Alerting is a helpful and important part of monitoring that informs you when there are issues, when values exceed thresholds or a sensor status has changed, for example. PRTG offers many ways to alert you about your monitoring data like internal sensor alerts, limits, and lookups. You can also create custom alerts and define trigger notifications as extensions to alerts. If a sensor does not have pre-configured limits, you can also configure them accordingly so that you are notified when something is wrong.

Internal Sensor Alerts

Some sensor types display internal alerts when there are errors. These default alerts range from disconnected probes to socket or timeout errors. For example, the HTTP sensors show pre-configured internal alerts according to certain HTTP status codes.

There are also sensor types that let you modify the internal sensor alerts. To see if you can modify an alert, check the sensor's settings for customizable options.

icon-book-bulbSee Which HTTP status code leads to which HTTP sensor status? for more information.

icon-book-bulbSee What does error code PExxx mean? for more information on error codes.

Here is an example of a sensor that is in Down status because of an internal sensor alert.

Probe Health Sensor with Disconnected Probe Alert
Probe Health Sensor with Disconnected Probe Alert

Standard Limits for Channels

Another type of alerts are alerts due to limits. These alerts are triggered when the measured value of a sensor is below or above a configured threshold limit. These limits change the sensor's status when they are breached. For example, you can set an SNMP CPU Load sensor to Warning status whenever it measures values that you consider critical. This sensor then shows up in the alarms list.

icon-i-roundThis type of alert is only displayed when the value breaches the configured limits. If the value is normal again in the next sensor scan, the sensor returns to the Up status.

  • To review or modify threshold limits for single sensors, open the sensor's channel settings via the gear icon in the respective channel gauge or in the channels table.
  • Enable alerting based on limits and specify your desired limits in the correct fields.
  • You can also optionally add messages that appear in the sensor message on the Overview tab or leave these fields empty to display the default message.

icon-i-roundThis limit only applies to the respective channel.

Setting Limits for Sensor Channels
Setting Limits for Sensor Channels

icon-playFor more information on setting limits, see this video on our website: How to Set Channel Limits

With the above configuration, the sensor turns to a Warning status (the gauge needle points to the yellow area in the example below) if the channel values exceed the upper warning limit of 60 percent and the Warning Limit Message is displayed in the sensor status. If the channel value exceeds the upper error limit of 80, the sensor will turn to a Down status (the gauge needle will point to the red area) and the Error Limit Message will be displayed.

CPU Load Sensor in Warning Status
CPU Load Sensor in Warning Status

You can use Multi-Edit if you want to apply the same limits for sensors of the same type in a batch operation.

  • To see all sensors of this type at a glance, just filter for it: From the PRTG main menu bar, choose Sensors | By Type | SNMP CPU Load.
  • Mark the checkboxes of the sensors you want to edit.
  • Click the wrench symbol in the menu.
  • Open the Channel Settings tab.
  • Select the channel you want to add a limit for; in this example we use the channel Total, which is common to all selected sensors.
  • Then Enable alerting based on limits and enter the number in the correct field as described above.

When you are done, Save these settings—the new limit is then applied to all selected channels from all selected sensors.

icon-i-roundMulti-edit is only available for channels that are common to all selected sensors.

Setting Channel Limits with Multi-Edit
Setting Channel Limits with Multi-Edit

icon-i-roundSome sensors have pre-defined limits in the sensor settings, for example the SNMP Linux Disk Free Sensor. Refer to the sensor's settings to adjust the limits (or behavior) for these sensors.

Absolute and Delta Values in Limits

icon-i-roundThe value that needs to be configured in the limits depends on the type of data that the sensor's channel delivers.

For channels that measure absolute values (for CPU load and memory usage, for example) limits will also be set using absolute values like in the example above.

You can also set limits for channels that measure delta (x.xx/sec) values. Let's say you have an SNMP Traffic Sensor and want to receive an alert when it reports errors. In this case, you need take into account that this is a delta measurement. In this example, you could set the following limits for the channel Errors in with a standard scanning interval (60 seconds).

  • Set the sensor to Warning status when 1 error occurs
  • Set the sensor to Down status when 30 errors occur

This example shows how to configure the limits for delta channels.

Setting Channel Limits with Delta Values
Setting Channel Limits with Delta Values

Because this channel uses per second (delta) measurements, a single error that occurs over a standard 60-second scanning interval is reported as 0.016 # per second. So the warning limit for one single RX error within an interval is 0.1 (errors/sec). To get an alert when there are 30 errors within a scanning interval, the limit needs to be 0.5 (errors/sec).

icon-i-blueIf no new errors occur in the next scanning interval, the sensor will turn back to the Up status. To ensure that you do not miss any notifications for this sensor, set a notification trigger with 0 seconds. For more information, see Triggers (Notifications) as Extensions to Alerts below.

Standard Lookups for Channels

PRTG also uses lookups for some sensor types. In general, lookups make data more human friendly because they map status values as returned by a device (usually integers) to more informative expressions in words that show you the status of a monitored device as a clear message.

Additionally, lookups can also define the sensor status that will be shown in correlation with certain status codes, just like sensor channel limits can define a sensor status, too. For example, PRTG can show a sensor in a gray Inactive status with channel values, provided by lookups, like Inactive instead of a numerical value like -1.

icon-i-roundThe various states displayed in gauges always follow the clockwise order Up (green) < Warning (yellow) < Down (red) < Unknown (Gray / Black).

SNMP HP LaserJet Hardware Gauge
SNMP HP LaserJet Hardware Gauge

Modifying a Standard Lookup: Example

You can also modify standard lookups to include new definitions according to your monitoring needs. The following code illustrates the lookup definition for the paper status of the SNMP HP LaserJet Hardware sensor:

<?xml version="1.0" encoding="UTF-8"?>
  <ValueLookup id="oid.paessler.hplaserjet.paperstatus" desiredValue="0"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation=
       <SingleInt state="Ok" value="0">
        Paper Okay
      <SingleInt state="Error" value="1">
        Out of Paper or No Cassette Loaded
      <SingleInt state="Error" value="2">
        Manual Paper Feed Required


Imagine you do not want to be alerted in case the printer is out of paper. You basically have to do the following:

  1. Copy the file oid.paessler.hplaserjet.paperstatus.ovl to the \lookups\custom subfolder of the PRTG program directory (ensure you do not change the file name!).
  2. Open this file with an editor.
  3. Replace the first SingleInt state "Error" with "Ok".
  4. Optionally replace the message "Out of Paper or No Cassette Loaded" to "Ok".
  5. Save the file.
  6. Reload the custom lookup folder in PRTG.

The channel gauge now displays the new state and the new message.oid

SNMP HP LaserJet Hardware Gauge Modified
SNMP HP LaserJet Hardware Gauge Modified

icon-book-arrowsFor more information on how lookups are defined, see section Define Lookups.

Creating a New Lookup

It is also possible to create new lookups to use with any custom or standard sensor that supports lookups. If you want to create a new lookup with the example above, you basically follow the same procedure except you save the lookup file in the \lookups\custom subfolder with a new name, for example oid.paessler.hplaserjet.mynewpaperstatus.ovl. Select the newly created lookup file in the sensor's channel settings.

Selecting a Newly Created Lookup
Selecting a Newly Created Lookup

Custom String Lookups

You can also set up custom string lookups, which can exclusively be used with the SNMP Custom String Lookup Sensor. This is useful if you are using an OID that returns a known string value stating possible states or values. To inform PRTG of the possible statues or values, you have to create a new lookup file defining them.

icon-book-bulbSee the Knowledge Base article Monitor and "Lookup" a SNMP String value for more information.

Triggers (Notifications) as Extensions to Alerts

The status or the data of a sensor can trigger notifications. Using this mechanism, you can configure external alerting tailored to your needs. Which trigger types are available depends on the kind of object you edit. You can define triggers that are activated by an 'on change' event. Some sensors offer the option to send such a trigger whenever sensor values have changed.

Create Triggers to Alert You When a Value Changes

icon-i-blueBefore you set up a change trigger, make sure that the Trigger 'change' notification setting is enabled in the sensor's settings, otherwise the notification will never be sent.

Trigger 'Change' Notification Setting
Trigger 'Change' Notification Setting

Select the Notifications tab to create a change trigger for this sensor. Hover over plus_button and select Add Change Trigger from the menu to add a new trigger, or click the Edit button next to an existing notification to change it. Every trigger will provoke one or more notification(s) to be executed.

Adding a Change Trigger Notification
Adding a Change Trigger Notification

Save the trigger. You will now be notified when sensor values change.

icon-book-arrowsFor more information on triggers, see section Sensor Notifications Settings.

Create Triggers to Match Your Limits/Lookups (status-based)

After you have set up alerts using limits or lookups, you can complement them with state-based triggers. On the Notifications tab, hover over plus_button and select Add State Trigger from the menu to add a new trigger.

Adding a State Trigger Notification
Adding a State Trigger Notification

Save the trigger. You will now be notified according to the notification settings.

icon-i-roundIf you want to receive alerts for more than one status, you have to add a trigger for each status.

icon-i-roundA sensor's status can change for other reasons such as internal sensor alerts. If you configure state-based triggers, you will be notified of these changes as well.


Knowledge Base: Monitor and "Lookup" a SNMP String value

Knowledge Base: Which HTTP status code leads to which HTTP sensor status?

Knowledge Base: What does error code PExxx mean?

Video Tutorial: How to Configure Lookups in PRTG Network Monitor


Understanding Basic Concepts—Topics