limiting number of requests of a single hosts

Fr34k freaknetboy at
Fri Jun 15 13:03:08 UTC 2012


You may wish to read ISC/BIND's ARM about these settings (i.e., what they do, how they work, what the defaults are, etc):

        recursive-clients N;
        tcp-clients M;
        clients-per-query P;
        max-clients-per-query R;

where N, M, P, and R are numbers appropriate for your environment for each respective option.
See BIND v9.x ARM at


> From: Holemans Wim <wim.holemans at>
>To: "'bind-users at'" <bind-users at> 
>Sent: Friday, June 15, 2012 4:25 AM
>Subject: limiting number of requests of a single hosts
>We have a problem with one of our firewalls caused by DNS peaks. Once or twice a day a DNS burst (20K requests/15sec) kills all connections on the firewall.
>The firewall is due for replacement but in the mean time we would like to stop these peaks at their origin or at least try to limit their impact.
>We have 6 dns servers (bind) on our campus, that are all authoritative for our domains and also resolver for our campus hosts.
>Most of our clients however use our AD/LDAP/DNS Microsoft servers as their resolver, which on their turn contact our 6 dns servers for further resolving.
>What we figured out by packet capturing, is that at a certain point in time these AD/LDAP/DNS servers start ‘collecting’ dns requests without sending them further and then in a burt pass them on to our 6 dns servers which try to resolve these queries. Due to the fact that one request of a client mostly results in several queries of our dns servers to the outside world (root server contact, NS record resolving,..) , this results in a burst of dns requests through our firewalls, killing them.
>I have 2 questions, one, is there a way  to rate-limit the amount of request a single client (the AD servers in this case) can have standing out against a bind server ? Kind of rate-limiting parameter for bind name server.
>Two, has anyone already seen this type of behavior on a Microsoft AD/LDAP/DNS server and has a clue what could cause this stalling ? Solving that would be the best solution.
>Thanks in advance for any suggestion, answer,
>Wim Holemans
>Netwerkdienst Universiteit Antwerpen
>Network Services University of Antwerp
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