DHCP Server Options
How to use DHCP Servers to specify time server addresses to Domain Time Client
Domain Time II Client using automatic discovery mode can be configured to check for the presence of a
DHCP server on the local subnet. If a DHCP server is found, the DHCP options 004 and/or 042 can be examined
to provide the IP address(es) of time servers for the Client to use.
Note: It is not necessary to set the TCP/IP protocol stack to get its IP address from a DHCP server
in order for the Domain Time II Client to get a time server address. The Client uses its own independent inquiry of the
DHCP server to discover the time server options. Therefore DHCP discovery of time servers can be used
on a machine with either a static or a DHCP-assigned IP address.
An automatic Domain Time client with DHCP enabled will broadcast to locate a DHCP Server. Note that DHCP broadcasts usually do not cross routers.
If DHCP options 004 or 042 are configured, the DHCP server will respond with the IP address of the time server.
The client then uses the IP address provided by the DHCP server to contact and sync with the designated time server, even across a router.
The DHCP Time Server Options and Discovery Order
These are the DHCP options that can be set to provide time server addresses. If enabled, they are examined in the order listed:
Option 004 ("Time Servers") can contain a list of servers supplying the DT2 protocol.
If a server is listed in option 004 that doesn't support DT2 UDP, it will be ignored.
Note: That this is a change in behavior from Domain Time v4.1, where Option 004 was used to
specify servers providing the TIME/ITP protocol. TIME/ITP service has been deprecated in v5.1 and later Clients.
Option 042 ("NTP Servers") can contain a list of servers supplying either the NTP or DT2 protocol.
If a server is listed in option 042, it will be checked for NTP first. If NTP fails, it will be checked for DT2 UDP.
If it does not provide time under either of these two protocols, it will be ignored.
Note: That this is a change in behavior from Domain Time v4.1, where Option 042 was evaluated for the DT2
protocol before NTP. The new behavior more accurately reflects the purpose off option 042 (to specify NTP servers)
and also allows administrators to set Clients to prefer NTP. You should now use option 004 if you want to prefer
the DT2 protocol.
Use the Client Discovery Options section of the
Obtain the Time property page on the Client Control Panel Applet to select which DHCP options are enabled.