In situations where you need to restrict the use of specific characters in a host name for DDNS updates, you can configure a hostname rewrite policy. Such policy accepts certain characters and replaces others in host names specified in IPv4 DHCP requests. When you create a hostname rewrite policy, you enter a regular expression matching invalid characters that the application replaces in the host name. You also specify a character that the application uses to replace invalid characters.
When you enable a hostname rewrite policy, the application replaces host names with the newly translated host name when it issues DHCP leases and sends DDNS updates for IPv4 DHCP clients. Before you enable a hostname rewrite policy, consider the following:
You must enable DDNS updates before the hostname rewrite policy can take effect.
The policy supports only IPv4 DHCP clients.
If DHCP option 81 support is enabled and updating DDNS is in the request, the application sends updates for A records directly to the DNS server and DHCP only updates the PTR record. When this happens, there can be a mismatch in the host name between the A and PTR records.
Changes made to a hostname rewrite policy apply only to subsequent DDNS updates.
When an IPv4 DHCP client requests an IP address, it includes its host name in DHCP option 12. If you enable a hostname rewrite policy, the application uses the newly translated host name when it issues a lease to the client.
The client can also include a FQDN in option 81, in which it instructs the server whether to perform DDNS updates. If the client sends a FQDN in option 81, the application replaces the entire FQDN based on the policy. For example, if the FQDN in option 81 is dev.bldg12.corpxyz.com, the application replaces invalid characters in the entire FQDN even though the host name can be dev or dev.bldg12. For example, if your hostname rewrite policy specifies valid characters as a-z and the replacement character is -, the newly translated FQDN is dev.bldg--.corpxyz.com. For information about client FQDN in option 81, see Enabling DDNS for IPv4 Clients#About the Client FQDN Option.
Note that when multiple IPv4 DHCP clients specify host names that map to the same translated host name, the application allocates leases to all clients, but it only sends DDNS updates to the first client request. When it tries to update DNS for subsequent clients, the updates fail. You can override this policy at the DNS server level.