Changes between Version 3 and Version 4 of doc/usage/external/LN3/nsca

Timestamp:

01/19/09 09:08:20 (4 years ago)

Author:

mickem

Comment:

--

doc/usage/external/LN3/nsca

@@ -10 +10 @@
 <p>Nagios also offers another way to work with the statuses of hosts and services. It is possible to configure Nagios so that it will receive status information sent over a command pipe. In such a case, checks are done by other programs, and their results are sent to Nagios. Nagios will still handle all notifications, event handlers, and dependencies between hosts and services.</p>
 <p>Active checks are most common in the Nagios world. They have a lot of advantages and some disadvantages. One of the problems is that such checks can take only a couple of seconds to complete—a typical timeout for an active check to complete is 10 or 30 seconds. In many cases, the time taken is not enough, as some checks need to be performed over a longer period of time to have satisfactory results. A good example might be running a check that takes several hours to complete—in this case, it does not make sense to raise the global <i>service_check_timeout</i> option, but rather to schedule these checks outside of Nagios and only report the results back to it.</p>
-<p>There are also different types of checks including external applications or devices that want to report information directly to Nagios. This can be done to gather all critical errors to a single, central place. These types of checks are called <b>Passive Checks</b>.</p><p>For example, when a web application cannot connect to the database, it will let Nagios know about it immediately. It can also send reports after a database recovery, or periodically, even if connectivity to the database has been consistently available, so that Nagios has an up-to-date status. This can be done in addition to active checks, to identify critical problems earlier.</p><p>Another example is where an application already processes information such as network bandwidth utilization. In such a case, adding a module that reports current utilization along with the <i>OK/WARNING/CRITICAL</i> state to Nagios seems much easier than using active checks for the same job.</p><p>Often, there are situations where active checks obviously fit better. In other cases, passive checks are the way to go. In general, if a check can be done quickly and does not require long running processes, it should definitely be done as an active service. If the situation involves reporting problems from other applications or machines, it is definitely a use case for a passive check. In cases where the checks require the deployment of long-running processes or monitoring information constantly, this should be done as a passive service.</p><p>Another difference is that active checks require much less effort to be set up when compared to passive checks. In the first case, Nagios takes care of the scheduling, and the command only needs to perform the actual checks and mark the results as <i>OK/WARNING/CRITICAL</i> based on how a check command is configured. Passive checks require all the logic related to what should be reported and when it should be checked to be put in an external application. This usually calls for some effort.</p><p>The following diagram shows how both active and passive checks are performed by Nagios. It shows what is performed by Nagios in both cases and what needs to be done by the check command or an external application for passive checks.</p><p style="text-align: center;"><img src="http://www.packtpub.com/files/images/nagios-article-image1.PNG"></p><p>Nagios also offers a way of combining the benefits of both active and passive checks. Often, you have situations where other applications can report if a certain service is working properly or not. But if the monitoring application is not running or some other issue prevents it from reporting, Nagios can use active checks to keep the service status up–to-date.</p><p>A good example would be a server that is a part of an application, processing job queues using a database. It can report each problem when accessing the database. We want Nagios to monitor this database, and as the application is already using it, we can add a module that reports this to Nagios.</p><p>The application can also periodically let Nagios know if it succeeded in using the database without problems. However, if there are no jobs to process and the application is not using it, Nagios will not have up-to-date information about the database.</p><h1>Configuring Passive Checks</h1><p>The first thing that needs to be done in order to use passive checks for your Nagios setup is to make sure that you have the following options in your main Nagios configuration file:</p><pre style="margin-left: 40px;">accept_passive_service_checks=1<br>accept_passive_host_checks=1<br></pre><p>It would also be good to enable the logging of incoming passive checks—this makes determining the problem of not processing a passive check much easier. The following directive allows it:</p><pre style="margin-left: 40px;">log_passive_checks=1<br></pre><p>Setting up hosts or services for passive checking requires an object to be defined and set up so as not to perform active checks. The object needs to have the <i>passive_checks_enabled</i> option set to 1 for Nagios to accept passive check results over the command pipe.</p><p>The following is an example of the required configuration for a host that accepts passive checks and has active checks disabled:</p><pre style="margin-left: 40px;">define host<br>{<br>use generic-host<br>host_name linuxbox01<br>address 10.0.2.1<br>active_checks_enabled 0<br>passive_checks_enabled 1 <br>}<br></pre><p>Configuring services is exactly the same as with hosts. For example, to set up a very similar service, all we need to do is to use the same parameters as those for the hosts:</p><pre style="margin-left: 40px;">define service<br>{<br>use ping-template<br>host_name linuxbox01<br>service_description PING<br>active_checks_enabled 0<br>passive_checks_enabled 1<br>}<br></pre><p>In this case, Nagios will never perform any active checks on its own and will only rely on the results that are passed to it.</p><p>We can also configure Nagios so that if no new information has been provided within a certain period of time, it will use active checks to get the current status of the host or service. If up-to-date information has been provided by a passive check during this period, then it will not perform active checks.</p><p>In order to do this, we need to enable active checks by setting the <i>active_checks_enabled</i> option to 1 without specifying the <i>normal_check_interval</i> directive. For Nagios to perform active checks when there is no up-to-date result from passive checks, you need to set the <i>check_freshness</i> directive to 1 and set <i>freshness_threshold</i> to the time period after which a check should be performed. The time performed is specified in seconds.</p><p>The first parameter tells Nagios that it should check whether the results from the checks are up-to-date. The next parameter specifies the number of seconds after which Nagios should consider the results to be out of date. Attributes can be used for both hosts and services.</p><p>A sample definition for a host that runs an active check if there has been no result provided within the last two hours:</p><pre style="margin-left: 40px;">define host<br>{<br>use generic-host<br>host_name linuxbox02<br>address 10.0.2.2<br>check_command check-host-alive<br>check_freshness 1<br>freshness_threshold 7200<br>active_checks_enabled 1<br>passive_checks_enabled 1<br>}<br></pre><p>The following is an illustration showing when Nagios would invoke active checks:</p><p style="text-align: center;"><img src="http://www.packtpub.com/files/images/nagios-article-image2.PNG"></p><p>Each time there is at least one passive check result that is still valid (i.e., was received within the past two hours), Nagios will not perform any active checks. However, two hours after the last passive or active check result was received, Nagios would perform an active check to keep the results up-to-date.</p><h1>Passive Checks—Hosts</h1><p>Nagios allows applications and event handlers to send out passive check results for host objects. In order to use them, the host needs to be configured to accept passive checks results.</p><p>In order to be able to submit passive check results, we need to configure Nagios to allow the sending of passive check results, and set the host objects to accept them.</p><p>Submitting passive host check results to Nagios requires sending a command to the Nagios external command pipe. This way, the other applications on your Nagios server can report the status of the hosts.</p><p>The command to submit passive checks is <i>PROCESS_HOST_CHECK_RESULT</i> (visit <i>http://www.nagios.org/developerinfo/externalcommands/commandinfo.php?command_id=115</i>). This command accepts the host name, status code, and the textual output from a check. The host status code should be 0 for an <i>UP</i> state, 1 for <i>DOWN</i> and 2 for an <i>UNREACHABLE</i> state.</p><p>The following is a sample script that will accept the host name, status code, and output from a check and will submit these to Nagios:</p><pre style="margin-left: 40px;">#!/bin/sh<br><br>NOW='date +%s'<br>HOST=$1<br>STATUS=$2<br>OUTPUT=$3<br><br>echo "[$NOW] PROCESS_HOST_CHECK_RESULT;$HOST;$STATUS;$OUTPUT" <br> &gt;/var/nagios/rw/nagios.cmd<br><br>exit 0<br></pre><p>As an example of the use of this script, the command that is sent to Nagios for <i>host01</i>, status code 2 (<i>UNREACHABLE</i>) and output <i>router 192.168.1.2</i> down would be as follows:</p><pre style="margin-left: 40px;">[1206096000] PROCESS_HOST_CHECK_RESULT;host01;2;router<br>192.168.1.2 down<br></pre><p>When submitting results, it is worth noting that Nagios might take some time to process them, depending on the intervals between Nagios' checks of the external command pipe.</p><p>Unlike active checks, Nagios will not take network topology into consideration by default. This is very important in situations where a host behind a router is reported to be down because the router is actually down.</p><p>By default, Nagios handles results from active and passive checks differently. When Nagios plans and receives results from active checks, it takes the actual network topology into consideration and performs a translation of the states based on this. This means that if Nagios receives a result indicating that a host is <i>DOWN</i>, it assumes that all child hosts are in an <i>UNREACHABLE</i> state.</p><p>When a passive result check comes in to Nagios, Nagios expects that the result already has a network topology included. When a host is reported to be <i>DOWN</i> as a passive check result, Nagios does not perform a translation from <i>DOWN</i> to <i>UNREACHABLE</i>. Even if its parent host is currently <i>DOWN</i>, the child host state is also stored as <i>DOWN</i>.</p><p>The following illustration shows how results from active and passive checks are treated differently by Nagios:</p><p style="text-align: center;"><img src="http://www.packtpub.com/files/images/nagios-article-image3.PNG"></p><br><br><hr size="1" color="#ff9933" noshade="noshade"><br><div class="header">Learning Nagios 3.0</div><div style="line-height: 0.4em;">&nbsp;</div> <table width="100%" cellpadding="0" cellspacing="0"><tbody><tr><td valign="top" width="99"> <a href="http://www.packtpub.com/guide-for-learning-nagios-3/book/bh/nagios-abr/1108"><img title="Learning Nagios 3.0" class="left" alt="Learning Nagios 3.0" src="http://images.packtpub.com/images/100x123/1847195180.png" width="99" border="0" height="123"></a> </td> <td valign="top">A comprehensive configuration guide to monitor and maintain your network and systems <ul><li>Secure and monitor your network system with open-source Nagios version 3</li><li>Set up, configure, and manage the latest version of Nagios</li><li>In-depth coverage for both beginners and advanced users</li></ul> <a href="http://www.packtpub.com/guide-for-learning-nagios-3/book">http://www.PacktPub.com/passive-checks-nsca-nagios-service-check-acceptor</a></td></tr></tbody></table><br><hr size="1" color="#ff9933" noshade="noshade"><p>In both the cases, a check result stating that the host is down is received by Nagios. When it comes in as a passive check, no state translation is done and Nagios stores the host and all child nodes being down. When it is an active check result, Nagios takes the fact that <i>switch1</i> is down into account and maps the child node's result into an <i>UNREACHABLE</i> state.</p><p>How Nagios process handles passive check results can be defined in the main Nagios configuration file. In order to make Nagios treat passive host check results in the same way as active check results, we need to enable the following option:</p><pre style="margin-left: 40px;">translate_passive_host_checks=1</pre><p>By default, Nagios treats host results from passive checks as hard results. This is because, very often, passive checks are used to report host and service statuses from other Nagios instances. In such cases, only reports regarding hard state changes are propagated across Nagios servers.</p><p>If you want Nagios to treat all passive check results for hosts as if they were soft results, you need to enable the following option in the main Nagios configuration file:</p><pre style="margin-left: 40px;">passive_host_checks_are_soft=1<br></pre><h1>Passive Checks—Services</h1><p>Passive service checks are very similar to passive host checks. In both the cases, the idea is that Nagios receives information about host statuses over the external commands pipe.</p><p>As with passive checks of hosts, all that is needed is to enable the global Nagios option to accept passive check results, and also enable this option for each service that should allow the passing of passive check results.</p><p>The results are passed to Nagios in the same way as they are passed for hosts. A command to submit passive checks is <i>PROCESS_SERVICE_CHECK_RESULT</i> (visit <a target="_blank" href="" http:="" www.nagios.org="" developerinfo="" externalcommands="" commandinfo.php?command_id="114&quot;">http://www.nagios.org/developerinfo/externalcommands/commandinfo.php?command_id=114</a>). This command accepts the host name, service description, status code, and the textual output from a check. Service status codes are the same as those for active checks—0 for <i>OK</i>, 1 for <i>WARNING</i>, 2 for <i>CRITICAL</i>, and 3 for an <i>UNKNOWN</i> state.</p><p>The following is a sample script that will accept the host name, status code, and output from a check and will submit these to Nagios:</p><pre style="margin-left: 40px;">#!/bin/sh<br><br>CLOCK='date +%s'<br>HOST=$1<br>SVC=$2<br>STATUS=$3<br>OUTPUT=$4<br><br>echo "[$CLOCK] PROCESS_SERVICE_CHECK_RESULT;$HOST;$SVC;$STATUS;<br> $OUTPUT"<br> &gt;/var/nagios/rw/nagios.cmd<br><br>exit 0<br></pre><p>As an example of the use of this script, the command that is sent to Nagios for <i>host01</i>, service <i>PING</i>, status code 0 (<i>OK</i>) and output <i>RTT=57 ms</i> is as follows:</p><pre style="margin-left: 40px;">[1206096000] PROCESS_SERVICE_CHECK_RESULT;host01;PING;0;RTT=57 ms<br></pre><p>A very common scenario for using passive checks is a check that takes a very long time to complete.</p><p>As with submitting host check results, it is worth mentioning that Nagios will take some time to process passive check results as they are polled periodically from the external commands pipe.</p><p>A major difference between hosts and services is that service checks differentiate between soft and hard states. When new information regarding a service gets passed to Nagios via the external commands pipe, Nagios treats it the same way as if it had been received by an active check.</p><p>If a service is set up with a <i>max_check_attempts</i> directive of 5, then the same number of passive check results would need to be passed in order for Nagios to treat the new status as a hard state change.</p><p>Passive service checks are often used to report the results of long lasting tests asynchronously. A good example of such a test is checking whether there are bad blocks on a disk. This requires trying to read the entire disk directly from the block device (such as <i>/dev/sda1</i>) and checking if the attempt has failed. This can't be done as an active check as reading the device takes a lot of time to complete—larger disks might require several hours to complete.</p><p>For this reason, the only way to perform such a check is to schedule them from the system—for example, using the <i>cron</i> daemon (visit <i>http://man.linuxquestions.org/index.php?query=cron</i>). The script should then post results to the Nagios daemon.</p><p>The following is a script that runs the <i>dd</i> system command (visit <i>http://man.linuxquestions.org/index.php?query=dd</i>) to read an entire block device. Based on whether the read was successful or not, the appropriate status code, along with plugin output, is sent out.</p><pre style="margin-left: 40px;">#!/bin/sh<br><br> SVC=$1<br> DEVICE=$2<br> TMPFILE=/tmp/ddlog.$$<br> NOW='date +%s'<br> PREFIX="['date +%s'] [$NOW] PROCESS_SERVICE_CHECK_<br>RESULT;localhost;$SVC"<br> <br> # try to read the device<br> dd if=$DEVICE of=/dev/null &gt;$TMPFILE 2&gt;&amp;1<br> CODE=$?<br> RESULT='grep copied &lt;$TMPFILE'<br> rm $TMPFILE<br><br> if [ $CODE == 0 ] ; then<br> echo "$PREFIX;0;$RESULT"<br> else<br> echo "$PREFIX;2;Error while checking device $DEVICE"<br> fi<br><br> exit 0<br></pre> 
+<p>There are also different types of checks including external applications or devices that want to report information directly to Nagios. This can be done to gather all critical errors to a single, central place. These types of checks are called <b>Passive Checks</b>.</p><p>For example, when a web application cannot connect to the database, it will let Nagios know about it immediately. It can also send reports after a database recovery, or periodically, even if connectivity to the database has been consistently available, so that Nagios has an up-to-date status. This can be done in addition to active checks, to identify critical problems earlier.</p>
+<p>Another example is where an application already processes information such as network bandwidth utilization. In such a case, adding a module that reports current utilization along with the <i>OK/WARNING/CRITICAL</i> state to Nagios seems much easier than using active checks for the same job.</p>
+<p>Often, there are situations where active checks obviously fit better. In other cases, passive checks are the way to go. In general, if a check can be done quickly and does not require long running processes, it should definitely be done as an active service. If the situation involves reporting problems from other applications or machines, it is definitely a use case for a passive check. In cases where the checks require the deployment of long-running processes or monitoring information constantly, this should be done as a passive service.</p>
+<p>Another difference is that active checks require much less effort to be set up when compared to passive checks. In the first case, Nagios takes care of the scheduling, and the command only needs to perform the actual checks and mark the results as <i>OK/WARNING/CRITICAL</i> based on how a check command is configured. Passive checks require all the logic related to what should be reported and when it should be checked to be put in an external application. This usually calls for some effort.</p>
+<p>The following diagram shows how both active and passive checks are performed by Nagios. It shows what is performed by Nagios in both cases and what needs to be done by the check command or an external application for passive checks.</p>
+<p style="text-align: center;"><img src="http://www.packtpub.com/files/images/nagios-article-image1.PNG"></p>
+<p>Nagios also offers a way of combining the benefits of both active and passive checks. Often, you have situations where other applications can report if a certain service is working properly or not. But if the monitoring application is not running or some other issue prevents it from reporting, Nagios can use active checks to keep the service status up–to-date.</p>
+<p>A good example would be a server that is a part of an application, processing job queues using a database. It can report each problem when accessing the database. We want Nagios to monitor this database, and as the application is already using it, we can add a module that reports this to Nagios.</p>
+<p>The application can also periodically let Nagios know if it succeeded in using the database without problems. However, if there are no jobs to process and the application is not using it, Nagios will not have up-to-date information about the database.</p>
 
-
-<p>If the check fails, then a <i>critical</i> status, along with text stating that there was a problem checking the specific device, is sent out to Nagios. If the check was successful, an output mentioning number of bytes and the speed of transfer is sent out to Nagios. A typical output would be something like this:</p>
+<h1>Configuring Passive Checks</h1>
+<p>The first thing that needs to be done in order to use passive checks for your Nagios setup is to make sure that you have the following options in your main Nagios configuration file:</p>
 <pre style="margin-left: 40px;">
-254951424 bytes (255 MB) copied, 9.72677 seconds, 26.2 MB/s
+accept_passive_service_checks=1
+accept_passive_host_checks=1
 </pre>
-<p>The host name is hardcoded to localhost. Using this script requires configuring a service to have active checks disabled and passive checks enabled. As the checks will be done quite rarely, it's recommended to set <i>max_check_attempts</i> to 1.</p>
-
-<h1>Troubleshooting Passive Checks</h1>
-<p>It's not always possible to set up passive checks correctly the first time. In such cases, it is a good thing to try to debug the issue one step at a time in order to find any potential problems. Sometimes the problem could be a configuration issue, while in other cases, it could be an issue such as the mistyping of the host or service name.</p>
-<p>One thing worth checking is whether the Web UI shows changes after you have sent the passive result check. If it doesn't, then at some point, things are not working correctly.</p>
-<p>The first thing you should start with is enabling the logging of external commands and passive checks. To do this, make sure that the following values are enabled in the main Nagios configuration file:</p>
+<p>It would also be good to enable the logging of incoming passive checks—this makes determining the problem of not processing a passive check much easier. The following directive allows it:</p>
 <pre style="margin-left: 40px;">
-log_external_commands=1
 log_passive_checks=1
 </pre>
-<p>In order for the changes to take effect, a restart of the Nagios process is needed. After this has been done, Nagios will log all commands passed via the command pipe and log all of the passive check results it receives.</p>
-<p>The first issue, a common problem, is that an application or script cannot write data to the Nagios command pipe. In order to test this, simply change to the user your scripts are running as, and try the following command:</p>
+<p>Setting up hosts or services for passive checking requires an object to be defined and set up so as not to perform active checks. The object needs to have the <i>passive_checks_enabled</i> option set to 1 for Nagios to accept passive check results over the command pipe.</p>
+<p>The following is an example of the required configuration for a host that accepts passive checks and has active checks disabled:</p>
 <pre style="margin-left: 40px;">
-user@ubuntuserver:~$ echo "TEST" &gt;/var/nagios/rw/nagios.cmd
-</pre>
-<p>If the command above runs fine, and no errors are reported, then your permissions are set up correctly. If an error shows up, you should add the user to the <i>nagioscmd</i> group.</p>
-<p>The next thing to do is to manually send a passive check result to the Nagios command pipe and check whether the Nagios log file was received and parsed correctly. To test this, run the following command:</p>
-<pre style="margin-left: 40px;">
-echo "['date +%s'] PROCESS_HOST_CHECK_RESULT;host1;2;test" 
-&gt;/var/nagios/rw/nagios.cmd
-</pre>
-<p>The name, <i>host1</i>, needs to be replaced with an actual host name from your configuration. A few seconds after running this command, the Nagios log file should reflect the command that we have just sent. You should see the following lines in your log:</p>
-<pre style="margin-left: 40px;">
-EXTERNAL COMMAND: PROCESS_HOST_CHECK_RESULT;host1;2;test
-[1220257561] PASSIVE HOST CHECK: host1;2;test
-</pre>
-<p>If only the first line is present, then it means that either the global option to receive passive host check results is disabled, or it is disabled for this particular object. The first thing you should do is to make sure that your main Nagios configuration file contains the following line:</p>
-<pre style="margin-left: 40px;">
-accept_passive_host_checks=1
-</pre>
-<p>Next, you should check your configuration to see whether the host definition has passive checks enabled as well. If not, simply add the following directive to the object definition:</p>
-<pre style="margin-left: 40px;">
-passive_checks_enabled 1
-</pre>
-<p>If you have misspelled the name of the host object, then the following will be logged:</p>
-<pre style="margin-left: 40px;">
-Warning: Passive check result was received for host ‘host01',
-but the host could not be found!
-</pre>
-<p>In this case, make sure that your host name is correct.</p>
-<p>Similar checks can also be done for services. You can run the following command to check if a passive service check is being handled correctly by Nagios:</p>
-<pre style="margin-left: 40px;">
-echo "['date +%s'] PROCESS_SERVICE_CHECK_RESULT;host1;APT;0;test" 
- &gt;/var/nagios/rw/nagios.cmd
-</pre>
-<p>Again, <i>host1</i> should be replaced by the actual host name, and <i>APT</i> needs to be an existing service for that host. After a few seconds, the following entries in Nagios log file would indicate the result has been successfully parsed:</p>
-<pre style="margin-left: 40px;">
-EXTERNAL COMMAND: PROCESS_SERVICE_CHECK_RESULT;host1;APT;0;test
-PASSIVE SERVICE CHECK: host1;APT;0;test
-</pre>
-<p>If the second line is not in the log file, either the option to accept service passive checks is disabled on a global basis, or this particular service has the option to accept passive check results disabled. You should start by making sure that your main Nagios configuration file contains the following line:</p>
-<pre style="margin-left: 40px;">
-accept_passive_service_checks=1
+define host
+{
+use generic-host
+host_name linuxbox01
+address 10.0.2.1
+active_checks_enabled 0
+passive_checks_enabled 1 
+}
 </pre>