Determine what a Splunk forwarder is forwarding

I recently came across a need to determine exactly what is logging to a forwarder in Splunk. I had a hard time finding out what to search for so I thought I’d share what I found.

The key to discovering where data is coming in from is in Splunk’s metrics.log files. Searching these files gives us what we need. This search reveals anything coming in over UDP (you can also change it to be TCP if desired) and totals it by host (forwarder.)

source=*metrics.log group=udpin_connections | stats count by host

Oddly enough Splunk doesn’t have a field extracted for its own metrics.log. A key useful field is missing – sourceHost. I used the field extraction tool to create it and it generated this field extraction:

^[^,\n]*,\s+(?P<sourceHost>\d+\.\d+\.\d+\.\d+)

Field extraction in hand, I was able to generate the report I was looking for: devices actively sending logs to my forwarder over UDP.

source=*metrics.log group=udpin_connections  host=splunk | stats count by sourceHost sourcePort

where host is the forwarder you want to investigate. Useful.

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