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Date: Wed, 31 Aug 2022 21:32:02 -0400
From: Jeffrey Walton <noloader@...il.com>
To: musl@...ts.openwall.com
Subject: Re: musl resolver handling of "search ." in /etc/resolv.conf
On Wed, Aug 31, 2022 at 7:59 PM Rich Felker <dalias@...c.org> wrote:
>
> On Wed, Aug 31, 2022 at 10:33:05AM -0700, Dalton Hubble wrote:
> > Hey folks,
> >
> > I wanted to flag a possible issue with musl handling of DNS "search ." in
> > /etc/resolv.conf.The easiest way I have to repro and consume musl is
> > starting an alpine or busybox musl container image.
> >
> > podman run -it docker.io/alpine:3.16.2 /bin/ash
> >
> > Edit /etc/resolv.conf to the following (not the "." at the end of search):
> >
> > ```
> > search default.svc.cluster.local .
> > nameserver 8.8.8.8
> > options ndots:5
> > ```
> >
> > ```
> > wget www.google.com
> > wget: bad address 'www.google.com'
> > ```
> >
> > Remove the "." from search and wget will work fine again.
> >
> > https://github.com/coreos/fedora-coreos-tracker/issues/1287 has some great
> > details showing DNS packet capture and a malformed packet.
> >
> > Broader context is that systemd and recently Kubernetes start adding
> > "search ." to resolv.conf in certain scenarios, which seems to break
> > musl-based resolvers.
> > - https://github.com/systemd/systemd/pull/17201
> > - https://github.com/kubernetes/kubernetes/pull/109441
> > - https://github.com/kubernetes/kubernetes/issues/112135
>
> Uhg. It was not forseen that . would be put in the search domains
> list, and putting it there, especially anywhere but the final position
> in the list, recreates a bad behavior that we explicitly tried to
> avoid having in musl.
>
> The mechanism of the failure is that malformed DNS queries are sent
> with a literal . at the end of the name. This probably also happens if
> the domains in the search list end in dot. Since the queries are
> malformed, they don't get responses (or ServFail) and then the search
> cannot continue.
>
> This can be fixed by properly stripping the final dot in search
> entries, and skipping ones that are otherwise malformed. Then we need
> to decide what to do with the empty (root) search suffix. 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.
>
> If it weren't for systemd and other things creating searches for .
> without the user's intent, I think the first option would clearly be
> the most reasonable. It provides a way to explicitly "get back" the
> functionality musl omits, on an opt-in basis. And maybe systemd is
> only emitting it as "search .", not putting . in the middle of other
> search domains?
>
> One of the other options might be a more conservative choice to make
> now, to avoid creating a new "feature" without thinking through what
> consequences it might have. We could always allow searching root
> later after there's been time to think through the consequences,
> rather than rushing is as part of a bugfix.
>
> Anyone care strongly about this one way or another?
Forgive my ignorance Rich. What, exactly, does 'search .' mean? Does
that mean lookup a single-label hostname in the TLD context?
If so, that's not supposed to happen:
https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/
. I would reject a 'search .' as malformed.
And be careful of systemd and its networking implementation. The
systemd folks do some shady things, like stripping a trailing dot from
a fqdn when setting a hostname. Stripping that dot means it is no
longer fully qualified.
Jeff
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