Title: Extending fail2ban on NixOS
Author: Solène
Date: 02 October 2022
Tags: linux nixos fail2ban security
Description: This article shows how to create new filters in fail2ban
on NixOS.

# Introduction

Fail2ban is a wonderful piece of software, it can analyze logs from
daemons and ban them in the firewall.  It's triggered by certain
conditions like a single IP found in too many lines matching a pattern
(such as a login failure) under a certain time.

What's even cooler is that writing new filters is super easy!  In this
text, I'll share how to write new filters for NixOS.

fail2ban GitHub project page

NixOS official website

# Terminology

Before continuing, if you are not familiar with fail2ban, here are the
few important keywords to understand:

* action: what to do with an IP (usually banning an IP)
* filter: set of regular expressions and information used to find bad
actors in logs
* jail: what ties together filters and actions in a logical unit

For instance, a sshd jail will have a filter applied on sshd logs, and
it will use a banning action.  The jail can have more information like
how many times an IP must be found by a filter before using the action.

# Configuration

## Enabling fail2ban

The easiest part is to enable fail2ban.  Take the opportunity to
declare IPs you don't want to block, and also block IPs on all ports if
it's something you want.

```
  services.fail2ban = {
    enable = true;
    ignoreIP = [
      "192.168.1.0/24"
    ];
  };

  # needed to ban on IPv4 and IPv6 for all ports
  services.fail2ban = {
    extraPackages = [pkgs.ipset];
    banaction = "iptables-ipset-proto6-allports";
  };
```

## Creating new filters

A filter is composed of one/many regex, and also a systemd journal unit
in case you are pulling information from it instead of a log file.

We will use the module `environment.etc` to create files in
`/etc/fail2ban/filter.d/` directory, so they can be used in the jails.

These are examples of filters you may want to use.  They target very
large, this may not be ideal for your use case, but can serve as a good
start.

```
  environment.etc = {
    "fail2ban/filter.d/molly.conf".text = ''
      [Definition]
      failregex = \s+(31|40|51|53).*$
    '';

    "fail2ban/filter.d/nginx-bruteforce.conf".text = ''
      [Definition]
      failregex = ^.*GET.*(matrix/server|\.php|admin|wp\-).* HTTP/\d.\d\" 404.*$
    '';

    "fail2ban/filter.d/postfix-bruteforce.conf".text = ''
      [Definition]
      failregex = warning: [\w\.\-]+\[\]: SASL LOGIN authentication failed.*$
      journalmatch = _SYSTEMD_UNIT=postfix.service
    '';
  };
```

## Defining the jails using our new filters

Now we can declare fail2ban jails with each filter we created.  If you
use a log file, make sure to have `backend = auto`, otherwise the
systemd journal is used and this won't work.

The most important settings are:

* filter: choose your filter using its filename minus the `.conf` part
* maxretry: how many times an IP should be reported before taking an
action
* findtime: how long should we keep entries to match in maxretry

```
  services.fail2ban.jails = {

    # max 6 failures in 600 seconds
    "nginx-spam" = ''
      enabled  = true
      filter   = nginx-bruteforce
      logpath = /var/log/nginx/access.log
      backend = auto
      maxretry = 6
      findtime = 600
    '';

    # max 3 failures in 600 seconds
    "postfix-bruteforce" = ''
      enabled = true
      filter = postfix-bruteforce
      findtime = 600
      maxretry = 3
    '';

    # max 10 failures in 600 seconds
    "molly" = ''
      enabled = true
      filter = molly
      findtime = 600
      maxretry = 10
      logpath = /var/log/molly-brown/access.log
      backend = auto
    '';
  };
```

# Creating filters

It's actually easy to create filters, fail2ban provides a good
framework like automatic date and host detection, which make creating
regex very easy.

You can use the command `fail2ban-regex` to experiment with regexes on
some logs.

Here is an example of a log file that would contain an IP and an error
message:

```
fail2ban-regex /var/log/someservice.log " ERROR"
```

Here is an example of a systemd unit log that would contain an IP, then
a space and a 403 error:

```
fail2ban-regex -m _SYSTEMD_UNIT=someservice.service systemd-journal " 403"
```

You can analyze what lines matched or not with the flags
`--print-all-matched` and `--print-all-missed`.

I recommend you to read fail2ban man pages and --help output if you
want to create filters.

# Conclusion

Fail2ban is a fantastic tool to easily create filtering rules to ban
the bad actors.  It turned out most rules didn't work out of the box,
or were too narrow for my use case, so extending fail2ban was quite
straightforward.