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-   -   IP header record route option (http://www.linuxquestions.org/questions/linux-networking-3/ip-header-record-route-option-4175451087/)

narender14 02-21-2013 05:56 AM

IP header record route option
 
If i specify Record Route option in IP header, it will record the ip address of at most 9 intermediate hops. My question is how this packet with ip Record Route option enabled reaches to the destination host get processed.

I used ICMP ping with record route option enabled and the destination sends a normal ICMP echo reply in which the recorded route is copied back. But if i want to send a UDP or TCP and not ICMP echo request with Record Route option then how the destination will send the recorded route back to source?

acid_kewpie 02-21-2013 12:08 PM

it doesn't send it back, there's nothing else happening other than the intermediate routers adding their details to the packet as it passes.

blip 08-14-2013 07:07 PM

It is possible to have the packet sent back by setting the TTL value (Time To Live as number of router hops) to an appropriately low value. In this case, the reply is an ICMP Time Exceeded message with a code indicating that the TTL expired in transit. Your original packet, including IP header and options (Record Route), will be encapsulated in this ICMP message.

In practice however, I found that TCP SYNs with the Record Route option are often silently dropped unlike ICMP echo requests. When combining all three (ICMP Echo, low TTL, and Record Route), I was able to detect routers that did not decrement the TTL and routers that forwarded the Record Route option unaltered.


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