TCP alias problem

[David A. Bandel]

On Wed, 23 Apr 2003 15:54:47 -0700
Bill Campbell <bill at celestial.com> wrote:

> I'm having a very ``interesting'' problem with a Caldera eDesktop 2.4
> system that's causing problems with NFS mounts from Apple iMacs
> running OS X.  Looking at tcpdump output I see the NFS request from
> the Mac going to the primary interface, eth0, on the NIC, but the
> response is going back from Caldera box from another interface,
> eth0:3.  The Mac doesn't see this as an appropriate reply since it's
> coming from the wrong IP address, hence the mount fails.  I have
> manually mounted using the IP address of eth0:3 which allows the mount
> to succeed, but there are other problems.
> 
> I've tried ssh to various systems on the LAN, and they also appear to
> originate from the eth0:3 interface.  On the other hand, I've tried
> ssh via an IPSec VPN tunnel to our private network here, and the IP
> address comes back as eth0.
> 
> Why would outgoing connections and replies appear to originate from
> the aliased interface instead of from eth0?

can you provide the ip/mask?

Generally, you'll see this not as an IP/interface problem, but as a
route problem.  The kernel picks the first route (generally the smallest
mask, i.e., least generic, most specific route) that gets the packet to
the host.  Look at your routing table.  I'll bet a virtual beer the
route being used is the first match in your routing table (and if your
masks are not identical, this will be the "smallest" mask).

To solve the problem, make eth0 your smallest mask and your aliases your
larger masks.

Now if your masks are the same, it goes in order from lowest network
number to the highest.  Solution is the same, manipulate which interface
gets which IP to make eth0 (not eth0:3) the first one.

Ciao,

David A. Bandel
-- 
Focus on the dream, not the competition.
		Nemesis Racing Team motto
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