net: allow lookup of _ names

[gopherbot]

by bortzmeyer:

What steps will reproduce the problem?

[This is actually a problem in a comment.]


Which operating system are you using?

Debian "squeeze"

Which revision are you using?  (hg identify)

0dc6198e4ff3 tip

Please provide any additional information below.

The source code says:

func isDomainName(s string) bool {
    // Requirements on DNS name:
    //  * must not be empty.
    //  * must be alphanumeric plus - and .
    //  * each of the dot-separated elements must begin
    //    and end with a letter or digit.
    //    RFC 1035 required the element to begin with a letter,
    //    but RFC 3696 says this has been relaxed to allow digits too.
    //    still, there must be a letter somewhere in the entire name.

But this is quite misleading:

1) RFC 1035 never *required* names to be "alphanumeric plus - and ." RFC 1035,
section 2.3.1 "Preferred name syntax" just says it is the *preferred* syntax. 

2) RFC 2181 made very clear in section 11 "Name syntax" that domain names can
be anything (for instance, the _ is quite common, I do not even mention IDN)

3) The relaxation you mention (allowing a leading digit) was in RFC 1123, section 2.1
"Host Names and Numbers" and is about *HOST* names, not *DOMAIN* names. Host
names have much stricter rules.

[rsc]

Comment 1:

http://golang.org/cl/2290044

Owner changed to r...@golang.org.

Status changed to Started.

[rsc]

Comment 2:

This issue was closed by revision a0a7768.

Status changed to Fixed.

[gopherbot]

Comment 3 by bortzmeyer:

This is not "comment pedantry", it is an error in the code logic.
Currently, isDomainName returns false for a domain name like
_jabber._tcp.gmail.com whih does exist. I pointed the comments which
were misleading but there is a deeper issue in the code. Now that the
comments are gone, it is difficult to know what isDomainNameis
supposed to do but it is certainly not faithful to its name.

[rsc]

Comment 4:

If there is a problem in the code, it would have
helped not to start the bug report with
"This is actually a problem in a comment."
What do you suggest changing?

Status changed to Accepted.

[rsc]

Comment 5:

Is this sufficient?
http://golang.org/cl/2353043

[rsc]

Comment 6:

Issue #1168 has been merged into this issue.

[gopherbot]

Comment 7 by bortzmeyer:

The patch solves the problem of _ names (by far the most common) but isDomainName is
still not compliant with the standard since it rejects legitimate domain names (although
they are not frequent in the wild), for instance domain names with UTF-8 chars, with %
or &, etc

[rsc]

Comment 8:

The standard does say that clients can impose whatever 
restrictions they choose.  It only requires servers to
allow arbitrary syntax.
I'd like to avoid sending crazy requests on to DNS
and incurring the associated delays, failure modes, and so on.
I don't want to pass on UTF-8 because what if it's Latin-1
or what if it's supposed to be Punycode?  I don't think % and &
are going to arise in practice because they would be invalid
in a URL or email host name.

[rsc]

Comment 9:

This issue was closed by revision 2408a4b.

Status changed to Fixed.