x/net/dns: add LookupCNAME

[therealjoesmith]

What version of Go are you using (go version)?

$ go version
go version go1.11.1 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What did you do?

https://play.golang.org/p/HTHVSd81SPJ

What did you expect to see?

I wanted to see the chain of CNAME records for www.microsoft.com.

What did you see instead?

Go returns only the last record. That's correct (it is the canonical name after all), but there are situations where it's interesting to probe the chain. On the command line, try:

$ host www.microsoft.com
www.microsoft.com is an alias for www.microsoft.com-c-3.edgekey.net.
www.microsoft.com-c-3.edgekey.net is an alias for www.microsoft.com-c-3.edgekey.net.globalredir.akadns.net.
www.microsoft.com-c-3.edgekey.net.globalredir.akadns.net is an alias for e13678.dspb.akamaiedge.net.
e13678.dspb.akamaiedge.net has address 23.211.9.92
e13678.dspb.akamaiedge.net has IPv6 address 2a02:26f0:d3:484::356e
e13678.dspb.akamaiedge.net has IPv6 address 2a02:26f0:d3:48c::356e

I'd like functions that return a []string of the intermediaries instead of just the final CNAME.

[ianlancetaylor]

This may be more appropriate for golang.org/x/net/dns, or similar, rather than the standard library.

[andybons]

@therealjoesmith how do you feel about @ianlancetaylor’s suggestion to have this in /x/net/dns?

[therealjoesmith]

I don't have any convincing argument either way. I appreciate the standard library is already complicated as it is. I'll leave it to more informed contributors to make that decision.

[ianlancetaylor]

Is there any reason that the new functionality could not go in x/net/dns?

[rsc]

If you want detailed DNS information, the right thing to do is to use a DNS-specific package. The routines in package net are for getting basic stuff done, not advanced queries. It's probably fine to add to x/net/dns but that doesn't need to be a proposal. /cc @miekg

[odeke-em]

Some more DNS goodies for you @iangudger, if interested.

[iangudger]

To clarify, the proposal that was accepted was adding LookupCNAME to x/net/dns? In order to do that, we are going to need a DNS client there.

[joedian]

Implemented.