Abhinav's Notes
2022-08-31 Edit

Setting up Fail2Ban with Nginx and Cloudflare on NixOS

Fail2Ban is a service to scan the log files of various services on servers to find malicious IPs (by matching log lines to predefined regular expressions), and ban such IPs using various firewalls like iptables. One of its use is to ban malicious IPs that issue bad requests on NGINX servers.

However, in my case, the NGINX server is fronted by Cloudflare (CF) for resource caching etc. This means that the IPs that the NGINX server sees are that of the CF proxies, not the real visitors. If we use Fail2Ban on them, we’ll just end up banning the CF IPs, thus breaking the websites proxied by CF.

So, we need a smarter setup than the run-of-the-mill one. Also, I automate my server setup and config with NixOS.

The first thing to do is to discern the requests coming through CF. CF lists its proxy IPs publicly. We consume the list and use it to configure NGINX.

{ modulesPath, pkgs, lib, ... }:

let
  cloudflareIPs = builtins.fetchurl "https://www.cloudflare.com/ips-v4";
  setRealIpFromConfig =
    lib.concatMapStrings (ip: "set_real_ip_from ${ip};\n") 
      (lib.strings.splitString "\n" (builtins.readFile "${cloudflareIPs}"));
in {
  services.nginx = {
    enable = true;
    appendHttpConfig = ''
      ${setRealIpFromConfig}
      real_ip_header CF-Connecting-IP;
    '';
  };
}
nginx.nix

We also set the real_ip_header setting so that NGINX extracts the real visitor IPs from the CF-Connecting-IP header that CF sends, and prints them in the access log instead of the CF proxy IPs.

Fail2Ban now sees the real visitor IPs through the access log. However, banning those IPs in the server does no good because they are being proxied through CF. We need to ban them at CF as well.

CF comes with a tool called WAF for the same, and it can be accessed via HTTP APIs. Even better, Fail2Ban already comes with a cloudflare action with the API calls builtin. All we need to do is to configure our Fail2Ban jails to use this action.

{ lib, config, pkgs, ... }:

let
  cfEmail = "cfadmin@example.net";
  cfApiKey = "xxxxxx";
in {
  services.fail2ban = {
    enable = true;
    extraPackages = [ pkgs.curl ];

    jails.DEFAULT = ''
      bantime  = 1d
      findtime = 1h
    '';

    jails.nginx-noagent = ''
      enabled  = true
      port     = http,https
      filter   = nginx-noagent
      backend  = auto
      maxretry = 1
      logpath  = %(nginx_access_log)s
      action   = cloudflare[cfuser="${cfEmail}", cftoken="${cfApiKey}"]
                 iptables-multiport[port="http,https"]
    '';

    environment.etc."fail2ban/filter.d/nginx-noagent.conf".text = ''
      [Definition]

      failregex = ^<HOST> -.*"-" "-"$

      ignoreregex =
    '';
  };
}
fail2ban.nix

The code above shows an example jail called nginx-noagent that bans all IPs requesting without an HTTP user-agent. We use the cloudflare action (with CF account’s email and API key) to ban in the CF WAF, and the iptables-multiport action to ban in the server iptables. The curl package is needed to make the CF API calls.

That’s it! You can add more jails that use different regexes to catch different malicious actions, and ban the bad IPs. Safe secure and serve (HTTP requests).