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Would the location of an Intrusion Detection (or Packet Analysis) sensor on an internal network impact whether the external host's Source IP (assuming inbound TCP traffic) is retrievable from a packet?

Put differently, is there a common deployment scenario in which sensor placement would mean constantly seeing an internal IP as the source rather than the actual external host from which the traffic originated?

(yes, I know how dicey attribution gets--I mean the last sending host).

  • This looks not really like a question regarding IDS but more like a general question on how networks work and what part of packets gets changed due to routing etc. Also a question regarding the source address is more at the level of packet filters and not the deeper inspection done by IDS. Thus I think this is off-topic here. – Steffen Ullrich Mar 1 '16 at 20:22
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It depends on how your network is setup; but usually the IP is not stripped and the packet headers contain the original source and destination.

Generally, a core switch comes after a router, the router is likely on an enterprise network to be doing a lot of NAT translations. In which case, you may see the NAT IP of a given packet be referenced (e.g instead of some public ip, you see 192.168.x.x). So it's likely you will see the internal IP as a source of anything outbound, because, that's the only IP the network device knows about -- the external IP is applied by the router doing NAT.

However unless you're doing some strange (e.g. Cisco fix-up) packet-level inspection and changing, the packet headers should remain the same and indicate the source and destination IP of the endpoints -- not of any intermediate network devices that carry the traffic. Those devices only have management IPs -- and don't tend to (although this can happen in sophisticated layer 3 switching setups which are uncommon) alter the packet headers.

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