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Consider the Longest Prefix Matching (LPM) mechanism that routers use to decide which data link the packet should be sent next. Assuming that the router tables don't change, shouldn't the path the packet takes (hop-by-hop) be the same for the same source and destination address pairs? If it is, then isn't it a security vulnerability?

PS: I am new to this, so bear with me :) Thanks!

  • The reason for the existence of IP and routing protocols is that the Internet is dynamic. The DoD funded the development of IP because it can dynamically reroute around damage. – Ron Maupin Sep 23 '18 at 9:49
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Assuming that the router tables don't change, shouldn't the path the packet takes (hop-by-hop) be the same for the same source and destination address pairs?

For solely prefix-based routing, the next hop is selected based on the destination address. The path stays the same unless routing entries change somewhere. Note that routing entries may change and a forwarding decision may be policy-based as well (based on load, protocol, ports, QoS, ...).

If you don't know all parameters for each routing decision the path may appear random to you.

If it is, then isn't it a security vulnerability?

No. Why do you think so? Security isn't based on obscuring data paths in a network but on using trustworthy devices and encryption.

  • Thanks for the answer! I have another question. No. Why do you think so? Isn't it the reason why source routing is prohibited? – yobro97 Sep 23 '18 at 9:55
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    Source routing was a concept to let the source decide the path (instead of the hops) which opens up various attack vectors. There's no problem in having stable paths, as long as you don't have someone else decide about them. Segment routing is a concept that's currently being explored for IPv6 - let's see how that turns out. – Zac67 Sep 23 '18 at 10:14
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Assuming that the router tables don't change, shouldn't the path the packet takes >(hop-by-hop) be the same for the same source and destination address pairs?

There are other factors that determine which path is chosen, such as traffic.

You might find this page: Computer Networks | Longest Prefix Matching in Routersn helpful.

If it is, then isn't it a security vulnerability?

I assume you're thinking of something along the lines of poisoning the route decision to send packets where you want them to go, sort of like ARP poisoning in a LAN.

While I don't know of any way to do this using Longest Prefix Matching specifically, I'm no expert in LPM, there do exist methods for hijacking routes in the exterior routing space. For example: IP Prefix Spoofing Attacks.

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