net: allow TCP only DNS requests in the pure Go resolver on unix

[jfbus]

There is a DNS resolution bug in Kubernetes (UDP response packets get dropped by conntrack, causing timeouts in DNS queries).

This enables configuration of the pure Go resolver to use TCP, either using the GODEBUG env var (GODEBUG=netdns=go+tcp) or using the Resolver.PreferTCP flag.

Fixes #29358

[gopherbot]

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[gopherbot]

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Patch Set 1:

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[gopherbot]

Message from Brad Fitzpatrick:

Patch Set 1:

This would need test and docs.

But you should start with a proposal bug to get approval on such an API change before we discuss code.

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[gopherbot]

Message from Jean-François Bustarret:

Patch Set 1:

Patch Set 1:

This would need test and docs.

Will be added.

But you should start with a proposal bug to get approval on such an API change before we discuss code.

Is the github issue enough ? #29358

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[gopherbot]

Message from Brad Fitzpatrick:

Patch Set 1:

Patch Set 1:

Patch Set 1:

This would need test and docs.

Will be added.

But you should start with a proposal bug to get approval on such an API change before we discuss code.

Is the github issue enough ? #29358

Yup, thanks. I converted it into a proposal.

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[gopherbot]

This PR (HEAD: 541f2d0) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/go/+/155378 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info

[gopherbot]

Message from Jean-François Bustarret:

Patch Set 2:

Unit test & godoc added.

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[gopherbot]

Message from Jean-François Bustarret:

Patch Set 2:

An alternate CL with a smaller impact is here : https://go-review.googlesource.com/c/go/+/156366

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[jfbus]

Replaced by #29594