You must use 
  tethereal instead of ethereal. There's an autostop condition –a and you can 
  fix a limit in time or size. For example, to capture just 1kB in "file.pcap" 
  from the default interface use :
  tethereal -a 
  filesize:1 -w file.pcap
   
  see http://www.ethereal.com/tethereal.1.html 
  for more info
   
   
  -----Original 
  Message-----
From: Venkatesh, 
  Susheel [mailto:susheel@xxxxxx] 
Sent: Wednesday, June 25, 
  2003 3:19 
  PM
To: 'ethereal-dev@xxxxxxxxxxxx'; 
  'ethereal-users@xxxxxxxxxxxx'
Subject: [Ethereal-dev] 
  Help!!
   
  Hi, 
  I am trying to develop a Automated 
  test-suite which would require controlling Ethereal. I was just wondering if I 
  can get custom-made executable that would allow me to control Ethereal (start, 
  stop and save ethereal capture in text format from an external application) or 
  if someone could suggest how to add this functionality into 
  Ethereal.
  The sequence of steps during a 
  typical test is as follows. These steps are to be executed using an external 
  application (the automation test tool):-
  1) Launch ethereal and start (this 
  can be done from the command-line and there is no problem with this 
  step) 
2) Run a test-script 
  
3) Stop capture and save the 
  capture in text-format (there is a problem here; there is no way I can control 
  ethereal from an external application and there is no command-line option to 
  stop and save capture in text-format)
  I am using Windows 2K. What all do 
  I require to take the ethereal source code and compile it on my 
  PC? 
  I would appreciate some help on 
  this. 
  Thanks, 
Susheel