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You are here: Home arrow Forum arrow Ethical Hacking Discussions and Related Certificationsarrow Network Pen Testingarrow NACK data packer - Questions from a certification program
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January 09, 2009, 10:26:01 PM *
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Author Topic: NACK data packer - Questions from a certification program  (Read 2561 times)
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Manu Zacharia (-M-)
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« on: July 07, 2006, 12:40:02 PM »
Hi Friends,

I am posting more questions that I have faced during my course. Please discuss the same. The next question is:

Code:
Assume that system X sends a NACK data packet to system Y. What does the packet mean?
1. It means that X does not want to establish a connection.
2. It means that X did not understand what Y is saying.
3. It means that X wants to end the connection with Y.
4. It means that Y wants to end a connection with X.
5. None of the above.

Regards,

« Last Edit: January 28, 2007, 11:28:40 AM by The Morpheus » Logged
Manu Zacharia
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Negrita
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« Reply #1 on: July 07, 2006, 02:50:26 PM »
This is a trick question. If you're rushing through your exam, and don't concentrate you may be tempted to answer no. 1.

The correct answer is;
5.   None of the above.

In this situation system Y is sending packets to system X using a TCP session. System X is acknowliging the receipt of these packets by replying with ACK's. If system X fails to receive a packet it will send a NACK to system Y to request a resend of the lost packet.
« Last Edit: July 07, 2006, 02:59:27 PM by Negrita » Logged
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Manu Zacharia (-M-)
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« Reply #2 on: July 07, 2006, 09:26:02 PM »
Hi Negrita,

Thanks for the answer, however, the question is not talking about any TCP session. And I recently came across a defenition of NACK :
Quote
A NACK or a non-acknowledgment message is meant for ending the connection, resulting in no further communication between the two systems involved.

Is the above definition is true, and if it is true, then why cant we select the option 3 as the answer.

Thanks for the reply.

Regards,

The Morpheus
« Last Edit: January 28, 2007, 11:29:01 AM by The Morpheus » Logged
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pcsneaker
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« Reply #3 on: July 08, 2006, 05:02:39 AM »
To answer that question it would be necessary to know what application/protocol is sending that NACK packet.

At least with TCP/IP (TCP and UDP) there is no "NACK data packet", it can be some upper layer protocol/application (some multicasting data streams use NACKs, DHCP has NACK, etc...)
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« Reply #4 on: July 08, 2006, 04:13:28 PM »
The Morpheus, a TCP session can be ended using either a RST or a FIN packet, depending on the situation.
Read here for more info.

pcsneaker, a NACK packet is part of an experimental multicast streaming protocol called Negative-acknowledgment (NACK) Oriented Reliable Multicast protocol or NORM. It has been defined in RFC 3940 and in RFC 3941 and does what I said in my previous post. As you say, there are also DHCP NACKs'.
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There are 10 kinds of people, those that understand binary, and those that don't.
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