add a rule to the firewall

netfilter/iptables is the mostly widely used software to create firewalls for Linux systems. If you've got one running and you want to add a rule 'on the fly', use a command similar to the following

/usr/sbin/iptables -I INPUT -p tcp --syn -s X.X.X.X/32 \
--destination-port 22 -j ACCEPT

This lets a particular IP login with SSH, which runs on port 22.

Change the -I to -D to delete a particular rule

To shut a particular IP out of your network, the following rule will suffice:

/sbin/iptables -I INPUT -p tcp --syn -s X.X.X.X/32 -j DROP

md5 checksums

If you develop software, one way to insure a good measure of security is by creating md5 checksums for your packages.

First, create the checksum file like so:

md5sum file.tar.gz > file.md5

Then, to check it, run this command:

md5sum -c file.md5

md5sum and passwords

Remembering passwords can be a problem. A password that's easy to remember may be insecure because it's easily guessed by others. On the other hand, a password that's virtually impossible to guess, say 'mR5YYt7Q' may also be virtually impossible to remember. There is a way to have your cake and eat it too, as they say. You can pipe an ordinary word to 'md5sum' and then pipe to 'cut' to get the result down to the first 8 characters (or more if you want). From now on, all you have to do is remember your special word.

echo Jennifer | md5sum | cut -b 1-8

This will make an 8 character password from the word 'Jennifer'. A couple of important notes:

• 'Jennifer' with a lower case 'j' or with only one 'N' will give you a different result (try it and you'll see), so your word must always be the same.
• Though not as insecure as writing your password down, this command will get into your bash shell history every time you use it. Please take the necessary precautions if you're sharing you computer with others.

install rkhunter

rkhunter stands for 'root kit hunter'. It's a utility written in Perl that looks for known root kits, trojans and other malware that may have ended up on your machine.

It can be downloaded from: http://www.rootkit.nl/. Once you've downloaded the tarball, installation is fairly straightforward. Just run the installer.sh script to install it.

The basic syntax is: /usr/local/bin/rkhunter -c --createlogfile

This will create a log file called rkhunter.log in /var/log/

Encryption

basic use of GnuPG

GnuPG is a Free Software version of Phil Zimmerman's famous PGP (Pretty Good Privacy) program for personal encryption

First, you need to generate a private key

gpg --gen-key

This will ask you some questions. Once you've done this, you can then work with it.

First you may want to create a public key that you'll give to your contacts

gpg --export -a --output mykey.asc username

Here's how you encrypt and sign a file:

gpg -sear username file_to_encrypt

Here's how you decrypt an encrypted file:

gpg -d file_to_decrypt

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