The NOTIFY mechanism (was Re: short ttl and servermmirroring)

[Jim Reid]

>>>>> "john" == hilgart  <hilgart at netscape.net> writes:

    john> I have a related question.  I'm really eager to use the BIND
    john> 8 NOTIFY FEATURE but in my tests between to BIND 8.2.2 p 5
    john> servers, it's taking longer than I'd like - anywhere from 30
    john> seconds to a few minutes. That is, the primary server is
    john> waiting that long before initiating a NOTIFY to the
    john> secondary when a domain has changed.  The secondary picks up
    john> the new zone instantly once it has been NOTIFY'd.

    john> I was hoping for a 1- or 2-second value for this feature to
    john> kick in.  These timings are key to enable rapid failover to
    john> back-up servers.

I think you are mistaken. The NOTIFY mechanism allows for fast
propagation of fresh zone data. This has nothing to do with "rapid
failover to back-up servers". What if the master server dies while a
zone transfer is in progress to one of your so-called back-up servers?
A faster NOTIFY is pointless once that happens. It doesn't matter how
many seconds after the load that the transfer started: it still
happens when the server crashes. BTW the DNS protocol says that name
servers automatically try the other NS records for some zone if the
first one they use doesn't respond.

    john> What are other people's experiences?  What might be slowing
    john> this down?

BIND8 deliberately introduces a small, random delay after loading a
zone before sending out NOTIFYs. This prevents the name servers and
network from being saturated with zone transfers whenever the master
loads a whole bunch of zones at once: when the name server is started
for instance. Somebody posted here last week that they had over
200,000 zones on their name server. Just imagine the problems if there
were simultaneous zone transfers for all of them!

The upper limit on this delay is 15 minutes IIRC. This should be good
enough for most environments. The actual delay is roughly proportional
to the number of master zones on the server. [Read the code.] So if
you want/need faster NOTIFYs, put less zones on your name server.