[conspire] DNS vulnerability details

Ruben Safir ruben at mrbrklyn.com
Wed Jul 23 21:41:04 PDT 2008

> Good, right?  Except, then, the firewall appliance's network address
> translation / port address translation (NAT/PAT) algorithm kicks in, and
> rewrites the outbound traffic.  The originating port was random, so the
> firewall's rewritten version of that same packet should likewise have a
> random source port, right?  Because all $40 cheap plastic appliances
> have excellent random number generators, right?  Oops.  Sorry, your
> originating port assignment probably doesn't end up being quite so
> random, any more.  See:
> http://www.circleid.com/posts/87143_dns_not_a_guessing_game/  Basically,
> a typical firewall box makes a rather efficient de-randomiser.

If the name server is using random ports how does the resolver know
where to find it.  I'm not likely to rewrite firefox.


> Testing your nameserver's randomness of source port selection:
> Do:
> $  dig [namserver IP or hostname] porttest.dns-oarc.net in txt
> The result string will include a editorial comment like "GOOD", "FAIR",
> or "POOR" about randomness quality.
> Or use this Web facility:
> https://www.dns-oarc.net/oarc/services/dnsentropy
> You really do want to attend to this now.  It's not Somebody Else's
> Problem.
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