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How does a computer create a 'Response' packet? (infosec.pub)
submitted 1 month ago by driftWood@infosec.pub to c/networking@infosec.pub
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Consider a Ping Request packet arriving on a computer with 2 NICs (multi-homed PC). The packet is received on 1 of the interfaces. Now the computer has to send the Ping Response packet. To fill the source IP and source MAC address the computer does which of the following?

  • Computer first determines which interface should be used as the egress interface by looking at the Destination IP address. Destination IP address was taken from source IP address field of Ping Request packet. Once it determines egress port, it will enter that interface's IP and MAC address in the Ping Response packet.
  • Computer takes the destination IP and MAC address of the Ping Request packet and just flips them over to fill source IP and MAC address in Ping Response packet.
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[-] driftWood@infosec.pub 1 points 1 month ago* (last edited 1 month ago)

I recently tested this using wireshark. When I run packet capture on nic1 of dstPC I see ping request packets coming, but no response packets leaving the interface. On nic2 I don't see any packets leaving either. So kind of stumped what is happening. It seems the computer just drops the response packet and it never makes it till any nic. But still don't have a good explanation of WHY the packet gets dropped.

this post was submitted on 27 Jul 2024
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