June 17, 2004

DNS lookups under Fedora Core 2

If you use Mozilla under Fedora Core 2, then this'll interest you.

Users of Mozilla on Fedora Core 2 may experience unusually long delays in resolving hostnames. This results from the fact that IPv6 is enabled by default in Fedora Core 2. If you do not need IPv6 support (which is most likely the case), then it is advised that you disable it in the kernel. To do this run the following command as root:

  echo "alias net-pf-10 off" >> /etc/modprobe.conf

You will need to reboot to have this take effect (or simply unload the ipv6 kernel module).

See bug 239358 for details.

NOTE: Disabling IPv6 via the Mozilla preference "network.dns.disableIPv6" is not sufficient. The problem is caused by a change in the behavior of getaddrinfo under Fedora Core 2. When asked to provide the canonical hostname, getaddrinfo performs a reverse lookup on all returned IP addresses. The result is 10 DNS queries for www.yahoo.com instead of just 1. When IPv6 is disabled in the operating system, Mozilla falls back to using good old gethostbyname, and the problem goes away.

WinXP and older versions of GLIBC (such as those used by FC1 and RH9) do not exhibit this behavior.

Posted by darin at June 17, 2004 12:00 PM
Comments

In general, you can check if IPv6 is enabled in the Linux kernel by checking the output of "lsmod"

lsmod | grep ipv6

If you get a hit, then IPv6 is probably enabled.

Posted by: Darin Fisher at June 17, 2004 12:47 PM

Holy shit, this really is a *lot* faster! Thanks Darin!

Posted by: bsander at June 18, 2004 1:42 AM

What if user does have ipv6 tunnel? What would happen? Also what about user that uses dns namecaching, will this affect them? I have both ipv6 tunnel and caching nameserver. Some sites seems to resolve quite quickly while others are quick.

Posted by: Asun at July 2, 2004 11:36 AM

Opps, some are quick and some are slow. My mistake.

Posted by: Asun at July 2, 2004 11:37 AM

> What if user does have ipv6 tunnel? What would
> happen? Also what about user that uses dns
> namecaching, will this affect them? I have both
> ipv6 tunnel and caching nameserver. Some sites
> seems to resolve quite quickly while others are
> quick.

If you are using IPv6, then I think you are out of luck :-/

A caching nameserver should definitely help.

Posted by: Darin Fisher at July 8, 2004 9:01 AM

OK, I'm a real greenhorn here. I have the
"resolving host" issue with both Mozilla and Netscape, though it seems worse on Mozilla. I would truly appreciate a remedial description of how I might disable IPv6. I don't know what 'disabling in the kernel' means. thanks

Posted by: sinc at July 19, 2004 6:50 PM

sinc, assuming you are running Linux, you can try disabling IPv6 using the command I gave above, i.e.:

echo "alias net-pf-10 off" >> /etc/modprobe.conf

You'll need to be logged in as root to run this command. After running this command, you'll need to reboot your system.

Posted by: Darin Fisher at July 19, 2004 9:51 PM

My mistake. I am running windows98. I didn't realize this was a linux forum. I simply did a search on "resolving host", and this thread came up.
Any help w/ windows?

Posted by: sinc at July 20, 2004 8:56 AM

Sinc,

Win98 does not have any ipv6 stack. Unless you have gone through all the trouble installing ipv6 stack for win98, slow resolve is not caused by ipv6.

Posted by: Asun at July 20, 2004 4:07 PM

Asun,
Thanks for that info. What causes the same symptoms on my system that are described above?

Posted by: sinc at July 20, 2004 5:04 PM