Linux Directory Structure

/bin - This directory contains most of your non-privileged system commands such as ls, mkdir, rm, etc.

/boot - Contains the systems boot image, bootloader, and the kernel

/dev - Symbolic links to system devices such as optical and removable drives

/etc - Contains all system configuration files and most configurations for installed packages

/home - Contains a directory for each user and contains profile information

/lib - Contains dynamic libraries and modules for the Linux system and installed packages

/media - Contains mount points for optical drives and removable media

/mnt - Used as a location for mounted drives and shares

/opt - Contains user installed packages and custom software not handled by the system or package manager

/proc - An interface between the kernel and the system, useful for diagnostics and system information

/root - The root superuser's home directory

/sbin - Contains privileged commands that are usually run as superuser (root/sudo)

/sys - An interface between the kernel and the system, used for modifying system settings

/tmp - A location for temporary files such as sessions on a web server

/usr - Contains most installed packages that are not part of the system, user installed programs

/usr/bin - Contains commands related to user installed packages in /usr

/usr/sbin - Contains privileged commands related to user installed packages in /usr

/var - Contains files that change often or accessed frequently

/var/log - Contains all system logs and most logs generated by installed packages

Linux NAT

If you are running a recent 2.6 Linux Kernel this four step process should work for you. This has been specifically tested on Fedora Core 3, 4, 5, and 6, but should work on any modern Linux distribution. All of these commands must be executed as the root user. First you need to tell your kernel that you want to allow IP forwarding.

echo 1 > /proc/sys/net/ipv4/ip_forward

Then you'll need to configure iptables to forward the packets from your internal network, on /dev/eth1, to your external network on /dev/eth0. You do this will the following commands:

/sbin/iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
/sbin/iptables -A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT
/sbin/iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT

You should now be NATing. You can test this by pinging an external address from one of your internal hosts. The last step is to ensure that this setup survives over a reboot. Obviously you should only do these last two steps if your test is a success.

Search Engine Installation and Configaration

Introduction
htdig is a webpage search engine licensed under the GNU Public License. It uses a very simple configuration file to allow it to search only the webpages you specify. For example, you can exclude the cgi-bin or a testing directory from the search engine. In addition to installing it on a webserver, some programs use it as a search engine plugin such as Glade, the GTK+ User Interface Builder. In addition, it will create a searchable database of any website. You just supply to URL.
Installing htdig

1. Download the latest version from the htdig ftp server.
2. tar -xvfz htdig-3.1.5.tar.gz
3. cd htdig-3.1.5
4. ./configure
5. make
6. make install

Configuring htdig

Once you have htdig installed, you must make a few changes to the configuration file and the HTML templates into which the search results are embedded.

Configuration File

The configuration file for htdig is located at /opt/www/htdig/conf/htdig.conf. It is pretty self-explanitory. The main attributes you need to configure are as follows. It will work if you leave the defaults for the other options or change them if you wish.

Attribute	Value	Example
start_url	URL of your site	http://www.mywebsite.com
exclude_urls	Directories you do not want searched separated by white spaces	/cgi-bin/ /testing/
adminstrator	Email address of administrator	admin@mywebsite.com
search_results_header	HTML file to be used as header of search results. Only use this if you don’t want to use the default location for the header file: /opt/www/htdig/common/header.html	/home/httpd/search/header.html
search_results_footer	HTML file to be used as footer of search results. Only use this if you don’t want to use the default location for the header file: /opt/www/htdig/common/footer.html	/home/httpd/search/footer.html
nothing_found_file	HTML file to be displayed if there is no match to search string entered. Only use this if you don’t want to use the default location for the header file: /opt/www/htdig/common/nomatch.html	/home/httpd/search/nomatch.html
syntax_error_file	HTML file to be displayed if there is a syntax error in the search string entered. Only use this if you don’t want to use the default location for the header file: /opt/www/htdig/common/syntax.html	/home/httpd/search/syntax.html

HTML Templates

If you don’t want to use the default look-and-feel of htdig, you can edit the following files to use the look-and-feel of your website. The paths may be different if you choose to change the paths of them in your configuration file.

• /opt/www/htdig/common/header.html
• /opt/www/htdig/common/footer.html
• /opt/www/htdig/common/nomatch.html
• /opt/www/htdig/common/syntax.html

Post-installation and configuration

1. Next, you must setup the search database by running the script /opt/www/htdig/bin/rundig.
2. Copy the default search.html and images from /opt/www/htdocs/htdig to a directory named htdig off of your webRoot. If the images are not in this directory, they will not appear unless you configure it otherwise it htdig.conf.
3. Copy /opt/www/cgi-bin/htsearch to the cgi-bin for your webserver.
4. Test the search engine by opening search.html in your browser and entering a search string.
5. Because the search engine uses a database to return results, the database must be rebuilt with the rundig command used in step 1 every time any pages are added to the website.
6. If you want to configure anything else, refer the the htdig website. Pretty much everything is configurable with htdig.

Monitor ALL eth0 Traffic Except My Own SSH Session tcpdump:

I'm using tcpdump to dump, debug and monitor traffic on a network. However, there is lots of noise and I would like to exclude ssh from my dumps. How do I monitor all traffic except my ssh session?

The tcpdump command displays out the headers of packets on a network interface that match the boolean expression. In other words you can use boolean expression to drop ssh traffic from dumping and monitoring operation using the following syntax:

tcpdump -i eth0  -s 1500 port not 22

You can skip additional ports too:

tcpdump -i eth0  -s 1500 port not 22 and port not 53

You can also use ip or hostname:

tcpdump -i eth0 port not 22 and host 1.2.3.4

See also:

man tcpdump