Mail servers sharing an ip

[Mightytater]

Hey all, I need ideas.

In an internal network, I need to configure a secondary virtual mail server for high availability. Problem is, DNS is not used, so a bunch of clients are configured to use the current virtualized mail server by IP address. Thanks for your ideas!

[Irithori]

There is a way of moving an IP between hosts, but the scenario you describe calls for a better and easier solution:
- Introduce DNS
- Communicate that
- Set a deadline for the clients.

Good mailservice cannot and should not be done without DNS.
For a start, look at the Priority argument of the MX resource record.

[kados]

Well, I may be just talking nonsense, but I have a simple solution.
I came up with this while setting up a minecraft server.

Ok, since email NORMALLY runs on these ports, on any given IP address (weather it be an internal 192.* or an external)

POP3 - port 110
IMAP - port 143
SMTP - port 25
HTTP - port 80
Secure SMTP (SSMTP) - port 465
Secure IMAP (IMAP4-SSL) - port 585
IMAP4 over SSL (IMAPS) - port 993
Secure POP3 (SSL-POP) - port 995

Now, theoretically, you should be able to run multiple instances by just changing the port of the mail server. So, if your normal email server is on say 192.168.100.100:110 (pop3) you could also run an additional server on 192.168.100.100:120, you of course would need separate installs for each server. You would just have to change what port the email software the client is attempting to connect at.

[Mudgen]

If it's just an smtp relay, it's not too tough to have each host with individual ip on eth0:1 and switchable ip on eth0:2, which is not started at boot (ONBOOT=NO in ifcfg-eth0:2). Each starts checking the switchable ip at boot and ongoing, and if not pingable, brings up its eth0:2. If it's a full blown mail server, delivering to boxes and serving pop/imap clients, you also need clustered storage and replication of the client credentials.

But there's a lot of potential for trainwrecks, ways for both servers to be trying to use the ip at once. I've tried it (relay only) and it worked just well enough to drive me half nuts. Irithori gives good advice, as usual.

[Mightytater]

Irithori, I know, and wish it were that simple. Unfortunately, DNS is performed inside a closed system.

Mudgen, a coworker described that case to me, and I'll most likely give it a shot. I'll probably also go mad.

Thanks for the suggestions!