I've been using Debian now for a few days, so I'm sure this APT stuff will start to make sense eventually, but I'm stumped on this one:

Googling around, I found nagios packages at:

http://natasha.stmarytx.edu/debian/pool/main/n/nagios/


Which is convenient, as I wish to install Nagios, and already have the following line in my sources.list:

deb http://natasha.stmarytx.edu/debian/ stable main non-free contrib


So when I do 'apt-cache search nagios' I get no packages.

As other packages in .../main/n are in my cache, why isn't nagios?

To answer the usual first questions:
It's a clean install of Debian from the 'compact' floppy images and I'm running many packages which were installed using apt.

'apt-get update' was performed this morning

What am I missing here?

Nothing, I added that entry to my sources.list and I can't find nagios either.
I'd send a mail to the owner of that site.

I've since changed mirrors to several others including debian.org and found the same problem.

I'm asking the mirror for 'stable main non-free and contrib' are there other groups which nagios could be in?

I've also sent an e-mail to the maintainer or the package, and received no response.

It depends which branch the package is for, stable, testing or unstable.

Also, did you do "apt-get update" after updating sources.list?

You could just download the *.debs you require and install them yourself.

I updated after every change to the sources list.

How do I tell which branch a package is for?

Don't know! I suppose it would tell you on the website of the particular package.

I think your best bet is to download the debs and install them yourself.

For other sources check out apt-get.org. The search utility gave three sources for nagios.