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diff --git a/docs/configuration/service/eventhandler.md b/docs/configuration/service/eventhandler.md index 6413c24d..ab813b93 100644 --- a/docs/configuration/service/eventhandler.md +++ b/docs/configuration/service/eventhandler.md @@ -1,129 +1,186 @@ +--- +myst: + html_meta: + description: | + The event handler watches the system log and runs a user-supplied + script when a preconfigured event occurs. Configure a pattern, + optionally a syslog identifier, and a script path per event. + keywords: event-handler, syslog, log monitoring, script, automation +--- + (event-handler)= -# Event Handler +# Event handler -## Event Handler Technology Overview +The event handler watches the system log and runs a user-supplied +script when a preconfigured event occurs. For each event, you configure +a pattern to look for in the log, optionally restrict matching to a +specific syslog identifier, and a script to run when a match occurs. -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. +Upon a match, the event handler launches the script. The script can +read the matched log entry from an environment variable named +`message` (for example, to extract an IP address or include the log +line in a notification). You can also pass your own environment +variables or a fixed argument string to the script. -## How to configure Event Handler +## Configuration -> [1. Create an event handler](#create-an-event-handler) -> -> [2. Add regex to the script](#add-regex-to-the-script) -> -> [3. Add a full path to the script](#add-a-full-path-to-the-script) -> -> [4. Add optional parameters](#add-optional-parameters) +Each event must be configured with a name, a pattern, and a script +path. All other settings are optional. -## Event Handler Configuration Steps +### Mandatory settings -### 1. Create an event handler +```{cfgcmd} set service event-handler event \<name\> filter pattern \<regex\> -```{cfgcmd} set service event-handler event \<event-handler name\> +**Configure a regular expression to match against log entries for the +specified event.** -This is an optional command because the event handler will be -automatically created after any of the next commands. +The regular expression must match the entire log entry, not just a +part of it. For example, to match any log entry that contains `eth0`, +wrap it as `.*eth0.*`. Matching is case-sensitive. ``` +Example: -### 2. Add regex to the script - -```{cfgcmd} set service event-handler event \<event-handler name\> filter pattern \<regex\> +```none +set service event-handler event LINK-DOWN filter pattern '.*eth0.*,RUNNING,.*->.*' +``` -This is a mandatory command. Sets regular expression to match -against log string message. +```{cfgcmd} set service event-handler event \<name\> script path \<path\> -:::{note} -The regular expression matches if and only if the entire -string matches the pattern. -::: +**Configure the path to the script run for the specified event.** ``` +```{note} +Event-handler scripts must be executable (see +{ref}`command-scripting`). Storing them under `/config/scripts/` +ensures they are preserved across image upgrades. +``` -### 3. Add a full path to the script +```{warning} +Event-handler scripts run with root privileges. Review them carefully +before use. +``` -```{cfgcmd} set service event-handler event \<event-handler name\> script path \<path to script\> +Example: -This is a mandatory command. Sets the full path to the script. -The script file must be executable. +```none +set service event-handler event LINK-DOWN script path /config/scripts/link-down.py ``` +### Optional settings -### 4. Add optional parameters +```{cfgcmd} set service event-handler event \<name\> filter syslog-identifier \<identifier\> -```{cfgcmd} set service event-handler event \<event-handler name\> filter syslog-identifier \<syslogid name\> +**For the specified event, match the regular expression only against +log entries produced by the specified process.** +``` -This is an optional command. Filters log messages by -syslog-identifier. +Example: + +```none +set service event-handler event LINK-DOWN filter syslog-identifier kernel ``` -```{cfgcmd} set service event-handler event \<event-handler name\> script environment \<env name\> value \<env value\> +```{cfgcmd} set service event-handler event \<name\> script environment \<env-var\> value \<value\> -This is an optional command. Adds environment and its value to -the script. Use separate commands for each environment. +**Add an environment variable, with the specified name and value, to +the script's execution environment.** -One implicit environment exists. +Repeat the command for each variable. +``` -* ``message``: Full message that has triggered the script. +```{note} +The variable name `message` is reserved for the matched log entry and +cannot be reused. +``` + +Example: + +```none +set service event-handler event LINK-DOWN script environment interface_name value eth0 ``` -```{cfgcmd} set service event-handler event \<event-handler name\> script arguments \<arguments\> +```{cfgcmd} set service event-handler event \<name\> script arguments \<arguments\> -This is an optional command. Adds arguments to the script. -Arguments must be separated by spaces. +**Append an argument string to the script invocation.** -:::{note} -We don't recommend using arguments. Using environment variables is preferable. -::: +The string is appended verbatim after the script path. To pass +multiple arguments, separate them with spaces within a single string. ``` +```{note} +Prefer environment variables for passing data to the script. +``` + +Example: + +```none +set service event-handler event LINK-DOWN script arguments '--notify admin@example.com' +``` ## Example -Event handler that monitors the state of interface eth0. +The following example configures an event handler that reacts to +link-state messages emitted by `netplugd` for `eth0` and runs a Python +script. When triggered, the script writes a line containing the +original log entry and the configured `interface_name` value to +`/tmp/link-state.log`. + +### Configuration ```none -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 'eth0' -set service event-handler event INTERFACE_STATE_DOWN script path '/config/scripts/eventhandler.py' +set service event-handler event LINK-DOWN filter pattern '.*eth0.*,RUNNING,.*->.*' +set service event-handler event LINK-DOWN filter syslog-identifier 'netplugd' +set service event-handler event LINK-DOWN script environment interface_name value 'eth0' +set service event-handler event LINK-DOWN script path '/config/scripts/link-down.py' ``` -Event handler script +### Event handler script + +Save the following as `/config/scripts/link-down.py`: ```none #!/usr/bin/env python3 # # VyOS event-handler script example + +from datetime import datetime from os import environ -import subprocess +from pathlib import Path from sys import exit -# Perform actions according to requirements +LOG_FILE = Path('/tmp/link-state.log') + + 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) + # 'message' is set by the event handler and holds the log entry + # that matched the configured pattern. + log_line = environ.get('message', '') + # User-defined environment variables are read the same way. + interface = environ.get('interface_name', '') + + timestamp = datetime.now().isoformat() + with LOG_FILE.open('a') as f: + f.write( + f'{timestamp} link-state change on {interface}: {log_line}\n' + ) + 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}') + # Script stdout is discarded by the event handler, but stderr + # is captured and surfaces in the system journal. + from sys import stderr + print(f'Error running script: {err}', file=stderr) exit(1) ``` +After creating the script, make it executable: + +```none +sudo chmod +x /config/scripts/link-down.py +``` |