I recently had a chance to set up and then work with a console server. This was a first for me and so useful a device that I've become convinced that any systems administrator with more than a dozen systems to manage should know something about how console servers work and the service they can provide.

A console server is pretty much like what it sounds like. It's a device that consolidates console access to as many servers as the unit has ports. The particular unit that I set up and use is a Cyclades AlterPath with 48 serial ports, often referred to as an ACS48. With this box set up in the lab in which I work, 48 servers -- all "headless" (lacking consoles) -- are now accessible in single user mode or otherwise console-only states.

How are Servers Connected?

The console server, which has the general apearance of a router or switch, occupies one unit of rack space and connects to the local network in the same manner as all the other network nodes. Each of its 48 serial ports connects through a serial cable to the console (serial) port on a server. If a server is in a rack at the far end of the lab, the serial cable might be run through a set of patch panels. In other words, one serial cable might go from the console port on the server to a nearby patch panel and another might go from a patch panel near the console server to a particular port on the console server. Whether directly connected or not, the serial cable needs to be a "crossover" cable, similar to the null modem cables I used to build as needed.

How is Access Provided?

After associating every port on the new console server with a unique IP address, I could then open an ssh session to any of the assigned IP addresses and, after providing a username and password, I could work on the server console -- whether I was sitting in my office or working from another building, even from home.

The connection between me and the console server is always a TCP/IP connection while the connection between the console server and the target console is serial. This allows a large distance to exist between me and the console -- as long as nearly all of it is between me and the console server (serial connections are subject to length limitations that constrain the distance between a console server and the console ports that it supports).

How Do Console Servers Help?

Within a few days of setting up the new console server, I had two great opportunities to make use of it. In the first of these, the target server was sitting at the ok prompt, waiting for an fsck to clear up some file system damage. I logged in to the console and issued the required "fsck -y" and ^D and had the system up without having to leave my

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