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Old 02-14-2008, 08:42 AM   #1
paragkalra
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Using tcpdump ?


Hello all,

I am trying to perform network installation using PXE boot. But the machine which I want to install is showing following error:
[PXE-E51: No DHCP or proxyDHCP offers were received ]

To diagnose this issue I want to use "tcpdump" but the command looks quite complex.

Can anybody please put some light on how to use the tcpdump command so that I can analyse my problem.
 
Old 02-14-2008, 09:08 AM   #2
acid_kewpie
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well tcpdump doesn't need any options at all by default, just run it, but i'd wonder how comfortable you would be with the data that you will get from it in the first place. you can look at a tutorial like this: http://www2.papamike.ca:8082/tutorials/pub/tcpdump.html but again it's about the data from it, not the application that matters. if you have xwindows on that machine, you'll probably have a better time using a gui tool like wireshark, but again you're still going to need to understand the output.
 
Old 02-14-2008, 09:09 AM   #3
MS3FGX
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You just need to give tcpdump the interface for a basic sniff, which should be all you need for what you are doing.

Try simply running:

Code:
tcpdump -i eth0
That will show you all the traffic that the machine is receiving over the network.
 
Old 02-14-2008, 09:23 AM   #4
paragkalra
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So I believe I just need to execute "#tcpdump" on the server and it will take care of all the incoming and outgoing packet information. Actually my main concern is that "#tcpdump" keeps on displaying the messages on the console. It Never seems to halt.
 
Old 02-14-2008, 09:26 AM   #5
MS3FGX
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Well, that is because the packets keep coming. It is going to display them as long as they are coming and going on that interface.

As acid_kewpie said, your biggest hurtle here might be interpreting the actual output of tcpdump. Do you know what you are looking for, and what to do with said information once you have found it? There may be easier ways to go about this.
 
Old 02-14-2008, 09:44 AM   #6
paragkalra
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My main issue is to diagnose the problem. Can anyone please suggest some easier tools to determine whats going wrong and what I should look for in them.
 
Old 02-14-2008, 10:18 AM   #7
acid_kewpie
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packet sniffing is an awesome thing to do with just about any network touching problem, but doesn't necessarily get you as close as you'd like to be to resolve it. it's more a divide and conquer method than a way to actually find the problem. the other part of the scenario is to look at the pxe server, but the point of packet sniffing if to see 1) if packets are hitting the machine for dhcp and 2) if they are valid and 3) if they are getting a valid reply.
 
Old 02-15-2008, 12:34 AM   #8
paragkalra
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Any other utility similar to tcpdump and easy to use..????
 
Old 02-15-2008, 07:17 AM   #9
Kropotkin
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Quote:
Originally Posted by paragkalra View Post
Any other utility similar to tcpdump and easy to use..????
Ya, wireshark. It should be in your distro's repos.
 
Old 02-17-2008, 09:02 PM   #10
paragkalra
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I believe Ethereal and Wireshark are same?
 
Old 02-17-2008, 09:24 PM   #11
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Quote:
Originally Posted by paragkalra View Post
I believe Ethereal and Wireshark are same?
Yes - have a look at the wireshark site for the latest version if you don't have a package.
 
  


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