source: Extensions/Fast-Ping-Check.md

## Fast up/down checking

Normally, LibreNMS sends an ICMP ping to the device before polling to check if it is up or down.
This check is tied to the poller frequency, which is normally 5 minutes.  This means it may take up to 5 minutes
to find out if a device is down.

Some users may want to know if devices stop responding to ping more quickly than that. LibreNMS offers a ping.php script
to run ping checks as quickly as possible without increasing snmp load on your devices by switching to 1 minute polling.

> **WARNING**: If you do not have an alert rule that alerts on device status, enabling this will be a waste of resources.
> You can find one in the [Alert Rules Collection](../Alerting/Rules.md#Alert Rules Collection).
                         


### Setting the ping check to 1 minute

1. Change the ping_rrd_step setting in config.php
    ```php
    $config['ping_rrd_step'] = 60;
    ```

2. Update the rrd files to change the step (step is hardcoded at file creation in rrd files)
    ```bash
    ./scripts/rrdstep.php -h all
    ```

3. Add the following line to /etc/cron.d/librenms.nonroot.cron to allow 1 minute ping checks

```
*    *    * * *   librenms    /var/www/librenms/ping.php >> /dev/null 2>&1
```

#### Sub minute ping check

Cron only has a resolution of one minute, so we have to use a trick to allow sub minute checks.
We add two entries, but add a delay before one.

>Alerts are only run every minute, so you will have to modify them as well. Remove the original alerts.php entry.

1. Set ping_rrd_step
    ```php
   $config['ping_rrd_step'] = 30;
   ```
   
2. Update the rrd files
    ```bash
    ./scripts/rrdstep.php -h all
    ```

3. Update cron (removing any other ping.php or alert.php entries)

```
*    *    * * *   librenms    /var/www/librenms/ping.php >> /dev/null 2>&1
*    *    * * *   librenms    sleep 30 && /var/www/librenms/ping.php >> /dev/null 2>&1
*    *    * * *   librenms    sleep 15 && /var/www/librenms/alerts.php >> /dev/null 2>&1
*    *    * * *   librenms    sleep 45 && /var/www/librenms/alerts.php >> /dev/null 2>&1
```

### Device dependencies

The ping.php script respects device dependencies, but the main poller does not (for technical reasons).
However, using this script does not disable the icmp check in the poller and a child may be reported as
down before the parent.

### Settings

ping.php uses much the same settings as the poller fping with one exception: retries is used instead of count.
ping.php does not measure loss and avg response time, only up/down, so once a device responds it stops pinging it.

```
$config['fping_options']['retries'] = 2;
$config['fping_options']['timeout'] = 500;
$config['fping_options']['interval'] = 500;
```
