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 your machine like this: http://pcN.ws.nsrc.org/nagios3/ - 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. Remember to refer to the Network Diagram for our classroom if you get confused. We recommend creating instances like this: rtr (the gateway router: 10.10.0.254) sw (the gateway switch: 10.10.0.253, parent: rtr) rtr1 (group 1 router: 10.10.1.254, parent: sw) rtr2 (group 2 router: 10.10.2.254, parent: sw)) rtr3 (group 3 router: 10.10.3.254, parent: sw)) rtr4 (group 4 router: 10.10.4.254, parent: sw)) rtr5 (group 5 router: 10.10.5.254, parent: sw)) rtr6 (group 6 router: 10.10.6.254, parent: sw)) pc1 (pc in group 1: 10.10.1.1, parent: rtr1) . pc2 (pc in group 2: 10.10.2.2, parent: rtr2) . pc9 (pc in group 3: 10.10.3.9, parent: rtr3) . pc10 (pc in group 4: 10.10.4.10, parent: rtr4) . pc17 (pc in group 5: 10.10.5.17, parent: rtr5) . pc18 (pc in group 6: 10.10.6.18, parent: rtr6) . . pc26 s1 (on backbone: 10.10.0.241, parent: sw) s2 (on backbone: 10.10.0.242, parent: sw) noc (on backbone: 10.10.0.250, parent: sw) ap1 (on backbone: 10.10.0.251, parent: sw) ap2 (on backbone: 10.10.0.252, parent: sw) We recommend placing these items in the files: routers.cfg (rtr, rtr1...rtr6) switches.cfg (sw) pcs.cfg (pc1...pc26, s1, s2, noc, ap1, ap2) 1. First we need to tell Nagios to monitor the gateway router for our classroom which is 10.10.0.254: # cd /etc/nagios3/conf.d/ Create the routers gateway like this: # vi routers.cfg define host { use generic-host host_name rtr alias Gateway Router address 10.10.0.254 } In the same file create the 6 entries for the group routers: define host { use generic-host host_name rtr1 alias Group 1 Router address 10.10.1.254 parents sw } repeate this for ap1, ap2, rtr2, rtr3, rtr4, rtr5, rtr6, rtr7, rtr8, rtr9. note ap1 IP - 10.10.0.244 note ap2 IP - 10.10.0.245 Note that the entry for "sw" our gateway switch has not yet been created. That is next. Exit and save this file. 2. Create a file called switches.cfg and add an entry for this item: # vi switches.cfg define host { use generic-host host_name sw alias Backbone Switch address 10.10.0.253 parents rtr } 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 servers s1, s2 and the noc: Be sure you add in a proper "parents" entry for each host. Remember, if you don't understand the parent relations in our network you can review the logical network diagram here: http://noc.ws.nsrc.org/wiki/wiki/NetworkDiagram Note the Nagios parent bullet points: Nagios Parent Relationships 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 some warnings like : Warning: Host 'rtr' has no services associated with it! Warning: Host 'sw' has no services associated with it! etc.... ... Total Warnings: N 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 the classroom PCs 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. ***** Let us first describe the machines in our noc which at the moment comprises of one machine # vi noc.cfg # Our classroom NOC define host { use generic-host host_name noc alias Workshop NOC machine address 10.10.0.254 parents sw } Save and exit the file. Next, create a file to describe the devices in group one as follows: # vi group1pcs.cfg # PCs define host { use generic-host host_name pc1 alias pc1 address 10.10.1.1 parents rtr1 } define host { use generic-host host_name pc3 alias pc3 address 10.10.1.3 parents rtr1 } Do this for all the PCs in group1, note that the parent is maintained. When done, save and exit the file. Do the same for group2 pcs: # vi group2pcs.cfg define host { use generic-host host_name pc2 alias pc2 address 10.10.2.2 parents rtr2 } define host { use generic-host host_name pc4 alias pc4 address 10.10.2.4 parents rtr2 } Do this for all the PCs in group2, note that the parent is maintained. When done, save and exit the file. Pay attention to the parent entries and the IP addresses. Take the three entries above and expand to create the remaining entries organized per group for all active PCs. That is, fill in for PCs 1 through 39 (rememember to skip your PC) per group As before, repeat steps 2a-2c to verify your configuration, correct any errors, and activate it. 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 # vi 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...,pc26,....ap1,ap2,s1,s2,noc,rtr1 } 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. To continue with hostgroups you can add additional groups for later use, such as all our virtual servers. Go ahead and edit the file hostgroups_nagios2.cfg again: # vi hostgroups_nagios2.cfg and add the following to the end of the file: # A list of our virtual routers define hostgroup { hostgroup_name cisco7200 alias Cisco 7200 Routers members rtr1,rtr2,rtr3,rtr4,rtr5,rtr6,rtr7,rtr8,rtr9 } Save and exit from the file. Verify that everything is OK: # nagios3 -v /etc/nagios3/nagios.cfg If everything looks good, then restart Nagios # /etc/init.d/nagios3 stop # /etc/init.d/nagios3 start 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. Create a file to store the information, lets call it afnog.cfg # vi afnog.cfg 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 } Save and exit the file and of course you'll need to create a hostgroup called nagios-servers to link to this service check. # vi hostgroups_nagios2.cfg At the bottom add the following: define hostgroup { hostgroup_name nagios-servers alias Servers running Nagios members localhost,pc1,pc2,pc3,pc4 } Add all the servers in the room (upto 39) 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. # vi /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. - Remember to skip your pc entry as it is represented by the localhost entry. 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/nagios3/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/nagios3/htdocs/images/logos/base/debian.* /usr/share/nagios3/htdocs/images/logos/cook/router.* /usr/share/nagios3/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. # vi /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: # vi /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".