PRTG Manual: DHCP Sensor

Network Interfaces

Select the network interfaces that you want to monitor. PRTG creates one sensor for each network interface that you select.

Choose the network card on the probe system that is used to send the broadcast message.

Enable check boxes in front of the respective lines to select the items. Use the check box in the table header to select all items or to cancel the selection. In large tables, use the search function in the upper-right corner.

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.

If the name contains angle brackets (<>), PRTG replaces them with braces ({}) for security reasons. For more information, see the Knowledge Base: What security features does PRTG include?

Parent Tags

Shows tags that the sensor inherits from its parent device, parent group, and parent probe.

This setting is for your information only. You cannot change it.

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.

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

For performance reasons, it can take some minutes until you can filter for new tags that you added.

The sensor has the following default tags that are automatically predefined in the sensor's settings when you add the sensor:

• dhcpsensor

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 () to the highest priority ().

MAC Address

Shows the MAC address of the network adapter that sends the broadcast message to the network.

PRTG shows this value for reference purposes only. If you need to change this value, add the sensor anew.

Client IP Address

Specify if you want to check the returned client IP address with a regular expression (regex):

• Do not check the IP address using a regular expression: The IP address only appears in the sensor message without further processing.
• Check the IP address using a regular expression: Enter the regex that you want to use below.

Client IP Address Must Include (Down Status if Not Included)

This setting is only visible if you enable Check the IP address using a regular expression above. In the response of the DHCP server, search by using a regex. If the response for the client IP address does not contain this string, the sensor shows the Down status.

For example, enter 10\.0\.5\..* to make sure that any answering DHCP server returns any client IP address starting with 10.0.5.. If it does not, the sensor shows the Down status. Leave this field empty if you do not want to use it.

PRTG supports Perl Compatible Regular Expression (PCRE) regex. For more details, see section Regular Expressions.

Client IP Address Must Not Include (Down Status if Included)

This setting is only visible if you enable Check the IP address using a regular expression above. In the response of the DHCP server, search by using a regex. If the response for the client IP address contains this string, the sensor shows the Down status. See the example above. Leave this field empty if you do not want to use it.

PRTG supports Perl Compatible Regular Expression (PCRE) regex. For more details, see section Regular Expressions.

Server IP Address

Specify if you want to check the returned server IP address with a regex:

• Do not check the IP address using a regular expression: The IP address only appears in the sensor message without further processing.
• Check the IP address using a regular expression: Enter the regex that you want to use below.

Server IP Address Must Include (Down Status if Not Included)

This setting is only visible if you enable Check the IP address using a regular expression above. In the response of the DHCP server, search by using a regex. If the response for the server IP address does not contain this string, the sensor shows the Down status. See the example above. Leave this field empty if you do not want to use it.

PRTG supports Perl Compatible Regular Expression (PCRE) regex. For more details, see section Regular Expressions.

Server IP Address Must Not Include (Down Status if Included)

This setting is only visible if you enable Check the IP address using a regular expression above. In the response of the DHCP server, search by using a regex. If the response for the server IP address contains this string, the sensor shows the Down status. See the example above. Leave this field empty if you do not want to use it.

PRTG supports Perl Compatible Regular Expression (PCRE) regex. For more details, see section Regular Expressions.

Timeout (Sec.)

Enter a timeout in seconds for the request. Enter an integer value. The maximum timeout value is 900 seconds (15 minutes).

If the reply takes longer than this value, the sensor cancels the request and shows a corresponding error message.

DHCP Server Change

If there is more than one DHCP server in the network that can respond to the broadcast message, the sensor can receive a response from a different DHCP server compared to the last scan of the sensor. In this case, PRTG can write an entry to the system logs. Specify how PRTG handles DHCP server changes:

• Ignore: Do not write a log entry if the DHCP server changes.
• Write log entry: Write an entry to the system logs whenever the DHCP server changes between two sensor scans.

Regardless of this setting, entries are always added to the sensor log.

Offered IP Address Change

If the IP address offered by the DHCP server changes between two sensor scans, PRTG can write an entry to the system logs. Specify how PRTG handles IP address changes:

• Ignore: Do not write a log entry if the offered IP address changes.

• Write log entry: Write an entry to the system logs whenever the DHCP server offers a different IP address compared to the last sensor scan.

Regardless of this setting, entries are always added to the sensor log.

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.

You can set a different primary channel later by clicking below a channel gauge on the sensor's Overview tab.

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.
  You cannot use this option in combination with manual Vertical Axis Scaling (available in the channel settings).

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.

Scanning Interval

Select a scanning interval from the dropdown list. The scanning interval determines the amount of time that the sensor waits between two scans. Choose from:

• 30 seconds
• 60 seconds
• 5 minutes
• 10 minutes
• 15 minutes
• 30 minutes
• 1 hour
• 4 hours
• 6 hours
• 12 hours
• 24 hours

You can change the available intervals in the system administration of PRTG Network Monitor.

If a Sensor Query Fails

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

• Set sensor to down immediately: Set the sensor to the Down status immediately after the first request fails.
• Set sensor to warning for 1 interval, then set to down (recommended): Set the sensor to the Warning status after the first request fails. If the second request also fails, the sensor shows the Down status.
• Set sensor to warning for 2 intervals, then set to down: Set the sensor to the Down status only after the third request fails.
• Set sensor to warning for 3 intervals, then set to down: Set the sensor to the Down status only after the fourth request fails.
• Set sensor to warning for 4 intervals, then set to down: Set the sensor to the Down status only after the fifth request fails.
• Set sensor to warning for 5 intervals, then set to down: Set the sensor to the Down status only after the sixth request fails.

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

If you define error limits for a sensor's channels, the sensor immediately shows the Down status. None of the interval options apply.

If a channel uses lookup values, the sensor immediately shows the Down status. None of the interval options apply.

Schedule

Select a schedule from the list. You can use schedules to monitor during a certain time span (days or hours) every week. Choose from:

• None
• Saturdays
• Sundays
• Weekdays
• Weekdays Eight-To-Eight (08:00 - 20:00)
• Weekdays Nights (17:00 - 09:00)
• Weekdays Nights (20:00 - 08:00)
• Weekdays Nine-To-Five (09:00 - 17:00)
• Weekends

You can create schedules, edit schedules, or pause monitoring for a specific time span. For more information, see section Schedules.

Maintenance Window

Select if you want to set up a one-time maintenance window. During a maintenance window, monitoring stops for the selected object and all child objects. They show the Paused status instead. Choose between:

• Do not set up a one-time maintenance window: Do not set up a one-time maintenance window. Monitoring is always active.
• Set up a one-time maintenance window: Set up a one-time maintenance window and pause monitoring. You can define a time span for the pause below.

To terminate an active maintenance window before the defined end date, change the time entry in Maintenance Ends to a date in the past.

Maintenance Begins

This setting 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 one-time maintenance window.

Maintenance Ends

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

Dependency Type

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

• Use parent: Use the dependency type of the parent object.
• Select a sensor: Use the dependency type of the parent object. Additionally, pause the current object if a specific sensor is in the Down status or in the Paused status because of another dependency.

• Master sensor for parent: Make this sensor the master object for its parent device. The sensor influences the behavior of its parent device: If the sensor is in the Down status, the device is 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 is paused if the parent group is paused by another dependency.

To test your dependencies, select Simulate Error Status from the context menu of an object that other objects depend on. A few seconds later, all dependent objects are paused. You can check all dependencies under Devices | Dependencies in the main menu bar.

Dependency

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

Dependency Delay (Sec.)

This setting is only visible if you select Select a sensor above. Define a time span in seconds for the dependency delay.

After the master sensor for this dependency returns to the Up status, PRTG additionally delays the monitoring of the dependent objects by the time span you define. This can prevent false alarms, for example, after a server restart or to give systems more time for all services to start. Enter an integer value.

This setting is not available if you set this sensor to Use parent or to be the Master sensor for parent. In this case, define delays in the parent device settings or in its parent group settings.

User Group Access

Define the user groups that have access to the sensor. You see a table with user groups and group access rights. The table contains all user groups in your setup. For each user group, you can choose from the following group access rights:

• Inherited: Inherit the access rights settings of the parent object.
• No access: Users in this user group cannot see or edit the sensor. The sensor neither shows up in lists nor in the device tree.
• Read access: Users in this group can see the sensor and view its monitoring results. They cannot edit any settings.
• Write access: Users in this group can see the sensor, view its monitoring results, and edit its settings. They cannot edit its access rights settings.
• Full access: Users in this group can see the sensor, view its monitoring results, edit its settings, and edit its access rights settings.

For more details on access rights, see section Access Rights Management.

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 in percent.

Lease Time

The lease time reported by the server

Response Time

The response time in msec

This channel is the primary channel by default.