RE: tcpdump endianness

"Viral Mehta" <[email protected]> Fri, 18 Aug 2006 19:16:22 +0530
Newsgroups gmane.network.ethereal.devel
Message-ID <[email protected]>
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The Ethereal project is being continued at a new site.  Please go to
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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: [email protected]
[mailto:[email protected]] On Behalf Of Guy Harris

Sent: Thursday, August 17, 2006 11:13 PM

To: Ethereal development

Subject: Re: [Ethereal-dev] tcpdump endianness

 

-------------------

The Ethereal project is being continued at a new site.  Please go to
http://www.wireshark.org <http://www.wireshark.org/>  and subscribe to
[email protected].

Don't forget to unsubscribe from this list at
http://www.ethereal.com/mailman/listinfo/ethereal-dev

-------------------

 

[email protected] 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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http://www.ethereal.com/mailman/listinfo/ethereal-dev

 

 

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<p class=3DMsoNormal><font size=3D2 color=3Dnavy face=3DArial><span =
style=3D'font-size:
10.0pt;font-family:Arial;color:navy'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'>Hi<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>Thanks for your =
valuable
input..<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>Now I m stuck up =
with
&quot;what is Link Layer Type&quot;. 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?<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>Secondly, Why do we =
have
TimezoneOffset and TimeStampAccuracy fields if they are not used at =
all?<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>What is a =
per-packet header?
Will we have it when we don&#8217;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&#8217;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)<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'>Thanks,<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><st1:PersonName =
w:st=3D"on"><font
 size=3D2 face=3D"Courier New"><span =
style=3D'font-size:10.0pt;font-family:"Courier New"'>Viral
 Mehta</span></font></st1:PersonName><font size=3D2 face=3D"Courier =
New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>Embedded =
Engineer,<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>eInfochips =
Ltd.<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>-----Original =
Message-----<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>From:
[email protected] [<a
href=3D"mailto:[email protected]">mailto:ethereal-dev-bou=
[email protected]</a>]
On <st1:PersonName w:st=3D"on"><st2:GivenName =
w:st=3D"on">Behalf</st2:GivenName> <st2:middlename
 w:st=3D"on">Of</st2:middlename> <st2:middlename =
w:st=3D"on">Guy</st2:middlename> <st2:Sn
 =
w:st=3D"on">Harris</st2:Sn></st1:PersonName><o:p></o:p></span></font></p>=


<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>Sent: Thursday, =
<st1:date
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2006</st1:date> <st1:time
Minute=3D"13" Hour=3D"23" w:st=3D"on">11:13 =
PM</st1:time><o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>To: <st1:PersonName =
w:st=3D"on"><st2:GivenName
 w:st=3D"on">Ethereal</st2:GivenName> <st2:Sn =
w:st=3D"on">development</st2:Sn></st1:PersonName><o:p></o:p></span></font=
></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>Subject: Re: =
[Ethereal-dev]
tcpdump endianness<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'>-------------------<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>The Ethereal =
project is
being continued at a new site.&nbsp; Please go to <a
href=3D"http://www.wireshark.org/">http://www.wireshark.org</a> and =
subscribe to
[email protected].<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>Don't forget to =
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from this list at <a
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/p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'>-------------------<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'>[email protected]
wrote:<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>&gt; I am analyzing =
tcpdump
file.<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>&gt; I want to know =
how
endianness can affect when file is being =
written.<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>&gt; Or rather, who =
affects?
Application which is writing? or Network byte =
<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>&gt; order? or =
what?<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>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.<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>&gt; I read =
somewhere that
magic number of a tcpdump file can be =
either<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>&gt; 0xa1b2c3d4 or
0xd4c3b2a1. How can it be?<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>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.&nbsp; 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.<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>&gt; If endianness =
is
architecture<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>&gt; dependant then =
how
would I know that I have to swap 2 bytes or 4 =
bytes?<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>You have to swap if =
the
magic number is 0xa1b2c3d4.<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>You have to swap 2 =
bytes if
you have to swap and you're processing a 2-byte =
quantity.<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>You have to swap 4 =
bytes if
you have to swap and you're processing a 4-byte =
quantity.<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>Note, by the way, =
that this
applies *ONLY* to the per-file and per-packet headers.&nbsp; It does not =
apply
to the packet data, which is in whatever byte order it was put into on =
the wire.<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>&gt; really, A bit
confusing...<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>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.<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>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.<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>&gt; One more =
thing, does
this file have any extension? .tcpdump or =
.cap<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>No, there is no =
standard
extension for those files.&nbsp; (It originated on UN*X, and UN*X =
systems are,
by and large, not as insistent on standard extensions as, say, =
Windows.)<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>&gt; I mean there =
are so
many possible or there is not anything like that.. =
<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>&gt; one can =
recognise file
from its magic number only...<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier =
New"'><o:p>&nbsp;</o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
face=3D"Courier New"><span
style=3D'font-size:10.0pt;font-family:"Courier New"'>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 &quot;.pdf&quot; in order to be recognized as a =
PDF
file).<o:p></o:p></span></font></p>

<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
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style=3D'font-size:10.0pt;font-family:"Courier =
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<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
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<p class=3DMsoNormal style=3D'text-autospace:none'><font size=3D2 =
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