Figures, I sit down to finally debug this and find that glibc
2.9-0ubuntu12 was released yesterday; it contains a patch which appears
to... well, at least patch the issue. Lookups, even with apt, appear to
be working with this version!
So is the fix appropriate? From the changelog:
glibc (2.9-0ubuntu12) jaunty; urgency=low
* debian/patches/all/fedora-nss_dns-gethostbyname4-disable.diff: Patch
from Fedora 2.9-3 to temporarily disable _nss_dns_gethostbyname4_r,
which caused problems for systems with broken IPv6 connectivity
(LP: #313218, https://bugzilla.redhat.com/show_bug.cgi?id=459756).
I am having trouble finding this particular .diff - where am I supposed
to look? - but assume it is substantially similar to:
http://pasky.or.cz/~pasky/dev/glibc/glibc-2.10-dns-no-
gethostbyname4.diff
found via Google.
...so apparently parallel lookups were codified as
_nss_dns_gethostbyname4_r. Fair enough.
That version bears the following comment: "This should work in theory,
but it turns out that many cheap DSL modems and similar devices have
buggy DNS servers - if the AAAA query arrives too quickly after the A
query, the server will generate only a single reply with the A query id
but returning an error for the AAAA query; we get stuck waiting for the
second reply."
This blames "cheap DSL modems and similar devices," but If I understand
my own dumps (see: tcpdump_snippet - search race.txt for example) with
the "broken" resolver, this was not the case for my configuration:
separate queries were issued from the same source port but with
different IDs, the nameserver properly responded to both, then for
inexplicable reasons the resolver reissued the same queries (reusing the
same IDs, but a new source port) a second time, before blindly trying a
third set of requests with the search domain appended. Rather than
"getting stuck waiting," it rapidly repeated itself.
Clearly something was screwy with the resolver algorithm, rather than
the particular DNS server, unless I have overlooked some subtle
noncompliance in the responses. If I get a chance, I hope to explore
the "_nss_dns_gethostbyname4_r" behavior in greater depth and come to an
absolute and reasoned conclusion. :}
To reiterate, though: In the meantime, this patch, reverting to the
earlier strategies, does seem to "fix everything."
--
[Jaunty] Multiple DNS resolver issues on ia32 and AMD64
https://bugs.launchpad.net/bugs/326718
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