Ubuntu Manpage: gdnsd-plugin-extmon - gdnsd plugin for monitoring via external commands

NAME

       gdnsd-plugin-extmon - gdnsd plugin for monitoring via external commands

SYNOPSIS

       Example plugin config:

         service_types => {
           wget_ssl_index => {
             plugin => "extmon",
             timeout => 15,
             interval => 30,
             cmd => ["/usr/bin/wget", "-T", "3", "-q", "-O", "/dev/null", "https://%%ITEM%%/index.html"]
           },
           svc_alwaysok => {
             plugin => "extmon",
             timeout => 1,
             direct => true,
             cmd => ["/bin/sh", "-c", "exit 0"]
           },
           svc_alwaysfail_via_timeout => {
             plugin => "extmon",
             timeout => 2,
             max_proc => 10,
             cmd => ["/bin/sh", "-c", "sleep 5"]
           }
         }

         # optional, if installed path confuses the daemon...
         plugins => {
           extmon => { helper_path => "/usr/lib/x86_64-linux-gnu/gdnsd/gdnsd_extmon_helper" }
         }

DESCRIPTION

       gdnsd-plugin-extmon allows you to configure external commands to provide monitoring feedback to the
       dynamic address plugins.

CONFIGURATION - GLOBAL

These are plugin-global settings which are configured within "plugins => { extmon => { ... } }".

helper_path
String pathname, optional.

Normally, this plugin will successfully locate its helper binary gdnsd_extmon_helper in the
installation libexec directory at /usr/lib/x86_64-linux-gnu/gdnsd/gdnsd_extmon_helper. If for some
reason it can't do so, e.g. due to a relocated installation, you can set an explicit full pathname to
the helper here. Note that in this case you're probably also having to set the daemon-global config
option "plugin_search_path" as well, since locating plugin objects uses similar mechanisms to find
the installation's plugin library path (default /usr/lib/x86_64-linux-gnu/gdnsd) as well.

helper_failure_action
String, either "stasis" or "kill_daemon". Default is "stasis".

This configures the behavior of the plugin if the helper process gdnsd_extmon_helper unexpectedly
dies, which prevents the gathering of any further legitimate monitoring state updates.

Regardless of this setting, there will be at least one syslog message indicating the failure.

If set to "stasis", all monitored states are left as-is with no further updates. The downside is
that unless someone notices the syslog message, this failure mode is hard to notice.

If set to "kill_daemon", the whole gdnsd daemon will fail fatally.

CONFIGURATION - PER-SERVICE-TYPE

The universal, plugin-neutral service_type parameters all apply in their usual sense: "up_thresh",
"ok_thresh", "down_thresh", "interval", "plugin", and "timeout". See gdnsd.config(5) for basic
information on these. The following are specific to this "extmon" plugin:

cmd Array of one or more strings, required.

This sets the command and arguments to execute for the monitoring check. The array is passed
directly to "execv()" for execution (with re-use of the first element as the pathname to execute).
If you need to use shell facilities, start the argument list with e.g. ""/bin/sh", "-c", ...".

All argument strings are searched for the magic string "%%ITEM%%" (which can appear multiple times
per argument). Everywhere this magic string is found, the IP address or CNAME text of the resource
being monitored will be substituted in its place.

The command must exit with an exit value of zero for success or non-zero for failure.

direct
Boolean, default false.

Normally extmon's results are processed by the same anti-flap state machine used for traditional
monitors like the HTTP and TCP monitoring plugins. This means the results are subject to the various
thresholds ("ok_thresh", "up_thresh", "down_thresh") and it may take some time for a new persistent
state to actually affect resolution.

If "direct" is set to "true", the results of extmon's monitoring are applied directly and immediately
as the final state of the monitored resources after every monitoring check. If your command flaps
back and forther between success and failure on every run, so will the internal state and so will the
resulting changes in resolution. Useful if extmon is actually gathering state from an external
complex monitoring environment that has already taken care of things like anti-flap measures.

max_proc
Interger, default 0 (unlimited).

The maximum number of concurrent processes to spawn.

This sets the limit on the number of concurrent commands that will be run. If the limit is exceeded,
excess commands are rescheduled for 0.1 seconds later. After a few runs, the processes will be spread
out enough to run without running into the limit.

EXECUTION ENVIRONMENT

       The plugin launches a helper binary gdnsd_extmon_helper during daemon startup, and this helper does the
       actual runtime command executions.  It manages the timeouts and intervals and feeds results back to the
       plugin over a pipe.  This communication is one-way at runtime (no daemon -> helper traffic) for security
       reasons (to help ensure that a compromised daemon can't easily leverage the helper to muck with the
       execution of the external commands).  This separation also prevents a class of functional and security
       bugs related to mixing runtime pthreads with routine forking to exec the child processes.

       The executed scripts will run with the same userid the daemon normally drops privileges to.

       The stdout, stdin, and stderr descriptors will usually be set to /dev/null.  stdout and stderr may be
       open to the current tty if the main daemon was started in foreground debugging mode via "-f start".

COPYRIGHT AND LICENSE

       Copyright (c) 2012 Brandon L Black <blblack@gmail.com>

       This file is part of gdnsd.

       gdnsd is free software: you can redistribute it and/or modify it under the terms of the GNU General
       Public License as published by the Free Software Foundation, either version 3 of the License, or (at your
       option) any later version.

       gdnsd is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the
       implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
       License for more details.

       You should have received a copy of the GNU General Public License along with gdnsd.  If not, see
       <http://www.gnu.org/licenses/>.