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[Bug 530440] mysterious delay in processing of DNS replies in aria2c
  • From: bugzilla_noreply@xxxxxxxxxx
  • Date: Mon, 17 Aug 2009 09:19:10 -0600
  • Message-id: <20090817151910.396F0245513@xxxxxxxxxxxxxxxxxxxxxx>
http://bugzilla.novell.com/show_bug.cgi?id=530440

User tatsuhiro.t@xxxxxxxxx added comment
http://bugzilla.novell.com/show_bug.cgi?id=530440#c12





--- Comment #12 from Tatsuhiro Tsujikawa <tatsuhiro.t@xxxxxxxxx> 2009-08-17
09:19:09 MDT ---
Thanks for quick response, Stephan.
I agree that this is a general issue of 11.2. There are nothing we application
developer can do here.
I am still concerned about IPv6/DNS lookup and found an interesting page:

http://udrepper.livejournal.com/20948.html

The interesting part is:

"In glibc 2.9 I already implemented an improvement to the DNS NSS module which
optimizes the lookup of IPv4 and IPv6 addresses for the same host.
(snip)
"The problem with this change was that there are broken DNS servers and broken
firewall configurations which prevented the two results from being received
successfully. Some broken DNS servers (especially those in cable modems etc)
only send one reply. For this reason Fedora had this change disabled in F10.

"The drawback is that a timeout is needed to detect the broken servers or
firewalls. This delay is experienced once per process start and could be
noticeable. But the broken setups of the few people affected must not prevent
the far larger group of people with working setups to experience the advantage
of the parallel lookup.

I think this is possibly a cause of the issue: In previous 2 tcpdump logs, I
observed that IPv4 and IPv6 DNS lookups were done but the name server responded
only IPv4 request and there were delays. 11.2 uses glibc2.10 since Milestone 3.

The above article says workaround for the delays:

* Install a caching name server on this machine or somewhere on the local
network. bind is known to work correctly.
* Run nscd on the local machine. In this case the delay is incurred once per
system start (i.e., at the first lookup nscd performs).
* Add “single-request” to the options in /etc/resolv.conf. This selects the
compatibility mode from the start.

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