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Re: [Plugins-writers] recursive DNS issue
From: Paul Johnston <paul () westpoint ltd uk>
Date: Thu, 04 Mar 2004 16:37:39 +0000


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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