Reference: man tcpdump
tcpdump - dump traffic on a network
tcpdump [options] [expression]
-i Listen on interface
-w Write the raw packets to file rather than parsing and printing them out.
-r Read packets from file.
-s Snarf snaplen bytes of data from each packet rather than the default of 65535 bytes. Setting snaplen to 0 sets it to the default of 65535.
-S Print absolute, rather than relative, TCP sequence numbers.
-nn Don't convert host addresses to names. This can be used to avoid DNS lookups.
The filter expression consists of one or more primitives. Primitives usually consist of an id (name or number) preceded by one or more qualifiers. There are three different kinds of qualifier:
Possible types are host, net , port and portrange.
Possible directions are src, dst, src or dst, src and dst, ra, ta, addr1, addr2, addr3, and addr4.
Possible protos are: ether, fddi, tr, wlan, ip, ip6, arp, rarp, decnet, tcp and udp.
'udp port 53' 'dst 126.96.36.199 (port 21 or 20'