mailing list of musl libc
 help / color / mirror / code / Atom feed
From: Rich Felker <dalias@libc.org>
To: Luca BRUNO <lucab@lucabruno.net>
Cc: musl@lists.openwall.com
Subject: Re: [musl] musl resolver handling of "search ." in /etc/resolv.conf
Date: Thu, 1 Sep 2022 14:01:53 -0400	[thread overview]
Message-ID: <20220901180153.GB21934@brightrain.aerifal.cx> (raw)
In-Reply-To: <20220901160318.67d15b3f@ephyra>

On Thu, Sep 01, 2022 at 04:03:18PM +0000, Luca BRUNO wrote:
> On Thu, 1 Sep 2022 08:45:12 -0400
> Rich Felker <dalias@libc.org> wrote: 
> 
> > "search ." by itself is a semantically a no-op. It specifies a single
> > search domain that's the DNS root, which is exactly what gets queried
> > with no search at all. systemd is writing this into resolv.conf
> > because of a glibc "misbehavior" (to put it lightly) where, in the
> > absence of any search directive, it defaults to searching the domain
> > of the system hostname (so hostname=foo.example.com would implicitly
> > search example.com, which is obviously wrong to do, and systemd is
> > trying to suppress that). But it would also cause failing lookups to
> > be performed in duplicate, unless there's logic to suppress the final
> > non-search lookup when root was already searched explicitly.
> 
> While tracking down this musl bug, I empirically observed from
> network traces that glibc does apply such de-duplication logic under the
> same configuration.
> That is, it performs the root-anchored query in the specified order, and
> in case of a negative response it does *not* perform the query again as
> it would otherwise do for the final fallback case.

Thanks! This is good to know.

> > > > There are 3 options I see:
> > > >
> > > > - Actually support it as a search. This is *bad* behavior, but at
> > > >   least unlike the version of this behavior musl explicitly does
> > > > not implement, it was explicitly requested by the user. Except
> > > > that it wasn't, because systemd is just putting it in everyone's
> > > >   resolv.conf..
> > > >
> > > > - Skip it completely. Never search root; wait for the end of the
> > > >   search list and query root as always.
> > > >
> > > > - End search on encountering it and go directly to the post-search
> > > >   query at root.
> > > >
> > > > Anyone care strongly about this one way or another?  
> 
> From my observations, option 1 is consistent with other libc's behavior.
> But it has the above caveat that it needs additional caching to
> avoid duplicate root-queries on negative responses.
> If it isn't too invasive to implement, that would be my preferred one.

I'm not clear what additional caching you have in mind. AFAICT the
search loop can just set a flag if it searched root already, and the
final root query can be skipped if it's reached and the flag is set.

> Option 2 looks somehow reasonable too. The skewed order would be
> a bit surprising, but it can be documented and it's unlikely to affect
> many real-world usages.

If we go this route, I think the way to document it would be that
search list entries are strings of one or more label, and that
malformed ones (including zero-length, over-length, etc.) are ignored.

Rich

  reply	other threads:[~2022-09-01 18:02 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-08-31 17:33 Dalton Hubble
2022-08-31 23:59 ` Rich Felker
2022-09-01  1:32   ` Jeffrey Walton
2022-09-01 12:45     ` Rich Felker
2022-09-01 16:03       ` Luca BRUNO
2022-09-01 18:01         ` Rich Felker [this message]
2022-09-02  8:09           ` Luca BRUNO
2022-09-19 17:18           ` Rich Felker

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20220901180153.GB21934@brightrain.aerifal.cx \
    --to=dalias@libc.org \
    --cc=lucab@lucabruno.net \
    --cc=musl@lists.openwall.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
Code repositories for project(s) associated with this public inbox

	https://git.vuxu.org/mirror/musl/

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).