Re: [PATCH v3 23/30] ipv6: defrag: drop non-last frags smaller than min mtu

Michal Kubecek
2019-01-11T14:21:28Z

On Friday, 11 January 2019 14:26 Timothy Winters wrote:
> Hi Eric,
>
> So I understand correctly the attack that you are trying to prevent is
> many small fragments from different IPs?
>
> The 6MAN working group has had some discussion about this topic, if
> you want read some IPv6 networking prospectives.
>
> https://mailarchive.ietf.org/arch/browse/ipv6/?gbt=1&index=zsYUwQTt0mA
> dXdk9MN4dP-uiiag

I haven't read all mails in that discussion but most seem to be missing
the point. The problem is not memory consumption, we have a (runtime
configurable) limit for that. The problem is that by sending many small
(8 bytes) fragments of a large (up to 64 KB) packet but never finishing
it, an attacker can force receiving host into using quite a lot of CPU
time just by looking up the fragment queue of the partially reassembled
packet.

Currently, IPv6 reassembly uses a simple linear list which is fine with
1280 byte long fragments (up to ~50 of them) but not with 8 byte long
ones (there could be as many as ~8000). The IPv4 reassembly code (where
we cannot assume minimal size of non-last fragment) switched to rbtree
(with logarithmic lookup time) exactly for this reason.

> What about lowering the value of accepted fragments? to something
> like 1280/2?

That would be probably sufficient to mitigate the DoS attacks.

Michal Kubecek