Bug#414569: [Pkg-utopia-maintainers] Bug#414569: avahi-daemon: delay on resolving IP addresses when mdns is specified in /etc/nsswitch.conf

[Sjoerd Simons]

severity 414569 normal
thanks,

On Mon, Mar 12, 2007 at 04:57:08PM +0100, Martin Steigerwald wrote:
> Today I spent 5 hours or more for debugging a problem with slowness of
> almost all running applications. I first found that IMAP access with
> KMail was unusable slow, even after I installed dovecot locally. ssh and
> most network related commands took 5 seconds delay after being started
> before anything happened.
> 
> Well finally I found it times out on a request to resolve an IP address
> with avahi. I stopped avahi and ssh worked instantly and IMAP access via
> KMail accelerated immediately.
> 
> I started avahi-daemon again and the delays occured again.
> 
> I found that instead of stopping avahi-daemon I could also change
> /etc/nsswitch.conf by removing any reference to mdns:
> 
> #hosts:          files mdns4_minimal [NOTFOUND=return] dns mdns4
> hosts:          files dns
> 
> This happened on a NFS based workstation with Debian Etch. And then
> after an update to second workstation it happened also there. However I
> am not sure which package update triggered this problem (see package
> list below).
> 
> I am setting this to critical as it makes a NFS based network
> workstation next to unusable. Feel free to lower the priority as you see
> fit. Maybe it just happened here. But if not it IMHO should be fixed
> before Etch release.

It looks like you don't have proper reverse resolving setup. So if an
application wants to do reverse resolving on your network, it ends up using the
mdns4 in your nsswitch. Which in turn triggers avahi to try and resolve the
ip. If there is nobody that can answer the lookup on mdns, the request times
out after a while and thus the failure is reported to the app..  Now what your
seeing is that the timeout is (and afaik is supposed to be) a few seconds.

I'm not sure why this makes your NFS based workstation unusable.. Surely NFS
shouldn't do reverse lookups that often.

Also i'm not sure if this is really a bug.. An unsuccessfull rev. dns lookup on
mdns takes a few seconds by design, so not much we can do about this. For your
local setup you can either add proper rev. resolving to your network or disable
the final mdns fallback so it only ever uses mdns4_minimal.

  Sjoerd
-- 
Life can be so tragic -- you're here today and here tomorrow.