If the wireshark text file is simply stored as a rolling text file (i.e. more data is appended to file, and not stored in a new file). I would set the input up as a "file monitor". I would like to know how do set the input file as a "file monitor"?
I think you may need to sit down and read through alot of the documentation to become completely familiar with the product. Have you seen the videos on the Splunk site? they sometimes hold free webinars of basic training.
In the meantime, I would suggest that running TCPDUMP as a scripted input would be a better solution.
