

cause => quic.DialAddrEarlyContext also uses net.ResolveUDPAddr thru dialAddrContext

[marten-seemann]

If you're setting the Dial option yourself, you can implement a similar workaround as in this PR.

Issue for the fix of DialAddr here: #3772.

[kgersen]

Does this means quic.DialAddrEarlyContext and quic.DialAddrContext will always prefer IPv4 ?

I've updated h3dial with a use case of a custom http3.RoundTripper.Dial (which should be called DialContext ?)

[marten-seemann]

Does this means quic.DialAddrEarlyContext and quic.DialAddrContext will always prefer IPv4 ?

It's also using ResolveUDPAddr, so I'd assume so. See #3772.

[lucas-clemente]

Feels unfortunate to have a workaround here rather than fix this in the stdlib, but I agree with the agenda of pushing ipv6 ;)

[marten-seemann]

The problem now is that if you do a Get("https://localhost:1234"), it only does IPv6, which doesn't work if you're just listening on IPv4.

[bt90]

Isn't the fallback mechanism defined by the IETF?

https://en.m.wikipedia.org/wiki/Happy_Eyeballs

[bt90]

https://datatracker.ietf.org/doc/rfc8305/

[bt90]

This is also what curl is doing as explained in @bagder blog post:

https://daniel.haxx.se/blog/2023/01/12/

[kgersen]

Fallback (happy eyeball) is another issue. see #3772

[bt90]

Strictly speaking, the preference of IPv6 is part of happy eyeballs.