Please Whitelist This Site? I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :) If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads. If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK. Thanks for your understanding! Sincerely, Charles Kozierok Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited. If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  Name Systems and TCP/IP Name Registration and Name Resolution            9  TCP/IP Name Systems: Host Tables and Domain Name System (DNS)                 9  TCP/IP Domain Name System (DNS)                      9  DNS Name Servers and Name Resolution                           9  DNS Name Server Concepts and Operation

DNS Name Server Types and Roles: Primary/Master, Secondary/Slave and Caching-Only Servers 1 2 3 DNS Root Name Servers

Zone Transfer Mechanism

When a zone transfer is required, it is accomplished using a DNS query sent to the master server using the regular DNS query/response messaging method used for name resolution (discussed in the next section). A special DNS question type, called AXFR (address transfer) is used to initiate the zone transfer. The server will then transfer the resource records for the zone using a series of DNS response messages (assuming that the server that requested the transfer is authorized to do so). Since it's important that zone transfers be received reliably, and since the amount of data to be sent is large and needs to be managed, a TCP session must first be established and used for zone transfers. This is in contrast to the simpler UDP transport used for regular DNS messages. This is discussed in the topic discussing the use of UDP and TCP for DNS.

Once the zone transfer is complete, the slave name server will update its database and return to regular operation. It will continue to perform regular polls of the master server every Refresh seconds. If it has a problem with a regular poll, it will try again after Retry seconds. Finally, if an amount of time equal to Expires seconds elapses, the master name server will stop serving data from the zone until it re-establishes contact with the primary name server.

Note that the DNS Notify feature is an enhancement to the basic zone status check / zone transfer model. It allows the master server to notify a slave server when the master’s database has changed. Another new feature allows only part of a zone to be transferred instead of the entire zone. See the topic on DNS name server enhancements for more.

DNS Name Server Types and Roles: Primary/Master, Secondary/Slave and Caching-Only Servers 1 2 3 DNS Root Name Servers