Feasible TTL.

[Kevin Darcy]

The refresh value governs how often stealth slaves check the zone for
changes. Thus it has impact on both the speed of change propagation and the
amount of traffic the master has to deal with in terms of serial-number
queries. Note that I said "stealth slaves". Registered slaves should be
doing serial-number checks and zone transfers based on the NOTIFY messages
which the master sends them whenever the zone changes.

The retry value governs how often slaves retry in case of failure of a
serial-number query to the master. If you have unreliable connections and
tight change-propagation requirements, you may want to set this to a small
number. On the other hand, if your slaves handle a lot of zones, setting
this number too low could cause them to "thrash".

The expire value is a way of eventually getting rid of a zone from a
secondary if you forget to explicitly deconfigure the zone from its
named.conf after removing it from the master. If you set this value too
low, then a zone can "expire" just because of a temporary inability to do a
serial-number check to the master. If you set it too high, the slave could
be giving out stale data authoritatively for much too long.

The last field is *not* the "minimum TTL" as your comment indicates. See
RFC 2308. It now governs the TTL of negative cache entries for names in the
zone. In other words, if I query the non-existent name "foo.bar.com" then
if the last field of the "bar.com" SOA specifies 1h, I'll remember the fact
that "foo.bar.com" doesn't exist for 1 hour, and after that I'll query it
again.

Note that "default TTL" is not specified in the SOA record. It is specified
in the $TTL directive, which is available in any secure version of BIND 8
or in BIND 9. A low TTL generates more traffic (because names expire more
frequently from cache), but permits faster change-propagation. It's the
eternal tradeoff...


- Kevin

Salman Ahmed Hashmi. wrote:

> Hi guys,
> Can anybody suggest a feasible TTL value for each of the following ?
> This question may look ridiculous but i have been thinking over it and
> even found that it is up to the administrator either to decide a default
> value for all of his domains or give an individual value to each and
> every domain.
> The format is given below.
>
>                                 900531422       ;Serial
>                                 3600            ; Refresh every 1 hour
>                                 900             ; Retry 15 minutes
>                                 2592000         ; Expire 30 days
>                                 3600 )          ; Minimum TTL of 1 hour
>
> Furthermore While defining ttl values for a domain,
> 1)Is there any relation b/w the available bandwidth (i.e link capacity
> )to nslookups ?....
> And,
> 2)How can an adminstrator know which domain needs a faster update and
> which domain is comparatively dormant.
>
> Regards