The lease time in effect says how long the PC/Server may retain the IP address before it must request another one or renew its lease.
Because the IP addresses are being allocated dynamically by the DHCP server, this poses a problem for any DNS configuration which needs to keep track of both the forward map (name to IP address) and possibly the reverse map (IP address to name).
Transactions 5 and 6: show an alternative method of providing the same DNS update service directly from the DNS client.
This normally occurs in cases where DHCP is not used such as IPv6 using SLAAC though a number of DHCP clients elect to do this even when using DHCP (includes most Windows clients).
These additional elements are known as options in the DHCP jargon.
In general, when a PC/server, configured to use DHCP (a DHCP Client), is booted it attempts to make contact with a DHCP server by sending a UDP DHCPDISCOVER message (to port 67) using the local network broadcast address (though RFC 2131 explicitly uses the value 255.255.255.255).
DHCP provides a lot of flexibility using a bucket load of statements and many clauses to control its behavior.
These statements are defined in the configuration file.
There are a number of ways this can be done as shown in figure 1 below: Figure 1 - DHCP DNS Update Flow Transactions 1 and 2: show the DHCP Client/Server dialog.
The DDNS standards allow a further method based on an asymmetric cipher called SIG(0).
ISC DHCP does not (as of v4.x) support SIG(0) and it is not discussed further.
This example assumes that DDNS will only use IP security.
This is generally a trivial, but potentially - depending on the environment - insecure, form of configuration.