Introduction

While connecting to your server through SSH can be very secure, the SSH daemon itself is a service that must be exposed to the Internet to function properly. This comes with some inherent risk and offers a vector of attack for would-be assailants.
Any service that is exposed to the network is a potential target in this way. If you pay attention to application logs for these services, you will often see repeated, systematic login attempts that represent brute-force attacks by users and bots alike.
A service called Fail2ban can mitigate this problem by creating rules that automatically alter your iptables firewall configuration based on a predefined number of unsuccessful login attempts. This will allow your server to respond to illegitimate access attempts without intervention from you.
In this guide, we’ll cover how to install and use Fail2ban on a CentOS 7 server.

Install Fail2ban on CentOS 7

While Fail2ban is not available in the official CentOS package repository, it is packaged for the EPEL project. EPEL, standing for Extra Packages for Enterprise Linux, can be installed with a release package that is available from CentOS:

sudo yum install epel-release

You will be prompted to continue—press y, followed by Enter:

yum promptTransaction Summary
============================================================================
Install  1 Package

Total download size: 14 k
Installed size: 24 k
Is this ok [y/d/N]: y

Now we should be able to install the fail2ban package:

sudo yum install fail2ban

Again, press y and Enter when prompted to continue.
Once the installation has finished, use systemctl to enable the fail2ban service:

sudo systemctl enable fail2ban

Configure Local Settings

The Fail2ban service keeps its configuration files in the /etc/fail2ban directory. There, you can find a file with default values called jail.conf. Since this file may be overwritten by package upgrades, we shouldn’t edit it in-place. Instead, we’ll write a new file called jail.local. Any values defined in jail.local will override those in jail.conf.
jail.conf contains a [DEFAULT] section, followed by sections for individual services. jail.local may override any of these values. Additionally, files in /etc/fail2ban/jail.d/ can be used to override settings in both of these files. Files are applied in the following order:

/etc/fail2ban/jail.conf
/etc/fail2ban/jail.d/*.conf, alphabetically
/etc/fail2ban/jail.local
/etc/fail2ban/jail.d/*.local, alphabetically

Any file may contain a [DEFAULT] section, executed first, and may also contain sections for individual jails. The last vavalue set for a given parameter takes precedence.
Let’s begin by writing a very simple version of jail.local. Open a new file using nano (or your editor of choice):

sudo nano /etc/fail2ban/jail.local

Paste the following:
/etc/fail2ban/jail.local

[DEFAULT]
# Ban hosts for one hour:
bantime = 3600

# Override /etc/fail2ban/jail.d/00-firewalld.conf:
banaction = iptables-multiport

[sshd]
enabled = true

This overrides three settings: It sets a new default bantime for all services, makes sure we’re using iptables for firewall configuration, and enables the sshd jail.
Exit and save the new file (in nano, press Ctrl-X to exit, y to save, and Enter to confirm the filename). Now we can restart the fail2ban service using systemctl:

sudo systemctl restart fail2ban

The systemctl command should finish without any output. In order to check that the service is running, we can use fail2ban-client:

sudo fail2ban-client status


Status
|- Number of jail:      1
`- Jail list:   sshd

You can also get more detailed information about a specific jail:

sudo fail2ban-client status sshd

Explore Available Settings

The version of jail.local we defined above is a good start, but you may want to adjust a number of other settings. Open jail.conf, and we’ll examine some of the defaults. If you decide to change any of these values, remember that they should be copied to the appropriate section of jail.local and adjusted there, rather than modified in-place.

sudo nano /etc/fail2ban/jail.conf

Default Settings for All Jails

First, scroll through the [DEFAULT] section.

ignoreip = 127.0.0.1/8

You can adjust the source addresses that Fail2ban ignores by adding a value to the ignoreip parameter. Currently, it is configured not to ban any traffic coming from the local machine. You can include additional addresses to ignore by appending them to the end of the parameter, separated by a space.

bantime = 600

The bantime parameter sets the length of time that a client will be banned when they have failed to authenticate correctly. This is measured in seconds. By default, this is set to 600 seconds, or 10 minutes.

findtime = 600
maxretry = 3

The next two parameters that you want to pay attention to are findtime and maxretry. These work together to establish the conditions under which a client should be banned.
The maxretry variable sets the number of tries a client has to authenticate within a window of time defined by findtime, before being banned. With the default settings, Fail2ban will ban a client that unsuccessfully attempts to log in 3 times within a 10 minute window.

destemail = root@localhost
sendername = Fail2Ban
mta = sendmail

If you wish to configure email alerts, you may need to override the destemail, sendername, and mta settings. The destemail parameter sets the email address that should receive ban messages. The sendername sets the value of the “From” field in the email. The mta parameter configures what mail service will be used to send mail.

action = $(action_)s

This parameter configures the action that Fail2ban takes when it wants to institute a ban. The value action_ is defined in the file shortly before this parameter. The default action is to simply configure the firewall to reject traffic from the offending host until the ban time elapses.
If you would like to configure email alerts, you can override this value from action_ to action_mw. If you want the email to include the relevant log lines, you can change it to action_mwl. You’ll want to make sure you have the appropriate mail settings configured if you choose to use mail alerts.

Settings for Individual Jails

After [DEFAULT], we’ll encounter sections configuring individual jails for different services. These will typically include a port to be banned and a logpath to monitor for malicious access attempts. For example, the SSH jail we already enabled in jail.local has the following settings:
/etc/fail2ban/jail.local

[sshd]

port    = ssh
logpath = %(sshd_log)s

In this case, ssh is a pre-defined variable for the standard SSH port, and %(sshd_log)s uses a value defined elsewhere in Fail2ban’s standard configuration (this helps keep jail.conf portable between different operating systems).
Another setting you may encounter is the filter that will be used to decide whether a line in a log indicates a failed authentication.
The filter value is actually a reference to a file located in the /etc/fail2ban/filter.d directory, with its .conf extension removed. This file contains the regular expressions that determine whether a line in the log is bad. We won’t be covering this file in-depth in this guide, because it is fairly complex and the predefined settings match appropriate lines well.
However, you can see what kind of filters are available by looking into that directory:

ls /etc/fail2ban/filter.d

If you see a file that looks to be related to a service you are using, you should open it with a text editor. Most of the files are fairly well commented and you should be able to tell what type of condition the script was designed to guard against. Most of these filters have appropriate (disabled) sections in jail.conf that we can enable in jail.local if desired.
For instance, pretend that we are serving a website using Nginx and realize that a password-protected portion of our site is getting slammed with login attempts. We can tell Fail2ban to use the nginx-http-auth.conf file to check for this condition within the /var/log/nginx/error.log file.
This is actually already set up in a section called [nginx-http-auth] in our /etc/fail2ban/jail.conf file. We would just need to add an enabled parameter for the nginx-http-auth jail to jail.local:
/etc/fail2ban/jail.local

[DEFAULT]
# Ban hosts for one hour:
bantime = 3600

# Override /etc/fail2ban/jail.d/00-firewalld.conf:
banaction = iptables-multiport

[sshd]
enabled = true

[nginx-http-auth]
enabled = true

And restart the fail2ban service:

sudo systemctl restart fail2ban

Monitor Fail2ban Logs and Firewall Configuration

It’s important to know that a service like Fail2ban is working as-intended. Start by using systemctl to check the status of the service:

sudo systemctl status fail2ban

If something seems amiss here, you can troubleshoot by checking logs for the fail2ban unit since the last boot:

sudo journalctl -b -u fail2ban

Next, use fail2ban-client to query the overall status of fail2ban-server, or any individual jail:

sudo fail2ban-client status
sudo fail2ban-client status jail_name

Follow Fail2ban’s log for a record of recent actions (press Ctrl-C to exit):

sudo tail -F /var/log/fail2ban.log

List the current rules configured for iptables:

sudo iptables -L

Show iptables rules in a format that reflects the commands necessary to enable each rule:

sudo iptables -S

Conclusion

You should now be able to configure some basic banning policies for your services. Fail2ban is very easy to set up, and is a great way to protect any kind of service that uses authentication.
If you want to learn more about how Fail2ban works, you can check out our tutorial on how fail2ban rules and files work.