Wireshark · Ethereal-users: Re: [Ethereal-users] New to capturing, ? about http authorizations

[Guy Harris <gharris@xxxxxxxxx>]

Jonathan Sanders wrote:
> I do a
>
> tcpdump -nt -X -s1500 'filter expression here'
>
> for whenever I need to get the actual packet data from tcpdump....

Presumably by "actual packet data" you mean "full packet dissection" - 
if you mean "raw packet data", in a form Ethereal can use, you'd also 
use "-w {filename}" for a file that can be read by programs that can 
read libpcap files (tcpdump/WinDump, Ethereal/Tethereal, and a number of 
other programs that do various sorts of network analysis).

Note also that if you want to capture a full Ethernet packet, the 
argument to "-s" needs to be 1514 or greater or, in newer versions of 
tcpdump, 0 (which, in newer versions, means "65535", which is the 
largest snapshot length that some systems support).  The argument to 
"-s" is the largest packet length *including the link-layer header*, not 
the largest *payload* length - i.e., if you want all packets to be 
captured in full, it should *not* be the MTU for the network.

(Note also that the "link-layer header" might include various bits of 
"metadata", such as VPI/VCI and possibly packet type information for ATM 
and radio information such as signal strength for some 802.11 link-layer 
header types.)