Elwyn Davies
s3.2: I do not know enough about the details of NHDP and OLSRv2 to know if
this is a silly question: Would it be possible for a compromised node to
perform hop-limit or hop-count modification attacks even with RFC 6183
security in place just by modifying these fields and reforwarding the packet
even if it wasn't actually in the network topology? If so, it would be
desirable to mention this if it can do any harm.
No, not a silly question at all. But there are details that make the answer
longer than yes or no.
(Typo: RFC 6183 is RFC 7183.)
You need to distinguish packets from messages (this is RFC 5444 territory). And
NHDP doesn't matter here, as its message (HELLO) is not forwarded and any hop
count or limit is either ignored or possibly used as a reason to reject.
So OLSRv2 messages (TC) are forwarded, but at each hop they are put into a
packet. That packet is assembled from one or more messages, and at each hop it
is broken apart and a new packet formed. So the TC message may share a packet
with different other messages at each hop.
RFC 7183, which forwards to RFC 7182 where the actual work is defined, allows
you to protect either messages, or packets (or both). Packet protection
protects hop count and hop limit, but has other limitations (it is not end to
end). Message protection is applied to each message, and is end to end (or
rather, originator to each processing/forwarding router) but does not protect
hop count and hop limit.
So if using RFC 7183/7182 just to protect messages (it also covers sender
addresses) then there is an attack. Attacker receives packet, sends new packet
that resets hop count and limit in those messages it includes in a new packet
to only one more hop before end of life. Sends quickly (normal forwarding may
be delayed, especially if using RFC 5148) and possibly even elsewhere in
network (wormhole attack). This "penultimate hop" message poisons the real
message, if it arrives later, as it is seen before, and not forwarded, while
the penultimate hop message will go one hop and stop. (Can we do this with a
"last hop" message to poison even more successfully? That I would need to check
some details in RFC 7181 to determine.)
Could this be prevented? I can imagine a revision of RFC 7181's forwarding
rules that recorded hop count/limit, and if seeing a longer range message
decided to forward that even if seen before with a lower range. But that
introduces a new attack of creating a sequence of increasing range messages to
add to the traffic load. Or you could use both packet and message ICVs, which
does prevent this attack but increases overhead. Or (potential future that I
know someone is working on, but is not a solution now as far as I know) find a
form of aggregating signature that overcomes this problem efficiently.
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