Build a trigger to monitor responses to NTP monlist requests

Machines in your environment synchronize clocks through the Network Time Protocol (NTP), but NTP has some security vulnerabilities such as amplification attacks that lead to denial of service.

For example, an attacker can spoof the IP address of your NTP server, and then repeatedly send a monlist command through the spoofed address. The monlist command requests a list of the last 600 hosts that connected to the NTP server, but because the requesting IP address is spoofed, the server actually sends the list to the spoofed address. The response is considerably larger than the request, and the spoofed client becomes overloaded, which can lead to denial of legitimate requests.

In this walkthrough, you will write a trigger that checks UDP traffic on your NTP server for responses to monlist commands. The trigger also sends an alert-level message to a remote syslog server when a monlist response occurs.


Configure an open data stream to a syslog target

In the following steps, you will configure the host, port, and protocol for the open data stream target.

  1. Log in to the ExtraHop system that you want to send data from with an account that has unlimited privileges.
  2. Click the System Settings icon , and then click All Administration.
  3. In the System Configuration section, click Open Data Streams.
  4. Click Add Target.
  5. Select Syslog from the Target Type drop-down list.
  6. In the Name field, type NTP Syslog unless this is the first syslog target you have created. In that case, the target is automatically named "default" and cannot be renamed.
  7. In the Host field, type the IP address or hostname of the syslog server you want to send data to.
  8. In the Port field, type the port number you want to send data to.
  9. From the protocol list, select UDP.
    Tip:Click Test to establish a connection and send a test message from the ExtraHop system to the remote syslog server.
  10. Select Local if you want to send syslog information with timestamps in the local timezone of the ExtraHop system. Otherwise, timestamps are sent in GMT.
  11. Click Save.
    The target is added to the Syslog table on the Open Data Stream page.

Write a trigger to parse NTP payloads

In the following steps, you will write a trigger that specifies what data to examine from NTP server responses and whether to send the data to a remote syslog server.

  1. Click the ExtraHop logo in the upper left corner to return to the ExtraHop system.
  2. Click the System Settings icon , and then click Triggers.
  3. Click Create.
  4. In the Name field, type Parse UDP payload for NTP responses.
  5. Click Enable debug log to enable the debug log and trigger performance metrics.
  6. In the Events field, select UDP_PAYLOAD.
  7. Click Show advanced options and specify the following payload settings to look for NTP traffic only on UDP port 123:
    1. Select Run trigger on all UDP packets.
    2. In the Server Port Range Min field, type 123.
    3. In the Server Port Range Max field, type 123.
  8. In the right pane, add the following trigger code to enable access to the NTP server response payload:
    //Capture the NTP server response
    let buf = Flow.server.payload;
    //Exit the trigger if the NTP server response cannot be captured
    if (buf === null) {
  9. Add the following trigger code to the existing script to specify which fields the trigger will extract from the header of the payload and which fields to ignore:
    //Define the format of the NTP response
    let fmt = ('B' + // Flags (LI, Version, Mode)
               'x' + // Auth + Seq (ignore)
               'x' + // Implementations (ignore)
               'B' + // Request code
               'B'); // Error
  10. Add the following trigger code to the existing script to extract the fields from the payload:
    //Analyze the NTP response based on the defined format
    let values = buf.unpack(fmt);
    let mode = values[0] & 0x7;
  11. Add the following trigger code to the existing script to check the values of the following header fields:
    // Exit the trigger if the mode value is not 7.
    if (mode !== 7) {
    let reqCode = values[1];
    //Save the last four bits of the error code as a variable
    let errorCode = values[2] >> 4;

    The mode, located in the last three bits of the field, indicates the NTP mode of operation. A value of 7 specifies that the NTP server is responding to a private mode command, which includes the monlist command.

    The request code field indicates the request type. A value of 20 or 42 specifies a monlist request.

    The error code field, located in the last four bits, indicates the error type. A value of 0 specifies that the response is not an error.

  12. Add the following trigger code to the existing script to send an alert-level message to the remote syslog server if the NTP server is responding to a monlist command and if the response is not an error.
    //Check that there is no error and that the monlist command has been run
    if ((errorCode === 0) && ((reqCode === 20) || (reqCode === 42))) {
        //If the monlist command has been run, send an alert level message with
        //the NTP server IP address to the Syslog server 
        Remote.Syslog('NTP Syslog').alert('monlist enabled on ' + Flow.server.ipaddr);

    The trigger sends messages that include the NTP server's IP address to the remote syslog server that you configured earlier. If the target you configured was automatically named, replace 'NTP Syslog' with 'default' in the code.

  13. Add the following trigger code to the existing script to check whether debugging is enabled and send the specified output to the debug log.
    //Print the IP address, request code, and error code in the debug log
    debug('NTP Server ' + Flow.server.ipaddr +
          ' responded to mode 7 command ' + reqCode +
          ' with error code '  + errorCode + '.');
  14. Click Save.

Assign the UPA trigger to a device

Before the trigger can examine UDP response payloads, you must assign the trigger to at least one device. For this walkthrough, you will assign the trigger to NTP servers on your network.

Important:Assign triggers only to the specific devices that you need to collect metrics from to minimize the performance impact of your triggers on the ExtraHop system.
  1. Click Assets from the top menu.
  2. From the left pane, click Devices.
  3. In the Name column, locate at least one NTP server and select the checkbox.
  4. Click Assign Trigger at the top of the page.
  5. Click the checkbox next to the Parse UDP payload for NTP responses trigger, and then click Assign Triggers.
After the trigger is assigned, it runs continuously until disabled.

Check your syslog server and the debug log for trigger results

When a response to a monlist command is sent by the NTP server, the trigger sends an alert-level message to your remote syslog server. The message contains the IP address of the NTP server that sent the response, similar to the following message:

1 2017-01-11T22:14:15.003Z monlist enabled on

In addition, the trigger sends output to the debug log if debugging is enabled. To view the results of the debug statement, return to the Edit Trigger pane, click Edit Trigger Script, and click Debug Log. The output includes the IP address of the NTP server, the monlist request code, and the error code, similar to the following output:

NTP Server responded to mode 7 command 42 with error code 0.

If the trigger results indicate that your NTP server has responded to a monlist command, you can take one of the following actions:

  • Upgrade your NTP server to version 4.2.7 or later, which disallows monlist commands by default. Downloads are available from the NTP Software Downloads page at
  • Modify the ntp.conf file on the NTP server to disable the monitoring function that allows monlist commands. Instructions are available on the Access Restrictions page at
  • If your security and monitoring workflow requires that your NTP server responds to monlist commands, you can leverage this trigger to tighten controls around NTP responses. For example, you can create custom metrics based on information extracted with the trigger. With those custom metrics, you can create a dashboard to track NTP server activity or configure an alert that notifies you of responses to monlist commands.

If your NTP server is already configured to disallow monlist commands, you will not receive any syslog messages or see output in the debug log. You can still check that the trigger is running through one of the following actions:

  • Return to the Edit Trigger pane and view the Capture Trigger Load chart. The graph shows activity as long as there is UDP traffic on the NTP server.
  • View the Trigger Executes and Drops chart on the System Health dashboard. The chart shows activity that indicates the trigger is running.
  • Test for monlist commands from the client-side. Modify the trigger by setting the buf variable to Flow.client.payload, and then send a monlist command through a program such as ntpdc to the NTP server. This code change in conjunction with the monlist command extracts the request payload and the trigger sends a message to syslog and shows results in the output log.

By running this trigger, you learn whether your NTP servers are vulnerable to amplification attacks and what you can do to either monitor for attacks or disable the NTP commands that open the door to attacks.

Last modified 2023-11-07