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Articles taggués ‘fragmented ip packet forwarding’

Fragmented IP packet forwarding

02/11/2015 Comments off

About Fragmented IP packet forwarding.

I couldn’t really find a suitable topic for this post actually but I will try to find answers for the following questions:

  • How can we fragment an IP packet manually in scapy
  • How does a fragmented packet look like and how the transport layer (TCP/UDP) header is located
  • How do we forward fragmented packets, do we reassemle them?
  • If we don’t reassemble, can we force reassembly?

First of all a bit of a theory: if an incoming IP packet is to be forwarded to another next hop and the MTU of this new path is smaller than the packet to be transmitted, we must find a way to forward the packet. If the packet has DF (Don’t Fragment) bit on i.e we are instructed not to fragment the packet most probably by the source, then normally we are expected to send an ICMP packet with type “Fragmentation needed” and pray that on the way back to the source no devices block all ICMP type of traffic. Second scenario is that what if the source lets us fragment the packet. Then we need to fragment it and story from now on is about this part of the scenario and the topology we will use is something like below.

fragmented_packets

 

Scapy is a fantastic tool to generate your own packets. It is exremely flexible and in our example, we will perform the fragmentation of a packet via our script.

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