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Date: Thu, 1 Sep 2022 14:01:53 -0400
From: Rich Felker <dalias@...c.org>
To: Luca BRUNO <lucab@...abruno.net>
Cc: musl@...ts.openwall.com
Subject: Re: musl resolver handling of "search ." in /etc/resolv.conf
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@...c.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
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