[vox-tech] Multiple IPs on one system

[Jeff Newmiller]

On Fri, 3 May 2002, Marc Hasbrouck wrote:

> Here's a challenging question (for me at least),
> 
> I want to have server with two IP's on the same
> subnet. So far, my attemps with two NIC's don't work.
> To get any kind of connectivity, each NIC needs to
> talk to a differnt subnet.
> 
> What I want to do it this:
> 
> eth0 - dhcp to get my name isp's name servers from the
> firewall/dhcp server. No server service requests
> (Samba, NFS, or FTP) allowed in.
> 
> eth1 - in the same subnet, but a fixed address outside
> the dhcp range with server services running against
> it.
> 
> The idea was to set up a mirror of Redhat's updates
> files and update my systems locally with out having to
> pay for multiple RH network memberships. I plan to
> keep three to four systems updated this way. Each will
> have a different configuration (Server, laptop,
> workstation, etc.).
> 
> Questions:
> 
> 1. Is this possible?

The configuration you want is not practical.

The killer is getting your ISP to route packets to this other "fixed
address".  They provide this service for a fee... and it is higher than
(2-3 times) the dhcp level of service you pay for now.  If you do pay for
it, there won't be much point in keeping the dhcp service.

Even if you could get this configuration set up with your ISP, and you
then used IP Aliasing, Linux will accept packets coming in on either IP
number, but you may experience some difficulties with serving protocols
like FTP that expect the server to open connections going back to the
client. New outbound connections will usually pick up the source ip of the
default interface... and the client that requested the "return reply" can
get confused when it receives an "answer" from the wrong IP address.
Having one IP address on each network segment is really the most
straightforward solution.

> 2. Would it be easier to just learn how to use iptables?

Perhaps, but are apples easier than oranges?  I don't think the solution
lies thataway.

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