Miscellaneous Options

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Last update: 22:42 UTC Sunday, December 16, 2007

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broadcastdelay seconds

The broadcast and multicast modes require a special calibration to determine the network delay between the local and remote servers. Ordinarily, this is done automatically by the initial protocol exchanges between the client and server. In some cases, the calibration procedure may fail due to network or server access controls, for example. This command specifies the default delay to be used under these circumstances. Typically (for Ethernet), a number between 0.003 and 0.007 seconds is appropriate. The default when this command is not used is 0.004 seconds.

calldelay delay

This option controls the delay in seconds between the first and second packets sent in burst or iburst mode to allow additional time for a modem or ISDN call to complete.

driftfile driftfile { tolerance ]

This command specifies the complete path and name of the file used to record the frequency of the local clock oscillator. This is the same operation as the -f command linke option. If the file exists, it is read at startup in order to set the initial frequency and then updated once per hour with the current frequency computed by the daemon. If the file name is specified, but the file itself does not exist, the starts with an initial frequency of zero and creates the file when writing it for the first time. If this command is not given, the daemon will always start with an initial frequency of zero.

The file format consists of a single line containing a single floating point number, which records the frequency offset measured in parts-per-million (PPM). The file is updated by first writing the current drift value into a temporary file and then renaming this file to replace the old version. This implies that ntpd must have write permission for the directory the drift file is located in, and that file system links, symbolic or otherwise, should be avoided.

The parameter tolerance is the wander threshold to skip writing the new value. If the value of wander computed from recent frequency changes is greater than this threshold the file will be updated once per hour. If below the threshold, the file will not be written.

Provides a way to enable or disable various system options. Flags not mentioned are unaffected. Note that all of these flags can be controlled remotely using the ntpdc utility program.

auth: Enables the server to synchronize with unconfigured peers only if the peer has been correctly authenticated using either public key or private key cryptography. The default for this flag is enable.
bclient: Enables the server to listen for a message from a broadcast or multicast server, as in the multicastclient command with default address. The default for this flag is disable.
calibrate: Enables the calibrate feature for reference clocks. The default for this flag is disable.
kernel: Enables the kernel time discipline, if available. The default for this flag is enable if support is available, otherwise disable.
monitor: Enables the monitoring facility. See the ntpdc program and the monlist command or further information. The default for this flag is enable.
ntp: Enables time and frequency discipline. In effect, this switch opens and closes the feedback loop, which is useful for testing. The default for this flag is enable.
pps: Enables the pulse-per-second (PPS) signal when frequency and time is disciplined by the precision time kernel modifications. See the A Kernel Model for Precision Timekeeping page for further information. The default for this flag is disable.
stats: Enables the statistics facility. See the Monitoring Options page for further information. The default for this flag is disable

includefile includefile

This command allows additional configuration commands to be included from a separate file. Include files may be nested to a depth of five; upon reaching the end of any include file, command processing resumes in the previous configuration file. This option is useful for sites that run ntpd on multiple hosts, with (mostly) common options (e.g., a restriction list).

logconfig configkeyword

This command controls the amount and type of output written to the system syslog facility or the alternate logfile log file. All configkeyword keywords can be prefixed with =, + and -, where = sets the syslogmask, + adds and - removes messages. syslog messages can be controlled in four classes (clock, peer, sys and sync). Within these classes four types of messages can be controlled: informational messages (info), event messages (events), statistics messages (statistics) and status messages (status).

Configuration keywords are formed by concatenating the message class with the event class. The all prefix can be used instead of a message class. A message class may also be followed by the all keyword to enable/disable all messages of the respective message class. By default, logconfig output is set to allsync.

Thus, a minimal log configuration could look like this:

logconfig=syncstatus +sysevents

This would just list the synchronizations state of ntpd and the major system events. For a simple reference server, the following minimum message configuration could be useful:

logconfig=allsync +allclock

This configuration will list all clock information and synchronization information. All other events and messages about peers, system events and so on is suppressed.

logfile logfile This command specifies the location of an alternate log file to be used instead of the default system syslog facility. This is the same operation as the -l command line option.

phone dial1 dial2 ... This command is used in conjunction with the ACTS modem driver (type 18). The arguments consist of a maximum of 10 telephone numbers used to dial USNO, NIST or European time services. The Hayes command ATDT is normally prepended to the number, which can contain other modem control codes as well.

setvar variable [default]

This command adds an additional system variable. These variables can be used to distribute additional information such as the access policy. If the variable of the form name = value is followed by the default keyword, the variable will be listed as part of the default system variables (ntpq rv command). These additional variables serve informational purposes only. They are not related to the protocol other that they can be listed. The known protocol variables will always override any variables defined via the setvar mechanism. There are three special variables that contain the names of all variable of the same group. The sys_var_list holds the names of all system variables. The peer_var_list holds the names of all peer variables and the clock_var_list holds the names of the reference clock variables.

This command can be used to alter several system variables in very exceptional circumstances. It should occur in the configuration file before any other configuration options. The default values of these variables have been carefully optimized for a wide range of network speeds and reliability expectations. In general, they interact in intricate ways that are hard to predict and some combinations can result in some very nasty behavior. Very rarely is it necessary to change the default values; but, some folks can't resist twisting the knobs anyway and this command is for them. Emphasis added: twisters are on their own and can expect no help from the support group.

The variables operate as follows:

allan allan: The argument becomes the new value for the Allan intercept, which is a parameter of the PLL/FLL clock discipline algorithm. The value is in seconds with default 1500 s, which is appropriate for most computer clocks.
dispersion dispersion: The argument becomes the new value for the dispersion increase rate, normally .000015 s/s.
freq freq: The argument becomes the initial value of the frequency offset in parts-per-million. This overrides the value in the frequency file, if present, and avoids the initial training state if it is not.
huffpuff huffpuff: The argument becomes the new value for the experimental huff-n'-puff filter span, which determines the most recent interval the algorithm will search for a minimum delay. The lower limit is 900 s (15 m), but a more reasonable value is 7200 (2 hours). There is no default, since the filter is not enabled unless this command is given.
panic panic: The argument is the panic threshold, by default 1000 s. If set to zero, the panic sanity check is disabled and a clock offset of any value will be accepted.
step step: The argument is the step threshold, by default 0.128 s. It can be set to any positive number in seconds. If set to zero, step adjustments will never occur. Note: The kernel time discipline is disabled if the step threshold is set to zero or greater than the default.
stepout stepout: The argument is the stepout timeout, by default 900 s. It can be set to any positive number in seconds. If set to zero, the stepout pulses will not be suppressed.

tos [ ceiling ceiling | cohort {0 | 1} | floor floor | orphan orphan | maxdistance maxdistance | minclock minclock | minsane minsane ]

This command affects the clock selection and clustering algorithms. It can be used to select the quality and quantity of peers used to synchronize the system clock and is most useful in manycast mode. The variables operate as follows:

beacon beacon: The manycast server sends packets at intervals of 64 s if less than maxclock servers are available. Otherwise, it sends packets at the beacon interval in seconds. The default is 3600 s.
ceiling ceiling: Servers with stratum at or above ceiling will be discarded if there are at least minclock peers remaining. This value defaults to 15, but can be changed to any number from 1 to 15.
cohort { 0 | 1 }: This is a binary flag which enables (0) or disables (1) manycast server replies to manycast clients with the same stratum level. This is useful to reduce implosions where large numbers of clients with the same stratum level are present. The default is to enable these replies.
floor floor: Peers with strata below floor will be discarded if there are at least minclock peers remaining. This value defaults to 1, but can be changed to any number from 1 to 15.
orphan stratum: If stratum is set at some value less than 16 a special orphan mode is enterred when no outside source of synchronization is available. To use orphan mode a number of participants are identically configured both as broadcast client and as broadcast server. One or more participants are configured to use an outside source, either a reference clock or another Internet server. When the source or sources fail, the system stratum is set at stratum and a leader is elected to serve as the reference source. When an outside source of synchronization is again available, the orphan mode is disabled.
mindist mindistance: The slection algorithm normally pads each intersection a minimum of one millisecond to avoid needless classification. In some cases, such as reference clocks with high jitter and a PPS signal, it is useful to increase the padding. This command can be used for that purpose. As a general rule, set the mindistance to the maximum expected offset plus the maxiumum expected jitter, in seconds.
maxdist maxdistance: The selection algorithm accumulates a number of packets before setting the clock in order to use the best data available. The number is determined by the synchronization distance for each association and a limit called the distance threshold. The synchronization distance starts at 16, then drops by a factor of about two as each packet is received. The default distance threshold is 1.0, which usually results in four packets. Setting maxdistance to some value between 1 and 16 can be used to change the number of packets required. For instance, setting it to 16 will set the clock on the first packet received; howver, setting it to this value essentially disables the mitigation and grooming algorithms.
minclock minclock: The clustering algorithm repeatedly casts out outlyer associations until no more than minclock associations remain. This value defaults to 3, but can be changed to any number from 1 to the number of configured sources.
minsane minsane: This is the minimum number of candidates available to the clock selection algorithm in order to produce one or more truechimers for the clustering algorithm. If fewer than this number are available, the clock is undisciplined and allowed to run free. The default is 1 for legacy purposes. However, according to principles of Byzantine agreement, minsane should be at least 4 in order to detect and discard a single falseticker.

ttl hop ...

This command specifies a list of TTL values in increasing order. up to 8 values can be specified. In manycast mode these values are used in turn in an expanding-ring search. The default is eight multiples of 32 starting at 31.

trap host_address [port port_number] [interface interface_address]

This command configures a trap receiver at the given host address and port number for sending messages with the specified local interface address. If the port number is unspecified, a value of 18447 is used. If the interface address is not specified, the message is sent with a source address of the local interface the message is sent through. Note that on a multihomed host the interface used may vary from time to time with routing changes.

The trap receiver will generally log event messages and other information from the server in a log file. While such monitor programs may also request their own trap dynamically, configuring a trap receiver will ensure that no messages are lost when the server is started.

ttl hop ...

Files

ntp.drift frequency

Leapseconds File

The NIST provides a file documenting the epoch for all historic occasions of leap second insertion since 1972. The leapsecond table shows each epoch of insertion along with the offset of International Atomic Time (TAI) with respect to Coordinated Universal Time (UTC), as disseminated by NTP. The table can be obtained directly from NIST national time servers using ftp as the ASCII file pub/leap-seconds.

While not strictly a security function, the Autokey protocol provides means to securely retrieve the leapsecond table from a server or peer. Servers load the leapsecond table directly from the file specified in the crypto command, with default ntpkey_leap, while clients can obtain the table indirectly from the servers using the Autokey protocol. Once loaded, the table can be provided on request to other clients and servers.

As explained in the Authentication Options page, all cryptographically sound key generation schemes must have means to randomize the entropy seed used to initialize the internal random number generator. The OpenSSL library uses a designated random seed file for this purpose. The file must be available when starting the NTP daemon and the ntp-keygen program. If a site supports OpenSSL or its companion OpenSSH, it is very likely that means to do this are already available.

compensation (PPM)