Sending logs to Microsoft Sentinel with NXLog

The pivotal part of sending secure HTTPS requests to Azure is the authentication process. Azure validates the values of two custom HTTP headers, Authorization and x-ms-date along with the length of the data payload to determine if the request is authentic. The value assigned to the Authorization header is dynamically generated using a cryptographic hash. For details, see the Azure Monitor Authorization section in the Microsoft documentation.

NXLog can natively collect ETW logs without the need to capture the trace into an .etl file. Configuring an NXLog agent to capture Windows DNS Server events using the Event Tracing for Windows (im_etw) input module is fairly straightforward as illustrated here:

The Exec statement of the DNS_Logs input instance above invokes the to_json() procedure, which converts the Windows events to JSON records, as required by Azure’s HTTP Data Collector API.

Additionally, the Table variable is defined which allows for multiple inputs to be defined with their own table name while pointing towards a single output instance.

The output module is the part that connects directly to Azure. The first step in configuring the output instance is retrieving the Workspace ID and either the Primary key or the Secondary key (also referred to as the shared key). These keys can be found by navigating in the Azure portal to Log Analytics workspace > Settings > Agents management. The same set of keys can be viewed under either the Windows servers or Linux servers tab.

The next step is to add this information to the nxlog.conf file as constants making them accessible to the output instance.

The SUBDOMAIN, RESOURCE, and APIVER are used to construct the complete URL. The value for SIZELIMIT can be tuned to your needs. It represents the maximum size in bytes of the data payload for each batch of events. 65000 is the upper limit. The higher values mean better network efficiency. Lower values mean events can be received faster because they are not waiting for a large buffer to be full before they can be sent.

When looking at the entire output instance that uses the Microsoft Sentinel (om_azure) module, you can see how batches of events are buffered and then flushed:

Since all REST API events are categorized by Azure Monitor as Custom Logs, Azure appends _CL to the value of TableName in order to prevent naming conflicts with other Azure tables thus the name we originally chose, DNS_Logs, appears in Microsoft Sentinel as DNS_Logs_CL.

If analytical event logging is enabled, you can capture and view DNS Sever analytical events having EventIDs ranging from 256 to 286. Technically, no further changes are needed for logging and viewing both audit and analytical events in Azure Sentinel. However, there is one enhancement you might want to implement:

Enrich the schema with a new attribute: DNS_LogType. If you need to frequently differentiate between audit and analytical DNS Server events, querying for a range of values on a regular basis is not only tedious and makes queries less readable, but it can also be slower on large data sets. This is as simple as replacing the original Exec to_json(); with an Exec block that sets the new $DNS_LogType field to either Audit or Analytical depending on the value of EventID before calling the to_json() which will then enrich the schema with this new field.

Download/view the entire nxlog.conf configuration file.

Let’s take a look at the configuration file to see how the input module is configured and how the rules are defined.

Instead of defining a small set of audit rules within a Rules block diectly in the LinuxAudit input instance, we use the LoadRule directive to load a more comprehensive collection of rules in an audit rule file which is based on the ruleset maintained by the Best Practice Auditd Configuration project.

The xm_resolver module is needed for the ResolveValues directive in the audit input instance, where it is used for resolving some of the numeric values to more human-readable string values.

As with the Windows example, ensure that the required values are properly configured and proceed to name the Table accordingly, in our case we are using LinuxAudit as the name.

Download/view the nxlog.conf configuration file.

Once these changes have been implemented and the NXLog service has been restarted, events should be sent to the LinuxAudit_CL Microsoft Sentinel table. The following JSON event was triggered and captured according to the very last line in the im_linuxaudit.rules file.

Upon successful receipt in the Log Analytics workspace by Azure Monitor, events are further processed and finally ingested by Microsoft Sentinel where they can be viewed via user-defined queries.

After expanding the following event to reveal its columns and their values, it can be verified against the JSON formatted event above that was sent via the REST API.