Date: Mon, 14 May 2001 15:57:02 -0700 From: "Kevin Oberman" <oberman@es.net> To: Rick Duvall <maillist@coastsight.com> Cc: freebsd-questions@FreeBSD.ORG Subject: Re: Named definitions Message-ID: <200105142257.f4EMv3c15708@ptavv.es.net> In-Reply-To: Your message of "Mon, 14 May 2001 10:59:51 PDT." <Pine.BSF.4.21.0105141056580.25916-100000@ns1.coastsight.com>
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> Date: Mon, 14 May 2001 10:59:51 -0700 (PDT) > From: Rick Duvall <maillist@coastsight.com> > Sender: owner-freebsd-questions@FreeBSD.ORG > > What do the following mean, and what do they do? > > TTL > Refresh > Retry > Expire > > I *think* TTL is Time for DNS entry to live before it forgets the > entry. But if that's the case, then why is Expire there if TTL does the > same thing? Refresh I *think* is when it re-loads the DNS entry from the > main server, and retry I *think* is how often it tries to get a DNS entry > until successful. TTL is the time a non-authoritative server will cache a negative response. for re-use. (This is actually an over-loaded term. See below.) Expire the the time a slave server will continue to provide data for a zone without contact to with the master server. Refresh is how often a slave should check with the master for possible changes in the zone. Retry is the time a slave should wait after a failed refresh attempt before trying another refresh. Expire the the time a slave server will continue to provide data for a zone without contact to with the master server. More on TTL. The value in the SOA record is the MINIMUM TTL for cache entries to live. A separate TTL should be attached to each RR in the zone. That TTL is the one used to time out normal cache entries. The MINIMUM TTL in the SOA is used to set the time a negative cache entry is to be kept. Negative entries are things like non-existent domain messages from an authoritative server. This keeps the servers from being bombarded by broken software looking for something that is not in the zone. The TTL for negative cache is normally small (< 30 minutes) since the query may have simply been premature and the name might be created soon. Since there is no RR, the server can't provide TTL information, so the TTL in the SOA is used. As a matter of practice people do not enter a TTL for every RR. Instead, they use the $TTL declaration at the beginning of a zone to define a default TTL and that value will be attached to all RRs that do not contain and explicit TTL. Are you completely confused yet? R. Kevin Oberman, Network Engineer Energy Sciences Network (ESnet) Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab) E-mail: oberman@es.net Phone: +1 510 486-8634 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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