PRTG Manual: SNMP Custom String Sensor
The SNMP Custom String sensor monitors a string returned by a specific object identifier (OID) via the Simple Network Management Protocol (SNMP). It can check for keywords. If you want to set limits to the channel value, you can also extract a numeric value contained in the string.
In the sensor message, the sensor shows the string that you search for and the reason for the Warning or Down status.
The SNMP Library sensor automatically creates SNMP Custom String sensors when the Management Information Base (MIB) file that you import contains string values.
SNMP Custom String Sensor
For a detailed list and descriptions of the channels that this sensor can show, see section Channel List.
▪Dutch: SNMP Aangepaste Tekenreeks
▪French: Chaîne personnalisée (SNMP)
▪German: SNMP-Zeichenfolge
▪Japanese: SNMP カスタム文字列
▪Portuguese: Sequência de caracteres (customizado) (SNMP)
▪Russian: SNMP: строка ответа
▪Simplified Chinese: SNMP 自定义字符串
▪Spanish: Cadena (personalizado) (SNMP)
▪It might not work to query data from a probe device via SNMP (querying localhost, 127.0.0.1, 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.
▪For an example, see section Number Extraction with Regular Expression
▪This sensor supports the IPv6 protocol.
▪This sensor has a very low performance impact.
▪See the Knowledge Base: How do I find out which OID I need for an SNMP Custom sensor?
The Add Sensor dialog appears when you manually add a new sensor to a device. It only shows the settings that are required to create the sensor. You can change nearly all settings on the sensor's Settings tab after creation.
Click the Settings tab of a sensor to change its settings.
Basic Sensor Settings
Setting |
Description |
|---|---|
Sensor Name |
Enter a 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 the sensor inherits from its parent device, parent group, and parent probe.
|
Tags |
Enter one or more tags. Confirm each tag with the Spacebar key, a comma, or the Enter key. You can use tags to group objects and use tag-filtered views later on. Tags are not case-sensitive. Tags are automatically inherited.
The sensor has the following default tags that are automatically predefined in the sensor's settings when you add the sensor: ▪snmpcustomstringsensor |
Priority |
Select a priority for the sensor. This setting determines the position of the sensor in lists. The highest priority is at the top of a list. Choose from the lowest priority ( |
Usually, a sensor connects to the IP Address/DNS Name of the parent device. See the device settings for details. For some sensors, you can explicitly define the monitoring target in the sensor settings.
OID Settings
Setting |
Description |
|---|---|
OID |
Enter the OID of the SNMP object that you want to receive a string from.
|
Maximum String Length |
Define the maximum length of the string that PRTG receives from the SNMP object at the OID. If the string is longer than this value, the sensor shows the Down status. Enter an integer or leave the field empty. |
If Value Changes |
Define what the sensor does when the sensor value changes: ▪Ignore changes (default): Do nothing. ▪Trigger 'change' notification: Send an internal message indicating that the sensor value has changed. In combination with a change trigger, you can use this mechanism to trigger a notification whenever the sensor value changes. |
Keyword Search
Setting |
Description |
|---|---|
Response Must Include (Down Status if Not Included) |
Define the search string that must be part of the data that PRTG receives from the SNMP object at the OID. You can enter a simple string in plain text or a regular expression (regex).
|
Search Method |
Define the method with which you want to provide the search string. ▪Simple string search: Search for a simple string in plain text. ▪Regular expression: Search with a regex.
|
Response Must Not Include (Down Status if Included) |
Define the search string that must not be part of the data that PRTG receives form the SNMP object at the OID. You can enter a simple string in plain text or a regex.
|
Search Method |
Define the method with which you want to provide the search string. ▪Simple string search: Search for a simple string in plain text. ▪Regular expression: Search with a regex.
|
Response Must Include (Warning Status If Not Included) |
Define the search string that must be part of the data that PRTG receives from the SNMP object at the OID. You can enter a simple string in plain text or a regex.
|
Search Method |
Define the method with which you want to provide the search string. ▪Simple string search: Search for a simple string in plain text. ▪Regular expression: Search with a regex.
|
Response Must Not Include (Warning Status If Included) |
Define the search string that must not be part of the data that PRTG receives form the SNMP object at the OID. You can enter a simple string in plain text or a regex.
|
Search Method |
Define the method with which you want to provide the search string. ▪Simple string search: Search for a simple string in plain text. ▪Regular expression: Search with a regex.
|
Extended Processing
Setting |
Description |
|---|---|
Interpret Result As |
Define the type of the received string: ▪String (default): Handle the result as a common string. ▪Bytes in hexadecimal notation: Handle the result as bytes in hexadecimal notation. For example, select this option if you monitor MAC addresses. ▪Bytes in decimal notation: Handle the result as bytes in decimal notation. For example, select this option if you monitor IP addresses. |
Numeric Value Extraction |
Define if you want to filter out a numeric value from the received string: ▪Do not use extraction: Do not extract a float value. Use the result as a string value. ▪Use a regular expression for extraction: Use a regular expression (regex) to identify a numeric value in the string and to convert it to a float value to use it, for example, with channel limits. Define below. See also the example. |
Regular Expression |
This setting is only visible if you select Use a regular expression for extraction above. Enter a regular expression to identify the numeric value that you want to extract from the string returned by the SNMP object at the specified OID. You can use capturing groups.
|
Index of Capturing Group |
This setting is only visible if you select Use a regular expression for extraction above. If your regex uses capturing groups, specify which one captures the number. Enter an integer or leave the field empty. |
Decimal Separator |
This setting is only visible if you select Use a regular expression for extraction above. Define the character for the decimal separator of the number. Enter a string or leave the field empty. |
Thousands Separator |
This setting is only visible if you select Use a regular expression for extraction above. Define the character for the thousands separator of the number. Enter a string or leave the field empty. |
Sensor Display
Setting |
Description |
|---|---|
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.
|
Graph Type |
Define how different channels are 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 generates a graph that visualizes the different components of your total traffic. |
Stack Unit |
This setting is only visible if you enable Stack channels on top of each other as Graph Type. Select a unit from the list. All channels with this unit are 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. |
By default, all of these settings are inherited from objects that are higher in the hierarchy. We recommend that you change them centrally in the root group settings if necessary. To change a setting for this object only, click 
under the corresponding setting name to disable the inheritance and to display its options.
For more information, see section Inheritance of Settings.
Scanning Interval
For more information, see section Root Group Settings, section Scanning Interval.
Schedules, Dependencies, and Maintenance Window
You cannot interrupt the inheritance for schedules, dependencies, and maintenance windows. The corresponding settings from the parent objects are always active. However, you can define additional schedules, dependencies, and maintenance windows. They are active at the same time as the parent objects' settings.
Schedules, Dependencies, and Maintenance Window
For more information, see section Root Group Settings, section Schedules, Dependencies, and Maintenance Window.
Access Rights
For more information, see section Root Group Settings, section Access Rights.
Number Extraction with Regular Expression
If you want to extract a number in the response string via a regex, note that the index for captures in this sensor is based on 1 (not on 0). Furthermore, capturing groups are not automatically created. The example below illustrates this issue.
Consider the following string as returned by a request for CPU usage:
5 Sec (3.49%), 1 Min (3.555%), 5 Min (3.90%)
Assuming you would like to filter for the number 3.555, this is the percentage in the second parentheses. Enter the following regex in the Regular Expression field:
(\d+\.\d+).*?(\d+\.\d+).*?(\d+\.\d+)
As Index of Capturing Group, enter 3. This extracts the desired number 3.555.
The index must be 3 in this case because the capturing groups here are the following:
▪Group 1 contains 3.49%), 1 Min (3.555), 5 Min (3.90
▪Group 2 contains 3.49
▪Group 3 contains 3.555
▪Group 4 contains 3.90
Keep this note about index and capturing groups in mind when using number extraction.
It is not possible to match an empty string with the PRTG regex sensor search.
PRTG supports Perl Compatible Regular Expression (PCRE) regex. For more details, see section Regular Expressions.
Which channels the sensor actually shows might depend on the target device, the available components, and the sensor setup.
Channel |
Description |
|---|---|
Downtime |
In the channel table on the Overview tab, this channel never shows any values. PRTG uses this channel in graphs and reports to show the amount of time in which the sensor was in the Down status |
Extracted Value |
The value extracted from the string (optional) |
Response Time |
The response time
|
KNOWLEDGE BASE
How do I find out which OID I need for an SNMP Custom sensor?
▪https://kb.paessler.com/en/topic/903
What security features does PRTG include?
▪https://kb.paessler.com/en/topic/61108
My SNMP sensors don’t work. What can I do?
