1. Related Links
3. Server and Peer Addresses
Following is a description of the server configuration commands in NTPv4. There are two classes of commands, configuration commands that configure an association with a remote server, peer or reference clock, and auxiliary commands that specify environment variables that control various related operations.
The various modes described on the Association Management page are determined by the command keyword and the DNS name or IP address. Addresses are classed by type as (s) a remote server or peer (IPv4 class A, B and C or IPv6), or (b) the IPv4 broadcast address of a local interface.
If the Basic Socket Interface Extensions for IPv6 (RFC 2553) is detected, support for the IPv6 address family is generated in addition to the default IPv4 address family. IPv6 addresses can be identified by the presence of colons ":" in the address field. IPv6 addresses can be used almost everywhere where IPv4 addresses can be used, with the exception of reference clock addresses, which are always IPv4. Note that in contexts where a host name is expected, a -4 qualifier preceding the host name forces DNS resolution to the IPv4 namespace, while a -6 qualifier forces DNS resolution to the IPv6 namespace.
4. Association Commands
Unless noted otherwise, further information about these commands is at Automatic Server Discovery.
The www.pool.ntp.org page describes a compatible pool of public NTP servers, which are probably what you want to define associations with unless you specifically know otherwise.
In these commands, an address can be any of (a) an IPV4 address in a.b.c.d format, (b) an IPV6 address in [a:b:c:d:e:f:g] format, (c) a link-local IPV6 address with an interface specified in [a:b:c:d:e:f:g]%device format, or (d) a DNS hostname.
pool address [burst] [iburst] [version version] [prefer] [minpoll minpoll] [maxpoll maxpoll] [preempt]
server address [key key] [burst] [iburst] [version version] [prefer] [minpoll minpoll] [maxpoll maxpoll]
peer address [key key] [version version] [prefer] [minpoll minpoll] [maxpoll maxpoll]
- unpeer [address | associd | clock clocktype [ unit unitnum]]
These six commands specify the time server name or address to be used and the mode in which to operate. The address can be either a DNS name or an IP address in dotted-quad notation. If it is a refclock, it can be clock followed by a type-unit pair as in the refclock directive; a missing unit clause is interpreted as unit 0.
For server addresses, this command mobilizes a persistent client mode association with a number of remote servers. In this mode the local clock can synchronized to the remote server, but the remote server can never be synchronized to the local clock.
For server addresses, this command mobilizes a persistent client mode association with the specified remote server or local radio clock. In this mode the local clock can synchronized to the remote server, but the remote server can never be synchronized to the local clock.
NTP peer mode has been removed for security reasons. peer is now just an alias for the server keyword. See above.
This command removes a previously configured association. An address or association ID can be used to identify the association. Either an IP address or DNS name can be used. This command is most useful when supplied via ntpq runtime configuration commands config and config-from-file.
5. Server Command Options
Add the command argument, a floating-point value in seconds, to the time offset (θ) computed for this server; this may be useful if you are a client on a network connection such as an ADSL line where there is a predictable asymmetry between upstream and downstream flight times. One way you might see this is if you use a fixed set of others and one has a stable offset that is an outlier from the others; in that case, you might want to use bias to compensate out the offset.
When the server is reachable, send a burst of eight packets instead of the usual one. The packet spacing is normally 2 s; however, the spacing between the first and second packets can be changed with the calldelay command to allow additional time for a modem or ISDN call to complete; this is designed to improve timekeeping quality with the server command.
When the server is unreachable, send a burst of six packets instead of the usual one. The packet spacing is normally 2 s; however, the spacing between the first and second packets can be changed with the calldelay command to allow additional time for a modem or ISDN call to complete; this is designed to speed the initial synchronization acquisition with the server command, and when ntpd(8) is started with the -q option.
- key key
All packets sent to and received from the server or peer are to include authentication fields encrypted using the specified key identifier with values from 1 to 65535, inclusive. The default is to include no encryption field.
- minpoll minpoll
- maxpoll maxpoll
These options specify the minimum and maximum poll intervals for NTP messages, as a power of 2 in seconds. The maximum poll interval defaults to 10 (1,024 s), but can be increased by the maxpoll option to an upper limit of 17 (36.4 h). The minimum poll interval defaults to 6 (64 s), but can be decreased by the minpoll option to a lower limit of 0 (1 s).
- mode option
Pass the option to a reference clock driver. This option is valid only with refclock addresses.
Marks the server as unused, except for display purposes. The server is discarded by the selection algorithm.
Marks the server as preferred. All other things being equal, this host will be chosen for synchronization among a set of correctly operating hosts. See the "Mitigation Rules and the prefer Keyword" page for further information.
Mark the association to assume truechimer status; that is, always survive the selection and clustering algorithms. This option can be used with any association but is most useful for reference clocks with large jitter on the serial port and precision pulse-per-second (PPS) signals. Caution: this option defeats the algorithms designed to cast out falsetickers and can allow these sources to set the system clock. This option is valid only with the server command.
- version version
Specifies the version number to be used for outgoing NTP packets. Versions 1-4 are the choices, with version 4 the default.
6. Auxiliary Commands
Information on authentication for broadcast options can be found at Authentication Options.
- mdnstries number
If we are participating in mDNS, after we have synched for the first time we attempt to register with the mDNS system. If that registration attempt fails, we try again at one minute intervals for up to number times. After all, ntpd may be starting before mDNS. The default value for mdnstries is 5.