Tuesday, 16 October 2007

Secure SSH against brute force

I have certain measures to secure SSH against Brute force attack (what is Brute force click h3r3 ?)

I'm writing this post to reply a question on Shorewall mailing list, and I got the idea that it may be relevant to many others of us.

I recommend doing the following but please remember it is all relative measure on how you want to do it !
0) Disable root login and use "sudo" or "su"
1) Options like these help (in /etc/sshd_config):
MaxAuthTries 4
MaxStartups 1:3:6

2) Disable password authentication in ssh and enable Certificate based authentication.
3) BlockHosts

BlockHosts, is a script written in Python, is easier to set up, maintain, and configure. The idea behind BlockHosts is to continuously scan a syslog file for SSHD failed login attempts, and add the IP addresses listed there (after a predetermined number of attempts have been exceeded) to the system's /etc/hosts.deny file -- a different approach from that of Daemon Shield, which uses iptables to block connection attempts.

After installing the software, run the included setup script (as described in the INSTALL file). The setup script copies and installs all of the necessary BlockHosts files to their proper locations:

python setup.py install -force

Once you have the BlockHosts script installed, begin configuration by editing the /etc/blockhosts.cfg file. BlockHosts comes with a default configuration file with all options commented out. Edit this file and uncomment each line suitable for your installation. All of the options are well-documented in the comments, and can be uncommented by removing the "#" at the beginning of each line.

Once your configuration file is ready, the next step is to prepare the /etc/hosts.deny (or /etc/hosts.allow, depending on your installation) for BlockHosts by copying the following lines (in their entirety) to your hosts.{deny|allow} file:

#---- BlockHosts Additions
#---- BlockHosts Additions
sshd:ALL:spawn (/usr/bin/blockhosts.py --verbose >> /var/log/blockhosts.log 2>&1 )&:allow
proftpd:ALL: spawn (/usr/bin/blockhosts.py --verbose >> /var/log/blockhosts.log 2>&1 )&:allow

These instructions tell the system to automatically run (spawn) the BlockHosts script (/usr/bin/blockhosts.py) each time a user attempts to connect to your system via either SSH or ProFTP. The script will then determine if the connecting host should be allowed access or be blocked.

Once you have completed these steps, can begin watching for dictionary attacks. Each blocked address will be added to your hosts.{deny|allow} file and prevented from accessing your machine for the specified length of time (specified by AGE_THRESHOLD in the /etc/blockhosts.cfg file).

4) sshdfilter

sshdfilter, which blocks dictionary attackers using iptables, and is very efficient in how it detects them. The sshdfilter script starts the SSHD service itself, and instructs SSHD to output all log details to stdout (which is then captured by sshdfilter). In this way, the script can detect attacks as they happen, in real time, and significantly reduces the overhead involved in searching for offenders.

Unfortunately, the sshdfilter script is more complex to set up and install than the Daemon Shield software, partly because the author has made distribution-specific installation files that failed for my (non-included) Mandriva system. Out-of-the-box configurations include Red Hat 7.3 and 9.0, Fedora Core 3, and Debian 3.1. Details exist for users who want to attempt an install on an unsupported system, though they appear to be highly platform-specific.

Employing the basic practices and scripts above, you can harden your Linux machine against many of the dictionary SSH attacks that plague Linux systems today. Keeping your system's software up to date goes a long way toward protecting yourself against many common security vulnerabilities that automated scripts attempt to take advantage of. Don't let your system be the jumping-off point for spam, additional system attacks, or even blackmail -- protect yourself with these practices today.

1 comment:

Samer Azmy aka Kernel The Canine said...

I have been through this as well
fail2ban, it works nice
http://www.fail2ban.org/wiki/index.php/Main_Page

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