Setting up Nagios on a Debian Server to Remotely Monitor an OpenWrt Router

These instructions assume that you already have Nagios installed and configured on your Debian server.  At some point I may write instructions for doing so, but reading the documentation that comes with the Nagios packages and found on nagios.org should be sufficient.

These instructions use NRPE (Nagios Remote Plugin Executor) to monitor OpenWRT.  Other methods, including SNMP, are available.  I chose NRPE because I was already using it to monitor my Untangle servers.

Install NRPE on the OpenWrt Router

This can be done using either the web interface (LuCI) or SSH.  I went the LuCI route.  After you authenticate, go to Administration, System, Software.  Click the Update package lists link.  After they have updated, install nrpe, nagios-plugins, libwrap, and luci-app-initmgr. (Nrpe and nagios-plugins will pull in some other packages. Libwrap is needed by nrpe, but it isn't listed in the dependencies.  Luci-app-initmgr adds a configuration page to LuCI that controls which init scripts run automatically on reboot.  By default npre is turned off.  This allows us to turn it on.)

Configure NRPE on the OpenWrt Router

For this section, you'll need to connect to the OpenWrt box using SSH. Once you are connected, edit /etc/nrpe.cfg. I prefer to use nano, but you can use whatever editor you prefer. If you do chose to use nano, you'll need to install it.

opkg install nano

Then to edit /etc/nrpe.cfg:

nano /etc/nrpe.cfg

Set the server_address to the IP address you want NRPE to bind to on the OpenWrt router.  If you want it to bind to all addresses, comment this out. If you want it to bind to the WAN address, there are probably some additional steps to allow the packets through the firewall. In my case, I set it to the LAN IP address.

server_address=10.0.0.1

Change allowed_hosts to the IP address of the Nagios server:

allowed_hosts=10.0.0.20

Change the warning on check_total_procs to 35 and the critical to 40.  If you don't it will go critical every time you ssh into the box.  You may even need to set it higher if you are running a lot of extra processes.

command[check_total_procs]=/usr/libexec/nagios/check_procs -w 35 -c 40

Save nrpe.conf.  My resulting file looks like this:

#############################################################################
# Sample NRPE Config File
# Written by: Ethan Galstad (nagios@nagios.org)
#
# Last Modified: 03-09-2007
#
# NOTES:
# This is a sample configuration file for the NRPE daemon.  It needs to be
# located on the remote host that is running the NRPE daemon, not the host
# from which the check_nrpe client is being executed.
#############################################################################

# PID FILE
# The name of the file in which the NRPE daemon should write it's process ID
# number.  The file is only written if the NRPE daemon is started by the root
# user and is running in standalone mode.

pid_file=/var/run/nrpe.pid

# PORT NUMBER
# Port number we should wait for connections on.
# NOTE: This must be a non-priviledged port (i.e. > 1024).
# NOTE: This option is ignored if NRPE is running under either inetd or xinetd

server_port=5666

# SERVER ADDRESS
# Address that nrpe should bind to in case there are more than one interface
# and you do not want nrpe to bind on all interfaces.
# NOTE: This option is ignored if NRPE is running under either inetd or xinetd

server_address=10.0.0.1

# NRPE USER
# This determines the effective user that the NRPE daemon should run as.
# You can either supply a username or a UID.
#
# NOTE: This option is ignored if NRPE is running under either inetd or xinetd

nrpe_user=nagios

# NRPE GROUP
# This determines the effective group that the NRPE daemon should run as.
# You can either supply a group name or a GID.
#
# NOTE: This option is ignored if NRPE is running under either inetd or xinetd

nrpe_group=nagios

# ALLOWED HOST ADDRESSES
# This is an optional comma-delimited list of IP address or hostnames
# that are allowed to talk to the NRPE daemon.
#
# Note: The daemon only does rudimentary checking of the client's IP
# address.  I would highly recommend adding entries in your /etc/hosts.allow
# file to allow only the specified host to connect to the port
# you are running this daemon on.
#
# NOTE: This option is ignored if NRPE is running under either inetd or xinetd

allowed_hosts=10.0.0.20

# COMMAND ARGUMENT PROCESSING
# This option determines whether or not the NRPE daemon will allow clients
# to specify arguments to commands that are executed.  This option only works
# if the daemon was configured with the --enable-command-args configure script
# option.
#
# *** ENABLING THIS OPTION IS A SECURITY RISK! ***
# Read the SECURITY file for information on some of the security implications
# of enabling this variable.
#
# Values: 0=do not allow arguments, 1=allow command arguments

dont_blame_nrpe=0

# COMMAND PREFIX
# This option allows you to prefix all commands with a user-defined string.
# A space is automatically added between the specified prefix string and the
# command line from the command definition.
#
# *** THIS EXAMPLE MAY POSE A POTENTIAL SECURITY RISK, SO USE WITH CAUTION! ***
# Usage scenario:
# Execute restricted commmands using sudo.  For this to work, you need to add
# the nagios user to your /etc/sudoers.  An example entry for alllowing
# execution of the plugins from might be:
#
# nagios          ALL=(ALL) NOPASSWD: /usr/lib/nagios/plugins/
#
# This lets the nagios user run all commands in that directory (and only them)
# without asking for a password.  If you do this, make sure you don't give
# random users write access to that directory or its contents!

# command_prefix=/usr/bin/sudo

# DEBUGGING OPTION
# This option determines whether or not debugging messages are logged to the
# syslog facility.
# Values: 0=debugging off, 1=debugging on

debug=0

# COMMAND TIMEOUT
# This specifies the maximum number of seconds that the NRPE daemon will
# allow plugins to finish executing before killing them off.

command_timeout=60

# CONNECTION TIMEOUT
# This specifies the maximum number of seconds that the NRPE daemon will
# wait for a connection to be established before exiting. This is sometimes
# seen where a network problem stops the SSL being established even though
# all network sessions are connected. This causes the nrpe daemons to
# accumulate, eating system resources. Do not set this too low.

connection_timeout=300

# WEEK RANDOM SEED OPTION
# This directive allows you to use SSL even if your system does not have
# a /dev/random or /dev/urandom (on purpose or because the necessary patches
# were not applied). The random number generator will be seeded from a file
# which is either a file pointed to by the environment valiable $RANDFILE
# or $HOME/.rnd. If neither exists, the pseudo random number generator will
# be initialized and a warning will be issued.
# Values: 0=only seed from /dev/[u]random, 1=also seed from weak randomness

#allow_weak_random_seed=1

# INCLUDE CONFIG FILE
# This directive allows you to include definitions from an external config file.

#include=<somefile.cfg>

# INCLUDE CONFIG DIRECTORY
# This directive allows you to include definitions from config files (with a
# .cfg extension) in one or more directories (with recursion).

#include_dir=<somedirectory>
#include_dir=<someotherdirectory>

# COMMAND DEFINITIONS
# Command definitions that this daemon will run.  Definitions
# are in the following format:
#
# command[<command_name>]=<command_line>
#
# When the daemon receives a request to return the results of <command_name>
# it will execute the command specified by the <command_line> argument.
#
# Unlike Nagios, the command line cannot contain macros - it must be
# typed exactly as it should be executed.
#
# Note: Any plugins that are used in the command lines must reside
# on the machine that this daemon is running on!  The examples below
# assume that you have plugins installed in a /usr/local/nagios/libexec
# directory.  Also note that you will have to modify the definitions below
# to match the argument format the plugins expect.  Remember, these are
# examples only!

# The following examples use hardcoded command arguments...

command[check_users]=/usr/libexec/nagios/check_users -w 3 -c 5
command[check_load]=/usr/libexec/nagios/check_load -w 7,4,2 -c 10,5,3
command[check_tmp]=/usr/libexec/nagios/check_disk -w 50% -c 25% -p /tmp
command[check_zombie_procs]=/usr/libexec/nagios/check_procs -w 1 -c 3 -s Z
command[check_total_procs]=/usr/libexec/nagios/check_procs -w 35 -c 40

# The following examples allow user-supplied arguments and can
# only be used if the NRPE daemon was compiled with support for
# command arguments *AND* the dont_blame_nrpe directive in this
# config file is set to '1'.  This poses a potential security risk, so
# make sure you read the SECURITY file before doing this.

#command[check_users]=/usr/lib/check_users -w $ARG1$ -c $ARG2$
#command[check_load]=/usr/lib/check_load -w $ARG1$ -c $ARG2$
#command[check_disk]=/usr/lib/check_disk -w $ARG1$ -c $ARG2$ -p $ARG3$
#command[check_procs]=/usr/lib/check_procs -w $ARG1$ -c $ARG2$ -s $ARG3$

In the LuCI administration interface, go to System, Initscripts.  Check the box next to nrpe and click Submit.  Alternatively, you can edit the /etc/init.d/nrpe file and change the line that says

STOP=70

to say

START=70

Restart the NRPE service:

/etc/init.d/nrpe restart

Configure the (Debian) Nagios Server

Now it is time to configure the Nagios server.  These instructions were written specifically for Debian's Nagios packages version 3.0.6-3 with nagios-nrpe-plugin version 2.12-3 and nagios-plugins 1.4.12-5, but they should work resonably well with other versions.  As always, your mileage may vary.

Make sure you have nagios-nrpe-plugin installed.  This package provides /usr/lib/nagios/plugins/check_nrpe, which we will be using shortly.

Create a file named /etc/nagios3/conf.d/openwrt.cfg with the following contents:

#Host Definition
define host{
        use                     generic-host
        host_name               OpenWrt
        alias                   OpenWrt Wireless Router
        address                 10.0.0.2
        }

#Service Definitions
define service{
        use                     generic-service
        host_name               OpenWrt
        service_description     CPU Load
        check_command           check_nrpe_1arg!check_load
        }

#The OpenWrt nagios-plugins package currently doesn't contain the check_disk command.
#This is probably an oversight that will be fixed in the future.  Until then, this
#command will not work.
#define service{
#        use                     generic-service
#        host_name               OpenWrt
#        service_description     Free /tmp Space
#        check_command           check_nrpe_1arg!check_tmp
#        }

define service{
        use                     generic-service
        host_name               OpenWrt
        service_description     Number of Users
        check_command           check_nrpe_1arg!check_users
        }

define service{
        use                     generic-service
        host_name               OpenWrt
        service_description     Number of Zombie Processes
        check_command           check_nrpe_1arg!check_zombie_procs
        }

define service{
        use                     generic-service
        host_name               OpenWrt
        service_description     Total Processes
        check_command           check_nrpe_1arg!check_total_procs
        }

We are using check_nrpe_1arg to match the syntax in Debian's /usr/lib/nagios/plugins/check_nrpe.  check_nrpe pulls the arguments from the .cfg file and sends them to the monitored machine.  If you'll notice in the above nrpe.conf, we have that configuration disabled (default setting) and we have hardcoded in the arguments for each command.  Hence, we need to use check_nrpe_1arg or we will get an error message when we run the checks.

You may change the host_name to anything of your choice (you'll want to make sure they are all the same throughout the file). You'll also want to update the address in the define host section to match that of your OpenWrt server.

Edit /etc/nagios3/conf.d/hostgroups_nagios2.cfg and add the following section to define an OpenWrt Routers group.  This step is optional, but is nice if you are going to monitor more than one OpenWrt router.  (Also, you might want to rename the file to something more logical, like hostgroups.cfg.)

define hostgroup {
        hostgroup_name  openwrt
                alias           OpenWrt Routers
                members         OpenWrt
        }

If you have more than one OpenWrt Router in the group, it will look like the following:

define hostgroup {
        hostgroup_name  openwrt
                alias           OpenWrt Routers
                members         OpenWrt1, OpenWrt2, OpenWrt3
        }

Restart the Nagios service to apply the changes:

/etc/init.d/nagios3 restart

Load up the Nagios website and see if you managed to type everything correctly.  :)