Difference between revisions of "Network Time Protocol"

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[[Category:Networking (English)]]
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#REDIRECT [[Network Time Protocol daemon]]
[[Category:Daemons and system services (English)]]
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[[Category:HOWTOs (English)]]
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{{i18n|Network Time Protocol}}
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This article describes two different methods to synchronize the Linux software clock (system clock) to internet time servers. For more information on computer timekeeping, see [[Time]]. Traditional ntpd is explained first and then OpenNTPD (part of the OpenBSD project) is explained. Both daemons can sync the local clock and act as a time server if needed. OpenNTPD is designed to be simple and secure, while ntpd has a larger set of features. '''Note that OpenNTPD is not currently maintained for Linux (see [http://bbs.archlinux.org/viewtopic.php?id=68627 this thread]).
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==ntpd==
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===Installation===
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[http://www.ntp.org/ ntp] is available from [extra]:
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# pacman -S ntp
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===/etc/ntp.conf===
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The first line in your ntp.conf file should contain the following:
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restrict default kod nomodify notrap nopeer noquery
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This restricts everyone from modifying anything and prevents everyone from querying your time server. In the past "notrust" was used here too, but the function of this option has changed to mean authentication with a key is required.
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Following this line, you need to tell ntpd what to allow through into your server.
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restrict 127.0.0.1
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restrict 1.2.3.4 nomodify
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restrict 192.168.0.0 mask 255.255.255.0 nomodify notrap
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This tells ntpd that 1.2.3.4 and all IP addresses from the 192.168.0.0 range will be allowed to synchronize on this server, but they will not be allowed to modify anything. All other IP addresses in the world will still obey the default restrictions (the first line in the ntp.conf).
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Now, is where the stratum 2 servers that our server will synchronize with come into play. The lines in ntp.conf will be used to tell ntpd what servers we would like to use for synchronizing (these are just examples; use ntp servers that are closest to your location). Please see http://support.ntp.org/bin/view/Servers/NTPPoolServers for a list of closer servers. The iburst option is recommended, and sends a burst of packets if it cannot obtain a connection with the first attempt. The "burst" option should never be used without explicit permission and will likely result in blacklisting.
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server ntp1.cs.wisc.edu iburst
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server ntp3.cs.wisc.edu iburst
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server ntp3.sf-bay.org iburst
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Unless you have a good reason not to, it is advisable to use the pool.ntp.org servers: http://www.pool.ntp.org/.
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The only thing left to do is add the drift file (which keeps track of yours clocks time deviation) and the log file location:
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driftfile /var/lib/ntp/ntp.drift
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logfile /var/log/ntp.log
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In case our server loses internet access, we need to add localhost as a server or it will stop serving time to the network. We add localhost as a "stratum 10" server so that it will never be used unless internet access is lost.
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server  127.127.1.0
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fudge  127.127.1.0 stratum 10
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The complete file will look like this:
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<pre>
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## default restrictions
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restrict default kod nomodify notrap nopeer noquery
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## override the default restrictions here
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restrict 127.0.0.1
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## serve time to a local network
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#restrict 192.168.0.0 mask 255.255.255.0 nomodify notrap
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## public NTP servers to sync with (all stratum 2)
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## change these to servers near you or comment them out
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server ntp1.cs.wisc.edu iburst
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server ntp3.cs.wisc.edu iburst
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server ntp3.sf-bay.org iburst
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## ntp pool servers
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server 0.pool.ntp.org iburst
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server 1.pool.ntp.org iburst
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server 2.pool.ntp.org iburst
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server 3.pool.ntp.org iburst
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## local server
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server 127.127.1.0
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fudge  127.127.1.0 stratum 10
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## NTP drift file - corrects for hardware clock time deviation
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driftfile /var/lib/ntp/ntp.drift
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## NTP log file
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logfile /var/log/ntp.log
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</pre>
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===/etc/rc.conf===
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Add ntpd to the DAEMONS array in rc.conf to have it start at boot:
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DAEMONS=(syslog-ng network '''@ntpd''' ...)
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===Running as non-root user===
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When compiled with ''--enable-linux-caps'', ntp can be run as a non-root user for increased security (the vanilla Arch Linux package has this enabled).
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{{Note|Before attempting this, make sure ntp has already created {{Filename|/var/lib/ntp/ntp.drift}}.}}
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Create ntp group and ntp user:
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# groupadd ntp
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# useradd -r -d /var/lib/ntp -g ntp -s /bin/false ntp
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Change ownership of ntp directory to the ntp user/group:
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# chown -R ntp:ntp /var/lib/ntp
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Edit {{Filename|/etc/conf.d/ntp-client.conf}} and change
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NTPD_ARGS="-g"
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to
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NTPD_ARGS="-g -u ntp:ntp"
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== OpenNTPD ==
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=== Installation ===
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[http://www.openntpd.org/ OpenNTPD] can be installed from [community]:
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# pacman -S openntpd
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The default configuration is actually usable if all you want is to sync the time of the local computer. For more detailed settings, the {{Filename|/etc/ntpd.conf}} file must be edited:
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To sync to a particular server, uncomment and edit the "server" directive. You can find the server's URL in your area at [http://www.pool.ntp.org/zone/@ www.pool.ntp.org/zone/@].
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server ntp.example.org
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The "servers" directive works the same as the "server" directive, however, if the DNS name resolves to multiple IP address, ALL of them will be synced to.  The default, "pool.ntp.org" is working and should be acceptable in most cases.
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pool.ntp.org
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Any number of "server" or "servers" directives may be used.
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If you want the computer you run OpenNTPD on to also be a time server, simply uncomment and edit the "listen" directive.
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For example:
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listen on *
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will listen on all interfaces, and
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listen on 127.0.0.1
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will only listen on the loopback interface.
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Your time server will only begin to serve time after it has synchronized itself to a high resolution. This may take hours, or days, depending on the accuracy of your system.
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If you would like to run OpenNTPD at boot, add {{Codeline|openntpd}} the DAEMONS variable in your {{Filename|/etc/rc.conf}} following your network daemon.
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DAEMONS=(syslog-ng network '''openntpd''' ...)
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If openntpd is being used to set local system time only, it may be safely backgrounded.
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DAEMONS=(syslog-ng network '''@openntpd''' ...)
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To see the status of NTP syncing, visit {{Filename|/var/log/daemon.log}} and look for entries with "ntpd".
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OpenNTPD adjusts the clock by small amounts at a time. It is designed this way to prevent sudden, large time fluctuations in your system, which could adversely affect system services (e.g., cron jobs). Thus, it can take some time to correct the time.
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If your clock is off by more than 180 seconds you can try "{{Codeline|ntpd -s -d}}" in the console. If ntpd is already running, you can simply restart it with {{Codeline|sudo /etc/rc.d/openntpd restart}}, as the Arch openntpd package uses the "-s" flag by default. See {{Codeline|man ntpd}} for more info. You can also set the [[Time#Time_Set|system clock]] to as close to possible to the actual time and then let OpenNTPD fine tune the time.
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=== Making openntpd dependent upon network access ===
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If you have intermittent network access (you roam around on a laptop, you use dial-up, etc), it does not make sense to have {{Codeline|openntpd}} running as a system daemon on start up. Here are a few ways you can control {{Codeline|openntpd}} based on the presence of a network connection. These instructions should also work for {{Codeline|ntpd}} found further below. 
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==== Using netcfg ====
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If you are using netcfg, you can also start/stop openntpd as a POST_UP/PRE_DOWN command in your network profile:
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POST_UP="/etc/rc.d/openntpd start || true"
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PRE_DOWN="/etc/rc.d/openntpd stop || true"
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Of course, you will have to specify this manually for each network profile.
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==== Using NetworkManager dispatcher ====
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OpenNTPD can be brought up/down along with a network connection through the use of [[NetworkManager#Network Services with NetworkManager Dispatcher|NetworkManager's dispatcher scripts]]. Place the following script as {{Filename|/etc/NetworkManager/dispatcher.d/20_openntpd}}
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<pre>
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#!/bin/sh
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INTERFACE=$1 # The interface which is brought up or down
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STATUS=$2 # The new state of the interface
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case "$STATUS" in
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    'up') # $INTERFACE is up
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exec /etc/rc.d/openntpd start
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;;
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    'down') # $INTERFACE is down
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exec /etc/rc.d/openntpd stop # Disable if having more than one active interface might bring openntpd down unnecessarily
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;;
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esac
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</pre>
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{{Note|Make sure you give the script executable permissions or it won't work!
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# chmod +x /etc/NetworkManager/dispatcher.d/20_openntpd
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}}
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{{Note|This nice little script works with any daemon you wish to start! Just substitute "openntpd" for your desired daemon.}}
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{{Warning|if you connect to foreign or public networks, be aware of what services you are starting and what servers you expect to be available for them to connect to. You could make a security hole by starting the wrong services while connected to a public network.}}
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==== Using wicd ====
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These instructions require wicd 1.7.0 or later, which is available in the standard Arch repository.
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You will also need write access to {{Filename|/etc/wicd/scripts}}.
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{{Note|Remember to make these two scripts executable using {{Codeline|chmod}} }}
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Make one shell script inside {{Filename|/etc/wicd/scripts/postconnect/openntpd-start.sh}} with the following:
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<pre>
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#!/bin/sh
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/etc/rc.d/openntpd start
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</pre>
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Similarly, make another shell script inside {{Filename|/etc/wicd/scripts/predisconnect/openntpd-stop.sh}} with the following:
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<pre>
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#!/bin/sh
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/etc/rc.d/openntpd stop
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</pre>
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==== Using dhcpcd hooks ====
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Another possibility is to use dhcpcd hooks to start and stop openntpd.
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When dhcpcd detects a change in state it will run the following scripts: <br>
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{{Filename|/etc/dhcpcd.enter-hook}} <br>
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{{Filename|/usr/lib/dhcpcd/dhcpcd-hooks/*}} <br>
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{{Filename|/etc/dhcpcd.exit-hook}} <br>
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The following example uses {{Filename|/etc/dhcpcd.exit-hook}} to start and stop openntpd depending on dhcp status:
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<pre>
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[ "$interface" != "eth0" ] && exit 0
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if $if_up; then
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    pgrep ntpd &> /dev/null || /etc/rc.d/openntpd start
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elif $if_down; then
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    pgrep ntpd &> /dev/null && /etc/rc.d/openntpd stop
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fi
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</pre>
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=== Troubleshooting ===
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'''Error adjusting time'''
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If you find your time set incorrectly and in log you see:
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openntpd adjtime failed: Invalid argument
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Try:
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ntpd -s -d
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This is also how you would manually sync your system.
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'''Increasing time shift'''
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Starting ''openntpd'' in the background could lead to synchronization errors between the actual time and the time stored on your computer. If you recognize an increasing time difference between your desktop clock and the actual time, try to start the ''openntpd'' daemon normal and not in the background.
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'''Initialization Failure'''
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Openntpd may fail to initialize properly if it is started before the network is fully configured. In some cases you may want to remove {{Codeline|openntpd}} from the DAEMONS array in {{Filename|/etc/rc.conf}} and add the following line to {{Filename|/etc/rc.local}}:
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(sleep 300 && /etc/rc.d/openntpd start) &
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{{Note|This method is an alternative to the four methods listed [[#Making openntpd dependent upon network access|above]]. The other three methods are preferred and work better. Use this as a last resort.}}
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This will wait 5 minutes before starting openntpd, which should give the system sufficient time to set up the network properly. If your network settings change often, you may also consider restarting the daemon regularly with cron.
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==External links==
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* http://www.ntp.org/
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* http://twiki.ntp.org/bin/view/Main/WebHome
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* http://www.eecis.udel.edu/~mills/ntp/html/index.html
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* http://www.openntpd.org
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Revision as of 20:58, 11 February 2011