Initial Lookup Slowness BIND 9.2.4

Anthony Blalock anthony_blalock at
Wed Dec 31 14:51:35 UTC 2008

We had a similar problem when we moved some DNS servers to a DMZ that was
behind a firewall (a Cisco FWSM blade in a 6513 chassis.)  A packet capture
showed that the initial query from the DNS server had the EDNS flag set.  It
never got a response to that query, and would then resend it without the
EDNS flag and would get an immediate response.  I¹m not sure if the firewall
didn¹t like the query itself or the response, but there was definitely
something about EDNS that it didn¹t like.  We fixed the problem by disabling
the DNS application inspection that the firewall was doing  (³no fixup
protocol dns²).   Check your firewalls to see if they are dropping the EDNS
requests.  If so, you can try modifying your firewalls to allow the EDNS
queries, or if that isn¹t possible then you can try limiting the EDNS packet
size to 512 in your options:

  options {
    edns-udp-size 512;

-Anthony Blalock

>> > I have installed a caching only instance of BIND (9.2.4) on a CentOS
>> > machine on my internal network.  I have noticed that initial DNS requests
>> > against the server take a rather large amount of time (usually around 7
>> > seconds).  I have done some basic troubleshooting and I am coming up at a
>> > loss.  I think my ISP might be doing something "funny" but I am not sure
>> > how to test any further.
>> >
>> > I have captured BIND debug info at a trace level of 3 (posted bellow).  I
>> > have also captured snoop data via tcpdump.  >From what I can tell; it seems
>> > as if responses are taking a "long" time to come back.  The same behavior
>> > is exhibited for any domain or host I attempt to lookup.
>> >
>> > To be clear, everything is working, just much slower than it should for
>> > initial queries.  Any help troubleshooting would be greatly appreciated.

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