["Viral Mehta" <viral.mehta@xxxxxxxxxxxxxx>]

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Hi
Thanks for your valuable input..
Now I m stuck up with "what is Link Layer Type". I googled it and find some
number assigned to particular type and etc etc.. But what is it? Is it the
protocol I am using at DataLinkLayer?

Secondly, Why do we have TimezoneOffset and TimeStampAccuracy fields if they
are not used at all?

What is a per-packet header? Will we have it when we don't write information
to a file? As far as I know they give information about time after 1 Jan
1970 and such things.. But if I don't do tcpdump -w sample.cap and instead
give a simple command tcpdump sample.cap... Will I have all these
information? That is will I have per-packet header? (That is info about
time, packet length, and packet's actual length)



Thanks,
Viral Mehta
Embedded Engineer,
eInfochips Ltd.

-----Original Message-----
From: ethereal-dev-bounces@xxxxxxxxxxxx
[mailto:ethereal-dev-bounces@xxxxxxxxxxxx] On Behalf Of Guy Harris
Sent: Thursday, August 17, 2006 11:13 PM
To: Ethereal development
Subject: Re: [Ethereal-dev] tcpdump endianness

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http://www.wireshark.org and subscribe to wireshark-dev@xxxxxxxxxxxxx.
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viral.mehta@xxxxxxxxxxxxxx wrote:

> I am analyzing tcpdump file.
> I want to know how endianness can affect when file is being written.
> Or rather, who affects? Application which is writing? or Network byte
> order? or what?

The endianness of a libpcap-format file (the format used by tcpdump, 
Wireshark, etc.) is affected by the byte order of the machine writing it.

> I read somewhere that magic number of a tcpdump file can be either
> 0xa1b2c3d4 or 0xd4c3b2a1. How can it be?

It can be if the code that writes the magic number writes it out from a 
4-byte integral value of 0xa1b2c3d4 *without* putting it into network 
byte order first.  A machine with the same byte order as the machine 
that wrote the file will see a magic number of 0xa1b2c3d4; a machine 
with the opposite byte order will see a magic number of 0xd4c3b2a1.

> If endianness is architecture
> dependant then how would I know that I have to swap 2 bytes or 4 bytes?

You have to swap if the magic number is 0xa1b2c3d4.

You have to swap 2 bytes if you have to swap and you're processing a 
2-byte quantity.

You have to swap 4 bytes if you have to swap and you're processing a 
4-byte quantity.

Note, by the way, that this applies *ONLY* to the per-file and 
per-packet headers.  It does not apply to the packet data, which is in 
whatever byte order it was put into on the wire.

> really, A bit confusing...

Perhaps - the intent was, as I remember reading, to do as little work as 
possible when capturing packets (the less work done, the less likely you 
are to drop packets), and do all the byte-order work when reading the 
capture file.

That's why applications typically use libpcap to read the file, rather 
than reading it directly - it also means that the application won't have 
to change if the file format changes.

> One more thing, does this file have any extension? .tcpdump or .cap

No, there is no standard extension for those files.  (It originated on 
UN*X, and UN*X systems are, by and large, not as insistent on standard 
extensions as, say, Windows.)

> I mean there are so many possible or there is not anything like that.. one
> can recognise file from its magic number only...

That's how it'd be done on the free-software UN*X desktops (on KDE, for 
example, a PDF file doesn't have to have a name ending in ".pdf" in 
order to be recognized as a PDF file).
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