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How To Protect SSH with fail2ban on CentOS 6

Jun 14, 2012 Security, Monitoring CentOS

About Fail2Ban

Servers do not exist in isolation, and those servers with only the most basic SSH configuration can be vulnerable to brute force attacks. fail2ban provides a way to automatically protect the server from malicious signs. The program works by scanning through log files and reacting to offending actions such as repeated failed login attempts.

Step One—Install Fail2Ban

Because fail2ban is not available from CentOS, we should start by downloading the EPEL repository:

rpm -Uvh http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm

Follow up by installing fail2ban:

yum install fail2ban

Step Two—Copy the Configuration File

The default fail2ban configuration file is location at /etc/fail2ban/jail.conf. The configuration work should not be done in that file, however, and we should instead make a local copy of it.

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

After the file is copied, you can make all of your changes within the new jail.local file. Many of possible services that may need protection are in the file already. Each is located in its own section, configured and turned off.

Step Three—Configure defaults in Jail.Local

Open up the the new fail2ban configuration file:

vi /etc/fail2ban/jail.local

The first section of defaults covers the basic rules that fail2ban will follow. If you want to set up more nuanced protection for your virtual private server, you can customize the details in each section.

You can see the default section below.

[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  = 3600

# 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

Write your personal IP address into the ignoreip line. You can separate each address with a space. IgnoreIP allows you white list certain IP addresses and make sure that they are not locked out from your VPS. Including your address will guarantee that you do not accidentally ban yourself from your own virtual private server.

The next step is to decide on a bantime, the number of seconds that a host would be blocked from the server if they are found to be in violation of any of the rules. This is especially useful in the case of bots, that once banned, will simply move on to the next target. The default is set for 10 minutes—you may raise this to an hour (or higher) if you like.

Maxretry is the amount of incorrect login attempts that a host may have before they get banned for the length of the ban time.

Findtime refers to the amount of time that a host has to log in. The default setting is 10 minutes; this means that if a host attempts, and fails, to log in more than the maxretry number of times in the designated 10 minutes, they will be banned.

Step Four (Optional)—Configure the ssh-iptables Section in Jail.Local

The SSH details section is just a little further down in the config, and it is already set up and turned on. Although you should not be required to make to make any changes within this section, you can find the details about each line below.

[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 simply refers to the fact that SSH protection is on. You can turn it off with the word "false".

The filter, set by default to sshd, refers to the config file containing the rules that fail2banuses to find matches. The name is a shortened version of the file extension. For example, sshd refers to the /etc/fail2ban/filter.d/sshd.conf.

Action describes the steps that fail2ban will take to ban a matching IP address. Just like the filter entry, each action refers to a file within the action.d directory. The default ban action, "iptables" can be found at /etc/fail2ban/action.d/iptables.conf .

In the "iptables" details, you can customize fail2ban further. For example, if you are using a non-standard port, you can change the port number within the brackets to match, making the line look more like this:

 eg. iptables[name=SSH, port=30000, protocol=tcp]

You can change the protocol from TCP to UDP in this line as well, depending on which one you want fail2ban to monitor.

If you have a mail server set up on your virtual private server, Fail2Ban can email you when it bans an IP address. In the default case, the sendmail-whois refers to the actions located at /etc/fail2ban/action.d/sendmail-whois.conf.

log path refers to the log location that fail2ban will track.

The max retry line within the SSH section has the same definition as the default option. However, if you have enabled multiple services and want to have specific values for each one, you can set the new max retry amount for SSH here.

Step Five—Restart Fail2Ban

After making any changes to the fail2ban config, always be sure to restart Fail2Ban:

sudo service fail2ban restart

You can see the rules that fail2ban puts in effect within the IP table:

iptables -L
By Etel Sverdlov

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