Sshguard

From ArchWiki
Jump to: navigation, search

Related articles

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 attacks, 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

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 7 minutes, and doubles each time he or she 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 systemd is being used. To make the ban only affect port 22, simply do not send packets going to other ports through the "sshguard" chain.

You must configure a firewall to be used with sshguard in order for blocking to work.

UFW

Tango-view-refresh-red.pngThis article or section is out of date.Tango-view-refresh-red.png

Reason: Is the following warning still applicable? (Discuss in Talk:Sshguard#ufw issue)
Warning: Currently, ufw-033-3 in [community] is not compatible with the method shown below. Users must use ufw-bzrAUR from the AUR.
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
# hand off control for sshd to sshguard
-N sshguard
-A ufw-before-input -p tcp --dport 22 -j sshguard

Restart ufw after making this modification.

iptables

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 -p tcp --dport 22 -j sshguard
# iptables-save > /etc/iptables/iptables.rules

If you use IPv6:

# ip6tables -N sshguard
# ip6tables -A INPUT -p tcp --dport 22 -j sshguard
# ip6tables-save > /etc/iptables/ip6tables.rules

If you do not use IPv6, create and empty file "ip6tables.rules" with:

# touch /etc/iptables/ip6tables.rules

Finally, reload the iptables service.

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

To finish saving your iptables configuration. Repeat above steps with ip6tables to configure the firewall rules for IPv6 and save them with ip6tables-save to /etc/iptables/ip6tables.rules.

For more information on using iptables to create powerful firewalls, see Simple stateful firewall.

The sshguard setup page, here, under "Here is a sample ruleset that makes sense", lists a slightly more elaborate, slightly more efficient ruleset that only allows services the host supports to even enter the sshguard rule chain.

Usage

systemd

Enable and start the sshguard.service. The provided systemd unit uses a blacklist located at /var/db/sshguard/blacklist.db and pipes journalctl into sshguard for monitoring.

To add optional sshguard arguments, modify the provided service with drop-in snippets as described in systemd#Editing provided units.

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

Change danger level

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

Edit the provided systemd unit and change the ExecStart= line:

[Service]
ExecStart=
ExecStart=/usr/lib/systemd/scripts/sshguard-journalctl "-b 200:/var/db/sshguard/blacklist.db" SYSLOG_FACILITY=4 SYSLOG_FACILITY=10

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

Finally restart the sshguard.service unit.

Aggressive banning

For some users under constant attack, it may be beneficial to enable a more aggressive banning policy. If you can be reasonably sure that accidental failed logins are unlikely, then you can instruct SSHGuard to automatically ban hosts with a single failed login. Edit the provided systemd unit in the following way:

[Service]
ExecStart=
ExecStart=/usr/lib/systemd/scripts/sshguard-journalctl "-a 1 -b 10:/var/db/sshguard/blacklist.db" SYSLOG_FACILITY=4 SYSLOG_FACILITY=10

Finally restart the sshguard.service unit.

Tips and Tricks

Unbanning

If you get banned, you can wait to get unbanned automatically or use iptables to unban yourself. First check if your ip is banned by sshguard:

# iptables -L sshguard --line-numbers

Or this, which is a lot faster as it returns packet/byte counts and ip addresses instead of names (saves a lot of DNS lookups of probably bogus sites).

# iptables -L sshguard -v -n --line-numbers -v

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

# iptables -D sshguard <line-number>

Logging

If you aren't sure what is being passed to sshguard, you can add a tee command to the script in /usr/lib/system/scripts/sshguard-journalctl:

#!/bin/sh
SSHGUARD_OPTS=$1
shift
LANG=C /usr/bin/journalctl -afb -p info -n1 -o cat "$@" | tee -a /var/log/sshguard.log | /usr/bin/sshguard $SSHGUARD_OPTS

but use this with care, remember the warning about how a bad-guy can flood your system and potentially lock you out? This makes it a lot easier to do so.