{ config, pkgs, lib, ... }:
{
  # Mostly from: https://nixos.wiki/wiki/Matrix

  services.coturn = {
    enable = true;
    no-cli = true;
    no-tcp-relay = true;
    min-port = 49000;
    max-port = 50000;
    use-auth-secret = true;
    static-auth-secret = "Ieg2nai3oongah7gesh8jevuitie9j";
    realm = "${config.networking.fqdn}";
    cert = "${config.security.acme.certs.${config.services.coturn.realm}.directory}/full.pem";
    pkey = "${config.security.acme.certs.${config.services.coturn.realm}.directory}/key.pem";
    extraConfig = ''
      # for debugging
      verbose
      # ban private IP ranges
      no-multicast-peers
      denied-peer-ip=0.0.0.0-0.255.255.255
      denied-peer-ip=10.0.0.0-10.255.255.255
      denied-peer-ip=100.64.0.0-100.127.255.255
      denied-peer-ip=127.0.0.0-127.255.255.255
      denied-peer-ip=169.254.0.0-169.254.255.255
      denied-peer-ip=172.16.0.0-172.31.255.255
      denied-peer-ip=192.0.0.0-192.0.0.255
      denied-peer-ip=192.0.2.0-192.0.2.255
      denied-peer-ip=192.88.99.0-192.88.99.255
      denied-peer-ip=192.168.0.0-192.168.255.255
      denied-peer-ip=198.18.0.0-198.19.255.255
      denied-peer-ip=198.51.100.0-198.51.100.255
      denied-peer-ip=203.0.113.0-203.0.113.255
      denied-peer-ip=240.0.0.0-255.255.255.255
      denied-peer-ip=::1
      denied-peer-ip=64:ff9b::-64:ff9b::ffff:ffff
      denied-peer-ip=::ffff:0.0.0.0-::ffff:255.255.255.255
      denied-peer-ip=100::-100::ffff:ffff:ffff:ffff
      denied-peer-ip=2001::-2001:1ff:ffff:ffff:ffff:ffff:ffff:ffff
      denied-peer-ip=2002::-2002:ffff:ffff:ffff:ffff:ffff:ffff:ffff
      denied-peer-ip=fc00::-fdff:ffff:ffff:ffff:ffff:ffff:ffff:ffff
      denied-peer-ip=fe80::-febf:ffff:ffff:ffff:ffff:ffff:ffff:ffff
    '';
  };
  # open the firewall
  networking.firewall = {
    interfaces.enp2s0 = let
      range = with config.services.coturn; [ {
        from = min-port;
        to = max-port;
      } ];
    in
      {
        allowedUDPPortRanges = range;
        allowedUDPPorts = [ 3478 5349 ];
        allowedTCPPortRanges = [ ];
        allowedTCPPorts = [ 3478 5349 ];
      };
  };

  # get a certificate
  security.acme.certs.${config.services.coturn.realm} = {
    /* insert here the right configuration to obtain a certificate */
    postRun = "systemctl restart coturn.service";
    group = "turnserver";
  };

  # configure synapse to point users to coturn
  services.matrix-synapse = with config.services.coturn; {
    settings = {
      turn_uris = [
        "turn:${config.services.coturn.realm}:3478?transport=udp"
        "turn:${config.services.coturn.realm}:3478?transport=tcp"
      ];

      turn_shared_secret = config.services.coturn.static-auth-secret;
      settiturn_user_lifetime = "1h";
    };
  };

  users.users.nginx.extraGroups = [ "turnserver" ];
}
