IPNAT seems to affect network performance? of jails on lo0 ( - why?

Christopher J. Ruwe cjr at cruwe.de
Mon Jun 25 20:17:23 UTC 2012

On a KVM virtualized host, I run FreeBSD 8.3-RELEASE-p3 and some qjails,
8.3-RELEASE. The jails are connected all via lo0 on

While by the large working as expected, I have noticed one pecularity I
have failed to pinpoint: When launching processes with some network
interaction, like sshing into one of the jails from the platform or
launching emacs, the command spends ages ( ~(1-2) minutes) idling?
(nothing happens) before becoming interactive.

For reasons unreleated, I have enabled NAT with ipf for the jails on (to the external re0 IF and some IP) and, out of the blue,
logging into the jails or starting emacs became snappy again.

Why? Why does ipnatting jails which should be connected via the same lo0
on have any impact? Don't get me wrong, I am not complaining
and it solved an issue which gave me kind of headaches, but I would like
to understand. 

Thanks and cheers,
TZ: GMT + 2h
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