Configuring Fail2Ban on RHEL & CentOS


Introduction to Fail2Ban

By default, a client connects to SSH using port 22. Because this is a well-known port, the default configuration can be vulnerable to brute force attacks. Fail2Ban offers a solution to automatically protect a server from these types of attacks. Fail2Ban runs in background reviewing log files for access attempts. If it detects an IP(s) attacking, it uses iptables to automatically ban the attacking server.

Installing Fail2Ban

Let’s install Fail2Ban using the EPEL repository –

yum install epel-release
yum install fail2ban

Configuring Fail2Ban

vi /etc/fail2ban/jail.conf

Adjust values based on requirements.

[DEFAULT]

# "ignoreip" can be an IP address, a CIDR mask or a DNS host. Fail2ban will not
# ban a host which matches an address in this list. Several addresses can be
# defined using space separator.
ignoreip = 127.0.0.1

# "bantime" is the number of seconds that a host is banned.
bantime = 600

# A host is banned if it has generated "maxretry" during the last "findtime"
# seconds.
findtime = 600

# "maxretry" is the number of failures before a host get banned.
maxretry = 3

Explanation

  • ignoreip: Don’t ban hosts which match an address in this list. Several addresses can be defined using space separator. Write your personal IP on this line.
  • bantime: The number of seconds that a host is banned.
  • findtime: A host is banned if it has generated maxretry during the last findtime.
  • maxretry: The number of failures before a host get banned.

Configuring Fail2Ban to protect SSH

Create a local configuration file.

cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.local
vi /etc/fail2ban/jail.local

Copy the lines below and paste to /etc/fail2ban/jail.local in the “Jail” section

[ssh-iptables]

enabled  = true
filter   = sshd
action   = iptables[name=SSH, port=ssh, protocol=tcp]
#           sendmail-whois[name=SSH, dest=root, sender=fail2ban@example.com]
logpath  = /var/log/secure
maxretry = 5
  • enabled: Activate the protection. If you want to turn it off, change the value to false.
  • filter: By default, it is set to sshd which refers to the file /etc/fail2ban/filter.d/sshd.conf.
  • action: Fail2Ban will ban the IP that matches the filter /etc/fail2ban/action.d/iptables.conf. If you had changed the SSH port before, change port=sshto the new port, for example port=2222. If you are using port 22, you won’t need to change the value.
  • logpath: The path of the log file used by Fail2Ban.
  • maxretry: The maximum number of failed login attempts.

Starting Fail2Ban service

Enable Fail2Ban on reboot and start the service:

chkconfig --level 23 fail2ban on
service fail2ban start

Validate iptables for rules added by Fail2Ban.

iptables -L

The result will look similar to this output.

Chain INPUT (policy ACCEPT)
target prot opt source destination
f2b-SSH tcp -- anywhere anywhere tcp dpt:EtherNet/IP-1

Chain FORWARD (policy ACCEPT)
target prot opt source destination

Chain OUTPUT (policy ACCEPT)
target prot opt source destination

Chain f2b-SSH (1 references)
target prot opt source destination
RETURN all -- anywhere anywhere

How to track failed login attempts

You can use this command to check if your server has had failed login attempts (possible attacks).

cat /var/log/secure | grep 'Failed password'

 

To view which IPs have been banned, use the following command.

iptables -L -n

To delete an IP address from banned list, run the following command. Change banned_ip to the IP that you want to unban.

iptables -D f2b-SSH -s banned_ip -j DROP

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