sshguard

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sshguard is an intrusion prevention system that parses server logs, determines malicious activity, and uses the system firewall to block the IP addresses of malicious connections. sshguard is written in C so it does not tax an interpreter.

How it works

sshguard is a simple daemon that continuously tracks one or more log files. It parses the log events that daemons send out in case of failed login attempts and then blocks any further attempts from those connections by updating the system's firewall.

Unlike what the name implies, sshguard does not only parse SSH logs. It also supports many mail systems as well as a few FTP ones. A full listing of supported services can be found on the sshguard.net website.

Installation

Emerge

Install app-admin/sshguard:

root #emerge --ask app-admin/sshguard

Additional software

Depending on the init system and the desired firewall backend to be used by sshguard, additional software may be required to be emerged in order for sshguard to block malicious actors.

More information on various supported backends can be found by reading the setup manpage:

root #man 7 sshguard-setup

iptables

When iptables are being used as the system firewall.

root #emerge --ask net-firewall/iptables

More information about using and configuring IPtables can also be found on the IPtables article.

nftables

When nftables are being used as the system firewall:

root #emerge --ask net-firewall/nftables

Configuration

iptables backend

Preparing the firewall

When sshguard blocks any malicious users (by blocking their IP addresses), it will use the sshguard chain.

Prepare the chain with iptables and make sure it is also triggered when new incoming connections are detected:

root #iptables -N sshguard

root #iptables -A INPUT -j sshguard

Then verify that the appropriate path to the iptables backend library is set in /etc/sshguard.conf:

FILE /etc/sshguard.confSet iptables library for BACKEND

# Full path to backend executable (required, no default)
BACKEND="/usr/libexec/sshg-fw-iptables"

Watching logfiles

The basic idea behind sshguard is that the administrator passes on the log file(s) to watch as options to the application - there is no native sshguard configuration file.

On Gentoo, the options can be best configured in the /etc/sshguard.conf file:

FILE /etc/sshguard.confConfiguring sshguard to read /var/log/messages and /var/log/auth.log

# Space-separated list of log files to monitor. (optional, no default)
FILES="/var/log/messages /var/log/auth.log" 
#
# How many problematic attempts trigger a block
THRESHOLD=10
# Blocks last at least 24 hours (86400 seconds)
BLOCKTIME=86400
# Track IP addresses for 24 hours (86400 seconds)
DETECTION_TIME=86400
#
# IPv6 subnet size to block. Defaults to a single address, CIDR notation. (optional, default to 128)
IPV6_SUBNET=64
# IPv4 subnet size to block. Defaults to a single address, CIDR notation. (optional, default to 32)
IPV4_SUBNET=24
#
# Full path to PID file (optional, no default)
PID_FILE=/run/sshguard.pid

Make sure that the log files are accessible for the runtime user that sshguard uses.

Services

OpenRC

Have sshguard be started by default by adding it to the default runlevel, and then start it:

root #rc-update add sshguard default

root #rc-service sshguard start

systemd

Use systemd's conventional way to enable it, and then start it:

root #systemctl enable sshguard

root #systemctl restart sshguard

Blacklisting hosts

With the blacklisting option after a number of abuses the IP address of the attacker or a IP subnet will be blocked permanently. The blacklist will be loaded at each startup and extended with new entries during operation. sshguard inserts a new address after it exceeded a threshold of abuses.

Blacklisted addresses are never scheduled to be released (allowed) again.

To enable blacklisting, create an appropriate directory and file:

root #mkdir -p /var/lib/sshguard

root #touch /var/lib/sshguard/blacklist.db

While defining a blacklist it is important to exclude trusted IP networks and hosts in a whitelist.

To enable whitelisting, create an appropriate directory and file:

root #mkdir -p /etc/sshguard

root #touch /etc/sshguard/whitelist

The whitelist has to include the loopback interface, and should have at least 1 IP trusted network f.e. 192.0.2.0/24.

FILE /etc/sshguard/whitelistWhitelisting trusted networks

127.0.0.0/8
::1/128
192.0.2.0/24

Note
The 192.0.2.0/24 entry has to be adjusted to fit the own needs.

Add the BLACKLIST_FILE and WHITELIST_FILE file to the configuration. Example configuration listed blocks all hosts after the first login attempt. To setup a less agressive blocking policy, adjust the THRESHOLD and BLACKLIST_FILE integer, and set it to f.e. 10 instead of 2:

FILE /etc/sshguard.confConfiguring sshguard to blacklist abusers

BACKEND="/usr/libexec/sshg-fw-iptables"
FILES="/var/log/auth.log"
#
THRESHOLD=2
BLOCK_TIME=43200
DETECTION_TIME=604800
#
IPV4_SUBNET=24
IPV6_SUBNET=64
#
PID_FILE=/run/sshguard.pid
#
# Add following lines
BLACKLIST_FILE=2:/var/lib/sshguard/blacklist.db
WHITELIST_FILE=/etc/sshguard/whitelist

Restart the sshguard daemon to have the changes take effect. On OpenRC:

root #rc-service sshguard restart

Or on systemd:

root #systemctl restart sshguard

Troubleshooting

File '/var/log/auth.log' vanished while adding!

When starting up, sshguard reports the following error:

CODE Error message when trying to add a monitor for /var/log/auth.log

Sep 23 03:39:11 foo.bar.com sshguard[64933]: File '/var/log/auth.log' vanished while adding!

Such an error (the file path itself can be different) occurs when the target file is not available on the system. Make sure that it is created, or update the sshguard configuration to not add it for monitoring.

On a syslog-ng system with OpenRC, the following addition to syslog-ng.conf can suffice:

FILE /etc/syslog-ng/syslog-ng.confcreating auth.log file

log { source(src); destination(messages); };
log { source(src); destination(console_all); };
 
destination authlog {file("/var/log/auth.log"); };
filter f_auth { facility(auth); };
filter f_authpriv { facility(auth, authpriv); };
log { source(src);  filter(f_authpriv);  destination(authlog);  };

Reload the configuration for the changes to take effect:

root #rc-service syslog-ng reload

See also

• Fail2ban — a system denying hosts causing multiple authentication errors access to a service.
• Iptables — a program used to configure and manage the kernel's netfilter modules.

External resources

The sshguard documentation provides all the information needed to further tune the application.