Agenda: exercises-nagios.txt

Nagios Installation and Configuration

Notes:
------
* Commands preceded with "$" imply that you should execute the command as
  a general user - not as root.
* Commands preceded with "#" imply that you should be working as root.
* Commands with more specific command lines (e.g. "RTR-GW>" or "mysql>") 
  imply that you are executing commands on remote equipment, or within 
  another program.

Exercises
---------

Exercises Part I
----------------

0. Log in to your PC or open a terminal window as the sysadm user.

1. You may need to install Nagios version 3. You would do this as root or as the sysadmin
   user and use the "sudo" command. As sysadm:

   $ sudo apt-get install nagios3

   Unless you already have an MTA installed, nagios3 will install
   postfix as a dependency. Select "Internet Site" option. (If you had wanted
   to use a different MTA likely you'd install it before nagios3)

   You will be prompted for nagiosadmin password. Give it the normal
   workshop password.

   To get the documentation in /usr/share/doc/nagios3-doc/html/ (which
   can also be read via the nagios web interface), do:

    $ sudo apt-get install nagios3-doc


2. Look at the file which contains the password. It's hashed (encrypted)

    $ cat /etc/nagios3/htpasswd.users


3. You should already have a working Nagios!

    - Open a browser, and go to

    http://pcN.ws.nsrc.org/nagios3/

        Check with the instructor or your neighbor if you are in doubt.

    - At the login prompt, login as:

        user: nagiosadmin
        pass: 

    Browse to the "Host Detail" page to see what's already configured.


4. Let's look at the configuration layout... But, first, let's become the root
   user on your machine:

    $ sudo bash

    # cd /etc/nagios3
    # ls -l 

    -rw-r--r-- 1 root root    1882 2008-12-18 13:42 apache2.conf
    -rw-r--r-- 1 root root   10524 2008-12-18 13:44 cgi.cfg
    -rw-r--r-- 1 root root    2429 2008-12-18 13:44 commands.cfg
    drwxr-xr-x 2 root root    4096 2009-02-14 12:33 conf.d
    -rw-r--r-- 1 root root      26 2009-02-14 12:36 htpasswd.users
    -rw-r--r-- 1 root root   42539 2008-12-18 13:44 nagios.cfg
    -rw-r----- 1 root nagios  1293 2008-12-18 13:42 resource.cfg
    drwxr-xr-x 2 root root    4096 2009-02-14 12:32 stylesheets

    # cd conf.d
    # ls -l    

    -rw-r--r-- 1 root root 1695 2008-12-18 13:42 contacts_nagios2.cfg
    -rw-r--r-- 1 root root  418 2008-12-18 13:42 extinfo_nagios2.cfg
    -rw-r--r-- 1 root root 1152 2008-12-18 13:42 generic-host_nagios2.cfg
    -rw-r--r-- 1 root root 1803 2008-12-18 13:42 generic-service_nagios2.cfg
    -rw-r--r-- 1 root root  210 2009-02-14 12:33 host-gateway_nagios3.cfg
    -rw-r--r-- 1 root root  976 2008-12-18 13:42 hostgroups_nagios2.cfg
    -rw-r--r-- 1 root root 2167 2008-12-18 13:42 localhost_nagios2.cfg
    -rw-r--r-- 1 root root 1005 2008-12-18 13:42 services_nagios2.cfg
    -rw-r--r-- 1 root root 1609 2008-12-18 13:42 timeperiods_nagios2.cfg

    Notice that the package installs files with "nagios2" in their name.
    This is because they are the same files as were used for the Nagios
    version 2 Debian package. However there was a change made to the
    host-gateway configuration file, so this has a new name.


5. You have a config which is already monitoring your own system
(localhost_nagios2.cfg) and your upstream default gateway
(host-gateway_nagios3.cfg).

Have a look at the config file for the default gateway: it's very simple.
(Note: tab completion is useful here. Type cat host-g then hit tab; the
filename will be filled in for you)

    # cat host-gateway_nagios3.cfg

    # a host definition for the gateway of the default route
    define host {
            host_name   gateway
            alias       Default Gateway
            address     10.10.0.254
            use         generic-host
            }



PART II
Configuring Equipment
-----------------------------------------------------------------------------

0. Order of configuration

Conceptually we will build our configuration files from the "nearest" device
then the further away ones.

By going in this order you will have defined the devices that act as parents
for other devices.

The classroom GW router is already defined (10.10.0.254).

1. First we need to tell Nagios to monitor the gateway for the router instances,
   which is 10.10.254.254 or gw-254.ws.nsrc.org.

   # cd /etc/nagios3/conf./

Create the routers gateway like this:

   # editor routers.cfg

define host {
    use         generic-host
    host_name   gw-254
    alias       Routers Gateway
    address     10.10.254.254
    parents     gateway
}

Exit and save this file.

*NOTE* - "gateway" is the same machine as gw.ws.nsrc.org. Nagios has simply given
this machine the name "gateway". While it's nice to have the host_name mactch the 
name in DNS it is not strictly necessary. This will be our only exception.


2. the final parent we have in our network is our backbone switch. Create
   a file called switches.cfg and add an entry for this item:

   # editor switches.cfg

define host {
    use         generic-host
    host_name   sw
    alias       Backbone Switch
    address     10.10.0.253
    parents     gateway
}

At this point Nagios is configured to monitor whether our core hosts (the parents) 
are up on our classroom network. Your next steps are to add in the individual hosts
such as the classroom virtual PC images (pc1 to pc26), the Wireless Access Points
(ap1 and ap2), the virtual router images (r1 through r26) and the classroom noc
host.

Be sure you add in a proper "parents" entry for each host.

To understand the parent relationship in our network review the logical 
network diagram located here:

        http://nocws.nsrc.org/wiki/wiki/NetworkDiagram

Note the Nagios parent bullet points:

Nagios Parent Relationships 

Parents are "gw", "sw" and "gw-254". The parent relations are:

    * gw is the parent of sw and gw-254
    * gw-254 is the parent of r1 through r26
    * sw is the parent of s0, s1, s2, ap1, ap2, noc and pc1 through pc26 



STEPS 2a - 2c SHOULD BE REPEATED WHENEVER YOU UPDATE THE CONFIGURATION!
    

2a. Verify that your configuration files are OK:

    # nagios3 -v /etc/nagios3/nagios.cfg 

    ... You should get :
Warning: Host 'bb-sw' has no services associated with it!
Warning: Host 'bb-gw' has no services associated with it!
...
Total Warnings: 2
Total Errors:   0

Things look okay - No serious problems were detected during the check.
Nagios is saying that it's unusual to monitor a device just for its
existence on the network, without also monitoring some service.


2b. Reload/Restart Nagios

    # /etc/init.d/nagios3 restart

Not always 100% reliable to use the "restart" option due to a bug in the Nagios init script.
To be sure you may want to get used to doing:

    # /etc/init.d/nagios3 stop
    # /etc/init.d/nagios3 start


2c. Go to the web interface (http://pcN.ws.nsrc.org/nagios3) and check that the hosts 
   you just added are now visible in the interface. Click on the "Host Detail" item
   on the left of the Nagios screen to see this. You may see it in "PENDING"
   status until the check is carried out.


HINT: You will be doing this a lot. If you do it all on one line, like this,
then you can hit cursor-up and rerun all in one go:

    nagios3 -v /etc/nagios3/nagios.cfg && /etc/init.d/nagios3 restart

The '&&' ensures that the restart only happens if the config is valid.


3. Create entries for ther routers and PCs in the classroom

Now that we have our routers and switches defined it is quite easy to create
entries for all our PCs.  Think about the parent relationships:

Remember, if you do not understand the parent relationship refer back to the
classroom network diagram here:

        http://noc.ws.nsrc.org/wiki/wiki/NetworkDiagram

Below are three sample entries. One for the NOC, one for pc1 and one for
pc26.  You should be able to use this example to create entries for all
classroom PCs plus the NOC.

We could put these entries in to separate files, but as our network is small
we'll use a single file called pcs.cfg.

NOTE! You do not add in an entry for your own PC or router. This has already
been defined in the file /etc/nagios3/conf.d/localhost_nagios2.cfg.  This
definition is what defines the Nagios network viewpoint. So, when you come to
the spot where you might add an entry for your PC you should skip this and go
on to the next PC in the list.

        # editor pcs.cfg
        
# Our classroom NOC

define host {
    use         generic-host
    host_name   noc
    alias       Workshop NOC machine
    address     10.10.0.250
    parents     sw
}

# PCs

define host {
    use         generic-host
    host_name   pc1
    alias       pc1
    address     10.10.0.1
    parents     sw
}

define host {
    use         generic-host
    host_name   pc26
    alias       pc26
    address     10.10.0.26
    parents     sw
}

Take the three entries above and now expand this to create the remaining
entries for all active PCs. That is, fill in for PCs 2 through 25 (rememember to 
skip your PC).


Exit and save the file pcs.cfg

As before, repeat steps 2a-2c to verify your configuration, correct any
errors, and activate it.



4. Now configure Nagios to start monitoring the multiple router instances
   we have available. These are from 10.10.254.1 through 10.10.254.26.

Let's create the first router in our file called routers.cfg. Add this line
to the bottom of the file:

define host {
    use         generic-host
    host_name   r1
    alias       router 1
    address     10.10.254.1
    parents     gw-254
}

Now create the remaining routers 2-26. Or, just create a few if you don't
want to spend too long on this particular part of the exercise. But, remember
which router instances you have defined!

Second router:

define host {
    use         generic-host
    host_name   r2
    alias       router 2
    address     10.10.254.2
    parents     gw-254
}

Repeat this until router number 26:

define host {
    use         generic-host
    host_name   r26
    alias       router 26
    address     10.10.254.26
    parents     gw-254
}

Save the file.


5. Look at your Nagios instance on the web. Note that "Status Map" gives
you a graphical view of the parent-child relationships you have defined.


PART III
Configure Service check for the classroom NOC
-----------------------------------------------------------------------------

0. Configuring

Now that we have our hardware configured we can start telling Nagios what services to monitor
on the configured hardware, how to group the hardware in interesting ways, how to group
services, etc.

1. Associate a service check for our classroom NOC

    # joe hostgroups_nagios2.cfg

    - Find the hostgroup named "ssh-servers". In the members section of the defintion 
      change the line:

members                 localhost

    to

members                 localhost,noc

Exit and save the file. 

Verify that your changes are OK:

        # nagios3 -v /etc/nagios3/nagios.cfg
        
Restart Nagios to see the new service assocation with your host:

        # /etc/init.d/nagios3 restart

Click on the "Service Detail" link in the Nagios web interface to see your new entry.


PART IV
Defining Services for all PCs
-----------------------------------------------------------------------------

0. For services, the default normal_check_interval is 5 (minutes) in
   generic-service_nagios2.cfg. You may wish to change this to 1 to speed up
   how quickly service issues are detected, at least in the workshop.

1. Determine what services to define for what devices

   - This is core to how you use Nagios and network monitoring tools in 
     general. So far we are simply using ping to verify that physical hosts
     are up on our network and we have started monitoring a single service on
     a single host (your PC). The next step is to decide what services you wish
     to monitor for each host in the classroom.

   - In this particular class we have:

     routers:  running ssh and snmp 
     switches: running telnet and possibly ssh as well as snmp
     pcs:      All PCs are running ssh and http and should be running snmp
               The NOC is currently running an snmp daemon
              
     So, let's configure Nagios to check for these services for these 
     devices.

2.) Verify that SSH is running on the routers and workshop PCs images

   - In the file services_nagios2.cfg there is already an entry for the SSH 
     service check, so you do not need to create this step. Instead, you 
     simply need to re-define the "ssh-servers" entry in the file
     /etc/nagios3/conf.d/hostgroups_nagios2.cfg. The initial entry in the file
     looked like:

# A list of your ssh-accessible servers
define hostgroup {
        hostgroup_name  ssh-servers
                alias           SSH servers
                members         localhost,noc
        }

     What do you think you should change? Correct, the "members" line. You should
     add in entries for all the classroom pcs, routers and  the switches that run ssh. 
     With this information and the network diagram you should be able complete this entry.
     
     The entry will look something like this:

define hostgroup {
        hostgroup_name  ssh-servers
                alias           SSH servers
                members         localhost,pc1,pc2,pc3,pc4....,noc,ap1,ap2,r1,r2,r3....
        }

         Note: leave in "localhost" - This is your PC and represents Nagios' network point of
         view. So, for instance, if you are on "pc3" you would not include "pc3" in the list
         of all the classroom pcs as it is represented by the "localhost" entry.
         
         The "members" entry will be a long line and will likely wrap on the screen.

         Remember to include all your PCs and all your routers that you have defined. Do no
         include any entries if they are not already defined in pcs.cfg, switches.cfg or
         routers.cfg.

    - Once you are done, run the pre-flight check:

    # nagios3 -v /etc/nagios3/nagios.cfg

    If everything looks good, then restart Nagios 

    # /etc/init.d/nagios3 stop
    # /etc/init.d/nagios3 start

    and view your changes in the Nagios web interface.

3.) Check that http is running on all the classroom PCs. 

    - This is almost identical to the previous exercise. Just make the change to the
      HTTP service adding in each PC (no routers or switches). Remember, you don't need
      to add your machine as it is already defined as "localhost".      

4.)  OPTIONAL EXTRA: as opposed to just checking that a web server is
     running on the classroom PCs, you could also check that the nagios3
     service is available, by requesting the /nagios3/ path. This means
     passing extra options to the check_http plugin.

     For a description of the available options, type this:

      # /usr/lib/nagios/plugins/check_http
      # /usr/lib/nagios/plugins/check_http --help

     and of course you can browse the online nagios documentation or google
     for information on check_http. You can even run the plugin by hand to
     perform a one-shot service check:

     # /usr/lib/nagios/plugins/check_http -H localhost -u /nagios3/

     So the goal is to configure nagios to call check_http in this way.

define command{
        command_name    check_http_arg
        command_line    /usr/lib/nagios/plugins/check_http -H '$HOSTADDRESS$' $ARG1$
        }

define service {
        hostgroup_name                  nagios-servers
        service_description             NAGIOS
        check_command                   check_http_arg!-u /nagios3/
        use                             generic-service
}

     and of course you'll need to create a hostgroup called nagios-servers to
     link to this service check.

     Once you have done this, check that Nagios warns you about failing
     authentication (because it's trying to fetch the page without providing
     the username/password). There's an extra parameter you can pass to
     check_http_arg to provide that info, see if you can find it.

      WARNING: in the tradition of "Debian Knows Best", their definition of the
      check_http command in /etc/nagios-plugins/config/http.cfg
      is *not* the same as that recommended in the nagios3 documentation.
      It is missing $ARG1$, so any parameters to pass to check_http are
      ignored. So you might think you are monitoring /nagios3/ but actually
      you are monitoring root!

     This is why we had to make a new command definition "check_http_arg".
     You could make a more specific one like "check_nagios", or you could
     modify the Ubuntu check_http definition to fit the standard usage.



PART V
Create More Host Groups
-----------------------------------------------------------------------------

0. In the web view, look at the pages "Hostgroup Overview", "Hostgroup
   Summary", "Hostgroup Grid". This gives a convenient way to group together
   hosts which are related (e.g. in the same site, serving the same purpose).

1. Update /etc/nagios3/conf.d/hostgroups_nagios2.cfg

    - For the following exercises it will be very useful if we have created
      or update the following hostgroups:

      debian-servers
      routers
      switches
 
      If you edit the file /etc/nagios3/conf.d/hostgroups_nagios2.cfg you
      will see an entry for debian-servers that just contains localhost. 
      Update this entry to include all the classroom PCs, including the
      noc (this assumes that you created a "noc" entry in your pcs.cfg
      file). Remember to skip your PC entry as it is represented by the
      localhost entry.

    # editor /etc/nagios3/conf.d/hostgroups_nagios2.cfg

     Update the entry that says:


# A list of your Debian GNU/Linux servers
define hostgroup {
        hostgroup_name  debian-servers
                alias           Debian GNU/Linux Servers
                members         localhost
        }
      
      So that the "members" parameter contains something like this. Use your
      classroom network diagram to confirm the exact number of machines and names
      in your workshop.

                members         localhost,pc1,pc2,pc3,pc4,pc5,pc6,pc7,pc8,pc9
                                pc10,pc11,pc12,pc13,pc14,pc15,pc16,pc17,pc18,
                                pc19,pc20,pc21,pc22,pc23,pc24,pc25,pc26

        Be sure that the line wraps and is not on two separate lines. Otherwise
        you will get an error when you go to restart Nagios. Remember that
        your own PC is "localhost".

      - Once you have done this, add in two more host groups, one for routers and 
        one for switches. Call these entries "routers" and "switches".

      - When you are done be sure to verify your work and restart Nagios.
  
2. Go back to the web interface and look at your new hostgroups


PART VI
Extended Host Information ("making your graphs pretty")
-----------------------------------------------------------------------------

1. Update extinfo_nagios2.cfg 

    - If you would like to use appropriate icons for your defined hosts in
      Nagios this is where you do this. We have the three types of devices:

      Cisco routers
      Cisco switches
      Ubuntu servers

      There is a fairly large repository of icon images available for you to
      use located here:

      /usr/share/nagios/htdocs/images/logos/

      these were installed by default as dependent packages of the nagios3
      package in Ubuntu. In some cases you can find model-specific icons for
      your hardware, but to make things simpler we will use the following 
      icons for our hardware:

      /usr/share/nagios/htodcs/images/logos/base/debian.*
      /usr/share/nagios/htdocs/images/logos/cook/router.*
      /usr/share/nagios/htdocs/images/logos/cook/switch.*

    - The next step is to edit the file /etc/nagios3/conf.d/extinfo_nagios2.cfg
      and tell nagios what image you would like to use to represent your devices.

    # editor /etc/nagios3/conf.d/extinfo_nagios2.cfg

      Here is what an entry for your routers looks like (there is already an entry
      for debian-servers that will work as is). Note that the router model (3600) 
      is not all that important. The image used represents a router in general.

define hostextinfo {
        hostgroup_name   routers 
        icon_image       cook/router.png
        icon_image_alt   Cisco Routers (3600) 
        vrml_image       router.png 
        statusmap_image  cook/router.gd2
}

      Now add an entry for your switches. Once you are done check your
      work and restart Nagios. Take a look at the Status Map in the web interface.
      It should be much nicer, with real icons instead of question marks.


PART VII
Create Service Groups
-----------------------------------------------------------------------------

1. Create service groups for ssh and http for each set of pcs.

   - The idea here is to create three service groups. Each service group will
     be for a quarter of the classroom. We want to see these PCs grouped together
     and include status of their ssh and http services. To do this edit
     and create the file:

   # joe /etc/nagios3/conf.d/servicegroups.cfg

     Here is a sample of the service group for group 1:

define servicegroup {
        servicegroup_name       group1-servers
        alias                   group 1 servers
        members                 pc1,SSH,pc1,HTTP,pc2,SSH,pc2,HTTP,pc3,SSH,pc3,HTTP,pc4,SSH,pc4
        }

        - Note that the members line should wrap and not be on two lines.
        
        - Note that "SSH" and "HTTP" need to be uppercase as this is how the service_description is
          written in the file /etc/nagios3/conf.d/services_nagios2.cfg
          
        - You should create an entry for other groups of servers too

    - Save your changes, verify your work and restart Nagios. Now if you click on
      the Servicegroup menu items in the Nagios web interface you should see
      this information grouped together. 



PART VIII
Configure Guest Access to the Nagios Web Interface
-----------------------------------------------------------------------------

1. Edit /etc/nagios3/cgi.cfg to give read-only guest user access to the Nagios 
   web interface.

    - By default Nagios is configured to give full r/w access via the Nagios
      web interface to the user nagiosadmin. You can change the name of this
      user, add other users, change how you authenticate users, what users
      have access to what resources and more via the cgi.cfg file.

    - First, lets create a "guest" user and password in the htpasswd.users
      file.
      
    # htpasswd /etc/nagios3/htpasswd.users guest

      You can use any password you want (or none). A password of "guest" is 
      not a bad choice.

    - Next, edit the file /etc/nagios3/cgi.cfg and look for what type of access
      has been given to the nagiosadmin user. By default you will see the following 
      directives (note, there are comments between each directive):

      authorized_for_system_information=nagiosadmin
      authorized_for_configuration_information=nagiosadmin
      authorized_for_system_commands=nagiosadmin
      authorized_for_all_services=nagiosadmin
      authorized_for_all_hosts=nagiosadmin
      authorized_for_all_service_commands=nagiosadmin
      authorized_for_all_host_commands=nagiosadmin

      Now let's tell Nagios to allow the "guest" user some access to 
      information via the web interface. You can choose whatever you would
      like, but what is pretty typical is this:

      authorized_for_system_information=nagiosadmin,guest
      authorized_for_configuration_information=nagiosadmin,guest
      authorized_for_system_commands=nagiosadmin
      authorized_for_all_services=nagiosadmin,guest
      authorized_for_all_hosts=nagiosadmin,guest
      authorized_for_all_service_commands=nagiosadmin
      authorized_for_all_host_commands=nagiosadmin

    - Once you make the changes, save the file cgi.cfg, verify your 
      work and restart Nagios. 

    - To see if you can log in as the "guest" user you may need to clear 
      the cookies in your web browser. You will not notice any difference
      in the web interface. The difference is that a number of items that
      are available via the web interface (forcing a service/host check, 
      scheduling checks, comments, etc.) will not work for the guest 
      user.


OPTIONAL
--------

* Check that SNMP is running on the classroom NOC

    - First you will need to add in the appropriate service check for SNMP in the file
      /etc/nagios3/conf.d/services_nagios2.cfg. This is where Nagios is impressive. There
      are hundreds, if not thousands, of service checks available via the various Nagios
      sites on the web. You can see what plugins are installed by Ubuntu in the nagios3
      package that we've installed by looking in the following directory:

    # ls /usr/lib/nagios/plugins

      As you'll see there is already a check_snmp plugin available to us. If you are 
      interested in the options the plugin takes you can execute the plugin from the
      command line by typing:

    # /usr/lib/nagios/plugins/check_snmp
    # /usr/lib/nagios/plugins/check_snmp --help 

      to see what options are available, etc. You can use the check_snmp plugin and
      Nagios to create very complex or specific system checks.

    - Now to see all the various service/host checks that have been created using the
      check_snmp plugin you can look in /etc/nagios-plugins/config/snmp.cfg. You will
      see that there are a lot of preconfigured checks using snmp, including:

      snmp_load
      snmp_cpustats
      snmp_procname
      snmp_disk
      snmp_mem
      snmp_swap
      snmp_procs
      snmp_users
      snmp_mem2
      snmp_swap2
      snmp_mem3
      snmp_swap3
      snmp_disk2
      snmp_tcpopen
      snmp_tcpstats
      snmp_bgpstate
      check_netapp_uptime
      check_netapp_cupuload
      check_netapp_numdisks
      check_compaq_thermalCondition
      
      And, even better, you can create additional service checks quite easily.
      For the case of verifying that snmpd (the SNMP service on Linux) is running we
      need to ask SNMP a question. If we don't get an answer, then Nagios can assume
      that the SNMP service is down on that host. When you use service checks such as
      check_http, check_ssh and check_telnet this is what they are doing as well.

    - In our case, let's create a new service check and call it "check_system". This
      service check will connect with the specified host, use the private community 
      string we have defined in class and ask a question of snmp on that ask - in this
      case we'll ask about the System Description, or the OID "sysDescr.0" -

    - To do this start by editing the file /etc/nagios-plugins/config/snmp.cfg:

    # joe /etc/nagios-plugins/config/snmp.cfg

      At the top (or the bottom, your choice) add the following entry to the file:

# 'check_system' command definition
define command{
       command_name    check_system
       command_line    /usr/lib/nagios/plugins/check_snmp -H '$HOSTADDRESS$' -C
'$ARG1$' -o sysDescr.0
        }
      
      You may wish to copy and paste this vs. trying to type this out.

          Note that "command_line" is a single line. If you copy and paste in joe the line
          may not wrap properly and you may have to manually add the part:
          
                        '$ARG1$' -o sysDescr.0
                        
          to the end of the line.

    - Now you need to edit the file /etc/nagios3/conf.d/services_nagios2.cfg and add
      in this service check. We'll run this check against all our servers in the 
      classroom, or the hostgroup "debian-servers"

    - Edit the file /etc/nagios3/conf.d/services_nagios2.cfg

    # joe /etc/nagios3/conf.d/services_nagios2.cfg

      At the bottom of the file add the following definition:

# check that snmp is up on all servers
define service {
        hostgroup_name                  snmp-servers
        service_description             SNMP
        check_command                   check_system!xxxxxx
        use                             generic-service
        notification_interval           0 ; set > 0 if you want to be renotified
}

      The "xxxxxx" is the community string previously (or to be) defined in class.
     
      Note that we have included our private community string here vs. hard-coding
      it in the snmp.cfg file earlier. You must change the "xxxxx" to be the snmp
      community string given in class or this check will not work.
      
    - Now we must create the "snmp-servers" group in our hostgroups_nagios2.cfg file. 
      Edit the file /etc/nagios3/conf.d/hostgroups_nagios2.cfg and go to the end of the
      file. Add in the following hostgroup definition:
      
# A list of snmp-enabled devices on which we wish to run the snmp service check
define hostgroup {
           hostgroup_name       snmp-servers
                   alias        snmp servers
                   members      noc
          }
          
        - Note that for "members" you could, also, add in the switches and routers for
          group 1 and 2. But, the particular item (MIB) we are checking for "sysDescr.0"
          may not be available on the switches and/or routers, so the check would then fail.

    - Now verify that your changes are correct and restart Nagios.

    - If you click on the Service Detail menu choice in web interface you should see
      the SNMP check appear for the noc host.
      
    - After we do the SNMP presentation and exercises in class, then you could come
      back to this exercise and add in all the classroom PCs to the members list in the
      hostgroups_nagios2.cfg file, snmp-servers hostgroup definition. Remember to list
      your PC as "localhost".