[olug] DNS: dropping to secondary when primary doesn't respond

[Phil Brutsche]

Dave Thacker wrote:
> I have three DNS servers providing internal (inside the firewall) DNS.   All 
> servers on the network have entries by ip address for these three servers 
> in /etc/resolv.conf.   My question is:
> 
> 1) If the first server in the list can't be reached at all, how fast does the 
> request time out and move to the second server.  

Gut feeling is 30 seconds, man 5 resolv.conf says 5 seconds

> 2)If the first server will accept the request, but is slow in providing an 
> answer, how fast does the request time out and move to the second server?

IMO hardly any different from a down DNS server: 5 seconds

> 3)  Is that interval tunable?

Yes, man 5 resolv.conf on one of my Debian boxes says:

   options
      Options allows certain internal resolver variables to be modified.
      The syntax is

         options option ...

      where option is one of the following:

...

      timeout:n
         sets the amount of time the resolver will wait for a response
         from a remote name server before retrying the query  via  a
         different  name  server.   Measured in seconds, the default is
         RES_TIMEOUT (currently 5, see <resolv.h>).

      attempts:n
         sets the number of times the resolver will send a query to its
         name servers before giving up and  returning an error to the
         calling application.  The default is RES_DFLRETRY (currently 2,
         see <resolv.h>).

-- 

Phil Brutsche
phil at brutsche.us