openwrt-packages/net/ntpd

Configuring NTPD with UCI

Precedent

Sysntpd is the lightweight implementation of the NTP protocol under Busybox. It supports many (but not all) of the same parameters.

It is configured as a config timeserver ntp section in /etc/config/system, below.

Configuration

A sample configuration looks like:

/etc/config/system:

config timeserver ntp
	option enabled 1
	option enable_server 1
	list server tick.udel.edu
	list server tock.udel.edu
	list interface eth0
	list interface eth1
	list interface eth2

If you want to temporarily disable the service without deleting all of the configuration state, this is done by clearing the enabled parameter. If this parameter is 1 (the default), the service is enabled.

The service can run as a stand-alone client (enable_server 0, the default) or it can also operate as a server in turn to local clients, by setting this parameter to 1.

The parameter(s) server enumerate a list of servers to be used for reference NTP servers by the local daemon. At least one is required, and two or more are recommended (unless you have an extremely available local server). They should be picked to be geographically divergent, and preferably reachable via different network carriers to protect against network partitions, etc. They should also be high-quality time providers (i.e. having stable, accurate clock sources).

The interface parameter enumerates the list of interfaces on which the server is reachable (see enable_server 1 above), and may be a subset of all of the interfaces present on the system. For security reasons, you may elect to only offer the service on internal networks. If omitted, it defaults to all interfaces.

Differences with sysntpd

Busybox sysntpd supports configuring servers based on DHCP provisioning (option 6, per the DHCP and BOOTP Parameter list from IANA). This functionality is enabled (in Busybox) with the use_dhcp boolean parameter (default 1), and the dhcp_interface list parameter, which enumerates the interfaces whose provisioning is to be utilized.

Considerations for DHCP-provisioned NTP servers

Most terrestrial and satellite ISPs have access to very high-quality clock sources (these are required to maintain synchronization on T3, OC3, etc trunks or earth terminals) but seldom offer access to those time sources via NTP in turn to their clients, mostly from a misplaced fear that their time source might come under attack (a slave closely tied to the master could also provide extremely high-quality time without the risk of network desynchronization should it come under sophisticated attack).

As a result, the NTP servers that your ISP may point you at are often of unknown/unverified quality, and you use them at your own risk.

Early millennial versions of Windows (2000, XP, etc) used NTP only to initially set the clock to approximately 100ms accuracy (and not maintain synchronization), so the bar wasn't set very high. Since then, requirements for higher-quality timekeeping have arisen (e.g. multi-master SQL database replication), but most ISPs have not kept up with the needs of their users.

Current releases of Windows use Domain Controllers for time acquisition via the NT5DS protocol when domain joined.

Because of the unreliable quality of NTP servers DHCP-provisioned by ISPs, support for this functionality was deemed unnecessary.