forwarding algorithm and timeouts
kcd at daimlerchrysler.com
Mon Mar 26 23:55:42 UTC 2001
Bob Vance wrote:
> Thanks, Kevin.
> >is actually assuming the role of a "resolver",
> >basically a stub-resolver-with-caching.
> Well, surely it's not trying the list more than once? -- like the stub
> resolver algorithm described in 'DNS and BIND' and in contradiction to
> the documentation where it says that the "forwarders" are tried each
> only once.
Different stub resolver implementations can have different timeout/retry
strategies, as evidenced by, e.g. the disparity between nslookup's and most
system resolvers' timetout/retry strategies.
> > don't recall offhand if there
> >are any specific parameters in the RFCs regarding resolver timeouts...
> So, still, I'm wondering what that timeout is and the rationale for its
> value, whatever it may be.
Sorry, I don't know the specifics. I'd have to dig through the code for
that, and you can probably do that as easily as I....
> Tks | <mailto:BVance at sbm.com>
> BV | <mailto:BobVance at alumni.caltech.edu>
> Sr. Technical Consultant, SBM, A Gates/Arrow Co.
> Vox 770-623-3430 11455 Lakefield Dr.
> Fax 770-623-3429 Duluth, GA 30097-1511
> -----Original Message-----
> From: bind-users-bounce at isc.org [mailto:bind-users-bounce at isc.org]On
> Behalf Of Kevin Darcy
> Sent: Monday, March 26, 2001 5:00 PM
> To: bind-users at isc.org
> Subject: Re: forwarding algorithm and timeouts
> Bob Vance wrote:
> > What exactly is the forwarding algorithm, assuming that "forward only"
> > is set.
> > Or where can I read about it (besides the source code :) ?
> > I checked the ARM, but only found this brief statement:
> > "they are queried in turn until the list is exhausted or an answer
> > is found.
> > "
> > Hmmm. I just looked in 'DNS and BIND, 2nd' and it says:
> > "each forwarder is contacted only once, and it waits a short time
> > for the forwarder to respond.
> > "
> Actually, as of BIND 8.2.3, the forwarders are supposedly tried
> to historical RTT, not necessarily sequentially...
> > OK. What is the timeout used before going to the next one in the
> > I would assume that it would have to be ~5 since we know that the stub
> > resolvers wait this long on their initial try. Otherwise, we could
> > several outstanding request from resolvers before the first forwarder
> > try times out.
> > Do all resolvers follow the algorithm described in 'DNS and BIND' ?
> > I.e., do WinX or MAC clients also try their list initially with 5
> > seconds, then re-do the list with 10 seconds, etc.?
> > Is this algorithm a requirement described in an RFC?
> In RFC terms, a nameserver doing "forward only" isn't really acting as a
> "nameserver" _per_se_, but is actually assuming the role of a
> basically a stub-resolver-with-caching. I don't recall offhand if there
> are any specific parameters in the RFCs regarding resolver timeouts...
> - Kevin
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