I would like to develop some commercial tool in the area of processing data extracted out of a packet sniffer. I am aware that libpcap is available for that but I am quite sure that there are many restrictions due to licensing issues.
Suppose I don't link my application directly to libpcap libraries, but instead I use the output of tcpdump forwarded to a socket or to a pipe as a feed for my tool..do you think that I will be in trouble from a legal/licensing point of view?
Your best bet is to look at the license directly, which is a 3-clause BSD license. There is no restriction on the usage in commercial applications.
Reliable legal advice is most likely available from a lawyer of your choice.