Sshguard

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Warning: Using an IP blacklist will stop trivial attacks but it relies on an additional daemon and successful logging (the partition containing /var can become full, especially if an attacker is pounding on the server). Additionally, if the attacker knows your IP address, they can send packets with a spoofed source header and get you locked out of the server. SSH keys provide an elegant solution to the problem of brute forcing without these problems.

sshguard is a daemon that protects SSH and other services against brute-force attacts, similar to fail2ban.

sshguard is different from the other two in that it is written in C, is lighter and simpler to use with fewer features while performing its core function equally well.

sshguard is not vulnerable to most (or maybe any) of the log analysis vulnerabilities that have caused problems for similar tools.

Installation

First, install iptables so sshguard can block remote hosts:

# pacman -S iptables

Then, install sshguard:

# pacman -S sshguard

Configuration

The main configuration required is creating a chain named "sshguard" in the INPUT chain of iptables where sshguard automatically inserts rules to drop packets coming from bad hosts:

# iptables -N sshguard
# iptables -A INPUT -j sshguard
# /etc/rc.d/iptables save

If you do not currently use iptables and just want to get sshguard up and running without any further impact on your system, these commands will create and save an iptables configuration that does absolutely nothing except allowing sshguard to work:

# iptables -F
# iptables -X
# iptables -P INPUT ACCEPT
# iptables -P FORWARD ACCEPT
# iptables -P OUTPUT ACCEPT
# iptables -N sshguard
# iptables -A INPUT -j sshguard
# /etc/rc.d/iptables save

For more information on using iptables to create powerfull firewalls, see Simple Stateful Firewall.

Finally, add iptables and sshguard to the DAEMONS array in /etc/rc.conf:

DAEMONS=(... iptables sshguard ...)

In Arch Linux

By default, sshguard does not have its own configuration file: all options are supplied on the command line. However, Arch Linux uses the /etc/conf.d/sshguard configuration file, allowing additional arguments to be passed to the command line when sshguard is started. By default sshguard will use its built-in log reader, called Log Sucker, to read the logs:

/usr/sbin/sshguard -l /var/log/auth.log -b /var/db/sshguard/blacklist.db

The -l switch tells sshguard which log to watch. Note also the -b option is used, which makes some bans permanent. Records of permanent bans are then kept in /var/db/sshguard/blacklist.db to be remembered between restarts.

General Information

sshguard works by watching /var/log/auth.log for changes to see if someone is failing to log in too many times. It can also be configured to get this information straight from syslog-ng. After too many login failures (default 4) the offending host is banned from further communication for a limited amount of time. The amount of time the offender is banned starts at 7 minutes and doubles each time he is banned again. By default in the archlinux package, at one point offenders become permanently banned.

Bans are done by adding an entry into the "sshguard" chain in iptables that drops all packets from the offender. To make the ban only affect port 22, simply do not send packets going to other ports through the "sshguard" chain.

When sshguard bans someone, the ban is logged to syslog and ends up in /var/log/auth.log.

See also