Hot backup of active and standby DNS servers indicates that both
the active DNS server and the standby DNS server provide the domain
name resolution service and automatically synchronize data.

In the DNS networking, two DNS servers work in active/standby mode.
The active and standby DNS servers provide the domain name resolution
service, and synchronize data between each other based on the rules
configured for zones.
On the active DNS server, the IP address of the standby DNS server
and the information about zones managed by the active DNS server are
configured. If the resource records of these zones change, the active
DNS server instructs the standby DNS server
to update the records in real time.
On the standby DNS server, the IP address of the active
DNS server is set. The zones managed by the standby DNS server contain
the following attributes for controlling data synchronization:

• Refresh
  interval for the standby DNS server: indicates the interval at which
  the standby DNS server synchronizes data from the active DNS server. The
  default interval is three hours.
• Retry interval for the standby
  DNS server: indicates the interval at which the standby DNS server
  reattempts synchronization in case of a synchronization failure. The
  default interval is one hour.
• Expiration time for the standby
  DNS server: indicates the maximum validity period for the standby DNS
  server to provide the domain name resolution service in the case that
  the communication between the standby DNS server and the active DNS
  server
  fails. The default maximum validity period is seven day

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Default maximum validity period is seven day ..............