Re: [TCLUG:5871] resolving multihomed ips

["Mark A. Bentley" <bentlema@cs.umn.edu>]

On Fri, 7 May 1999, Clay Fandre wrote:
> /etc/hosts is empty except for the localhost, kramer, and DNS server. So
> it is resolving the name with the DNS server, but it is using the last
> IP it gets, instead of the first. Anyone have a multihomed server with
> this working? Both Slowaris and HS-UX works fine.
> 
> Clay
> 

Yes, I did get it working.  It seems odd that you'd always be getting the
same undesired IP from the list of addresses though.  Without sortlist
functionality you'd normally resolve IPs in a round-robin fashion.  For
example, using ping, which is linked against the libc6 that comes with
RedHat 5.2, ping caesar.cs.umn.edu and you'll see that the IP resolved
rotates:

   PING caesar.cs.umn.edu (160.94.239.160): 56 data bytes
   64 bytes from 160.94.239.160: icmp_seq=0 ttl=254 time=1.4 ms

   PING caesar.cs.umn.edu (128.101.224.60): 56 data bytes
   64 bytes from 128.101.224.60: icmp_seq=0 ttl=254 time=1.4 ms
 
   PING caesar.cs.umn.edu (128.101.228.60): 56 data bytes
   64 bytes from 128.101.228.60: icmp_seq=0 ttl=254 time=1.3 ms

   PING caesar.cs.umn.edu (128.101.248.6): 56 data bytes
   64 bytes from 128.101.248.6: icmp_seq=0 ttl=254 time=1.8 ms

   PING caesar.cs.umn.edu (128.101.230.60): 56 data bytes
   64 bytes from 128.101.230.60: icmp_seq=0 ttl=254 time=14.8 ms

   PING caesar.cs.umn.edu (160.94.239.160): 56 data bytes
   64 bytes from 160.94.239.160: icmp_seq=0 ttl=254 time=3.3 ms


If I ping a multi-homed host which has an interface on the same subnet I'm
on, we get:  (this is doing "ping mail.cs" from feyd.cs)

   PING mail.cs.umn.edu (128.101.229.42): 56 data bytes
   64 bytes from 128.101.229.42: icmp_seq=0 ttl=255 time=2.7 ms

   PING mail.cs.umn.edu (128.101.229.42): 56 data bytes
   64 bytes from 128.101.229.42: icmp_seq=0 ttl=255 time=4.6 ms

   PING mail.cs.umn.edu (128.101.229.42): 56 data bytes
   64 bytes from 128.101.229.42: icmp_seq=0 ttl=255 time=12.2 ms

For this to work, you need to put a line in your /etc/resolv.conf like
this:

   sortlist 128.101.229.0/255.255.255.0

...that's network/netmask.  The problem is RedHat didn't compile in
sortlist support when they built glibc. To fix the problem, I just added a
-DRESOLVSORT in the resolv/Makefile, rebuilt the RPMs and installed them
again (need to use --force.)

If you don't feel like rebuilding the RPMs, you can grab the ones I built
from:

   ftp://ftp.cs.umn.edu/users/bentlema/RedHat/5.2

See Bugzilla bug #1467:

  http://developer.redhat.com/bugzilla/show_bug.cgi?id=1467&BUGLIST=1467

RedHat claims that the sortlist code hasn't been tested, and they don't
intend to fix this bug.

--Mark

==========================================================================
Mark A Bentley                  Email:  bentlema@cs.umn.edu
Systems Staff, CSci Dept
University of Minnesota         URL:    http://www.cs.umn.edu/~bentlema/