ethernet aliases & kernel?

Hello group,

I have a question which caused me loads of grief during a server
migration. A server has 1 NIC and on this NIC 1 ethernet ip and 2
ethernet aliases are defined. So in total I have, let's say eth0,
eth0:2 and eth0:5. All three ip's are on the same network segment and
I need to specify, or be able to determine, which ip the network
traffic goes out from. Is there a way of setting that and if yes is it
in the kernel and where? The server is a Debian Etch (from now on I'll
name the server Deb) with 1 NIC with 1 plus the 2 (eth0:2 and eth0:5)
aliases.

The reason I want to be able to set the interface outgoing traffic is
going through is various services this server has to subscribe to. For
instance, a remote nfs server expects the server Deb to act as an nfs
client.
The nfs server expects Deb to connect to it, coming with a source ip
associated with, say, eth0, but a tcp dump shows Deb to be coming with
source ip associated with eth0:5. Consequently, Deb is denied access
to the nfs server.

Another senario would be if Deb acts as the domain MTA. i.e. as the MX
for the domain. If, say, eth0 ip address is registered with the DNS,
and all Deb's outgoing traffing goes through eth0:5, there is problem
cause receiving MTAs do a DNS lookup and find out that there is no MX
record associated the eth0:5 ip address, thus rejecting Deb's MTA
attempt to connect to them and deliver emails.

So I need to be able to specify the interface traffic is going out
from. A workaround in Debian Linux is to specify the interface, you
want to act as the outbound, last in the /etc/network/interfaces
config file.
But I suspect that this is not a very sound way of doing things and
I'm looking for a more stable way to do this. Kernel comes to mind but
I'm guessing here.
Pardon my english, they used to be better :D
thanks

.

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