x/net/dns: create package

[gopherbot]

by [email protected]:

What does 'go version' print?
go version go1.3.1 darwin/amd64

What steps reproduce the problem?
If possible, include a link to a program on play.golang.org.

1. Run this program locally https://play.golang.org/p/BMNcHrJ1D5
2. Run dig txt 20120113._domainkey.gmail.com

What happened?
The Go program prints:
2014/08/16 16:23:06 lookup 20120113._domainkey.gmail.com on [192.168.1.1]:53: no such
host
2014/08/16 16:23:06 []

What should have happened instead?
The Go program should have found something. 

Please provide any additional information below.
I looked at a packet dump, and it looks like the problem is that TXT record contains two
strings. (For this particular use, the strings within a record are supposed to be
concatenated[1]. I don't know if this is true in general. If not, I guess there may need
to be a [][]string version of net.LookupTXT or something.)

unpackRR[2] in dnsmsg.go returns only the header if it fails to parse the entire body.
It tries to parse a dnsRR_TXT, but that only contains a string rather than a []string
and its Walk method only pulls one string out[3].

Per [4], a TXT-DATA contains "One or more <character-string>s.".

[1] https://tools.ietf.org/html/rfc6376#section-3.6.2.2
[2]
https://code.google.com/p/go/codesearch#go/src/pkg/net/dnsmsg.go&;q=unpackRR&sq=package:go&l=706
[3]
https://code.google.com/p/go/codesearch#go/src/pkg/net/dnsmsg.go&;q=unpackRR&sq=package:go&l=299
[4] https://tools.ietf.org/html/rfc1035#section-3.3.14

[gopherbot]

Comment 1 by [email protected]:

> For this particular use, the strings within a record are supposed to be
> concatenated[1]. I don't know if this is true in general. 
No such luck.  RFC 1035 leaves it up to the application, and although DKIM, DMARC, IKE,
and SPF specify that the constituent strings of their TXT record should be concatenated,
DNS-SD specifies a different interpretation for its TXT record (RFC 6763 §6.3):
“Each key/value pair is encoded as its own constituent string within the DNS TXT
record, in the form "key=value" (without the quotation marks).”
So for the general case, we do need a [][]string.

[ianlancetaylor]

Comment 2:

Labels changed: added repo-main, release-go1.4.

[rsc]

Comment 3:

We should make a go.net/dns that has all the bells and whistles. This one is for basic
things, and for basic things I don't think we need to support this case.

Labels changed: added release-none, removed release-go1.4.

Status changed to Accepted.

[mikioh]

Comment 4:

Labels changed: added repo-net, removed repo-main.

[bradfitz]

This kinda happened. https://godoc.org/golang.org/x/net/dns/dnsmessage

/cc @iangudger @mdempsky

[iangudger]

I am working on improvements for x/net/dns/dnsmessage and a DNS server to go along with it.

[mikioh]

Merged into #16218.