Enables/disabled a dynamic DNS entry depending on the carp status of a specified interface (i.e. lan). Good for failover scenarios when you can't run carp on the WAN interface.

dyndns_monitor.php

#!/usr/local/bin/php-cgi -f
<?php

require_once('functions.inc');

$opts = get_options();

$dyndns = &$config['dyndnses']['dyndns'][get_dyndns_id_by_host($opts['dyndns'])];

// This system must be master to enable specified dyndns hostname.
if (!is_carp_master($opts['interface']) && array_key_exists('enable', $dyndns)) {
  if (!$opts['quiet'])
    echo('Interface "' . $opts['interface'] . '" is not the master. Disabling dyndns for host ' . $opts['dyndns'] . '.' . PHP_EOL);
  unset($dyndns['enable']);
  write_config('Dynamic DNS client disabled.');
  services_dyndns_configure();
}
elseif (is_carp_master($opts['interface']) && !array_key_exists('enable', $dyndns)) {
  if (!$opts['quiet'])
    echo('Enabling dyndns for host ' . $opts['dyndns'] . '.' . PHP_EOL);
  $dyndns['enable'] = true;
  $dyndns['force'] = true;
  write_config('Dynamic DNS client enabled.');
  services_dyndns_configure();
}

function get_dyndns_id_by_host ($host) {
  global $config;
  $dynarr = &$config['dyndnses']['dyndns'];

  for ($i = 0; $i < count($dynarr); $i++) {
    if (strtolower($dynarr[$i]['host']) == strtolower($host))
      $key_id = $i;
  }

  if (!is_null($key_id))
    return $key_id;
  else
    die('Could not find ' . $host . ' as a configured dyndns entry.' . PHP_EOL);
}

function get_interface_by_name ($interface) {
  foreach (get_configured_interface_with_descr() as $name => $desc) {
    if (strtolower($interface) == strtolower($desc)) {
      $arr = get_interface_info($name);
      $arr['name'] = $name;
      $arr['desc'] = $interface;
    }
  }

  if (is_null($arr))
    die('Interface "' . $interface . '" is not valid.' . PHP_EOL);
  elseif ($arr['status'] != 'up')
    die('Interface "' . $interface . '" is not up.' . PHP_EOL);
  else
    return $arr;
}

function is_carp_master ($interface) {
  if (!get_carp_status())
    die('CARP is not running on this system.' . PHP_EOL);

  global $config;
  $carp_interface = get_interface_by_name($interface);
  foreach ($config['virtualip']['vip'] as $vip) {
    if ($carp_interface['name'] == $vip['interface'] && $vip['mode'] == 'carp')
      $vip_status = get_carp_interface_status("_vip{$vip['uniqid']}");
  }

  if (!$vip_status)
    die('CARP is not configured for interface "' . $interface . '".' . PHP_EOL);
  elseif ($vip_status == 'MASTER')
    return true;
}

function get_options () {
  $help = (PHP_EOL .
    'Usage:  ' . basename(__FILE__) . ' [-i interface] [-d dyndns hostname]' . PHP_EOL
    . PHP_EOL
    . 'Watches CARP status on an interface. Disables specified service for the' . PHP_EOL
    . 'backup, and enables it for the master.' . PHP_EOL
    . PHP_EOL
    . '        -i, --interface string   CARP interface to monitor' . PHP_EOL
    . '        -d, --dyndns string      Dynamin DNS hostname to enable/disable' . PHP_EOL
    . '        -q, --quiet              Suppress output' . PHP_EOL
    . '        -h, --help               Print usage' . PHP_EOL
    . PHP_EOL
  );

  $opts = getopt(
    'i:d:h',
    array(
      'interface:',
      'dyndns:',
      'help',
    )
  );

  if (array_key_exists('h', $opts) || array_key_exists('help', $opts))
    exit($help);

  if (array_key_exists('i', $opts))
    $opts['interface'] = $opts['i'];

  if (array_key_exists('d', $opts))
    $opts['dyndns'] = $opts['d'];

  if (!array_key_exists('interface', $opts))
    die('ERROR: interface is a required parameter.' . PHP_EOL . $help);

  if (!array_key_exists('dyndns', $opts))
    die('ERROR: dyndns hostname is a required parameter.' . PHP_EOL . $help);

  // We don't need these anymore.
  unset (
    $opts['i'],
    $opts['d']
  );

  return $opts;
}

Uses pfSense functions. It does nothing silently if it's on the master with the dyndns host enabled, or if it's on the backup and dyndns is disabled, which is how things should normally be. You can call it from cron, or you can insert it into devd.