I'm in the process of getting a new test server installed in a rackin our machine room. This means we needed to set up some networkconnections for it, which involved someone dragging yet more cablearound our machine room and finding switch ports to plug into to getthe necessary networks (which turned out not to be an entirely trivialthing).

As a result, the whole thing got me thinking about the issue of the bestway to get all of your networks to where they need to be in

Here's a mistake I've made more than once:

tail -f active-log | grep big-filter | grep -v one-thing

(Here the big-filter is something that selects only a small amountof actual interesting logfile output, and then you want to throw awaya small bit more. For example, my most recent version of this wasmonitoring the exim log on our new mail system for failures and otheranomalies, and then wanting to throw away one particular known failure.)

Looking up reverse DNS to determine the hostname of an IP address thatyou are talking to is one of those almost simple things that softwarekeeps getting wrong .So here is how you do it, in two variants.

First, if you use DNS and have an (almost) empty /etc/hosts or justdon't care about looking up something in it, is the simple version:

• gethostbyaddr() the IP address. If it fails, you are done. Otherwiseyou have the claimed hostname .

  (

Once upon a time, I was at home and really needed to see somethingthat was displayed on my screen at work. This being Unix, there was noconvenient remote desktop add-on that would have let me mirror my workdisplay to home, but this being Unix, there are ways around that.

(Also, I was connecting over a slow dialup PPP connection , so a live remote desktop thing would havebeen difficult anyways.)

My first attempt was simple; ssh in to the work machine and do