[Bug network/19791] res_query.c:262 asserts when dns contains unreachable entries

["fweimer at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=19791

--- Comment #3 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to priyesh from comment #2)
> (In reply to Florian Weimer from comment #1)
> > (In reply to priyesh from comment #0)
> > 
> > > res_query.c:262: __libc_res_nquery: Assertion `(hp != ((void *)0)) && (hp2
> > > != ((void *)0))' failed.
> > 
> > What's the exact glibc version you tested?  Current master would print a
> > different assertion due to commit 8ba14398e629c1f63b9c91a59a47a713b3cce8bc.
> 
> 2.23 with no patches

Just to be sure, I compiled 2.23 with no patches and assertions enabled, and
libresolv.so.2 does not contain the string quoted above:

$ strings resolv/libresolv.so.2 | grep '!=' 
(hp != NULL) && (hp2 != NULL)
anscp != NULL || ansp2 == NULL

The first of the two strings macro-expands to the assertion failure message you
see.

This means what you are testing is definitely not 2.23.

Can you run your test under strace or with LD_DEBUG=all, to check which DSOs
are opened?

If you get that unexpected assertion message, maybe a wrong version of
libresolv.so.2 is being loaded.

> > > This can be triggered by disabling ipv6, then adding an ipv6 dns and using
> > > the proof of concept here:
> > > https://github.com/fjserna/CVE-2015-7547/blob/master/CVE-2015-7547-client.c
> > 
> > How did you disable IPv6?  There multiple ways to do this (at least
> > partially).
> add the following to /etc/sysctl.conf and do sysctl -p
> net.ipv6.conf.all.disable_ipv6 = 1
> net.ipv6.conf.default.disable_ipv6 = 1
> net.ipv6.conf.lo.disable_ipv6 = 1
> 
> make sure that dns has an ipv6 entry. in my test, ipv6 was the first entry

I have followed these steps, and I still cannot reproduce the assertion
failure.

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