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Query Timing

Depending on how many name servers you have defined (up to three), it can take 15 to 24 seconds before a look-up fails and returns a "Host Not Found" error. The first listed server is queried, if it times out it retries once, then tries the next server, again with one retry. The time out periods are progressively reduced as it goes through the name servers defined in /etc/resolv.conf. To follow the process for the maximum 3 name server definitions, read this chart from top to bottom moving left to right.

BIND 8.2.1 and newer 
   Retry         NS 1           NS 2            NS 3
   0             5 sec          (2x) 5 sec      (3x) 5 sec
   1            10 sec          (2x) 5 sec      (3x) 3 sec
   Total Time   15 seconds      20 seconds      24 seconds
O'Reilly DNS and BIND [2]

BIND 8.2.0 and earlier
   Retry        NS 1            NS 2            NS 3
   0             5 sec          (2x)  5 sec     (3x)  5 sec
   1            10 sec          (2x)  5 sec     (3x)  3 sec
   2            20 sec          (2x) 10 sec     (3x)  6 sec
   3            40 sec          (2x) 20 sec     (3x) 13 sec
   Total Time   75 seconds      80 seconds      81 seconds
O'Reilly DNS and BIND [3]

Note: in BIND versions prior to 8.2.1 the resolver sent 3 retries instead of just 1 (4 attempts including the initial request). Normally things resolve in a second or less but in a worst case scenario on an early versions of BIND Resolver it could take up to 81 seconds to give up.



Pete Nesbitt 2012-04-23