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We currently use prandom_u32() for allocation of ports in tcp bind(0) and udp code. In case of plain SNAT we try to keep the ports as is or increment on collision. SNAT --random mode does use per-destination incrementing port allocation. As a recent paper pointed out in [1] that this mode of port allocation makes it possible to an attacker to find the randomly allocated ports through a timing side-channel in a socket overloading attack conducted through an off-path attacker. So, NF_NAT_RANGE_PROTO_RANDOM actually weakens the port randomization in regard to the attack described in this paper. As we need to keep compatibility, add another flag called NF_NAT_RANGE_PROTO_RANDOM_FULLY that would replace the NF_NAT_RANGE_PROTO_RANDOM hash-based port selection algorithm with a simple prandom_u32() in order to mitigate this attack vector. Note that the lfsr113's internal state is periodically reseeded by the kernel through a local secure entropy source. More details can be found in [1], the basic idea is to send bursts of packets to a socket to overflow its receive queue and measure the latency to detect a possible retransmit when the port is found. Because of increasing ports to given destination and port, further allocations can be predicted. This information could then be used by an attacker for e.g. for cache-poisoning, NS pinning, and degradation of service attacks against DNS servers [1]: The best defense against the poisoning attacks is to properly deploy and validate DNSSEC; DNSSEC provides security not only against off-path attacker but even against MitM attacker. We hope that our results will help motivate administrators to adopt DNSSEC. However, full DNSSEC deployment make take significant time, and until that happens, we recommend short-term, non-cryptographic defenses. We recommend to support full port randomisation, according to practices recommended in [2], and to avoid per-destination sequential port allocation, which we show may be vulnerable to derandomisation attacks. Joint work between Hannes Frederic Sowa and Daniel Borkmann. [1] https://sites.google.com/site/hayashulman/files/NIC-derandomisation.pdf [2] http://arxiv.org/pdf/1205.5190v1.pdf Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org> Signed-off-by: Daniel Borkmann <dborkman@redhat.com> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org> |
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