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Date: Thu, 1 Sep 2022 16:03:18 +0000
From: Luca BRUNO <lucab@...abruno.net>
To: musl@...ts.openwall.com
Subject: Re: musl resolver handling of "search ." in /etc/resolv.conf
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.
> > > 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.
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.
Ciao, Luca
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