How to monitor SMTP with Nagios Core

Mail transport depends on an SMTP listener that accepts connections and returns a greeting quickly enough for other mail systems to continue delivery. Nagios Core can check that handshake as a service on the mail host, so a refused connection, slow banner, or wrong listener port becomes an alertable service state.

The check_smtp plugin opens the SMTP connection, checks the first response line, and reports response time as performance data. On Ubuntu and Debian package installs, the plugin path is /usr/lib/nagios/plugins/check_smtp and site-specific object snippets can be placed under /etc/nagios4/conf.d.

A host object should already represent the mail endpoint, such as mail01.example.net. A small command object with separate arguments keeps the port, expected greeting, warning threshold, and critical threshold visible in the service definition before the scheduler loads the new SMTP check.

Steps to monitor SMTP with Nagios Core:

Confirm that Nagios Core loads the custom object directory.
```
$ grep '^cfg_dir=' /etc/nagios4/nagios.cfg
cfg_dir=/etc/nagios-plugins/config
cfg_dir=/etc/nagios4/conf.d
```
Add a cfg_dir or cfg_file entry first when custom objects are stored somewhere else.
Related: How to add a Nagios Core object configuration directory
Run the SMTP plugin manually from the monitoring server.
```
$ sudo -u nagios /usr/lib/nagios/plugins/check_smtp -H mail01.example.net -p 25 --expect=220 --warning=2 --critical=5
SMTP OK - 0.011 sec. response time|time=0.011164s;2.000000;5.000000;0.000000
```
Use --starttls for submission ports that require STARTTLS, and use --ssl for implicit TLS listeners such as port 465. Keep the tested options aligned with the listener that mail clients or remote mail exchangers actually use.
Related: How to run a Nagios plugin manually
Create a command object snippet for the reusable SMTP check.
```
$ sudoedit /etc/nagios4/conf.d/check-smtp-port.cfg
```
Related: How to define a Nagios Core check command

Add the command definition.

define command {
    command_name    check_smtp_port
    command_line    $USER1$/check_smtp -H $HOSTADDRESS$ -p $ARG1$ --expect=$ARG2$ --warning=$ARG3$ --critical=$ARG4$
}

Separate arguments keep the service object readable and avoid hiding several plugin options inside one quoted command argument.

Create a service object snippet for the mail host.
```
$ sudoedit /etc/nagios4/conf.d/mail01-smtp.cfg
```
Add the SMTP service definition.
```
define service {
    use                    generic-service
    host_name              mail01.example.net
    service_description    SMTP
    check_command          check_smtp_port!25!220!2!5
}
```
The host_name value must match an existing host object. Replace 25 only when the service should monitor a different listener, such as a submission or implicit TLS port.
Related: How to add a service check in Nagios Core

Validate the Nagios Core configuration.

$ sudo nagios4 -v /etc/nagios4/nagios.cfg
Nagios Core 4.4.6
##### snipped #####
Reading configuration data...
   Read main config file okay...
   Read object config files okay...

Running pre-flight check on configuration data...

Checking objects...
	Checked 9 services.
	Checked 2 hosts.
	Checked 181 commands.
##### snipped #####
Total Warnings: 0
Total Errors:   0

Things look okay - No serious problems were detected during the pre-flight check

Do not reload Nagios Core while Total Errors is greater than 0; the new service can be rejected with the rest of the changed object files.

Reload Nagios Core to apply the service object.
```
$ sudo systemctl reload nagios4
```
Package installs on Ubuntu and Debian use the nagios4 unit. Use the local unit or init script name on source installs and other distributions.
Related: How to manage the Nagios Core system service
Confirm that the nagios4 service is active after the reload.
```
$ sudo systemctl is-active nagios4
active
```
Check /var/log/nagios4/nagios.log or the journal before retrying the reload if the service is not active.
Related: How to check Nagios Core logs

Check the SMTP service result after the next active check.

$ curl --silent --show-error 'http://monitor.example.net/nagios4/cgi-bin/statusjson.cgi?query=service&hostname=mail01.example.net&servicedescription=SMTP'
{
  "format_version": 0,
  "result": {
    ##### snipped #####
    "type_code": 0,
    "type_text": "Success",
    "message": ""
  },
  "data": {
    "service": {
      "host_name": "mail01.example.net",
      "description": "SMTP",
      "plugin_output": "SMTP OK - 0.001 sec. response time",
      "perf_data": "time=0.000571s;2.000000;5.000000;0.000000",
      "has_been_checked": true,
      "checks_enabled": true
    }
  }
}

Use the real Nagios Core URL and credentials when CGI authentication is enabled. If the service remains pending, force one service check or wait for the next scheduled interval.
Related: How to reschedule an active check in Nagios Core