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chrony(1) [debian man page]

CHRONY(1)							   User's Manual							 CHRONY(1)

chrony - programs for keeping computer clocks accurate SYNOPSIS
chrony is a pair of programs for keeping computer clocks accurate. chronyd is a background (daemon) program and chronyc is a command-line interface to it. Time reference sources for chronyd can be RFC1305 NTP servers, human (via keyboard and chronyc), or the computer's real- time clock at boot time (Linux only). chronyd can determine the rate at which the computer gains or loses time and compensate for it while no external reference is present. Its use of NTP servers can be switched on and off (through chronyc) to support computers with dial- up/intermittent access to the Internet, and it can also act as an RFC1305-compatible NTP server. USAGE
chronyc is a command-line interface program which can be used to monitor chronyd's performance and to change various operating parameters whilst it is running. chronyd's main function is to obtain measurements of the true (UTC) time from one of several sources, and correct the system clock accord- ingly. It also works out the rate at which the system clock gains or loses time and uses this information to keep it accurate between mea- surements from the reference. The reference time can be derived from either Network Time Protocol (NTP) servers, reference clocks, or wristwatch-and-keyboard (via chronyc). The main source of information about the Network Time Protocol is It is designed so that it can work on computers which only have intermittent access to reference sources, for example computers which use a dial-up account to access the Internet. Of course, it will work on computers with permanent connections too. In addition, for Linux 2.0.x (for x >= 32) or 2.2 onwards, chronyd can monitor the system's real time clock performance, so the system can maintain accurate time even across reboots. Typical accuracies available between 2 machines are On an ethernet LAN : 100-200 microseconds, often much better On a V32bis dial-up modem connection : 10's of milliseconds (from one session to the next) With a good reference clock the accuracy can reach one microsecond. chronyd can also operate as an RFC1305-compatible NTP server and peer. SEE ALSO
chronyc(1), chrony(1) AUTHOR
Richard Curnow <> This man-page was written by Jan Schaumann <> as part of "The Missing Man Pages Project". Please see for details. The complete chrony documentation is supplied in texinfo format. chrony December 04, 2009 CHRONY(1)

Check Out this Related Man Page

ntpd(8) 						      System Manager's Manual							   ntpd(8)

       ntpd - network time protocol (NTP) daemon

       /usr/etc/ntpd [ -a threshold ][ -c file ][ -d ][ -D level ][ -l ][ -n ][ -s ]

       The  University of Maryland's daemon synchronizes the local clock with a set of distributed time servers.  The daemon distributes accurate,
       reliable time from the best time source available at your site to hosts on wide area networks (WAN) and local  area  networks  (LAN).   The
       three  recommended  time  sources in decreasing order of accuracy are:  Internet NTP service, local radio clock, and wristwatch.  Note that
       the daemon does not require time servers to be on the same LAN as time clients, and does not create a heavy broadcast load on the  network.
       If  the	NTP servers are not on your Local Area Network (LAN), you must run the daemon before running the daemon.  To run remove the number
       signs (#) from in front of the following lines in your file:
       #[ -f /etc/routed ] && {
       #       /etc/routed & echo 'routed'	       >/dev/console
       The daemon will be invoked when you reboot your system.	To start without rebooting, type the following on the command line:
       For information on setting up the network time services, see the Guide to System and Network Setup.

       The daemon automatically splits the nodes running the daemon into a dynamically reconfigurable hierarchy of nodes.  The nodes  at  the  top
       level  of the hierarchy (low stratum numbers) are connected to the most accurate sources available.  This information is transferred to the
       lower-level nodes (higher stratum numbers) which set their clocks based on the calculated offset from a remote server, and then	distribute
       this time to lower levels of the hierarchy.

       The daemon provides a solution for distributing time to a large number of individual workstations.  It can also be used in conjunction with
       a master daemon to distribute NTP time to workstations running If is run with the and options on at least one system that is  also  running
       then  all other systems on the network running can receive time updates from a host running Although is easier to set up on clients, NTP is
       recommended because it is more accurate and more secure.

       Normally, the daemon is invoked at boot time from the file.  When is started, it reads configuration information from the file, unless  you
       have specified another configuration file with the option.  The configuration file either specifies the list of NTP servers with which this
       host should synchronize, or identifies this host as a local reference clock.  See the reference page for more information on the configura-
       tion file.

       The daemon uses the system call to gradually adjust the local clock for small clock offsets (< 0.128 seconds).  If the local clock lags the
       time on the server by more than 0.128 seconds, the system call is used to make a forward step adjustment of the local  clock.   Clocks  are
       never  stepped  backwards; they are adjusted gradually, which can take a very long time.  Therefore, it is important to initialize the time
       using the command before running the daemon.

       -a threshold
	    Sets the threshold (in seconds) which limits how far the daemon can change the local clock.  By default, the threshold  is	1000  sec-
	    onds.   This  is  set  to avoid propagating major mistakes throughout the network.	If you specify the string instead of a number, the
	    daemon can change the local clock by any amount.

       -c file
	    Specifies a configuration file for the daemon.  By default, the configuration file is

       -d   Increments the debug level by one.	The option can be specified more than once.  Higher debug levels provide more diagnostic  informa-

       -D level
	    Sets the debug level to the specified value.

       -l   Causes  the daemon to log a message each time the local clock is adjusted.	Specify this option only if you want to gather statistical
	    information to analyze the local clock behavior.  If the option is set, a message may be  logged  every  two  minutes.   Messages  are
	    logged to

       -n   Inhibits the program from being swapped out of memory.  Using the option is recommended for both time servers and time clients.

       -s   Prevents the daemon from altering the time on the local host.  The daemon participates as an NTP server with the flag set, but it does
	    not change the time of the local host.

       Before starting the daemon, either manually or from the file, you must edit the file with the appropriate information.  If your system is a
       client  you  must specify the time servers for it to query.  If it is a time server, you must specify the time servers with which it peers.
       See the reference page for more information.

       To start the daemon manually (on a time client), enter the following commands:
       # /etc/rdate -s
       # /usr/etc/ntp -s -f server1 server2 server3
       # /usr/etc/ntpd -n
       The command initializes your time to the average network time.  The command further refines the initial time to the NTP time.  The  servers
       specified on the command line are the same as those specified in the file.

       To  start  the daemon from the file (on a time client), place the following entries in the file.  Multiple servers are included in case one
       of the servers crashes, or is brought down.  The servers specified in the file are the same as those specified in the file.  These  entries
       should be placed after the entry:

       [-f /etc/syslog] && {
	    /etc/syslog   & echo -n ' syslog'		>/dev/console
       [-f /etc/rdate] && {
	    /etc/rdate -s   & echo -n ' rdate'		>/dev/console
       [-f /usr/etc/ntp] && {
	    /usr/etc/ntp -s -f server1 server2 server3 
			     & echo -n ' ntp'		>/dev/console
       [-f /usr/etc/ntpd] && {
	    /usr/etc/ntpd -n & echo -n ' ntpd'		>/dev/console


       The  daemon  logs errors, major state changes, and statistics reports using the daemon; the log entries appear in the file with the word on
       each   relevant line.  Normal log entries show when gains or loses synchronization with a lower-stratum host.  Also, once an hour issues  a
       entry that gives information about its state.

       Once  an  hour,	if is synchronized, it updates the file.  This file shows the estimated clock drift for each of the past 5 hours, with the
       most recent hour listed first.  (The 6th number in this file is the number of hours has been running).  To  convert  the  drift	values	to
       parts  per  million  (ppm), divide them by 4096 and multiply by 1000000.  For example, +0.0107001161 means that estimates that the clock is
       drifting by about 2.61 ppm, or is losing about 0.226 seconds per day.  [(2.61/1000000) * 24 * 60 * 60 = 0.226].	Negative drift values mean
       that the clock is gaining time.	If is working, your computer's clock should be accurate to within a few seconds per day.

       Another	diagnostic  tool is the command.  You can use this to look at any host running The following command line returns the state of the
       remote host's server:
       % /usr/etc/ntpdc hostname

       The value that is returned for the should contain values not greater than 100 milliseconds.  See the reference page for more information.

       If the daemon sets the time as frequently as every 10 minutes (indicated by messages in the file), then you should kill the daemon,  remove
       the file, run the command to initialize the time, and restart the daemon.

       If  your  system  clock	is ahead of the server time by more than 1 second, you should kill the daemon, remove the file, run the command to
       initialize the time, and restart the daemon.

       If your clock is more than 1000 seconds off from the server time, and you did not specify the option, will not  change  your  system  time.
       Rather, will repeatedly log messages to the file, indicating that the time is too far off to reset.

See Also
       ntp(1), adjtime(2), settimeofday(2), ntp.conf(5), ntpdc(8), timed(8)
       RFC 1129--Internet Time Synchronization:  the Network Time Protocol
       Guide to System and Network Setup
       Introduction to Networking and Distributed System Services

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