Fail2ban

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.

Fail2ban scans various textual log files and bans IP that makes too many password failures by updating firewall rules to reject the IP address.

Installation

First, install python2-pyinotify so that Fail2ban can detect modification to the log files:

# pacman -S python2-pyinotify

Then, install fail2ban:

# pacman -S fail2ban

If you want Fail2ban to send an email when someone has been banned, you have to configure SSMTP (for example). You will also have to install whois to get some information about the attacker.

# pacman -S whois

systemd

Use the service unit fail2ban.service, refer to systemd for instructions.

Note: Fail2ban currently does not support journald logging. This is because it parses textual files like /var/log/auth.log and journald uses its own binary format. You must use syslog-ng or similar to have textual logging.

Hardening

Currently, fail2ban requires to run as root, therefore you may wish to consider some additional hardening on the process with systemd. Ref:systemd for Administrators, Part XII

Capabilities

For added security consider limiting fail2ban capabilities by adding CapabilityBoundingSet under [Service] section of the systemd service file, e.g.:

CapabilityBoundingSet=CAP_DAC_READ_SEARCH CAP_NET_ADMIN CAP_NET_RAW

In the example above, CAP_DAC_READ_SEARCH will allow fail2ban full read access, and CAP_NET_ADMIN and CAP_NET_RAW allow setting of firewall rules with iptables. Additional capabilities may be required, depending on your fail2ban configuration. See man capabilities for more info.

Filesystem Access

Also considering limiting file system read and write access, by using ReadOnlyDirectories and ReadWriteDirectories, again under the under [Service] section. For example:

ReadOnlyDirectories=/
ReadWriteDirectories=/var/run/fail2ban /var/spool/postfix/maildrop

In the example above, this limits the file system to read-only, except for /var/run/fail2ban for pid and socket files, and /var/spool/postfix/maildrop for postfix sendmail. Again, this will be dependent on you system configuration and fail2ban configuration. Note that adding /var/log is necessary if you want fail2ban to log its activity.

SSH jail

Edit /etc/fail2ban/jail.conf and modify the ssh-iptables section to enable it and configure the action.

If your firewall is iptables:

[ssh-iptables]
enabled  = true
filter   = sshd
action   = iptables[name=SSH, port=ssh, protocol=tcp]                                         
           sendmail-whois[name=SSH, dest=your@mail.org, sender=fail2ban@mail.com]
logpath  = /var/log/auth.log                                                                    
maxretry = 5

If your firewall is shorewall:

[ssh-shorewall]
enabled  = true
filter   = sshd
action   = shorewall
           sendmail-whois[name=SSH, dest=your@mail.org, sender=fail2ban@mail.com]
logpath  = /var/log/auth.log                                                                    
maxretry = 5

Note: You can set BLACKLISTNEWONLY to No in /etc/shorewall/shorewall.conf otherwise the rule added to ban an IP address will affect only new connections.

Also do not forget to add/change:

LogLevel VERBOSE

in your /etc/ssh/sshd_config. Else, password failures are not logged correctly.

See also

• sshguard