Slow DNS, but _only_ in Firefox -- cause and workaround

[Matthew Morgan]

After installing Ubuntu, I had slow-DNS-in-Firefox problems similar to
what a few others have reported.  Here's what I discovered:

The symptoms: Firefox takes 20 seconds or more to do a DNS lookup, but
other programs that use DNS (ping, wget, apt-get, etc.) work just
fine.

The cause: Mozilla bug 239358 (at
https://bugzilla.mozilla.org/show_bug.cgi?id=239358 ) has all the gory
details.  It's an interaction between Mozilla's use of getaddrinfo,
glibc's implementation of same, and IPv6 being enabled.  A fix has
been checked in on the main Mozilla tree and the Firefox branch, so
once that propagates into Ubuntu's version, this problem should go
away.

The workaround: Disable IPv6 by creating a file in /etc/modprobe.d/
with the single line "alias net-pf-10 off" (w/o quotes) in it, and
reboot.  (The name of the file doesn't matter; I used "noipv6".)

- Matt
-- 
http://matthewmorgan.blogspot.com