Tcpdump

bash-2.05b$ man tcpdump | head -27



TCPDUMP(8)                                             TCPDUMP(8)


NAME
       tcpdump - dump traffic on a network

SYNOPSIS
       tcpdump [ -aAdeflLnNOpqRStuvxX ]
               [ -D datalinktype ] [ -c count ]
               [ -C file_size ] [ -F file ]
               [ -i interface ] [ -m module ] [ -r file ]
               [ -s snaplen ] [ -T type ] [ -w file ]
               [ -E algo:secret ] [ expression ]

DESCRIPTION
       Tcpdump  prints  out  the  headers of packets on a network
       interface that match the boolean expression.  It can  also
       be  run  with  the  -w  flag,  which causes it to save the
       packet data to a file for later analysis, and/or with  the
       -b  flag, which causes it to read from a saved packet file
       rather than to read packets from a network interface.   In
       all cases, only packets that match expression will be pro-
       cessed by tcpdump.

bash-2.05b$  tcpdump -?
tcpdump version 3.7.1
libpcap version 0.4
Usage: tcpdump [-aAdeflLnNOpqRStuvxX] [ -D datalinktype ] [ -c count ] [ -C file_size ]
                [ -F file ] [ -i interface ] [ -r file ] [ -s snaplen ]
                [ -T type ] [ -w file ] [ -E algo:secret ] [ expression ]
bash-2.05b$