Willy's words make sense to me and I agree that the existing fragments
should be evicted when the new one comes in and the cache is full.
Though the attacker can still leverage this to flush the victim's
cache, as mentioned previously, since fragments are likely to be
assembled in a very short time, it would be hard to launch the
attack(evicting the legit fragment before it's assembled requires a
large packet sending rate). And this seems better than the existing
solution (drop all incoming fragments when full).


On Sat, Apr 17, 2021 at 6:30 PM Matt Corallo
<netdev-l...@mattcorallo.com> wrote:
> See-also "[PATCH] Reduce IP_FRAG_TIME fragment-reassembly timeout to 1s, from 
> 30s" (and the two resends of it) - given
> the size of the default cache (4MB) and the time that it takes before we 
> flush the cache (30 seconds) you only need
> about 1Mbps of fragments to hit this issue. While DoS attacks are concerning, 
> its also incredibly practical (and I do)
> hit this issue in normal non-adversarial conditions.
> Matt
> On 4/17/21 03:50, Willy Tarreau wrote:
> > On Sat, Apr 17, 2021 at 12:42:39AM -0700, Keyu Man wrote:
> >> How about at least allow the existing queue to finish? Currently a tiny new
> >> fragment would potentially invalid all previous fragments by letting them
> >> timeout without allowing the fragments to come in to finish the assembly.
> >
> > Because this is exactly the principle of how attacks are built: reserve
> > resources claiming that you'll send everything so that others can't make
> > use of the resources that are reserved to you. The best solution precisely
> > is *not* to wait for anyone to finish, hence *not* to reserve valuable
> > resources that are unusuable by others.
> >
> > Willy
> >

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