I had this question a while back as well but did not see a way to do it so I ended up using this program (which has a few log file options):

http://www.gropp.org/?id=projects&sub=bwm-ng

2012/1/22 Robert WANG chunyu_5@msn.com

Hi, I am a server support engineer. Now I want to monitor the whole day or whole week network traffic throught the network port of my Sun Solaris server. I tried to use the iftop and I found it can show the network traffic in real time , it is cool. But how can I re-direct the iftop output to a log file? So that I can analysis some network performace issue. I have checked man page , did not find any clue. Hope for your reply. Thank you very much! Best regards, Robert WANG