[CentOS] (?) Mailman VERY slow with IPv6 (with work-around)

Thu Sep 17 02:16:21 UTC 2015
Jay Leafey <jay.leafey at mindless.com>

I recently stood up an EL7 box with Mailman for a few lists I run for
some friends.  My old install, on an EL6 system, ran with no issues for
several years but I was induced to upgrade by a "hardware casualty" on
the old system.  I was going to have to rebuild anyway, so why not take
it as an opportunity to try EL7?

The build went fine and I was able to migrate the lists over with no
issues, but once I got there just about everything to do with Mailman
operations were painfully slow.  For example, "list_lists" took 5
seconds of "real" time.  I was used to it taking _much_ less as I only
have about 6 lists.  This affected both the command-line Mailman tools
and the web interface.  My first inclination was to blame Python, but
other code executed just fine with it.

While testing I tried an strace of list_lists and found that it was
timing out on a read operation to a socket to the Avahi daemon
(/var/run/avahi-daemon/socket) while trying to resolve the link-local
IPv6 address.  Having flashbacks to Sendmail stalling on DNS issues I
decided to try fixing resolution first.

As a test I put the link-local address into my /etc/hosts file with a
localized name.  Running list_lists then took about 0.19 seconds "real"
time!  The web interface also changed from painfully slow to it's
previous behaviour on EL6.

I imagine just turning off IPv6 would work as well, but I have an actual
use case that is a lot easier with it turned on.  I don't know if
anybody else has seen this, but thought it might be handy for someone else.
Jay Leafey - Memphis, TN
jay.leafey at mindless.com