Difference between revisions of "Sshguard"

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(Aggressive banning: Added information regarding sshd_config and "MaxAuthTries 1")
m (Only one other in the previous sentence)
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[http://www.sshguard.net sshguard] is a daemon that protects [[SSH]] and other services against brute-force attacks, similar to [[fail2ban]].
 
[http://www.sshguard.net 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.
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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 [https://web.archive.org/web/20120625102244/http://www.ossec.net/main/attacking-log-analysis-tools vulnerabilities] that have caused problems for similar tools.
 
sshguard is not vulnerable to most (or maybe any) of the log analysis [https://web.archive.org/web/20120625102244/http://www.ossec.net/main/attacking-log-analysis-tools vulnerabilities] that have caused problems for similar tools.

Revision as of 22:03, 18 September 2017

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 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 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.

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:

# firewallctl zone "<zone name>" --permanent 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
-N sshguard
-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. See this example.

# iptables -A INPUT -p tcp --dport 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.

Usage

systemd

Enable and start the 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.

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

Change danger level

By default in the Arch-provided configuration file, 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.

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 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. Modify the parameters in the configuration file in the following way:

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

Finally restart the sshguard.service unit.

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

MaxAuthTries 1

You will have to restart the sshd.service unit for this change to take effect.

Tips and Tricks

Unbanning

If you yourself 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 --numeric

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

# iptables -D sshguard <line-number>

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

# sed -i '/<ip-address>/d' /var/db/sshguard/blacklist.db

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