# 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

    You likely want to have a device down alert rule to take advantage
    of Fast Ping checks. You can find one in the [Alert Rules
    Collection](../Alerting/Rules.md#alert-rules-collection).

## Setting the ping check to 1 minute

1: If you are using [RRDCached](../Extensions/RRDCached.md), stop the service.

!!! note

    This will flush all pending writes so that the lnms maintenance:rrd-step script can change the steps.

2: Change the `ping_rrd_step` setting in WebUI or CLI

!!! setting "poller/rrdtool"

    ```bash
    lnms config:set ping_rrd_step 60
    ```

3: Update the rrd files to change the step (step is hardcoded at file
creation in rrd files)

```bash
lnms maintenance:rrd-step all
```

4: Add the following line to `/etc/cron.d/librenms` to allow 1 minute
ping checks

```title="/etc/cron.d/librenms"
*    *    * * *   librenms    /opt/librenms/ping.php >> /dev/null 2>&1
```

5: If applicable: Start the [RRDCached](../Extensions/RRDCached.md) service

!!! note

    If you are using distributed pollers you can restrict a
    poller to a group by appending `-g` to the cron entry. Alternatively,
    you should only run `ping.php` on a single node.

## Sub minute ping check

Cron only has a resolution of one minute, so for sub-minute ping checks we need to adapt both `ping`
and `alerts` entries. We add two entries per function, but add a delay before one of these entries.

Remember, you need to remove the original `ping.php` and `alerts.php` entries in crontab before
proceeding!

1: Set `ping_rrd_step` setting in WebUI or CLI

!!! setting "poller/rrdtool"

    ```bash
    lnms config:set ping_rrd_step 30
    ```

2: Update the rrd files

```bash
lnms maintenance:rrd-step all
```

3: Update cron (removing any other `ping.php` or `alerts.php` entries)

```title="/etc/cron.d/librenms"
*    *    * * *   librenms    /opt/librenms/ping.php >> /dev/null 2>&1
*    *    * * *   librenms    sleep 30 && /opt/librenms/ping.php >> /dev/null 2>&1
*    *    * * *   librenms    sleep 15 && /opt/librenms/alerts.php >> /dev/null 2>&1
*    *    * * *   librenms    sleep 45 && /opt/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.

!!! setting "poller/ping"

    ```bash
    lnms config:set fping_options.retries 2
    lnms config:set fping_options.timeout 500
    lnms config:set fping_options.interval 500
    ```