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I am reviewing for my final exam tomorrow, I came across a past exam question.

Network configuration

An attacker wants to initiate a TCP handshake with a target to make him think E initiated it, attacker knows IP address of E

(B, C, D, R), which ones can attack if the target is A?

(A, B, D, R), which ones can attack if target is C?

(A, B, C, and R), which ones can attack if target is D?

Am pretty much stuck on how to go about this, I would assume A, B, C, and R would be able to attack themselves since they are on the same network. But E seems to be too important, I don't really know why.

  • The question is badly worded... Is it a matter of just sending a SYN packet or does the attacker need to complete the three-ways handshake... The answers will be very different... – Bruno Rohée May 13 '15 at 3:47
  • The attacker needs a complete three way handshake – user2977715 May 13 '15 at 6:00
  • Hint: Completion of a successful TCP handshake requires knowing what was in the ACK sent by the target. The attacker must be able to get that ACK data. – cpast May 13 '15 at 6:52
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(B, C, D, R), which ones can attack if the target is A?

B and R.

Both B and R can spoof the IP address of E and can see the SYN-ACK reply packet from A to get the sequence number for the final ACK.

C cannot because the ACK will be sent via R that will route the packet out onto the internet.

D cannot because it will not see the SYN-ACKs over the internet.

(A, B, D, R), which ones can attack if target is C?

R.

R has visibility of all packets on the network, so can view the SYN-ACK and sequence number.

A and B will only be able to view packets for 192.168.150.0/25, packets destined for C will be sent over the 192.168.150.128/25 network only.

D cannot because it will not see the SYN-ACKs over the internet.

(A, B, C, and R), which ones can attack if target is D?

None. They will not have visibility of the SYN-ACK to get the sequence number.

  • Thank you very much Mr. SilverlightFox for this analysis, this will help me a lot – user2977715 May 13 '15 at 13:51
  • @user2977715: No worries, good luck! – SilverlightFox May 14 '15 at 9:44

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