Triage report (2021-06-17)

[Sergio Durigan Junior]

On Thursday, June 17 2021, Paride Legovini wrote:

> https://bugs.launchpad.net/ubuntu/+source/postfix/+bug/1931801
> Title: Apt install postfix fails with bad demiliter Edit
>
> which I think it's a duplicate of LP: #1906970. The problem seems to
> be triggered by the
>
>   search .
>
> line being present in /etc/resolv.conf. That "search ." line is
> something I've seen before, and the reporter of LP: #1906970 claims
> it's valid syntax, but I don't know what it's supposed to do and I
> couldn't find it documented. While working on LP: #1906970 I'm going
> to treat it as invalid syntax, unless anybody has a pointer to what it
> means. In this case the postfix postinst should be fixed.

Hi Paride,

I think the reporter from #1906970 is right: it seems that "search ." is
indeed supported.

Interestingly, I could not find any documentation to truly back this up;
even though the manpage for resolv.conf(5) is well written (kudos to
Michael Kerrisk and the man-pages project, btw!), it is not
authoritative when it comes to defining what "search ." means.  So I had
to resort to the real source of knowledge here: glibc and its resolver.

Long story short, the resolver code lives under the "resolv/" directory
on glibc.  If you look at the res_search function (which performs DNS
queries taking "search" into account), you will find this excerpt here:

  https://sourceware.org/git/?p=glibc.git;a=blob;f=resolv/res_query.c;h=ebbe5a6a4ed86abe3fccd4a134bfcf6f613c9bbb;hb=HEAD#l385

When RES_DNSRCH is set, it means that the resolver will search for
hostnames in the current domain and in parent domains.  Particularly,
this excerpt is interesting:

	/* __res_context_querydoman concatenates name
	   with dname with a "." in between.  If we
	   pass it in dname the "." we got from the
	   configured default search path, we'll end
	   up with "name..", which won't resolve.
	   OTOH, passing it "" will result in "name.",
	   which has the intended effect for both
	   possible representations of the root
	   domain.  */
	if (dname[0] == '.')
		dname++;
	if (dname[0] == '\0')
		root_on_list++;

So you see that the code takes into account the case when "search ." is
used, and interprets it as representing the root domain indeed.

Cheers,

-- 
Sergio
GPG key ID: E92F D0B3 6B14 F1F4 D8E0  EB2F 106D A1C8 C3CB BF14