I'm dealing with two dedicated servers hosted at liquidweb. The basic idea is that one machine might not be enough to handle the traffic at launch so we have a second machine to shoulder the MySQL duties -- it seemed a natural way to divy up computation burdens.

The support staff at LW tells me that each machine has three network cards
1) Public-facing NIC through which all public visitors are served
2) NIC on LAN so that liquidweb can access the machine locally for maintenance and support
3) Third NIC so that the two machines can speak to each other rapidly on their own private network. Apparently the two machines are connected via crossover cable.

It's my understanding that machine 1 has IP of 192.168.0.1 on this third card and machine 2 has IP of 192.168.0.2 on its third card. For some reason, these machines cannot speak to each other using ping, telnet, ssh, http, or MySQL.

I've contacted tech support about this issue but was wondering how I might be sure that the MySQL traffic transits the third network card on each machine. I'm guessing that it's just a matter of using the third card's IP addresses but want to be sure. Is there any way to specify a NIC when connecting via MySQL ?

You will need to look at the routing table. Use the command 'route' (may require root privilege) and it will show you the path that it wants to take to the other machine, including the interface. What you will want to see is that traffic for 192.168.0.0 with an appropriate netmask, e.g. 255.255.255.0 is routed out the third NIC card. This means that all traffic for machines on that network segment will be handled by that interface.

Since you can't ping, it is possible that this may be the issue. On the other end, or rather on both ends, you will also need to make sure that you don't have filters in place on the firewall for that interface. Since it is a private interface, you may want to set it to accept all traffic on that interface.

Lastly, make sure that you have a running process on the 'other' machine that is listening for traffic. You may then need to look at the configuration for the process and make sure that it isn't bound to listen only on one of the other interfaces. This will be likely in the configuration files in the /etc drive.

I would start with SSH as this is a pretty easy one to tell if it is working or not. In its configuration, erase or comment out and interface binding. You can also code 'netstat -nta | grep 22' to see if it is listening on port 22 and this will also show you the IP that it is bound to. If it says 0.0.0.0, it indicates that it is listening on all addresses (interfaces).