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Re: [Plugins-writers] recursive DNS issue
From: John Lampe <jwlampe () nessus org>
Date: Thu, 4 Mar 2004 14:11:29 -0500 (EST)

I think another risk might be the ability to bounce DoS attacks through
someone else's network.  So, if I want to blast the widgets.com webserver,
I find it's IP and start spoofing DNS UDP queries from the widgets.com
webserver's IP to the DNS recursive server.  If I choose the right domain
to 'resolve', I might get a 3:1 or 4:1 return on my packet sizes (i.e. I send
50 or 60 bytes and, depending on the return record, may get 200 bytes sent
to the attacked host)...Now, what if the recursive server is used to
bounce an attack into a competitor of the company owning the recursive
server?  So, if SodaBrand_X starts sending massive amounts of DNS reply
data to their competitor, SodaBrand_Y...and SodaBrand_Z can play both
sides against the middle :-)

John Lampe
jwlampe -at- nessus.org

On Thu, 4 Mar 2004, Paul Johnston wrote:


I think there are three potential risks, none of them particularly major.

1) Information leakage
If an attacker issues non-recursive queries against the server, they can
see what domains have been looked up, and also infer when from the TTL.
In fact even if you disable recursion they may still be able to do this.
For BIND I advocate denying all queries at the top of the config file,
and then specifically allowing queries for each authorative zone.

2) Unauthorized use of resources
People can hijack you as a free resolver. This is unlikely to be much of
a problem in reality. Personally I always know a few open resolvers, so
I've got backup nameservers if the local ones are down.

3) Potential cache poisoning
This is more a theoretical risk than anything. All the easy cache
poisoning attacks (ID prediction, birthday, etc.) are vulnerabilities in
specific versions of software. Better servers like DJB or BIND 9
randomize both the transaction ID and the UDP source port, giving an
attacker about 2^26 space to brute force (and no handy shortcuts). This
is scarcely easier than brute forcing a TCP connection.



omifix omnifix wrote:

Hi all

can anybody explain me what the problem is when my
external DNS server supports recursive DNS queries?

People are telling me that a DNS server is prone to
cache poisoning when recursive DNS queries are

Paul Johnston
Internet Security Specialist
Westpoint Limited
Albion Wharf, 19 Albion Street,
Manchester, M1 5LN
Tel: +44 (0)161 237 1028
Fax: +44 (0)161 237 1031
email: paul () westpoint ltd uk
web: www.westpoint.ltd.uk

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