about cache nonexist record

Chris Thompson cet1 at cam.ac.uk
Mon Jul 20 10:56:28 UTC 2009


On Jul 20 2009, Tech W. wrote:

>I have Bind-9.6.1 running on our university environment, have been 
>using dynamic update.
>
>My question is, when other DNS query my named for a record, for example
>test.example.com, but this record doesn't exist. How long time will the
>remote DNS cache this nonexist record?

Read RFC 2308 "Negative Caching of DNS Queries". Roughly, the answer is
"(at most) the (now misnamed) MINIMUM field of the SOA record for the zone".

>I found the problem that if this nonexist record was cached by remote
>DNS, even if I added the corresponding record to named zone soon, the
>remote DNS can't find it quickly.
>
>This is not good for dynamic update IMO, for example, I need to change
>a record, then I have to do:
>
>nsupdate delete ....
>nsupdate add ....
>
>If the remote DNS query for this record just after 'nsupdate delete',
>then it will get nothing and cache this nonexist record for some time,
>even though the correct record has been added. How to deal with this case?

Do the delete and the add in the same update transaction:

  nsupdate <<EOF
  update delete ...
  update add ...
  send
  EOF

-- 
Chris Thompson
Email: cet1 at cam.ac.uk



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