909 lines
29 KiB
Plaintext
909 lines
29 KiB
Plaintext
.Dd March 21 2017
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.Dt NTPD 1ntpdmdoc User Commands
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.Os
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.\" EDIT THIS FILE WITH CAUTION (ntpd-opts.mdoc)
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.\"
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.\" It has been AutoGen-ed March 21, 2017 at 10:44:23 AM by AutoGen 5.18.5
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.\" From the definitions ntpd-opts.def
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.\" and the template file agmdoc-cmd.tpl
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.Sh NAME
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.Nm ntpd
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.Nd NTP daemon program
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.Sh SYNOPSIS
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.Nm
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.\" Mixture of short (flag) options and long options
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.Op Fl flags
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.Op Fl flag Op Ar value
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.Op Fl \-option\-name Ns Oo Oo Ns "=| " Oc Ns Ar value Oc
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[ <server1> ... <serverN> ]
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.Pp
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.Sh DESCRIPTION
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The
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.Nm
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utility is an operating system daemon which sets
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and maintains the system time of day in synchronism with Internet
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standard time servers.
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It is a complete implementation of the
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Network Time Protocol (NTP) version 4, as defined by RFC\-5905,
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but also retains compatibility with
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version 3, as defined by RFC\-1305, and versions 1
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and 2, as defined by RFC\-1059 and RFC\-1119, respectively.
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.Pp
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The
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.Nm
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utility does most computations in 64\-bit floating point
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arithmetic and does relatively clumsy 64\-bit fixed point operations
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only when necessary to preserve the ultimate precision, about 232
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picoseconds.
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While the ultimate precision is not achievable with
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ordinary workstations and networks of today, it may be required
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with future gigahertz CPU clocks and gigabit LANs.
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.Pp
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Ordinarily,
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.Nm
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reads the
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.Xr ntp.conf 5
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configuration file at startup time in order to determine the
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synchronization sources and operating modes.
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It is also possible to
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specify a working, although limited, configuration entirely on the
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command line, obviating the need for a configuration file.
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This may
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be particularly useful when the local host is to be configured as a
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broadcast/multicast client, with all peers being determined by
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listening to broadcasts at run time.
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.Pp
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If NetInfo support is built into
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.Nm ,
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then
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.Nm
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will attempt to read its configuration from the
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NetInfo if the default
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.Xr ntp.conf 5
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file cannot be read and no file is
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specified by the
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.Fl c
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option.
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.Pp
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Various internal
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.Nm
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variables can be displayed and
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configuration options altered while the
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.Nm
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is running
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using the
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.Xr ntpq 1ntpqmdoc
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and
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.Xr ntpdc 1ntpdcmdoc
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utility programs.
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.Pp
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When
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.Nm
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starts it looks at the value of
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.Xr umask 2 ,
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and if zero
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.Nm
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will set the
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.Xr umask 2
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to 022.
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.Sh "OPTIONS"
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.Bl -tag
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.It Fl 4 , Fl \-ipv4
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Force IPv4 DNS name resolution.
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This option must not appear in combination with any of the following options:
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ipv6.
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.sp
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Force DNS resolution of following host names on the command line
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to the IPv4 namespace.
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.It Fl 6 , Fl \-ipv6
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Force IPv6 DNS name resolution.
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This option must not appear in combination with any of the following options:
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ipv4.
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.sp
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Force DNS resolution of following host names on the command line
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to the IPv6 namespace.
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.It Fl a , Fl \-authreq
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Require crypto authentication.
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This option must not appear in combination with any of the following options:
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authnoreq.
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.sp
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Require cryptographic authentication for broadcast client,
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multicast client and symmetric passive associations.
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This is the default.
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.It Fl A , Fl \-authnoreq
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Do not require crypto authentication.
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This option must not appear in combination with any of the following options:
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authreq.
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.sp
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Do not require cryptographic authentication for broadcast client,
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multicast client and symmetric passive associations.
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This is almost never a good idea.
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.It Fl b , Fl \-bcastsync
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Allow us to sync to broadcast servers.
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.sp
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.It Fl c Ar string , Fl \-configfile Ns = Ns Ar string
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configuration file name.
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.sp
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The name and path of the configuration file,
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\fI/etc/ntp.conf\fP
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by default.
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.It Fl d , Fl \-debug\-level
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Increase debug verbosity level.
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This option may appear an unlimited number of times.
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.sp
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.It Fl D Ar number , Fl \-set\-debug\-level Ns = Ns Ar number
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Set the debug verbosity level.
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This option may appear an unlimited number of times.
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This option takes an integer number as its argument.
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.sp
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.It Fl f Ar string , Fl \-driftfile Ns = Ns Ar string
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frequency drift file name.
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.sp
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The name and path of the frequency file,
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\fI/etc/ntp.drift\fP
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by default.
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This is the same operation as the
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\fBdriftfile\fP \fIdriftfile\fP
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configuration specification in the
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\fI/etc/ntp.conf\fP
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file.
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.It Fl g , Fl \-panicgate
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Allow the first adjustment to be Big.
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This option may appear an unlimited number of times.
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.sp
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Normally,
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\fBntpd\fP
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exits with a message to the system log if the offset exceeds the panic threshold, which is 1000 s by default. This option allows the time to be set to any value without restriction; however, this can happen only once. If the threshold is exceeded after that,
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\fBntpd\fP
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will exit with a message to the system log. This option can be used with the
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\fB\-q\fP
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and
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\fB\-x\fP
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options.
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See the
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\fBtinker\fP
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configuration file directive for other options.
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.It Fl G , Fl \-force\-step\-once
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Step any initial offset correction..
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.sp
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Normally,
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\fBntpd\fP
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steps the time if the time offset exceeds the step threshold,
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which is 128 ms by default, and otherwise slews the time.
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This option forces the initial offset correction to be stepped,
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so the highest time accuracy can be achieved quickly.
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However, this may also cause the time to be stepped back
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so this option must not be used if
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applications requiring monotonic time are running.
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See the \fBtinker\fP configuration file directive for other options.
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.It Fl i Ar string , Fl \-jaildir Ns = Ns Ar string
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Jail directory.
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.sp
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Chroot the server to the directory
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\fIjaildir\fP
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.
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This option also implies that the server attempts to drop root privileges at startup.
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You may need to also specify a
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\fB\-u\fP
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option.
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This option is only available if the OS supports adjusting the clock
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without full root privileges.
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This option is supported under NetBSD (configure with
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\fB\-\-enable\-clockctl\fP) or Linux (configure with
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\fB\-\-enable\-linuxcaps\fP) or Solaris (configure with \fB\-\-enable\-solarisprivs\fP).
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.It Fl I Ar iface , Fl \-interface Ns = Ns Ar iface
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Listen on an interface name or address.
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This option may appear an unlimited number of times.
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.sp
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Open the network address given, or all the addresses associated with the
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given interface name. This option may appear multiple times. This option
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also implies not opening other addresses, except wildcard and localhost.
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This option is deprecated. Please consider using the configuration file
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\fBinterface\fP command, which is more versatile.
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.It Fl k Ar string , Fl \-keyfile Ns = Ns Ar string
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path to symmetric keys.
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.sp
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Specify the name and path of the symmetric key file.
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\fI/etc/ntp.keys\fP
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is the default.
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This is the same operation as the
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\fBkeys\fP \fIkeyfile\fP
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configuration file directive.
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.It Fl l Ar string , Fl \-logfile Ns = Ns Ar string
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path to the log file.
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.sp
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Specify the name and path of the log file.
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The default is the system log file.
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This is the same operation as the
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\fBlogfile\fP \fIlogfile\fP
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configuration file directive.
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.It Fl L , Fl \-novirtualips
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Do not listen to virtual interfaces.
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.sp
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Do not listen to virtual interfaces, defined as those with
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names containing a colon. This option is deprecated. Please
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consider using the configuration file \fBinterface\fP command, which
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is more versatile.
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.It Fl M , Fl \-modifymmtimer
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Modify Multimedia Timer (Windows only).
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.sp
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Set the Windows Multimedia Timer to highest resolution. This
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ensures the resolution does not change while ntpd is running,
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avoiding timekeeping glitches associated with changes.
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.It Fl n , Fl \-nofork
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Do not fork.
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This option must not appear in combination with any of the following options:
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wait\-sync.
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.sp
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.It Fl N , Fl \-nice
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Run at high priority.
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.sp
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To the extent permitted by the operating system, run
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\fBntpd\fP
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at the highest priority.
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.It Fl p Ar string , Fl \-pidfile Ns = Ns Ar string
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path to the PID file.
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.sp
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Specify the name and path of the file used to record
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\fBntpd\fP's
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process ID.
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This is the same operation as the
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\fBpidfile\fP \fIpidfile\fP
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configuration file directive.
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.It Fl P Ar number , Fl \-priority Ns = Ns Ar number
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Process priority.
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This option takes an integer number as its argument.
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.sp
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To the extent permitted by the operating system, run
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\fBntpd\fP
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at the specified
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\fBsched_setscheduler(SCHED_FIFO)\fP
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priority.
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.It Fl q , Fl \-quit
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Set the time and quit.
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This option must not appear in combination with any of the following options:
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saveconfigquit, wait\-sync.
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.sp
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\fBntpd\fP
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will not daemonize and will exit after the clock is first
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synchronized. This behavior mimics that of the
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\fBntpdate\fP
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program, which will soon be replaced with a shell script.
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The
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\fB\-g\fP
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and
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\fB\-x\fP
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options can be used with this option.
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Note: The kernel time discipline is disabled with this option.
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.It Fl r Ar string , Fl \-propagationdelay Ns = Ns Ar string
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Broadcast/propagation delay.
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.sp
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Specify the default propagation delay from the broadcast/multicast server to this client. This is necessary only if the delay cannot be computed automatically by the protocol.
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.It Fl \-saveconfigquit Ns = Ns Ar string
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Save parsed configuration and quit.
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This option must not appear in combination with any of the following options:
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quit, wait\-sync.
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.sp
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Cause \fBntpd\fP to parse its startup configuration file and save an
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equivalent to the given filename and exit. This option was
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designed for automated testing.
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.It Fl s Ar string , Fl \-statsdir Ns = Ns Ar string
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Statistics file location.
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.sp
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Specify the directory path for files created by the statistics facility.
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This is the same operation as the
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\fBstatsdir\fP \fIstatsdir\fP
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configuration file directive.
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.It Fl t Ar tkey , Fl \-trustedkey Ns = Ns Ar tkey
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Trusted key number.
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This option may appear an unlimited number of times.
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.sp
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Add the specified key number to the trusted key list.
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.It Fl u Ar string , Fl \-user Ns = Ns Ar string
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Run as userid (or userid:groupid).
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.sp
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Specify a user, and optionally a group, to switch to.
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This option is only available if the OS supports adjusting the clock
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without full root privileges.
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This option is supported under NetBSD (configure with
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\fB\-\-enable\-clockctl\fP) or Linux (configure with
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\fB\-\-enable\-linuxcaps\fP) or Solaris (configure with \fB\-\-enable\-solarisprivs\fP).
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.It Fl U Ar number , Fl \-updateinterval Ns = Ns Ar number
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interval in seconds between scans for new or dropped interfaces.
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This option takes an integer number as its argument.
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.sp
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Give the time in seconds between two scans for new or dropped interfaces.
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For systems with routing socket support the scans will be performed shortly after the interface change
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has been detected by the system.
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Use 0 to disable scanning. 60 seconds is the minimum time between scans.
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.It Fl \-var Ns = Ns Ar nvar
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make ARG an ntp variable (RW).
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This option may appear an unlimited number of times.
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.sp
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.It Fl \-dvar Ns = Ns Ar ndvar
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make ARG an ntp variable (RW|DEF).
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This option may appear an unlimited number of times.
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.sp
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.It Fl w Ar number , Fl \-wait\-sync Ns = Ns Ar number
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Seconds to wait for first clock sync.
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This option must not appear in combination with any of the following options:
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nofork, quit, saveconfigquit.
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This option takes an integer number as its argument.
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.sp
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If greater than zero, alters \fBntpd\fP's behavior when forking to
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daemonize. Instead of exiting with status 0 immediately after
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the fork, the parent waits up to the specified number of
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seconds for the child to first synchronize the clock. The exit
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status is zero (success) if the clock was synchronized,
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otherwise it is \fBETIMEDOUT\fP.
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This provides the option for a script starting \fBntpd\fP to easily
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wait for the first set of the clock before proceeding.
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.It Fl x , Fl \-slew
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Slew up to 600 seconds.
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.sp
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Normally, the time is slewed if the offset is less than the step threshold, which is 128 ms by default, and stepped if above the threshold.
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This option sets the threshold to 600 s, which is well within the accuracy window to set the clock manually.
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Note: Since the slew rate of typical Unix kernels is limited to 0.5 ms/s, each second of adjustment requires an amortization interval of 2000 s.
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Thus, an adjustment as much as 600 s will take almost 14 days to complete.
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This option can be used with the
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\fB\-g\fP
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and
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\fB\-q\fP
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options.
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See the
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\fBtinker\fP
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configuration file directive for other options.
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Note: The kernel time discipline is disabled with this option.
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.It Fl \-usepcc
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Use CPU cycle counter (Windows only).
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.sp
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Attempt to substitute the CPU counter for \fBQueryPerformanceCounter\fP.
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The CPU counter and \fBQueryPerformanceCounter\fP are compared, and if
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they have the same frequency, the CPU counter (RDTSC on x86) is
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used directly, saving the overhead of a system call.
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.It Fl \-pccfreq Ns = Ns Ar string
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Force CPU cycle counter use (Windows only).
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.sp
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Force substitution the CPU counter for \fBQueryPerformanceCounter\fP.
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The CPU counter (RDTSC on x86) is used unconditionally with the
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given frequency (in Hz).
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.It Fl m , Fl \-mdns
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Register with mDNS as a NTP server.
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.sp
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Registers as an NTP server with the local mDNS server which allows
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the server to be discovered via mDNS client lookup.
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.It Fl \&? , Fl \-help
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Display usage information and exit.
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.It Fl \&! , Fl \-more\-help
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Pass the extended usage information through a pager.
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.It Fl \-version Op Brq Ar v|c|n
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Output version of program and exit. The default mode is `v', a simple
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version. The `c' mode will print copyright information and `n' will
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print the full copyright notice.
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.El
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.Sh "OPTION PRESETS"
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Any option that is not marked as \fInot presettable\fP may be preset
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by loading values from environment variables named:
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.nf
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\fBNTPD_<option\-name>\fP or \fBNTPD\fP
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.fi
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.ad
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.Sh USAGE
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.Ss "How NTP Operates"
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The
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.Nm
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utility operates by exchanging messages with
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one or more configured servers over a range of designated poll intervals.
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When
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started, whether for the first or subsequent times, the program
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requires several exchanges from the majority of these servers so
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the signal processing and mitigation algorithms can accumulate and
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groom the data and set the clock.
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In order to protect the network
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from bursts, the initial poll interval for each server is delayed
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an interval randomized over a few seconds.
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At the default initial poll
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interval of 64s, several minutes can elapse before the clock is
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set.
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This initial delay to set the clock
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can be safely and dramatically reduced using the
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.Cm iburst
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keyword with the
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.Ic server
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configuration
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command, as described in
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.Xr ntp.conf 5 .
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.Pp
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Most operating systems and hardware of today incorporate a
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time\-of\-year (TOY) chip to maintain the time during periods when
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the power is off.
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When the machine is booted, the chip is used to
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initialize the operating system time.
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After the machine has
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synchronized to a NTP server, the operating system corrects the
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chip from time to time.
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In the default case, if
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.Nm
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detects that the time on the host
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is more than 1000s from the server time,
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.Nm
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assumes something must be terribly wrong and the only
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reliable action is for the operator to intervene and set the clock
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by hand.
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(Reasons for this include there is no TOY chip,
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or its battery is dead, or that the TOY chip is just of poor quality.)
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This causes
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.Nm
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to exit with a panic message to
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the system log.
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The
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.Fl g
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option overrides this check and the
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clock will be set to the server time regardless of the chip time
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(up to 68 years in the past or future \(em
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this is a limitation of the NTPv4 protocol).
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However, and to protect against broken hardware, such as when the
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CMOS battery fails or the clock counter becomes defective, once the
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clock has been set an error greater than 1000s will cause
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.Nm
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to exit anyway.
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.Pp
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Under ordinary conditions,
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.Nm
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adjusts the clock in
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small steps so that the timescale is effectively continuous and
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without discontinuities.
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Under conditions of extreme network
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congestion, the roundtrip delay jitter can exceed three seconds and
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the synchronization distance, which is equal to one\-half the
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roundtrip delay plus error budget terms, can become very large.
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The
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.Nm
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algorithms discard sample offsets exceeding 128 ms,
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unless the interval during which no sample offset is less than 128
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ms exceeds 900s.
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The first sample after that, no matter what the
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offset, steps the clock to the indicated time.
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In practice this
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reduces the false alarm rate where the clock is stepped in error to
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a vanishingly low incidence.
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.Pp
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As the result of this behavior, once the clock has been set it
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very rarely strays more than 128 ms even under extreme cases of
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network path congestion and jitter.
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Sometimes, in particular when
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.Nm
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is first started without a valid drift file
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on a system with a large intrinsic drift
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the error might grow to exceed 128 ms,
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which would cause the clock to be set backwards
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if the local clock time is more than 128 s
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in the future relative to the server.
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In some applications, this behavior may be unacceptable.
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There are several solutions, however.
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If the
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.Fl x
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option is included on the command line, the clock will
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never be stepped and only slew corrections will be used.
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But this choice comes with a cost that
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should be carefully explored before deciding to use
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the
|
|
.Fl x
|
|
option.
|
|
The maximum slew rate possible is limited
|
|
to 500 parts\-per\-million (PPM) as a consequence of the correctness
|
|
principles on which the NTP protocol and algorithm design are
|
|
based.
|
|
As a result, the local clock can take a long time to
|
|
converge to an acceptable offset, about 2,000 s for each second the
|
|
clock is outside the acceptable range.
|
|
During this interval the
|
|
local clock will not be consistent with any other network clock and
|
|
the system cannot be used for distributed applications that require
|
|
correctly synchronized network time.
|
|
.Pp
|
|
In spite of the above precautions, sometimes when large
|
|
frequency errors are present the resulting time offsets stray
|
|
outside the 128\-ms range and an eventual step or slew time
|
|
correction is required.
|
|
If following such a correction the
|
|
frequency error is so large that the first sample is outside the
|
|
acceptable range,
|
|
.Nm
|
|
enters the same state as when the
|
|
.Pa ntp.drift
|
|
file is not present.
|
|
The intent of this behavior
|
|
is to quickly correct the frequency and restore operation to the
|
|
normal tracking mode.
|
|
In the most extreme cases
|
|
(the host
|
|
.Cm time.ien.it
|
|
comes to mind), there may be occasional
|
|
step/slew corrections and subsequent frequency corrections.
|
|
It
|
|
helps in these cases to use the
|
|
.Cm burst
|
|
keyword when
|
|
configuring the server, but
|
|
ONLY
|
|
when you have permission to do so from the owner of the target host.
|
|
.Pp
|
|
Finally,
|
|
in the past many startup scripts would run
|
|
.Xr ntpdate 1ntpdatemdoc
|
|
or
|
|
.Xr sntp 1sntpmdoc
|
|
to get the system clock close to correct before starting
|
|
.Xr ntpd 1ntpdmdoc ,
|
|
but this was never more than a mediocre hack and is no longer needed.
|
|
If you are following the instructions in
|
|
.Sx "Starting NTP (Best Current Practice)"
|
|
and you still need to set the system time before starting
|
|
.Nm ,
|
|
please open a bug report and document what is going on,
|
|
and then look at using
|
|
.Xr sntp 1sntpmdoc
|
|
if you really need to set the clock before starting
|
|
.Nm .
|
|
.Pp
|
|
There is a way to start
|
|
.Xr ntpd 1ntpdmdoc
|
|
that often addresses all of the problems mentioned above.
|
|
.Ss "Starting NTP (Best Current Practice)"
|
|
First, use the
|
|
.Cm iburst
|
|
option on your
|
|
.Cm server
|
|
entries.
|
|
.Pp
|
|
If you can also keep a good
|
|
.Pa ntp.drift
|
|
file then
|
|
.Xr ntpd 1ntpdmdoc
|
|
will effectively "warm\-start" and your system's clock will
|
|
be stable in under 11 seconds' time.
|
|
.Pp
|
|
As soon as possible in the startup sequence, start
|
|
.Xr ntpd 1ntpdmdoc
|
|
with at least the
|
|
.Fl g
|
|
and perhaps the
|
|
.Fl N
|
|
options.
|
|
Then,
|
|
start the rest of your "normal" processes.
|
|
This will give
|
|
.Xr ntpd 1ntpdmdoc
|
|
as much time as possible to get the system's clock synchronized and stable.
|
|
.Pp
|
|
Finally,
|
|
if you have processes like
|
|
.Cm dovecot
|
|
or database servers
|
|
that require
|
|
monotonically\-increasing time,
|
|
run
|
|
.Xr ntp\-wait 1ntp\-waitmdoc
|
|
as late as possible in the boot sequence
|
|
(perhaps with the
|
|
.Fl v
|
|
flag)
|
|
and after
|
|
.Xr ntp\-wait 1ntp\-waitmdoc
|
|
exits successfully
|
|
it is as safe as it will ever be to start any process that require
|
|
stable time.
|
|
.Ss "Frequency Discipline"
|
|
The
|
|
.Nm
|
|
behavior at startup depends on whether the
|
|
frequency file, usually
|
|
.Pa ntp.drift ,
|
|
exists.
|
|
This file
|
|
contains the latest estimate of clock frequency error.
|
|
When the
|
|
.Nm
|
|
is started and the file does not exist, the
|
|
.Nm
|
|
enters a special mode designed to quickly adapt to
|
|
the particular system clock oscillator time and frequency error.
|
|
This takes approximately 15 minutes, after which the time and
|
|
frequency are set to nominal values and the
|
|
.Nm
|
|
enters
|
|
normal mode, where the time and frequency are continuously tracked
|
|
relative to the server.
|
|
After one hour the frequency file is
|
|
created and the current frequency offset written to it.
|
|
When the
|
|
.Nm
|
|
is started and the file does exist, the
|
|
.Nm
|
|
frequency is initialized from the file and enters normal mode
|
|
immediately.
|
|
After that the current frequency offset is written to
|
|
the file at hourly intervals.
|
|
.Ss "Operating Modes"
|
|
The
|
|
.Nm
|
|
utility can operate in any of several modes, including
|
|
symmetric active/passive, client/server broadcast/multicast and
|
|
manycast, as described in the
|
|
.Qq Association Management
|
|
page
|
|
(available as part of the HTML documentation
|
|
provided in
|
|
.Pa /usr/share/doc/ntp ) .
|
|
It normally operates continuously while
|
|
monitoring for small changes in frequency and trimming the clock
|
|
for the ultimate precision.
|
|
However, it can operate in a one\-time
|
|
mode where the time is set from an external server and frequency is
|
|
set from a previously recorded frequency file.
|
|
A
|
|
broadcast/multicast or manycast client can discover remote servers,
|
|
compute server\-client propagation delay correction factors and
|
|
configure itself automatically.
|
|
This makes it possible to deploy a
|
|
fleet of workstations without specifying configuration details
|
|
specific to the local environment.
|
|
.Pp
|
|
By default,
|
|
.Nm
|
|
runs in continuous mode where each of
|
|
possibly several external servers is polled at intervals determined
|
|
by an intricate state machine.
|
|
The state machine measures the
|
|
incidental roundtrip delay jitter and oscillator frequency wander
|
|
and determines the best poll interval using a heuristic algorithm.
|
|
Ordinarily, and in most operating environments, the state machine
|
|
will start with 64s intervals and eventually increase in steps to
|
|
1024s.
|
|
A small amount of random variation is introduced in order to
|
|
avoid bunching at the servers.
|
|
In addition, should a server become
|
|
unreachable for some time, the poll interval is increased in steps
|
|
to 1024s in order to reduce network overhead.
|
|
.Pp
|
|
In some cases it may not be practical for
|
|
.Nm
|
|
to run continuously.
|
|
A common workaround has been to run the
|
|
.Xr ntpdate 1ntpdatemdoc
|
|
or
|
|
.Xr sntp 1sntpmdoc
|
|
programs from a
|
|
.Xr cron 8
|
|
job at designated
|
|
times.
|
|
However, these programs do not have the crafted signal
|
|
processing, error checking or mitigation algorithms of
|
|
.Nm .
|
|
The
|
|
.Fl q
|
|
option is intended for this purpose.
|
|
Setting this option will cause
|
|
.Nm
|
|
to exit just after
|
|
setting the clock for the first time.
|
|
The procedure for initially
|
|
setting the clock is the same as in continuous mode; most
|
|
applications will probably want to specify the
|
|
.Cm iburst
|
|
keyword with the
|
|
.Ic server
|
|
configuration command.
|
|
With this
|
|
keyword a volley of messages are exchanged to groom the data and
|
|
the clock is set in about 10 s.
|
|
If nothing is heard after a
|
|
couple of minutes, the daemon times out and exits.
|
|
After a suitable
|
|
period of mourning, the
|
|
.Xr ntpdate 1ntpdatemdoc
|
|
program will be
|
|
retired.
|
|
.Pp
|
|
When kernel support is available to discipline the clock
|
|
frequency, which is the case for stock Solaris, Tru64, Linux and
|
|
.Fx ,
|
|
a useful feature is available to discipline the clock
|
|
frequency.
|
|
First,
|
|
.Nm
|
|
is run in continuous mode with
|
|
selected servers in order to measure and record the intrinsic clock
|
|
frequency offset in the frequency file.
|
|
It may take some hours for
|
|
the frequency and offset to settle down.
|
|
Then the
|
|
.Nm
|
|
is
|
|
stopped and run in one\-time mode as required.
|
|
At each startup, the
|
|
frequency is read from the file and initializes the kernel
|
|
frequency.
|
|
.Ss "Poll Interval Control"
|
|
This version of NTP includes an intricate state machine to
|
|
reduce the network load while maintaining a quality of
|
|
synchronization consistent with the observed jitter and wander.
|
|
There are a number of ways to tailor the operation in order enhance
|
|
accuracy by reducing the interval or to reduce network overhead by
|
|
increasing it.
|
|
However, the user is advised to carefully consider
|
|
the consequences of changing the poll adjustment range from the
|
|
default minimum of 64 s to the default maximum of 1,024 s.
|
|
The
|
|
default minimum can be changed with the
|
|
.Ic tinker
|
|
.Cm minpoll
|
|
command to a value not less than 16 s.
|
|
This value is used for all
|
|
configured associations, unless overridden by the
|
|
.Cm minpoll
|
|
option on the configuration command.
|
|
Note that most device drivers
|
|
will not operate properly if the poll interval is less than 64 s
|
|
and that the broadcast server and manycast client associations will
|
|
also use the default, unless overridden.
|
|
.Pp
|
|
In some cases involving dial up or toll services, it may be
|
|
useful to increase the minimum interval to a few tens of minutes
|
|
and maximum interval to a day or so.
|
|
Under normal operation
|
|
conditions, once the clock discipline loop has stabilized the
|
|
interval will be increased in steps from the minimum to the
|
|
maximum.
|
|
However, this assumes the intrinsic clock frequency error
|
|
is small enough for the discipline loop correct it.
|
|
The capture
|
|
range of the loop is 500 PPM at an interval of 64s decreasing by a
|
|
factor of two for each doubling of interval.
|
|
At a minimum of 1,024
|
|
s, for example, the capture range is only 31 PPM.
|
|
If the intrinsic
|
|
error is greater than this, the drift file
|
|
.Pa ntp.drift
|
|
will
|
|
have to be specially tailored to reduce the residual error below
|
|
this limit.
|
|
Once this is done, the drift file is automatically
|
|
updated once per hour and is available to initialize the frequency
|
|
on subsequent daemon restarts.
|
|
.Ss "The huff\-n'\-puff Filter"
|
|
In scenarios where a considerable amount of data are to be
|
|
downloaded or uploaded over telephone modems, timekeeping quality
|
|
can be seriously degraded.
|
|
This occurs because the differential
|
|
delays on the two directions of transmission can be quite large.
|
|
In
|
|
many cases the apparent time errors are so large as to exceed the
|
|
step threshold and a step correction can occur during and after the
|
|
data transfer is in progress.
|
|
.Pp
|
|
The huff\-n'\-puff filter is designed to correct the apparent time
|
|
offset in these cases.
|
|
It depends on knowledge of the propagation
|
|
delay when no other traffic is present.
|
|
In common scenarios this
|
|
occurs during other than work hours.
|
|
The filter maintains a shift
|
|
register that remembers the minimum delay over the most recent
|
|
interval measured usually in hours.
|
|
Under conditions of severe
|
|
delay, the filter corrects the apparent offset using the sign of
|
|
the offset and the difference between the apparent delay and
|
|
minimum delay.
|
|
The name of the filter reflects the negative (huff)
|
|
and positive (puff) correction, which depends on the sign of the
|
|
offset.
|
|
.Pp
|
|
The filter is activated by the
|
|
.Ic tinker
|
|
command and
|
|
.Cm huffpuff
|
|
keyword, as described in
|
|
.Xr ntp.conf 5 .
|
|
.Sh "ENVIRONMENT"
|
|
See \fBOPTION PRESETS\fP for configuration environment variables.
|
|
.Sh FILES
|
|
.Bl -tag -width /etc/ntp.drift -compact
|
|
.It Pa /etc/ntp.conf
|
|
the default name of the configuration file
|
|
.It Pa /etc/ntp.drift
|
|
the default name of the drift file
|
|
.It Pa /etc/ntp.keys
|
|
the default name of the key file
|
|
.El
|
|
.Sh "EXIT STATUS"
|
|
One of the following exit values will be returned:
|
|
.Bl -tag
|
|
.It 0 " (EXIT_SUCCESS)"
|
|
Successful program execution.
|
|
.It 1 " (EXIT_FAILURE)"
|
|
The operation failed or the command syntax was not valid.
|
|
.It 70 " (EX_SOFTWARE)"
|
|
libopts had an internal operational error. Please report
|
|
it to autogen\-users@lists.sourceforge.net. Thank you.
|
|
.El
|
|
.Sh "SEE ALSO"
|
|
.Xr ntp.conf 5 ,
|
|
.Xr ntpdate 1ntpdatemdoc ,
|
|
.Xr ntpdc 1ntpdcmdoc ,
|
|
.Xr ntpq 1ntpqmdoc ,
|
|
.Xr sntp 1sntpmdoc
|
|
.Pp
|
|
In addition to the manual pages provided,
|
|
comprehensive documentation is available on the world wide web
|
|
at
|
|
.Li http://www.ntp.org/ .
|
|
A snapshot of this documentation is available in HTML format in
|
|
.Pa /usr/share/doc/ntp .
|
|
.Rs
|
|
.%A David L. Mills
|
|
.%T Network Time Protocol (Version 1)
|
|
.%O RFC1059
|
|
.Re
|
|
.Rs
|
|
.%A David L. Mills
|
|
.%T Network Time Protocol (Version 2)
|
|
.%O RFC1119
|
|
.Re
|
|
.Rs
|
|
.%A David L. Mills
|
|
.%T Network Time Protocol (Version 3)
|
|
.%O RFC1305
|
|
.Re
|
|
.Rs
|
|
.%A David L. Mills
|
|
.%A J. Martin, Ed.
|
|
.%A J. Burbank
|
|
.%A W. Kasch
|
|
.%T Network Time Protocol Version 4: Protocol and Algorithms Specification
|
|
.%O RFC5905
|
|
.Re
|
|
.Rs
|
|
.%A David L. Mills
|
|
.%A B. Haberman, Ed.
|
|
.%T Network Time Protocol Version 4: Autokey Specification
|
|
.%O RFC5906
|
|
.Re
|
|
.Rs
|
|
.%A H. Gerstung
|
|
.%A C. Elliott
|
|
.%A B. Haberman, Ed.
|
|
.%T Definitions of Managed Objects for Network Time Protocol Version 4: (NTPv4)
|
|
.%O RFC5907
|
|
.Re
|
|
.Rs
|
|
.%A R. Gayraud
|
|
.%A B. Lourdelet
|
|
.%T Network Time Protocol (NTP) Server Option for DHCPv6
|
|
.%O RFC5908
|
|
.Re
|
|
.Sh "AUTHORS"
|
|
The University of Delaware and Network Time Foundation
|
|
.Sh "COPYRIGHT"
|
|
Copyright (C) 1992\-2017 The University of Delaware and Network Time Foundation all rights reserved.
|
|
This program is released under the terms of the NTP license, <http://ntp.org/license>.
|
|
.Sh BUGS
|
|
The
|
|
.Nm
|
|
utility has gotten rather fat.
|
|
While not huge, it has gotten
|
|
larger than might be desirable for an elevated\-priority
|
|
.Nm
|
|
running on a workstation, particularly since many of
|
|
the fancy features which consume the space were designed more with
|
|
a busy primary server, rather than a high stratum workstation in
|
|
mind.
|
|
.Pp
|
|
Please send bug reports to: http://bugs.ntp.org, bugs@ntp.org
|
|
.Sh NOTES
|
|
Portions of this document came from FreeBSD.
|
|
.Pp
|
|
This manual page was \fIAutoGen\fP\-erated from the \fBntpd\fP
|
|
option definitions.
|