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doc/fail2ban.md
ericgaspar 8acc960026
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2020-09-30 18:19:44 +02:00

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Fail2Ban

Fail2Ban is an intrusion prevention software that protects computer servers against brute-force attacks. It monitors certain logs and will ban IP addresses that show brute-force-like behavior.

In particular, Fail2Ban monitors SSH connection attempts. After 5 failed SSH connection attempts, Fail2Ban will ban the IP address from connecting via SSH for 10 minutes. If this address fails several times, it might get banned for a week.

Unban an IP address

To unblock an IP address, you must first access your server by some means (for example from another IP address or from another internet connection than the banned one).

Then, look at the Fail2Ban’s log to identify in which jail the IP address has been banned:

sudo tail /var/log/fail2ban.log
2019-01-07 16:24:47 fail2ban.filter  [1837]: INFO    [sshd] Found 11.22.33.44
2019-01-07 16:24:49 fail2ban.filter  [1837]: INFO    [sshd] Found 11.22.33.44
2019-01-07 16:24:51 fail2ban.filter  [1837]: INFO    [sshd] Found 11.22.33.44
2019-01-07 16:24:54 fail2ban.filter  [1837]: INFO    [sshd] Found 11.22.33.44
2019-01-07 16:24:57 fail2ban.filter  [1837]: INFO    [sshd] Found 11.22.33.44
2019-01-07 16:24:57 fail2ban.actions [1837]: NOTICE  [sshd] Ban 11.22.33.44
2019-01-07 16:24:57 fail2ban.filter  [1837]: NOTICE  [recidive] Ban 11.22.33.44

Here, the 11.22.33.44 IP address has been banned in the sshd and recidive jails.

Then deban the IP address with the following commands:

sudo fail2ban-client set sshd unbanip 11.22.33.44
sudo fail2ban-client set recidive unbanip 11.22.33.44

Whitelist an IP address

If you don’t want a "legitimate" IP address to be blocked by YunoHost anymore, then you have to fill it in the whitelist of the jail configuration file.

When updating the Fail2Ban software, the original /etc/fail2ban/jail.conf file is overwritten. So it is on a new dedicated file that we will store the changes. They will thus be preserved over time.

  1. Start by creating the new jail configuration file which will be called yunohost-whitelist.conf:

    sudo touch /etc/fail2ban/jail.d/yunohost-whitelist.conf

  2. Edit this new file with your favorite editor:

    sudo nano /etc/fail2ban/jail.d/yunohost-whitelist.conf

  3. Paste the following content into the file and adapt the IP address XXX.XXX.XXX.XXX:

    [DEFAULT]

ignoreip = 127.0.0.1/8 XXX.XXX.XXX.XXX #<= the IP address (you can put more than one, separated by a space) that you want to whitelist

  4. Save the file and reload the Fail2Ban configuration:

    sudo fail2ban-client reload

Congratulations, no more risks of banning yourself from your own YunoHost server!