This is a discussion on Re: Outstanding queries vs. source port randomization entropy - DNS ; > [I posted a variation on this message as a comment on Dan Kaminsky's > blog earlier today, but I'm skeptical he'll have the time to respond, so > I'm throwing it out to a wider audience.] > > So ...
> [I posted a variation on this message as a comment on Dan Kaminsky's
> blog earlier today, but I'm skeptical he'll have the time to respond, so
> I'm throwing it out to a wider audience.]
> So here's a question. With source port randomization, what prevents the
> following attack?
> Attacker sets up a nameserver for example.com that receives queries but
> sends no responses, with an appropriate delegation. Attacker then asks
> the victim nameserver ~2^16 distinct queries under example.com in a
> short time frame. This effectively consumes the entire source port range
> with outstanding queries. The example.com nameserver observes the source
> port for each inbound recursive query from the victim nameserver; the
> one remaining source port that was not observed is the only port left to
> the victim nameserver for its next query. Attacker then asks victim
> nameserver for the domain to be poisoned, and forges the poison response
> targeting the remaining port.
> I'm presenting an idealized attack here for the purpose of illustration.
> The principle is to deplete the entropy included in source port
> randomization by consuming the majority of available ports with
> outstanding queries. The attack would only be effective against a
> nameserver that allows close to 2^16 outstanding queries, i.e. high open
> file descriptor limit, FD_SETSIZE appropriately redefined, etc.; a
> nameserver that could have only 1024 outstanding requests, for example,
> would be DoSed before it would lose significant entropy. The outstanding
> requests could be distributed among many domains and distinct
> nameservers if necessary, as long as the nameservers coordinate. The
> attack could be partly mitigated on nameservers that allow ~2^16
> outstanding requests if the victim nameserver can reuse randomly chosen,
> already open sockets for multiple successive requests once ~2^16 ports
> are bound. The efficacy of this attack depends on the precise algorithm
> by which the victim uses new ports, and what limits are enforced.
> I have to wonder how this has been addressed in the various resolvers
> that implement source port randomization. Where has this scenario been
> discussed before? It's too obvious not to have been considered earlier,
> but I've done some Googling and skimmed some list archives and haven't
> come up with anything.
> Jefferson Ogata : Internetworker, Antibozo
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Use connected UDP socket then kernel can also use the source
address as a differentiator when delivering replies. You
can then support
x to an individual destination.
You still need to randomly select ports and randomly select
txid. When you have a collision you just need to repick
one and/or the other. A port collision alone lets you use
a existing socket.
For queries to another server you just set SO_REUSEADDR
when you have a port collision with a port you already have
open as the existing one is in a connected state. If it
is a port that is not already in use you don't set SO_REUSEADDR
so you detect collisions with other applications.
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742 INTERNET: Mark_Andrews@isc.org
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