Hello,
I have just switched from using snoop to tcpdump. I am
getting my hands around creating filters for tcpdump.
I am looking a filter such as this:


tcp and (tcp[13] & 3 != 0) 


Now I know that the 13 is they byte in the tcp header
at which either the syn,fin,ack,urg,push or restet
bits is set. However the "& 3 != 0" seem to make no
sense to me. could someone break this down for me? I
would really appreciate it as this seem to be the key
for creating filters based on flags. TIA

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