Linux Blog

HowTo run an Perl or shell script using procmail

This is a sample (example) of procmail configuaration and in this configuation we want to run some script

:0 * conditions, if any | your-script-here

Obviously, you are not restricted to Perl or shell scripts. Anything you can run from a Unix command prompt can be run from Procmail.

Are you an Ubuntu User? - Get Counted !!!

The Ubuntu Counter Project is like a miniature version of The Linux Counter. It aims to catalogue the number of registered machines using the numerous varients of the Ubuntu Linux distribution.

Why? Because, quite simply, Ubuntu rocks.

To register your Ubuntu machines, simply register yourself as a user, and add your machines to your account.

Once registered, you will have a user button (like the ones below) to place on your website, in forum signatures or wherever you want!

Testing firewall rules

Sometimes it is handy to check firewall rules without coordinating a test with the end user. For these tests, use the hping2 utility to "spoof" traffic coming from the source IP address(es) used in the firewall rules.

At the same time, monitor the internal and external network interfaces on the firewall to make sure traffic is reaching the firewall and allowed through the firewall. In order to do this, you must have root access on the firewall and on the machine running hping2.

Example firewall rule:
Permit source IP 192.168.1.1 to communicate with destination IP 10.0.0.1 over TCP port 1000.

To test the rule, issue the following hping2 command:

hping2 -a 192.168.1.1 10.0.0.1 -p 1000

At the same time, log into the firewall and run the following commands (example with internal network interface etho and external network interface eth1)

In window 1:
tcpdump -i eth0 host 192.168.1.1 and port 1000

In window 2:
tcpdump -i eth1 host 10.0.0.1 and port 1000

If you do not see any output in window 1, traffic is not reaching the firewall. A choke router or other packet-filtering device may not be allowing the traffic to reach the firewall.

If you see output in window 1 but not in window 2, traffic is not being allowed through the firewall. Check the firewall rulebase for any errors.

ctime, atime, and mtime

It is important to distinguish between a file or directory's change time (ctime), access time (atime), and modify time (mtime).

ctime -- In UNIX, it is not possible to tell the actual creation time of a file. The ctime--change time--is the time when changes were made to the file or directory's inode (owner, permissions, etc.). It is needed by the dump command to determine if the file needs to be backed up. You can view the ctime with the ls -lc command.

atime -- The atime--access time--is the time when the data of a file was last accessed. Displaying the contents of a file or executing a shell script will update a file's atime, for example. You can view the atime with the ls -lu command.

mtime -- The mtime--modify time--is the time when the actual contents of a file was last modified. This is the time displayed in a long directoring listing (ls -l).

In Linux, the stat command will show these three times.

Important parts of the kernel in form of picture

The Linux kernel consists of several important parts: process management, memory management, hardware device drivers, filesystem drivers, network management, and various other bits and pieces. Figure shows some of them.

Probably the most important parts of the kernel (nothing else works without them) are memory management and process management. Memory management takes care of assigning memory areas and swap space areas to processes, parts of the kernel, and for the buffer cache. Process management creates processes, and implements multitasking by switching the active process on the processor.