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Old 05-07-2004, 07:49 AM   #1
myself_rajat
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Registered: May 2004
Location: India
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Lightbulb Problem of blocking ICMP packets while calculating Path MTU


Dear All,

I am in process of implementing Path MTU detection technique. So for that I have to send ICMP ECHO request packets to destination host, and process the received ICMP ECHO reply packets.

I read that the ICMP server is embedded in the kernel of OS itself, so there is no need to write server code at all. So for sending and receiving of ICMP packets we will use raw sockets without specifying any port number.

But in between the network, some routers have some firewall configuration so that they will simply drop any incomming ICMP packet without sending any response to the sender. Hence originating host will not be aware of the status of destination host.
These firewall settings are due to some attacks like Ping of deth, Denial of request etc.

Can any one provide me any solution to overcome this problem.

Thanks in advance.
 
Old 05-07-2004, 10:32 AM   #2
jalal
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Hello, and Welcome to LQ !

You can try experimenting with the DF ( Don't Fragment ) bit in the IP header.

Send a normal IP packet ( not ICMP ), set the DF bit, and play with the packet size.

True, some routers/firewalls block incoming ICMP packets, but in this case, they will not be receiving one, instead they will be making one for you, and hopefully will send an "ICMP unreachable: fragmentation required".
Hope this helps.
 
Old 05-10-2004, 10:23 AM   #3
myself_rajat
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Registered: May 2004
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Hi Jalal,

Thanx for response. I tried the same as you suggested me. I sent some IP datagrams to a specified host in my local LAN but couldn't receive any response.

So buddy this thing din't work. Do u have some other alternatives, if then plz let me know.

Eagerly waiting for reply.
 
Old 05-11-2004, 01:47 AM   #4
jalal
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Hello again,

Do you know of any ready-made tool that successfully does what you are looking for? if so, see if it works in your setup, and if it is open-sourced, have a look at the code to see how did they do it.

If not, then I guess the only way left is to talk to the sys/net/security admins to see if they are blocking ICMP, and if so, ask them to remove it for a little while just for your testing - assuming you aren't the admin ofcourse

Otherwise, sorry, I don't know how this could be done. Maybe someone else reading this can help.
 
  


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