[avahi] using Avahi in unaware applications

[Lennart Poettering]

On Wed, 21.06.06 11:55, Norman Ramsey (nr at eecs.harvard.edu) wrote:

> 
>  > > I've doctored my slogin and ssh scripts so that if they see a hostname
>  > > of the form *.local, they will ask avahi-resolve to get the IP address.
>  > > But what I would really like to do is somehow change my system resolver,
>  > > so that *any* application will send .local requests to avahi and 
>  > > other requests to the standard DNS service.  Does anybody know
>  > > a way to change the resolver library, or to change the configuration
>  > > for named, to make this happen?
> 
>  > You are after 'libnss-mdns'
> 
> I have this installed, and I made the suitable change to my
> /etc/nsswitch.conf:
> 
>  > hosts:          files mdns4_minimal [NOTFOUND=return] dns mdns4

Please note that this line requires nss-mdns 0.8 to
work. Unfortunately, Debian unstable is still at 0.7. 

The Debian maintainer seems to be vanished without a trace. At least
he hasn't responded to any of my emails recently and ignored my bug
report:

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=369827

> And when I try 'getent hosts toller.local', as suggested in Lennart's
> documentation, it works nicely.
> 
> But unfortunately, many applications appear not to use the glibc NSS
> functionality: neither curl, nor ssh, nor firefox see
> 'toller.local'.

Works fine here. 

> Even worse, I must have my system configured badly (no cacheing?),
> because the time required to look up a *nonlocal* host is appalling.
> Notice the difference here between local and nonlocal lookup:
> 
>   : nr at toller 10706 ; time getent hosts toller.local
>   192.168.2.4     toller.local
> 
>   real    0m0.36s
>   user    0m0.00s
>   sys     0m0.00s
> 
>   : nr at toller 10705 ; time getent hosts toller
>   140.247.60.147  toller toller.eecs.harvard.edu
> 
>   real    0m10.01s
>   user    0m0.00s
>   sys     0m0.00s
> 
> Ten seconds to look up a hostname!  Whereas the same nonlocal lookup
> using host(1) is lightning fast (presumably exploiting a local
> cache).

Hmm, have you added .local to /etc/resolv.conf? If so every non-local
host name is first resolved with .local appended on mDNS. non-existing
mDNS hostnames will always cause the query to time out.

host(1) doesn't use any local cache. It uses unicast DNS, bypassing
NSS or anything.

> 
>   : nr at toller 10704 ; time host toller
>   toller.eecs.harvard.edu A       140.247.60.147
> 
>   real    0m0.08s
>   user    0m0.00s
>   sys     0m0.00s
> 
> This starts to look like a GNU NSS problem, not an Avahi problem, but
> I will refine my original question: is there some way to get my local
> DNS named (from bind9) to ask Avahi about hosts in the .local
> domain?

mDNS and unicast DNS are related but not really compatible. Such a
"link" between mDNS and unicast DNS would definitely be hack and
probably not worth the effort.

Lennart

-- 
Lennart Poettering; lennart [at] poettering [dot] net
ICQ# 11060553; GPG 0x1A015CC4; http://0pointer.net/lennart/