Understanding DDNS Updates from DHCP

DHCP supports several DNS-related options (such as options 12, 15, and 81 for IPv4, and options 23, 24, and 39 for IPv6). With DDNS (Dynamic DNS) updates, a DHCP server or client can use the information in these options to inform a DNS server of dynamic domain name-to-IP address assignments.

Note: DDNS updates is not supported by IPv6-only appliances.

To set up one or more NIOS appliances for DDNS updates originating from DHCP, you must configure at least one DHCP server and one DNS server. These servers might be on the same appliance or on separate appliances. Three possible arrangements for a DHCP server to update a DNS server are shown in Figure 21.1.
Figure 21.1 Relationship of DHCP and DNS Servers for DDNS Updates

Here is a closer look at one setup for performing DDNS updates from a DHCP server (the steps relate to Figure 21.2).

When an IPv4 DHCP client requests an IP address, the client sends its host name (DHCP option 12). The client also includes its MAC address in the ethernet frame header.
a. When the DHCP server responds with an IP address, it usually provides a domain name (DHCP option 15). The combined host name (from the client) and domain name (from the server) form an FQDN (fully qualified domain name), which the NIOS appliance associates with the IP address in the DHCP lease.
b. The DHCP server sends the A, TXT, and PTR records of the DHCP client to the primary DNS server to update its resource records with the dynamically associated FQDN + IP address.
The primary DNS server notifies its secondary servers of a change. The secondary servers confirm the need for a zone transfer, and the primary server sends the updated zone data to the secondary server, completing the update.

Note: For information about zone transfers, see Enabling Zone Transfers.

Figure 21.2 DDNS Update from a DHCP Server

To enable a DHCP server to send DDNS updates to a DNS server, you must configure both servers to support the updates. First, configure the DHCP server to do the following:

Provide what is needed to create an FQDN: add a server-generated host name to a server-provided domain name, add a server-provided domain name to a client-supplied host name, or permit the client to provide its own FQDN
Send updates to a DNS server

Then, configure the following on the DNS server:

Accept updates from the DHCP server, a secondary DNS server, or a DHCP client
If the DHCP server sends updates to a secondary DNS server, configure the secondary server to forward updates to the primary DNS server

When setting up DDNS, you can determine the amount of information that DHCP clients provide to a DHCP server — and vice versa — and where the DDNS updates originate. A summary of these options for IPv4 is shown in Figure 21.3. It is similar for IPv6, except that the DHCP client and server exchange Request and Reply messages, AAAA records are updated instead of A records, and the FQDN option is option 39.

Figure 21.3 DHCP Clients and Server Providing DNS Information and Updates

You can configure the DHCP and DNS settings for DDNS at the Grid level, member level, and network and zone level. By applying the inheritance model in the NIOS appliance, settings made at the Grid level apply to all members in the Grid. Settings you make at the member level apply to all networks and zones configured on that member. Settings made at the network and zone level apply specifically to just that network and zone. When configuring independent appliances (that is, appliances that are not in a Grid), do not use the member-level settings. Instead, configure DDNS updates at the Grid level to apply to all zones and, if necessary, override the Grid-level settings on a per zone basis.