tclpcap is a Tcl extension that provides access to the Pcap packet
capture library. This extension allows a programmer to capture and
process packets from a Tcl script. Packets can be either captured from
a network interface or read from a capture file from tcpdump. This
extension allows dump files to be created so interesting tools to
process capture files can be written purely with Tcl.

Note that this extension provides no decoding capabilities. It simply
provides the raw data from the network or capture file.
