Subject: guests to use dummy0 or eth0 ?
From: Declan Mullen <declan@jadplace.com>
Date: Mon, 05 Jan 2009 22:59:16 +1100

Hi

I want to do something which is probably very typical of virtual 
servers. I believe there are at least two techniques to do it (see 
below), but because I have not done it before I am not sure which 
technique is best or if both techniques are possible. I hope some of you 
can share your recommendations and why :

I wish to set up a virtual server host with several guests. Each guest 
is to run a network service and I am intending on using the host's 
iptables to limit access to and in between the host and the guests and 
the other systems in the network. The network is a private subnet and 
there is no shortage of available ip addresses. The host has only one 
NIC (eth0). The only reason for running the network services in separate 
guests rather than running them all in the host, is for security. The 
network services are to include; apache2 web server, exim4 email, 
apt-cacher, bit torrent, scalix email.

Initially I assumed the technique of building/configuring each guest to 
use the host's eth0 interface with their own unique ip address in the 
host's eth0 subnet.

Then having browsed the virtual-server.org site I found an alternative 
technique of creating the dummy0 interface in the host with a ip address 
in a subnet different to eth0's subnet, and then building/configuring 
each guest to use the dummy0 interface with their own unique ip address 
in the host's dummy0 subnet. And then allowing other systems in the 
network to access the network services either by using the host as a 
simple router or by setting up port/service forwarding from host to 
relevant guest.

Are there any particular reasons why I should use the dummy0 technique 
rather than the simpler eth0 technique ?

BTW, I'm using the current Debian Lenny distribution and its virtual 
server kernel image (version "2.6.26+17") with its virtual server 
version "2.3" and the "util-vserver" version is "0.30.216~r2772-4". The 
"uname -a" output is "Linux max 2.6.26-1-vserver-686 #1 SMP Mon Dec 15 
21:11:05 UTC 2008 i686 GNU/Linux".

Thanks,
Declan