Suspecious DNS queries dropped by Firewall

babu dheen babudheen at
Tue Dec 13 12:46:32 UTC 2011

Dear Anand,
In what situation, DNS packet size can exceed more than 512 bytes.  In fact, my gateway DNS server should not contact internal DNS server except internal domain name resolution if any user access any internal website through proxy. 
My proxy is using gateway DNS for name resolution. So if any users access internal website through proxy, proxy will send the name lookup to gateway DNS and gateway DNS will forward the request to internal DNS server.
In this case, will the internal domain DNS query exceed 512 bytes?
papdheen M

--- On Tue, 13/12/11, Anand Buddhdev <anandb at> wrote:

From: Anand Buddhdev <anandb at>
Subject: Re: Suspecious DNS queries dropped by Firewall
To: "babu dheen" <babudheen at>
Cc: bind-users at
Date: Tuesday, 13 December, 2011, 5:39 PM

On 13/12/2011 13:04, babu dheen wrote:

> Hi,
> Our company users are using internal DNS servers for name resolution
> and internal DNS servers are configured to forward the DNS query to
> company gateway DNS servers for external queries
> User --> internal DNS server ---> gateway DNS server ---> internet
> But when i look at the firewall hit , i can see gateway DNS server is
> again sending DNS query to internal DNS server and the same is denied in
> firewall with below error
> Dropped UDP DNS reply from OUTSIDE:<gateway-dns-ip>/53 to
> DMZ50:<internal-dns-ip>/63953; packet length 526 bytes exceeds
> configured limit of 512 bytes

Your firewall is misconfigured. Who said DNS reply packets cannot be
bigger than 512 bytes? You need to reconfigure your firewall, and remove
that 512-byte limit for DNS queries and responses.
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