Setup fail2ban¶

Requirements¶

To install fail2ban you will need the following:

• a installed and supported operating system (e.g. CentOS 9.x)
• root-access
• a fast internet connection
• EPEL

Preliminary Note¶

This procedure is based on a documentation taken from:

• https://www.howtoforge.com/how-to-install-and-configure-fail2ban-on-fedora-33-centos-8/
• https://www.cyberciti.biz/faq/how-to-protect-ssh-with-fail2ban-on-centos-8/

Most Linux servers offer an SSH login via Port 22 for remote administration purposes. This port is a well-known port, therefore, it is often attacked by brute force attacks. Fail2ban is a software that scans log files for brute force login attempts in real-time and bans the attackers with firewalld or iptables. Fail2ban recognizes unwanted access or security breach efforts to the server within the administrator set time frame and blocks the IP addresses which show signs of brute force attacks or dictionary attacks. This program works in the background and continuously scans the log files for unusual login patterns and security breach attempts.

Install¶

dnf install fail2ban

if you are into noise ...

dnf install whois

Start/Enable Service¶

systemctl enable fail2ban
systemctl start fail2ban

When you are not running the CentOS Firewall yet, then start it:

systemctl enable firewalld
systemctl start firewalld

config¶

The /etc/fail2ban/jail.local file overrides defaults set in /etc/fail2ban/jail.conf file. Therefore, create or edit the jail.local

[DEFAULT]
# Ban IP/hosts for 24 hour ( 24h*3600s = 86400s):
bantime = 86400

# An ip address/host is banned if it has generated "maxretry" during the last "findtime" seconds.
findtime = 600
maxretry = 3

# "ignoreip" can be a list of IP addresses, CIDR masks or DNS hosts. Fail2ban
# will not ban a host which matches an address in this list. Several addresses
# can be defined using space (and/or comma) separator. For example, add your 
# static IP address that you always use for login such as 103.1.2.3
#ignoreip = 127.0.0.1/8 ::1 103.1.2.3

# Call iptables to ban IP address
banaction = firewallcmd-rich-rules[actiontype=<multiport>]

# Enable sshd protection
[sshd]
enabled = true

For each jail there may be a local configuration in /etc/fail2ban/jail.d/

Add a jail file to protect SSH.¶

Create a new file with the editor

vim /etc/fail2ban/jail.d/sshd.local

To the above file, add the following lines of code.

[sshd]
enabled = true
findtime = 3600
maxretry = 3
bantime = -1

Parameter enabled is set to true, in order to provide protection, to deactivate protection, it is set to false.

The parameter action is used to derive the IP address which needs to be banned using the filter available from /etc/fail2ban/action.d/firewallcmd-rich-rules.conf.

Port parameter may be changed to a new value such as port=1212, as is the case. When using port 22, there is no need to change this parameter.

Maxretry is used to set the maximum limit for failed login entries.

Bantime parameter is used to set the duration of seconds for which a host needs to be banned.

sendmail-action override to work with postfix-smarthost¶

cp /etc/fail2ban/action.d/sendmail-common.conf /etc/fail2ban/action.d/sendmail-common.local

change in section init sender from "fail2ban" to "root"

[Init]
sender = root

error in log like 'exec: ports="1:65535"; for p in $(echo $ports | tr ", " " "); do firewall-cmd --add-rich-rule'¶

change port-definition from

1:65535

to

1-65535

cp /etc/fail2ban/action.d/firewallcmd-common.conf /etc/fail2ban/action.d/firewallcmd-common.local
vim /etc/fail2ban/action.d/firewallcmd-common.local

[Init]
port = 1-65535

Status¶

fail2ban-client status
fail2ban-client status <jail>
fail2ban-client status sshd

Checking the banned IPs by Fail2Ban¶

firewall-cmd --list-all

Unbanning an IP address¶

In order to remove an IP address from the banned list, parameter IPADDRESS is set to appropriate IP which needs unbanning. The name "sshd" is the name of the jail, in this case the "sshd" jail that we configured above. The following command does the job.

fail2ban-client set <jail> unbanip <ip>
fail2ban-client set sshd unbanip 123.123.123.123