Active Directory 2008: Zone Transfer Facts…

Zone Transfer Facts

Replication of zone data between primary and secondary zones takes place through zone transfers. You should know the following facts about zone transfers:

  • Each      secondary server is pointed to one or more master servers. A master      server is the server from which the secondary copies the zone data. The      master server can be the primary server or another secondary server.
  • The      zone serial number keeps track of changes to the zone. When you make      changes to the zone, the serial number is incremented.
  • Zone      transfers can copy all records or only changed records:
    • A full zone transfer (AXFR) copies all of the zone       data with each zone transfer.
    • A partial (or incremental) zone transfer (IXFR) copies       only the changed records. This is the default method on Windows Server       2008.
  • By      default, zone transfer in Windows Server 2008 is disabled for security      reasons. To use zone transfers, manually enable the feature in the DNS settings      in Server Manager.
  • You      can restrict the servers to which zone transfers are allowed. There are      two ways of doing this:
    • Allow zone transfers only to servers that are listed       as name servers.
    • Allow zone transfers only to servers you specifically       identify.
  • Zone      transfer is always initiated by a secondary server.
    • The secondary server contacts the master server and       compares the serial number on the master with the serial number in its       copy.
    • If the serial number on the master is greater, the       secondary initiates zone transfer.
    • If the serial number is the same (or lower) on the       master, no zone transfer takes place.
  • Windows      DNS servers support the use of DNS Notify. With DNS Notify, master servers      are configured with a list of slave DNS servers.
    • When a change takes place, the master notifies the       slave servers that the zone has changed.
    • The secondary server then initiates zone transfer,       first checking the serial number, then requesting changes.

You can allow notification for all name servers, or only for listed servers.

  • You      can improve DNS performance by placing multiple DNS servers on your      network. For example, you can place a secondary server on the other side      of a WAN link to reduce WAN traffic caused by name resolution. However,      zone replication traffic must still cross the WAN link.
  • A      caching only server runs DNS but has no zones configured. Use a caching      only server to improve performance while eliminating zone transfers.
  • An      Active Directory-integrated zone stores DNS information in Active Directory      rather than in a zone file. Zone information is copied automatically when      Active Directory replicates.
  • If      a zone is Active Directory-integrated and has no secondary servers, you      can disable zone transfers. Zone data will continue to be replicated through      Active Directory.
  • Active      Directory replication traffic is automatically secured. To secure zone      transfers to secondary servers, use IPsec between servers.

Normally, zone transfers happen automatically at periodic intervals. You can force an update of zone data through the DNS console or by using the Dnscmd command. The following table lists some actions you can take to refresh zone data manually.

DNS Console Action Dnscmd Option Result
Reload Dnscmd /ReloadZone The server reloads zone data from   its local copy (it reads the data back in from the zone file on the hard   disk).
Transfer from Master Dnscmd /Refresh Initiates a normal zone transfer.   The DNS server compares its version number with the version of the zone   master. If the version numbers are the same, no zone transfer takes place.
Reload from Master N/A The DNS server dumps its copy of   the data and reloads the entire data from the master server.
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