Fail2Ban

Warning Warning: These instructions were only tested on Debian. It will probably work for other Linux distributions, but you might need to adapt the provided instructions.

Fail2ban is a program that parses logs and and block servers that try to abuse your system. While it doesn't replace a firewall, it's a good complement as it prevents people from trying thousands of password on your server.

Prerequisite

This guide will configure Fail2Ban to work with nftables.

Installation

# apt install fail2ban iptables-

Note: Debian Stretch (currently in testing) contain a much nicer version of fail2ban than Jessie (current stable). Configuration has been simplified a lot between the two releases and installing the version from stretch will save you from migration pain later. Make sure you configure stretch source before running the command bellow.

# apt install fail2ban/stretch iptables-

Note2: On systems with both 64bits and 32bits architectures enabled, you might need the following command to avoid installing iptables

# apt install fail2ban iptables- iptables:i386-

Configuration

After you change configuration, or add a new jail, don't forget to restart fail2ban

# service fail2ban restart

nftables

nftables support was added in release 0.9.4. If you have an older release, you can copy the 3 nftables-* files from the official repository and add them to /etc/fail2ban/action.d.

Create table

Create file /etc/nftables/fail2ban.conf

#!/usr/sbin/nft -f

# Use ip as fail2ban doesn't support ipv6 yet
table ip fail2ban {
        chain input {
                # Assign a high priority to reject as fast as possible and avoid more complex rule evaluation
                type filter hook input priority 100;
        }
}

Then add line include "/etc/nftables/fail2ban.conf" in /etc/nftables.conf.

Finally activate your rule in nftables

# nft -f /etc/nftables/fail2ban.conf

Set table in Fail2Ban

Create file /etc/fail2ban/action.d/nftables-common.local

[Init]
# Definition of the table used
nftables_family = ip
nftables_table  = fail2ban

# Drop packets 
blocktype       = drop

# Remove nftables prefix. Set names are limited to 15 char so we want them all
nftables_set_prefix =

Defaults

Create file /etc/fail2ban/jail.local

[DEFAULT]
# Destination email for action that send you an email
destemail = fail2ban@mydomain.example

# Sender email. Warning: not all actions take this into account. Make sure to test if you rely on this
sender    = fail2ban@mydomain.example

# Default action. Will block user and send you an email with whois content and log lines.
action    = %(action_mwl)s

# configure nftables
banaction = nftables-multiport
chain     = input

Recidive

The recidive rule ban users for a longer period if they have been banned multiple time in a row.

Create file /etc/fail2ban/jail.d/recidive.conf

# Jail for more extended banning of persistent abusers
# !!! WARNINGS !!! 
# 1. Make sure that your loglevel specified in fail2ban.conf/.local
#    is not at DEBUG level -- which might then cause fail2ban to fall into
#    an infinite loop constantly feeding itself with non-informative lines
# 2. If you increase bantime, you must increase value of dbpurgeage
#    to maintain entries for failed logins for sufficient amount of time.
#    The default is defined in fail2ban.conf and you can override it in fail2ban.local
[recidive]
enabled   = true
logpath   = /var/log/fail2ban.log
banaction = nftables-allports
bantime   = 86400 ; 1 day
findtime  = 86400 ; 1 day 
maxretry  = 3 
protocol  = 0-255

Other rules

Rules specific to one program are documented on the program page. You can see the list on the fail2ban category page.