Event Handler

Event Handler Technology Overview

Event handler allows you to execute scripts when a string that matches a regex or a regex with a service name appears in journald logs. You can pass variables, arguments, and a full matching string to the script.

How to configure Event Handler

1. Create an event handler

2. Add regex to the script

3. Add a full path to the script

4. Add optional parameters

Event Handler Configuration Steps

1. Create an event handler

set service event-handler event <event-handler name>

This is an optional command because the event handler will be automatically created after any of the next commands.

2. Add regex to the script

set service event-handler event <event-handler name> filter pattern <regex>

This is a mandatory command. Sets regular expression to match against log string message.

Note

The regular expression matches if and only if the entire string matches the pattern.

3. Add a full path to the script

set service event-handler event <event-handler name> script path <path to script>

This is a mandatory command. Sets the full path to the script. The script file must be executable.

4. Add optional parameters

set service event-handler event <event-handler name> filter syslog-identifier <sylogid name>

This is an optional command. Filters log messages by syslog-identifier.

set service event-handler event <event-handler name> script environment <env name> value <env value>

This is an optional command. Adds environment and its value to the script. Use separate commands for each environment.

One implicit environment exists.

message: Full message that has triggered the script.

set service event-handler event <event-handler name> script arguments <arguments>

This is an optional command. Adds arguments to the script. Arguments must be separated by spaces.

Note

We don’t recomend to use arguments. Using environments is more preffereble.

Example

Event handler that monitors the state of interface eth0.

set service event-handler event INTERFACE_STATE_DOWN filter pattern '.*eth0.*,RUNNING,.*->.*'
set service event-handler event INTERFACE_STATE_DOWN filter syslog-identifier 'netplugd'
set service event-handler event INTERFACE_STATE_DOWN script environment interface_action value 'down'
set service event-handler event INTERFACE_STATE_DOWN script environment interface_name value 'eth2'
set service event-handler event INTERFACE_STATE_DOWN script path '/config/scripts/eventhandler.py'

Event handler script

#!/usr/bin/env python3
#
# VyOS event-handler script example
from os import environ
import subprocess
from sys import exit

# Perform actions according to requirements
def process_event() -> None:
    # Get variables
    message_text = environ.get('message')
    interface_name = environ.get('interface_name')
    interface_action = environ.get('interface_action')
    # Print the message that triggered this script
    print(f'Logged message: {message_text}')
    # Prepare a command to run
    command = f'sudo ip link set {interface_name} {interface_action}'.split()
    # Execute a command
    subprocess.run(command)

if __name__ == '__main__':
    try:
        # Run script actions and exit
        process_event()
        exit(0)
    except Exception as err:
        # Exit properly in case if something in the script goes wrong
        print(f'Error running script: {err}')
        exit(1)