Keywords: NTP, BOOTP, DHCP
In the NTP FAQ is the following unanswered question
(http://www.eecis.udel.edu/~ntp/ntpfaq/NTP-s-config.htm#AEN3407:
6.2.5.1.2. How do I use information about NTP servers given in a BOOTP reply?
If you are using Windows or Macintosh, you have to check whether your DHCP client software or your NTP software contains such a feature already. If not, you are probably out of luck.
If you are using a Unix system, you may be able to install the needed code yourself. Of course, exactly what is needed depends on your particular software, but if you are comfortable editing the initialization scripts, it shouldn't be too hard.
In general, Unix systems implement BOOTP/DHCP via a client daemon which handles the interaction with the DHCP server. But often, the daemon does not itself set up the network interface's address or do any other work based on the DHCP reply. Instead, it stores the information in some suitable place, and activates an initialization script to use that information to adjust the computer's configuration.
Your first job is to identify the correct script. Start by tracking down the script that brings up the interface during normal startup. That script will have code which activates the DHCP client, and thus must also arrange to activate the script that implements the configuration provided by DHCP.
For instance, in Red Hat Linux 8.0, a network interface is brought up by /etc/sysconfig/network-scripts/ifup, which calls the DHCP client (/sbin/dhclient) at line 188. When dhclient receives a response from the DHCP server, it executes a program specified by the "-sf" argument, or by default, /sbin/dhclient-script. It is that script which adjusts the computer's configuration. In this case, the script is already set up (line 152) to change ntp.conf if the DHCP server provides NTP server information.
In Red Hat Linux 7.x, a network interface is also brought up by /etc/sysconfig/network-scripts/ifup, which calls the DHCP client (/sbin/pump or /sbin/dhcpcd) near line 127. We see from the dhcpcd(8) manual page that dhcpcd revises resolv.conf itself (unless "-R" is specified), but that other DHCP configuration information can be implemented by the script /etc/dhcpc/dhcpcd-{interface}.exe.
[People should contribute advice about other Unix systems.]
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