2005/03/31 IPP

Internet Printing Protocol which is what CUPS uses. You may know that if you fire up your web browser and point it at http://localhost:631 you get the CUPS administration screen, but did you realize that CUPS itself sends print and status requests to the same port?

There's nothing secret about this: CUPS documentation (which you can find at http://localhost:631/ipp.html ) explains all of it. If you wanted to, you could control CUPS directly with HTTP requests. We can demonstrate that with a little tom-foolery: let's create a little shell script that we'll run on port 631. First the script, which logs attempted connections.

#!/bin/bash
echo "Connection" >> /tmp/fakecupsd
date >> /tmp/fakecupsd
read line
echo $line >> /tmp/fakecupsd

We turn that on by creating a file in /etc/xinetd, stopping the real cups, and telling xinetd to start using ours for port 631 connections.

# cat /etc/xinetd.d/fakecups
service ipp
{
        socket_type     = stream
        wait            = no
        user            = root
        server          = /root/fakecups
}
/etc/init.d/cups stop
kill -1 `cat /var/run/xinetd.pid`

Now do an "lpstat -v" - it will hang, so ctrl-c it, and look in /tmp/fakecupsd. You'll see something like this repeated multiple times:

Connection
Sat Mar 26 05:36:30 EST 2005
POST / HTTP/1.1

The reason lpstat hangs is that it is sending more data with that http post, which the real cups daemon would process and acknowledge.

Many of the products and books I review are things I purchased for my own use. Some were given to me specifically for the purpose of reviewing them.

I resell or can earn commissions from the sale of some of these items. Links within these pages may be affiliate links that pay me for referring you to them. That's mostly insignificant amounts of money; whenever it is not I have made my relationship plain. I also may own stock in companies mentioned here. If you have any question, please do feel free to contact me.

Unix and Linux Support