[Xen-users] Multiple VMs - one static routable IP address

[xensource@xxxxxxxxxxxxxxxxxxx]

I have a new server and am looking into using XEN. Looks like everything I 
need is there, and gives me the security and power I've found frustrating 
or lacking in VMWare... I have one problem tho... I want to have each VM 
be able to completely host it's own domain name and it's own services. I 
only have one routable IP address however. I need to service ports: 22, 
25, 80, 110, and 443
I have no worldly idea how to route the packets to each respective VM when 
everybody is trying to use the same ports, and are all sitting behind the 
firewall effectively running non-routable IP addresses.
Example:
Domain-0 will host mail services on port 25
Domain-1 will also...
Domain-2 as well
Domain-3 again...

For port 80 and 443 traffic, I suppose I could always just stick apache in 
proxy mode and route to the respective VM running on the 10-net behind the 
firewall, and I can run SSHD on a non-standard port for each one I suppose 
as well... but with everybody running their own sendmail and needing to 
receive mail (sending shouldn't be a problem in this configuration near as 
I can tell anyway) everybody pig piles in on port 25 and only Domain-0 
wins. I could hack up some sort of MTA forwarding I suppose, but there HAS 
to be some way that this is done such that I don't have to special case 
every port. I just don't know what that is it seems.
So, when a packet comes in to the DHCP as asks for the IP address for one 
(of seven) of the domain names I host. They will all resolve to the same 
single routable IP address I have. I suspect I need some sort of 
soft-router running on Domain 0 to see if the packet is destined for one 
of the VMs and if so route to the 10.0.0.X address accordingly.
I'm completely new to bridging, tho am by no means new to Linux systems 
administration. All VMs, including Domain-0, are running CentOS 5.  My 
_guess_ is I need to get a fourth  DomU installed and running and have 
THAT run the routable IP address, and then forward all traffic into 
Domain-0, or the respective other VM domains, switching based on the 
domain name the traffic is trying to go to...  I wouldn't be able to 
connect to any of the VMs but that forward facing one by using the IP 
address, but then, I seldom to never do that anyway...  plus that way I 
could hide Domain-0 a bit better anyway.  I just have no idea what bits 
I'd need to install and run to get that working.
Any help pointing me in the right direction would be greatly appreciated. 
This can't be a unique problem... I'm sure the S390 guys are running 
thousands of VMs per system, and suspect there is no way all of those VMs 
are also running routable IP addresses. I just don't know how to get the 
packets to the right VM when I've only got the one externally routable IP 
address.
Help Help Help!

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