Fail2ban

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Fail2ban scans log files (e.g. /var/log/httpd/error_log) and bans IPs that show the malicious signs like too many password failures, seeking for exploits, etc. Generally Fail2ban is then used to update firewall rules to reject the IP addresses for a specified amount of time, although any other arbitrary action (e.g. sending an email) could also be configured.

Warning: Using an IP banning software will stop trivial attacks but it relies on an additional daemon and successful logging. Additionally, if the attacker knows your IP address, they can send packets with a spoofed source header and get your IP address banned. Make sure to specify your IP in ignoreip.

Installation

Install fail2ban.

Usage

Configure Fail2ban and enable/start fail2ban.service.

fail2ban-client

The fail2ban-client allows monitoring jails (reload, restart, status, etc.), to view all available commands:

$ fail2ban-client

To view all enabled jails:

# fail2ban-client status

To check the status of a jail, e.g. for sshd:

# fail2ban-client status sshd

Status for the jail: sshd
|- Filter
|  |- Currently failed: 1
|  |- Total failed:     9
|  `- Journal matches:  _SYSTEMD_UNIT=sshd.service + _COMM=sshd
`- Actions
   |- Currently banned: 1
   |- Total banned:     1
   `- Banned IP list:   0.0.0.0

For a compact version for all jails, including banned IPs:

# fail2ban-client banned

[{'sshd': ['192.168.100.50']}, {'apache-auth': []}]

Configuration

Due to the possibility of Pacnew and Pacsave files being created for /etc/fail2ban/jail.conf during an upgrade, jail.conf(5) § CONFIGURATION FILES FORMAT recommends that users create a /etc/fail2ban/jail.local file to "ease upgrades".

For example, to change the default ban time to 1 day:

/etc/fail2ban/jail.local

[DEFAULT]
bantime = 1d

Or create separate name.local files under the /etc/fail2ban/jail.d directory, e.g. /etc/fail2ban/jail.d/sshd.local.

Reload fail2ban.service to apply the configuration changes.

Enabling jails

By default all jails are disabled. Append enabled = true to the jail you want to use, e.g. to enable the OpenSSH jail:

/etc/fail2ban/jail.local

[sshd]
enabled = true

See #Custom SSH jail.

Receive an alert e-mail

If you want to receive an e-mail when someone has been banned, you have to configure an SMTP client (e.g. msmtp) and change default action, as given below.

/etc/fail2ban/jail.local

[DEFAULT]
destemail = yourname@example.com
sender = yourname@example.com

# to ban & send an e-mail with whois report to the destemail.
action = %(action_mw)s

# same as action_mw but also send relevant log lines
#action = %(action_mwl)s

Firewall and services

By default, Fail2ban uses iptables. However, configuration of most firewalls and services is straightforward. For example, to use nftables:

/etc/fail2ban/jail.local

[DEFAULT]
banaction = nftables
banaction_allports = nftables[type=allports]

See /etc/fail2ban/action.d/ for other examples, e.g. ufw.conf.

Tips and tricks

Custom SSH jail

Warning: If the attacker knows your IP address, they can send packets with a spoofed source header and get your IP address locked out of the server. SSH keys provide an elegant solution to the problem of brute forcing without these problems.

Edit /etc/fail2ban/jail.d/sshd.local, add this section and update the list of trusted IP addresses in ignoreip:

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

[sshd]
enabled   = true
filter    = sshd
banaction = iptables
backend   = systemd
maxretry  = 5
findtime  = 1d
bantime   = 2w
ignoreip  = 127.0.0.1/8

Note:

It may be necessary to set LogLevel VERBOSE in /etc/ssh/sshd_config to allow full fail2ban monitoring as otherwise password failures may not be logged correctly.
Fail2ban has IPv6 support since version 0.10. Adapt your firewall accordingly, e.g. start/enable ip6tables.service.
When using journal namespaces (by adding LogNamespace=something to a unit file), you can make fail2ban read those logs by setting backend like this: backend = systemd[journalfiles="/var/log/journal/*.something/system.journal"].

Tip:

If using iptables front-ends like ufw, one can use banaction = ufw instead of using iptables.
When using Shorewall, one can use banaction = shorewall and also set BLACKLIST to ALL in /etc/shorewall/shorewall.conf, otherwise the rule added to ban an IP address will affect only new connections.

Systemd backend: journald filtering

When using the systemd backend to improve performance, configure a filter with journalmatch. For example, to parse only kernel-level log messages:

/etc/fail2ban/filter.d/fwdrop.local

[Definition]
failregex = ^.*DROP_.*SRC=<ADDR> DST=.*$
journalmatch = _TRANSPORT=kernel

Service hardening

Currently, Fail2ban must be run as root. Therefore, you may wish to consider hardening the process with systemd.

Create a drop-in configuration file for fail2ban.service:

/etc/systemd/system/fail2ban.service.d/override.conf

[Service]
PrivateDevices=yes
PrivateTmp=yes
ProtectHome=read-only
ProtectSystem=strict
ReadWritePaths=-/var/run/fail2ban
ReadWritePaths=-/var/lib/fail2ban
ReadWritePaths=-/var/log/fail2ban
ReadWritePaths=-/var/spool/postfix/maildrop
ReadWritePaths=-/run/xtables.lock
CapabilityBoundingSet=CAP_AUDIT_READ CAP_DAC_READ_SEARCH CAP_NET_ADMIN CAP_NET_RAW

The CapabilityBoundingSet parameters CAP_DAC_READ_SEARCH will allow Fail2ban full read access to every directory and file. CAP_NET_ADMIN and CAP_NET_RAW allow Fail2ban to operate on any firewall that has command-line shell interface. See capabilities(7) for more info.

By using ProtectSystem=strict the filesystem hierarchy will only be read-only, ReadWritePaths allows Fail2ban to have write access on required paths.

Create /etc/fail2ban/fail2ban.local with the correct logtarget path:

/etc/fail2ban/fail2ban.local

[Definition]
logtarget = /var/log/fail2ban/fail2ban.log

Create the /var/log/fail2ban/ directory as root.

Finally, reload systemd daemon to apply the changes of the unit and restart fail2ban.service.