Finding out the offset of a filtered field in tethereal

Hi,
 I am using tethereal to capture a packet with a
specific field (eg. Ip address of 1.1.1.1 ) and then
modify the captured packet to send it out. I want this
to be an automated script that i can use for various
types of scnerios.

I can easily capture the packet and filter it based on
my field ( eg. Ip Address). But to modify the packet,
I need to know the offset in the packet, where the
field starts ( eg. src Ip address starts at byte 26 in
a typical case).

I want to avoid hardcoding the offset, since it can
easily lead to wrong results  (eg. if Vlan tags are
enabled then everything shifts by 4 ).

Would someone know of a way to obtain the offset of a
certain field (eg. rsvp.msg == 1 returns the right
packet, but how do i know which byte in the packet is
rsvp.msg comparing against ? )

thanks,
Rohit

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