sshguard

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, with the knowledge of your IP address, the attacker can send packets with a spoofed source header and get you locked out of the server. SSH keys provide an elegant solution to brute forcing without these problems.

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

sshguard is different from the latter 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

Install the sshguard package.

Setup

sshguard works by monitoring /var/log/auth.log, syslog-ng or the systemd journal for failed login attempts. For each failed attempt, the offending host is banned from further communication for a limited amount of time. The default amount of time the offender is banned starts at 120 seconds, and is increases by a factor of 1.5 every time it fails another login. sshguard can be configured to permanently ban a host with too many failed attempts.

Both temporary and permanent bans are done by adding an entry into the "sshguard" chain in iptables that drops all packets from the offender. The ban is then logged to syslog and ends up in /var/log/auth.log, or the systemd journal if the latter is being used.

You must configure one of the following firewalls to be used with sshguard in order for blocking to work.

firewalld

sshguard can work with firewalld. Make sure you have firewalld enabled, configured and setup first. To make sshguard write to your zone of preference, issue the following commands:

# firewall-cmd --permanent --zone=public --add-rich-rule="rule source ipset=sshguard4 drop"

If you use ipv6, you can issue the same command but substitute sshguard4 with sshguard6. Finish with

# firewall-cmd --reload

You can verify the above with

# firewall-cmd --info-ipset=sshguard4

Finally, in /etc/sshguard.conf, find the line for BACKEND and change it as follows

BACKEND="/usr/lib/sshguard/sshg-fw-firewalld"

UFW

If UFW is installed and enabled, it must be given the ability to pass along DROP control to sshguard. This is accomplished by modifying /etc/ufw/before.rules to contain the following lines which should be inserted just after the section for loopback devices.

Note: Users running sshd on a non-standard port should substitute that in the final line above (where 22 is the standard).

/etc/ufw/before.rules

# allow all on loopback
-A ufw-before-input -i lo -j ACCEPT
-A ufw-before-output -o lo -j ACCEPT

# hand off control for sshd to sshguard
:sshguard - [0:0]
-A ufw-before-input -p tcp --dport 22 -j sshguard

Restart ufw after making this modification.

iptables

Note: See iptables and Simple stateful firewall first to set up a firewall.

The main configuration required is creating a chain named sshguard, where sshguard automatically inserts rules to drop packets coming from bad hosts:

# iptables -N sshguard

Then add a rule to jump to the sshguard chain from the INPUT chain. This rule must be added before any other rules processing the ports that sshguard is protecting. Use the following line to protect FTP and SSH or see [1] for more examples.

# iptables -A INPUT -m multiport -p tcp --destination-ports 21,22 -j sshguard

To save the rules:

# iptables-save > /etc/iptables/iptables.rules

Note: For IPv6, repeat the same steps with ip6tables and save the rules with ip6tables-save to /etc/iptables/ip6tables.rules.

nftables

Edit /etc/sshguard.conf and change the value of BACKEND to the following:

/etc/sshguard.conf

BACKEND="/usr/lib/sshguard/sshg-fw-nft-sets"

When you start/enable the sshguard.service, two new tables named sshguard in the ip and ip6 address families are added which filter incoming traffic through sshguard's list of IP addresses. The chains in the sshguard table have a priority of -10 and will be processed before other rules of lower priority. See sshguard-setup(7) and nftables for more information.

Usage

systemd

Enable and start sshguard.service.

syslog-ng

If you have syslog-ng installed, you may start sshguard directly from the command line instead.

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

Configuration

Configuration is done in /etc/sshguard.conf which is required for sshguard to start. A commented example is located at /usr/share/doc/sshguard/sshguard.conf.sample or can also be found on Bitbucket sshguard.conf.sample.

Note: Piped commands and runtime flags in sshguard's systemd units are not supported. Such flags can be modified in the configuration file.

Blacklisting threshold

By default in the Arch-provided configuration file, offenders become permanently banned once they reach a "danger" level of 120 (or 12 failed logins; see attack dangerousness for more details). This behavior can be modified by prepending a danger level to the blacklist file.

BLACKLIST_FILE=200:/var/db/sshguard/blacklist.db

The 200: in this example tells sshguard to permanently ban a host after achieving a danger level of 200.

Finally restart sshguard.service

Moderate banning example

A slightly more aggressive banning rule than the default one is proposed here to illustrate various options:

It monitors sshd and vsftpd via logs from the systemd/Journal
It blocks attackers after 2 attempts (each having a cost of 10, explaining the 20 value of the THRESHOLD parameter) for 180 seconds with subsequent block time longer by a factor of 1.5. Note that this 1.5 multiplicative delay is internal and not controlled in the settings
Attackers are permanently blacklisted after 10 attempts (10 attempts having each a cost of 10, explaining the 100 value in the BLACKLIST_FILE parameter)
It blocks not only the attacker's IP but all the IPv4 subnet 24 (CIDR notation)

/etc/sshguard.conf

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

# Log reader command (optional, no default)
LOGREADER="LANG=C /usr/bin/journalctl -afb -p info -n1 -t sshd -t vsftpd -o cat"

# How many problematic attempts trigger a block
THRESHOLD=20
# Blocks last at least 180 seconds
BLOCK_TIME=180
# The attackers are remembered for up to 3600 seconds
DETECTION_TIME=3600

# Blacklist threshold and file name
BLACKLIST_FILE=100:/var/db/sshguard/blacklist.db

# 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

Aggressive banning

For some users under constant attack, a more aggressive banning policy can be adopted. If you are confident that accidental failed logins are unlikely, you can instruct SSHGuard to permanently ban hosts after a single failed login. Modify the parameters in the configuration file in the following way:

THRESHOLD=10
BLACKLIST_FILE=10:/var/db/sshguard/blacklist.db

Finally restart sshguard.service.

Also, to prevent multiple authentication attempts during a single connection, you may want to change /etc/ssh/sshd_config by defining:

MaxAuthTries 1

Restart sshd.service for this change to take effect.

Tips and Tricks

Unbanning

If you ban yourself, you can wait to get unbanned automatically or use iptables or nftables to unban yourself.

You will also need to remove the IP address from /var/db/sshguard/blacklist.db in order to make unbanning persistent.

iptables

First check if your IP is banned by sshguard:

# iptables --list sshguard --line-numbers --numeric

Then use the following command to unban, with the line-number as identified in the former command:

# iptables --delete sshguard line-number

nftables

Remove your IP address from the attackers set:

# nft delete element family sshguard attackers { ip_address }

where family is either ip or ip6.

Logging

To see what is being passed to sshguard, examine the script in /usr/lib/systemd/scripts/sshguard-journalctl and the systemd service sshguard.service. An equivalent command to view the logs in the terminal:

# journalctl -afb -p info SYSLOG_FACILITY=4 SYSLOG_FACILITY=10