Nagios plugin: check_rdns

check_rdns is a plugin for the system monitoring software Nagios, which allows to check reverse DNS entries using the tool dig.

The current version is available on

The plugin can be invoked with -h to get a list of options.

% ./check_rdns -h
check_rdns v0.1
Copyright (c) 2013 Stefan Huber <>
Usage: check_rdns -H ip-address [OPTIONS]

    -H, --address IP-address    The ip-addres on which reverse-DNS is performed.

    -h, --help                  Print this text.
    -a, --expect=HOST           The expected result.
    -s, --server=HOST           The DNS server to contact.
    -t, --timeout=SEC           Seconds before lookup times out. (Default: 10)
    -w, --warning=MSEC          Return warning if lookup time exceeds value.
    -c, --critical=MSEC         Return critical if lookup time exceeds value.
    -V, --version               Print version info.

In my case, I actually use Icinga instead of Nagios. Anyway, I added the following lines to ‘/etc/nagios-plugins/config/dns.cfg’ to make check_rdns available for service definitions:

# 'check_rdns' command definition
define command{
        command_name    check_rdns
        command_line    /usr/lib/nagios/plugins/check_rdns -H '$HOSTADDRESS$' \
-w 100 -c 1000 -a '$ARG1$'

In the above code, the backslash indicates that the line should be continued without a line break.