Track server errors with custom metrics and alerts

While the ExtraHop system offers over 5,000 built-in metrics, there are many situations in which it is more effective to track network issues with a custom metric. For example, while built-in metrics show you issues with HTTP responses and requests, a custom metric can identify 500-level server errors. These types of errors can indicate gateway issues, an overloaded server, or configuration issues.

In this walkthrough, you will learn how to write a trigger to collect custom metrics for server errors and how to create an alert that only sends an email notification when those specific errors occur. Then, you will be able to answer the following types of questions about server errors on your network:

  • Are my customers receiving 500-level server errors?
  • Which error codes occurred?
  • When did the errors occur?
  • What URI was the customer attempting to access?
  • What is the IP address of the client and server affected in the transaction?


  • You must have a user account with system and access administration privileges.
  • Your ExtraHop system must have network data with web server traffic.
  • Your ExtraHop system must be configured to send email notifications before you can send alert emails.
  • Familiarize yourself with the concepts in this walkthrough by reading Triggers and Alerts.
  • Familiarize yourself with the processes of creating triggers by reading Build a trigger.
  • It is helpful to have basic JavaScript knowledge.

Write a trigger to collect error data

First, let's create a trigger that monitors certain URIs for 500-level server errors. When errors occur, the trigger collects data such as error codes and server and client IP addresses and commits that data as custom metrics to an application.

  1. Log in to the ExtraHop system through https://<extrahop-hostname-or-IP-address>.
  2. Click the System Settings icon, and then click Triggers.
  3. Click Create.
  4. In the Name field, type a name for the trigger. For this walkthrough, type 500-level Server Errors.
  5. In the Description field, type information about the trigger. For this walkthrough, type Monitor specified URIs for 500-level server errors; application collects custom metrics upon errors.
  6. Click Enable debug log.
  7. Click in the Events field and select HTTP_RESPONSE from the list.
    The following figure displays the trigger settings we configured above:

  8. Click the Editor tab.
  9. In the Trigger Script editor, copy and paste the following code:
    // Edit this array with hosts and URIs to monitor
    var CHECK_URI_LIST = [
    // Edit this array with client IP addresses to ignore
    var IGNORE_CLIENT_IP = [
    // If client IP is in array above, end process
    if (IGNORE_CLIENT_IP.indexOf(Flow.client.ipaddr.toString()) > -1){
    //debug ('Ignoring client IP: ' + Flow.client.ipaddr);
    // If URI is empty, end process
    var uri = HTTP.uri ||;
    if (uri === null){//debug ('No URI Found, ending');
    // If URI/hostname does not match array above, end process
    var matches = CHECK_URI_LIST.filter(condition => uri.indexOf(condition) > -1);
    if (matches.length === 0) {
    //debug ('URI or host did not match: ' + uri);
    var app = "HTTP Server Errors",
    code = HTTP.statusCode,
    client = Flow.client.ipaddr.toString(),
    server = Flow.server.ipaddr.toString(),
    detail = 'Code: ' + code + ' || Server: ' + server + ' || Client: ' + client + ' || URI: '
              + uri;
    if (code < 500 || code > 600) {return}
    var code = HTTP.statusCode.toString();
    // If URI matches and is a 5xx error, commit custom metrics
    // to the application, which is created upon the initial event
    Application(app).metricAddCount('HTTP_error', 1);
    Application(app).metricAddDetailCount('HTTP_error_uri', uri, 1);
    Application(app).metricAddDetailCount('HTTP_error_serverIP', server, 1);
    Application(app).metricAddDetailCount('HTTP_error_clientIP', client, 1);
    Application(app).metricAddDetailCount('HTTP_error_allDetail', detail, 1);
    debug ('Detail: ' + detail);
  10. In the CHECK_URI_LIST variable array, replace with the hosts, URIs, and host-URI combinations that you want the trigger to monitor.
  11. In the IGNORE_CLIENT_IP variable array, replace the example IP addresses ( and with the IP addresses that you want the trigger to ignore.
    Delete or comment out this array if there are no IP addresses to ignore.
  12. Click Save and Close.
    The trigger editor validates the syntax of your script. Invalid actions, syntax errors, or deprecated elements will prevent you from saving the trigger until you fix the code or disable syntax validation.
After the trigger is assigned and running, it creates the following components that we will reference when configuring alert settings in the next sections:
HTTP Server Errors
The trigger commits data collected from the custom metrics to this application.
A custom count metric that collects the number of 500-level errors that occur.
A custom detail metric that collects the code number, URI, server IP and client IP on which each error occurred.

Assign the trigger to a device

Before the trigger can run, it must be assigned to at least one device. In this step, we will assign the trigger to one or more HTTP servers that support traffic over the URIs you specified in the trigger.

  1. Click Assetsfrom the top menu.
  2. From Devices by Protocol Activity, click HTTP Servers, and then click Devices from the left pane.
  3. From the device list, select the checkbox next to one or more devices that support traffic over the URIs.
  4. At the top of the page, click Assign Trigger to open a list of triggers.
  5. Select the trigger named 500-level Server Errors that we created in the previous section, and then click Assign Triggers.

Next steps

Tip:Assign triggers only to relevant devices to avoid unnecessary performance impact on the system. A good way to ensure that a trigger runs only on relevant devices is to create a device group and assign the trigger to that group.

Configure an alert to track a custom metric

Next, let's configure alert settings that will issue an alert and send an email notification each time a 500-level error occurs on the URIs watched by the trigger.

In the alert settings, we will reference the following custom metrics that we created in the trigger script:
The custom count metric that collects the number of 500-level errors that occur. We will configure alert settings to track this metric and issue an alert each time an error occurs.
The custom detail metric that collects the code number, URI, server IP address, and client IP address on which each error occurred. We will configure alert settings to display these error details in alert emails.

Before you begin

Your ExtraHop system must be configured for email notifications.
  1. Click the System Settings icon and then click Alerts.
  2. Click Create and then type a name for the alert in the Name field. For this walkthrough, type 500-level Server Errors.
  3. Type a description of the alert in the Description field. For this walkthrough, type Alert issued when a 500-level server error occurs on monitored URIs.
  4. In the Alert Type section, select Threshold Alert to issue an alert when the tracked metric event occurs.
  5. In the Monitored Metric field, type HTTP_error and then select Custom - HTTP_error from the search results.
  6. In the Alert Behavior section, select Alert once when alert condition is met to generate an alert for each occurrence of the tracked metric.
  7. In the Alert Condition section, specify the following condition to generate an alert if the tracked metric event occurs more than once in a 30-second time period: Alert when value > 1 during a 30s rollup
  8. In the Notifications section, type an email address that should receive alert notifications.
    Tip:The email notification groups drop-down list displays all email groups configured in the Administration settings. You can select one or more groups that should receive alert notifications.
  9. Click Show Advanced Options and then click Add Metric.
  10. In the search field, type HTTP_error_allDetail, and then select Custom - HTTP_error_allDetail from the search results.
  11. Click Save.

Assign the alert configuration to a source

Similar to triggers, the system does not generate alerts until the alert configuration is assigned to at least one metric source. In this step, we will assign the alert configuration to the application named HTTP Server Errors that we created with the trigger script. The custom metrics we want the alert to track are committed to this application.

  1. Click Assets from the top menu.
  2. Click Applications, and then select the HTTP Server Errors checkbox.
  3. Click Assign Alert from the top of the page to open a list of alert configurations that are eligible for assignment.
  4. Select the 500-level Server Errors alert, and then click Assign Alerts.

Check Alerts page and view email notifications

Now that we've configured the alert and assigned it to a source, we can check if the alert has issued any entries.

Click Alerts from the top menu to view the Alerts page and check for any alerts that were issued during the selected time interval, similar to the following figure:

When an alert is issued, a notification is sent to the specified email recipients The following email example shows that two HTTP error events occurred that met the conditions we set in the alert expression and provides additional information that helps us investigate the source of the errors:

In our example, we see that there were two 503 errors returned for the same URI across two different server IP addresses. A 503 status code might indicate an overloaded server that requires more CPU or memory resources to handle requests. By knowing the affected IP addresses you can immediately investigate potential problems on the listed servers.

Next steps

Last modified 2024-04-04