<%NUMBERING1%>.<%NUMBERING2%>.<%NUMBERING3%> PRTG Manual: SNMP Custom String Lookup Sensor

The SNMP Custom String Lookup sensor monitors a string that a specific Object Identifier (OID) returns via Simple Network Management Protocol (SNMP). It can map the string directly to a sensor status by using a defined lookup file. Basically, this sensor type does a "reverse lookup". You have to define all potential return strings in the lookup file as text values, each in one lookup entry. Graphs and data tables show the value to which the string is mapped, usually an integer (lookup type SingleInt). For more information, see section SNMP Custom String Lookup Sensor—Example.

The sensor can show the following:

  • A retrieved string value and its status, as defined in the corresponding lookup file
SNMP Custom String Lookup Sensor

SNMP Custom String Lookup Sensor

Sensor in Other Languages

Dutch: SNMP Aangepaste Tekst Lookup, French: Requête de chaîne personnalisée SNMP, German: SNMP-Zeichenfolge mit Lookup, Japanese: SNMP カスタム文字列検索, Portuguese: Pesquisa da sequência de caracteres customizada SNMP, Russian: Подстановка пользовательской строки по SNMP, Simplified Chinese: SNMP 自定义字符串查询, Spanish: Búsqueda de cadena personalizada de SNMP


  • It might not work to query data from a probe device via SNMP (querying localhost,, or ::1). Add this device to PRTG with the IP address that it has in your network and create the SNMP sensor on this device instead.
  • This sensor type uses lookups to determine the status values of one or more sensor channels. This means that possible states are defined in a lookup file. You can change the behavior of a channel by editing the lookup file that the channel uses. For details, see section Define Lookups.
  • For a general introduction to the technology behind Simple Network Management Protocol (SNMP), see section Monitoring via SNMP.
  • See manual section SNMP Custom String Lookup Sensor—Example for a sample lookup definition for this sensor type.
  • Knowledge Base: How do I find out what OID I need to use for a custom sensor?
  • icon-betaCurrently, this sensor type is in beta status. The methods of operating can change at any time, as well as the available settings. Do not expect that all functions work properly, or that this sensor works as expected at all. Be aware that this type of sensor can be removed again from PRTG at any time.

Add Sensor

The Add Sensor dialog appears when you manually add a new sensor to a device. It only shows the setting fields that are required for creating the sensor. Therefore, you will not see all setting fields in this dialog. You can change (nearly) all settings in the sensor's Settings tab later.

The following settings in the Add Sensor dialog differ in comparison to the sensor's Settings tab.

OID Values

Channel Name

Enter a name for the channel in which the sensor shows the results at the given Object Identifier (OID).

Enter a string.

You can change this value later in the respective channel settings of this sensor.


Select a lookup file that you stored in the \lookups\custom subfolder of your PRTG installation. This lookup file must contain all potential strings that the monitored Object Identifier (OID) can return.

Sensor Settings

On the details page of a sensor, click the Settings tab to change its settings.

icon-i-roundUsually, a sensor connects to the IP Address or DNS Name of the parent device on which you created the sensor. See the Device Settings for details. For some sensor types, you can explicitly define the monitoring target in the sensor settings. See below for details on available settings.

Basic Sensor Settings

Sensor Name

Enter a meaningful name to identify the sensor. By default, PRTG shows this name in the device tree, as well as in alarms, logs, notifications, reports, maps, libraries, and tickets.

Parent Tags

Shows Tags that this sensor inherits from its parent device, group, and probe.

This setting is shown for your information only and cannot be changed here.


Enter one or more Tags, separated by spaces or commas. You can use tags to group sensors and use tag–filtered views later on. Tags are not case sensitive.

We recommend that you use the default value.

There are default tags that are automatically predefined in a sensor's settings when you add a sensor. See section Default Tags below.

You can add additional tags to the sensor if you like. Other tags are automatically inherited from objects further up in the device tree. These are visible above as Parent Tags.

icon-i-roundIt is not possible to enter tags with a leading plus (+) or minus (-) sign, nor tags with parentheses (()) or angle brackets (<>).


Select a priority for the sensor. This setting determines where the sensor is placed in sensor lists. A sensor with a top priority is at the top of a list. Choose from one star (low priority) to five stars (top priority).

Default Tags


OID Values

OID Value

Enter the Object Identifier (OID) of the Simple Network Management Protocol (SNMP) object that you want to receive a string from.

icon-i-roundMost Object Identifier (OID)s begin with However, entering Object Identifier (OID)s starting with 1.0, or 1.1, or 1.2 is also allowed. If you want to entirely disable the validation of your entry, add the string norfccheck: at the beginning of your Object Identifier (OID), for example, norfccheck:


Shows the lookup file that this sensor uses. Once you have created a sensor, you cannot change this value. It is shown for reference purposes only. If you need to change this value, please add the sensor anew.

If Value Changes

Define what this sensor will do when the sensor value changes. Choose between:

  • Ignore changes (default): The sensor takes no action on change.
  • Trigger 'change' notification: The sensor sends an internal message indicating that its value has changed. In combination with a Change Trigger, you can use this mechanism to trigger a notification whenever the sensor value changes.

Sensor Display

Primary Channel

Select a channel from the list to define it as the primary channel. In the device tree, the last value of the primary channel is always displayed below the sensor's name. The available options depend on what channels are available for this sensor.

icon-i-roundYou can set a different primary channel later by clicking the pin symbol of a channel on the sensor's Overview tab.

Graph Type

Define how different channels will be shown for this sensor.

  • Show channels independently (default): Show a graph for each channel.
  • Stack channels on top of each other: Stack channels on top of each other to create a multi-channel graph. This will generate a graph that visualizes the different components of your total traffic.
    icon-i-roundThis option cannot be used in combination with manual Vertical Axis Scaling (available in the Sensor Channel Settings settings).

Stack Unit

This field is only visible if you enable Stack channels on top of each other above. Select a unit from the list. All channels with this unit will be stacked on top of each other. By default, you cannot exclude single channels from stacking if they use the selected unit. However, there is an advanced procedure to do so.

Inherited Settings

By default, all of the following settings are inherited from objects that are higher in the hierarchy and should be changed there if necessary. Often, best practice is to change them centrally in the Root group's settings. For more information, see section Inheritance of Settings. To change a setting for this object only, disable inheritance by clicking the button next to inherit from under the corresponding setting name. You will then see the options described below.

Scanning Interval

Click inherited_settings_button to interrupt the inheritance. See section Inheritance of Settings for more information.

Scanning Interval

Select a scanning interval (seconds, minutes, or hours). The scanning interval determines the amount of time that the sensor waits between two scans. You can change the available intervals in the system administration on PRTG on premises installations.

If a Sensor Query Fails

Define the number of scanning intervals that the sensor has time to reach and check a device again in case a sensor query fails. Depending on the option that you select, the sensor can try to reach and check a device again several times before the sensor will show a Down status. This can avoid false alarms if the monitored device only has temporary issues. For previous scanning intervals with failed requests, the sensor will show a Warning status. Choose from:

  • Set sensor to down immediately: Set the sensor to a Down status immediately after the first failed request.
  • Set sensor to warning for 1 interval, then set to down (recommended): Set the sensor to a Warning status after the first failed request. If the following request also fails, the sensor will show an error.
  • Set sensor to warning for 2 intervals, then set to down: Set the sensor to a Down status only after three consecutively failed requests.
  • Set sensor to warning for 3 intervals, then set to down: Set the sensor to a Down status only after four consecutively failed requests.
  • Set sensor to warning for 4 intervals, then set to down: Set the sensor to a Down status only after five consecutively failed requests.
  • Set sensor to warning for 5 intervals, then set to down: Set the sensor to a Down status only after six consecutively failed requests.

icon-i-roundSensors that monitor via Windows Management Instrumentation (WMI) always wait at least one scanning interval before they show a Down status. It is not possible to immediately set a WMI sensor to a Down status, so the first option will not apply to these sensor types. All other options can apply.

icon-i-roundIf you define error limits for a sensor's channels, the sensor will immediately show a Down status. No "wait" option will apply.

icon-i-roundIf a channel uses lookup values, the sensor will immediately show a Down status. No "wait" options will apply.

Schedules, Dependencies, and Maintenance Window

icon-i-roundYou cannot interrupt the inheritance for schedules, dependencies, and maintenance windows. The corresponding settings from the parent objects will always be active. However, you can define additional settings here. They will be active at the same time as the parent objects' settings.


Select a schedule from the list. Schedules can be used to monitor for a certain time span (days or hours) every week.

icon-book-arrowsYou can create schedules, edit schedules, or pause monitoring for a specific time span. For more information, see section Account Settings—Schedules.

icon-i-roundSchedules are generally inherited. New schedules will be added to existing schedules, so all schedules are active at the same time.

Maintenance Window

Specify if you want to set up a one-time maintenance window. During a maintenance window, the current sensor and all child objects will not be monitored. They will be in a Paused status instead. Choose between:

  • Not set (monitor continuously): No maintenance window will be set and monitoring will always be active.
  • Set up a one-time maintenance window: Pause monitoring within a maintenance window. You can define a time span for a monitoring pause below and change it even for a currently running maintenance window.

icon-i-roundTo terminate a current maintenance window before the defined end date, change the time entry in Maintenance Ends to a date in the past.

Maintenance Begins

This field is only visible if you enable Set up a one-time maintenance window above. Use the date time picker to enter the start date and time of the maintenance window.

Maintenance Ends

This field is only visible if you enable Set up a one-time maintenance window above. Use the date time picker to enter the start date and time of the maintenance window.

Dependency Type

Define a dependency type. You can use dependencies to pause monitoring for an object depending on the status of another object. You can choose from:

  • Use parent: Use the dependency type of the parent device.
  • Select a sensor: Use the dependency type of the parent device. Additionally, pause the current sensor if another specific sensor is in a Down status or in a Paused status caused by another dependency. Select below.
  • Master sensor for parent: Make this sensor the master object for its parent device. The sensor will influence the behavior of its parent device: If the sensor is in a Down status, the device will be paused. For example, it is a good idea to make a Ping sensor the master object for its parent device to pause monitoring for all other sensors on the device in case the device cannot even be pinged. Additionally, the sensor will be paused if the parent group is paused by another dependency.

icon-i-roundTesting your dependencies is easy! Simply select Simulate Error Status from the context menu of an object that other objects depend on. A few seconds later, all dependent objects should be paused. You can check all dependencies in your PRTG installation by selecting Devices | Dependencies from the main menu bar.


This field is only visible if you enable Select a sensor above. Click the Search button and use the object selector to select a sensor on which the current sensor will depend.

Dependency Delay (Sec.)

This field is only visible if you enable Select a sensor above. Define a time span in seconds for dependency delay.

After the master sensor for this dependency comes back to an Up status, monitoring of the dependent objects will be additionally delayed by the defined time span. This can help avoid false alarms, for example, after a server restart, by giving systems more time for all services to start up.

Enter an integer value.

icon-i-round-redThis setting is not available if you set this sensor to Use parent or to be the Master sensor for parent. In this case, please define delays in the parent Device Settings or in its parent Group Settings.

Access Rights

Click inherited_settings_button to interrupt the inheritance. See section Inheritance of Settings for more information.

User Group Access

Define the user groups that will have access to the selected object. A table with user groups and types of access rights is shown. It contains all user groups from your setup. For each user group, you can choose from the following access rights:

  • Inherited: Use the access rights settings of the parent object.
  • None: Users in this group cannot see or edit the object. The object neither shows up in lists nor in the device tree. Exception: If a child object is visible to the user, the object is visible in the device tree but it is not accessible.
  • Read: Users in this group can see the object and review its monitoring results.
  • Write: Users in this group can see the object, review its monitoring results, and edit its settings. They cannot edit access rights settings.
  • Full: Users in this group can see the object, review its monitoring results, edit its settings, and edit access rights settings.

You can create new user groups in the System Administration—User Groups settings. To automatically set all objects further down in the hierarchy to inherit this object's access rights, set a check mark for the Revert children's access rights to inherited option.

icon-book-arrowsFor more details on access rights, see section User Access Rights.


icon-speechYou have to provide all possible return strings for this sensor in one lookup file. For example, consider an Object Identifier (OID) that can return one of the three strings Good, Deficient, or Bad. Then you have to define a lookup file for this sensor that contains all these possible string values as text, each text value in one lookup entry:

<?xml version="1.0" encoding="UTF-8"?>
<ValueLookup id="mylookupfile" desiredValue="0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="PaeValueLookup.xsd">
                      <SingleInt state="Ok" value="0">
                      <SingleInt state="Warning" value="1">
           <SingleInt state="Error" value="2">

If a retrieved string matches one of the text values, the sensor maps it into the defined integer value ("reverse lookup") that is shown, for example, in data graphs. Depending on the integer, the sensor shows the respective status and converts the integer back to the original string to show it as a channel value. If the Object Identifier (OID) returns a string that the lookup definition does not contain, the sensor will show a Down status with a corresponding error message.

For example, you create an SNMP Custom String Lookup sensor, apply the example lookup definition from above (store it into the \lookups\custom subfolder of your PRTG installation), and the given Object Identifier (OID) returns the string Good. Then the sensor maps the string Good to the integer value 0 that is shown in the live graph of the sensor, for example. According to the status definition state="Ok", the sensor status is Up in this case. The integer 0 is converted back to the string Good, which is shown as the channel value.

icon-i-roundThe string match is not case sensitive.

icon-i-round-redUse the lookup type SingleInt for this sensor. BitFields and ranges are not supported!

icon-book-arrowsIf you imported an SNMP library (this is an oidlib file) that contains lookups (you can see this in section Lookup in the MIB Importer), you can define your own sensor states for returning values. Use the lookupname of the imported SNMP library as id parameter in a custom lookup definition. This overrides the lookups that an oidlib might contain with your own status definitions. See section Define Lookups—Customizing Lookups for details about this mechanism.


Knowledge Base: How do I find out what OID I need to use for a custom sensor?

Knowledge Base: What SNMP sensors does PRTG offer?

Knowledge Base: My SNMP sensors don't work. What can I do?

Edit Sensor Channels

To change display settings, spike filtering, and limits, switch to the sensor's Overview tab and click the gear icon of a specific channel. For detailed information, see section Sensor Channel Settings.

Notification Triggers

Click the Notification Triggers tab to change notification triggers. For detailed information, see section Sensor Notification Triggers Settings.


For more general information about settings, see section Object Settings.

Sensor Settings Overview

For information about sensor settings, see the following sections: