Name resolution of dot terminated hostnames

[Florian Weimer]

* Etienne Buira:

> On Fri, May 24, 2019 at 09:20:23PM +0200, Florian Weimer wrote:
>> * Etienne Buira:
>> 
>> > On a Gentoo box (with Gentoo packaged glibc 2.29-r2), libc was unable to
>> > resolve 'localhost.' (with the trailing dot), although it can resolve
>> > 'localhost', and conf looks fine.
>> >
>> > With older libcs, (at least 2.28 based), libc can resolve the dot
>> > terminated hostname just fine.
>> 
>> What's your /etc/nsswitch.conf file?  What's in /etc/hosts?
>
> In nsswitch.conf, the only line regarding hosts is:
> hosts: file dns

Presumably this?

hosts: files dns

> In hosts, the only non-comment line is:
> 127.0.0.1 localhost
>
> Some nameservers are set in resolv.conf, but the box didn't send any dns
> query.

That's odd.  I think localhost. queries would have always gone out over
DNS unless localhost. (with the trailing dot) is listed in /etc/hosts.
We made quite a few changes to the hosts implementation of nss_files in
the recent times, but I don't remember removing the trailing-dot
stripping.  It's always been a straight strcasecmp (which is arguably
wrong for several reasons).

> The same conf with the older libc versions are resolving dotted form.

With or without DNS queries?

>> > hostname(7) man page reads 'If the input name ends with a trailing dot,
>> > the trailing dot is removed, and the remaining name is looked up with no
>> > further processing.'
>> 
>> This covers DNS processing (“Internet name resolver”) only.  A newer
>> version of the manpage should reflect that more
>
> Manpage took from the Linux man-pages project 5.01, currently online. But
> reading it again, it is clear it is about DNS-based name resolver. Are
> you aware of any normative reference about this dotted form of names
> which are not part of a dns hierarchy?

There is RFC 6761 for special-use names.  The trailing dot is mentioned
in RFC 3986.  The behavior regarding trailing dots is varied; one major
DNS implementation suppressed search list processing with the trailing
dot, but had a single cache entry for domain with and without the
trailing dot.

Thanks,
Florian