Chapter 14. Configuring time synchronization
Chapter 14. Configuring time synchronization Accurate timekeeping in an IT environment is important. A consistent time across all network devices improves the traceability of log files and certain protocols rely on synchronized clocks. For example, Kerberos uses time stamps to prevent replay attacks. The user space daemon updates the system clock running in the kernel. Starting with Red Hat Enterprise Linux 8, the NTP protocol is implemented by the chronyd daemon, available from the repositories in the chrony package. 14.1. Introduction to chrony suite The implementation of the Network Time Protocol (NTP) is chrony . You can use chrony : To synchronize the system clock with NTP servers To synchronize the system clock with a reference clock, for example a GPS receiver To synchronize the system clock with a manual time input As an NTPv4(RFC 5905) server or peer to provide a time service to other computers in the network chrony performs well in a wide range of conditions: including intermittent network connections heavily congested networks changing temperatures (ordinary computer clocks are sensitive to temperature) systems that do not run continuously, or run on a virtual machine. Typical accuracy between two machines synchronized over the Internet is within a few milliseconds, and for machines on a LAN within tens of microseconds. Hardware timestamping or a hardware reference clock may improve accuracy between two machines synchronized to a sub-microsecond level. chrony consists of chronyd , a daemon that runs in user space, and chronyc , a command line program which can be used to monitor the performance of chronyd and to change various operating parameters when it is running. The chronyd daemon can be monitored and controlled by the command line utility chronyc . This utility provides a command prompt which allows entering a number of commands to query the current state of chronyd and make changes to its configuration. By default, chronyd accepts only commands from a local instance of chronyc , but it can be configured to accept monitoring commands also from remote hosts. The remote access should be restricted. 14.2. Using chronyc to control chronyd You can control chronyd by using the chronyc command line utility. Procedure To make changes to the local instance of chronyd using the command line utility chronyc in interactive mode, enter the following command as root : chronyc must run as root if some of the restricted commands are to be used. The chronyc command prompt will be displayed as follows: To list all of the commands, type help . Alternatively, the utility can also be invoked in non-interactive command mode if called together with a command as follows: Note Changes made using chronyc are not permanent, they will be lost after a chronyd restart. For permanent changes, modify /etc/chrony.conf . 14.3. Migrating to chrony In Red Hat Enterprise Linux 7, users could choose between ntp and chrony to ensure accurate timekeeping. For differences between ntp and chrony , ntpd and chronyd , see Differences between ntpd and chronyd . Starting with Red Hat Enterprise Linux 8, ntp is no longer supported. chrony is enabled by default. For this reason, you might need to migrate from ntp to chrony . Migrating from ntp to chrony is straightforward in most cases. The corresponding names of the programs, configuration files and services are: Table 14.1. Corresponding names of the programs, configuration files and services when migrating from ntp to chrony ntp name chrony name /etc/ntp.conf /etc/chrony.conf /etc/ntp/keys /etc/chrony.keys ntpd chronyd ntpq chronyc ntpd.service chronyd.service ntp-wait.service chrony-wait.service The ntpdate and sntp utilities, which are included in the ntp distribution, can be replaced with chronyd using the -q option or the -t option. The configuration can be specified on the command line to avoid reading /etc/chrony.conf . For example, instead of running ntpdate ntp.example.com , chronyd could be started as: The ntpstat utility, which was previously included in the ntp package and supported only ntpd , now supports both ntpd and chronyd . It is available in the ntpstat package. 14.3.1. Migration script A Python script called ntp2chrony.py is included in the documentation of the chrony package ( /usr/share/doc/chrony ). The script automatically converts an existing ntp configuration to chrony . It supports the most common directives and options in the ntp.conf file. Any lines that are ignored in the conversion are included as comments in the generated chrony.conf file for review. Keys that are specified in the ntp key file, but are not marked as trusted keys in ntp.conf are included in the generated chrony.keys file as comments. By default, the script does not overwrite any files. If /etc/chrony.conf or /etc/chrony.keys already exist, the -b option can be used to rename the file as a backup. The script supports other options. The --help option prints all supported options. An example of an invocation of the script with the default ntp.conf provided in the ntp package is: The only directive ignored in this case is disable monitor , which has a chrony equivalent in the noclientlog directive, but it was included in the default ntp.conf only to mitigate an amplification attack. The generated chrony.conf file typically includes a number of allow directives corresponding to the restrict lines in ntp.conf . If you do not want to run chronyd as an NTP server, remove all allow directives from chrony.conf . 14.4. Using Chrony The following sections describe how to start, and stop chronyd , and how to check if chrony is synchronized. Sections also describe how to manually adjust System Clock. 14.4.1. Managing chrony You can start, stop, and check the status of chronyd . The chrony suite is installed by default on Red Hat Enterprise Linux. To ensure that it is, run the following command as root : The default location for the chrony daemon is /usr/sbin/chronyd . The command line utility will be installed to /usr/bin/chronyc . To check the status of chronyd , issue the following command: To start chronyd , issue the following command as root : To ensure chronyd starts automatically at system start, issue the following command as root : To stop chronyd , issue the following command as root : To prevent chronyd from starting automatically at system start, issue the following command as root : 14.4.2. Checking if chrony is synchronized You can check if chrony is synchronized with the use of the tracking , sources , and sourcestats commands. Procedure To check chrony tracking, enter: The chronyc sources command displays information about the current time sources that chronyd is accessing. You can specify the optional -v argument to print more verbose information. In this case, extra caption lines are shown as a reminder of the meanings of the columns. The sourcestats command displays information about the drift rate and offset estimation process for each of the sources currently being examined by chronyd . To check chrony source statistics, issue the following command: The optional argument -v can be specified, meaning verbose. In this case, extra caption lines are shown as a reminder of the meanings of the columns. Additional resources chronyc(1) man page on your system 14.4.3. Manually adjusting the System Clock You can manually adjust the System Clock. Procedure To step the system clock immediately, bypassing any adjustments in progress by slewing, enter: Important If the rtcfile directive is used, the real-time clock should not be manually adjusted. Random adjustments would interfere with chrony 's need to measure the rate at which the real-time clock drifts. 14.4.4. Disabling a chrony dispatcher script The chrony dispatcher script manages the online and offline state of the NTP servers. As a system administrator, you can disable the dispatcher script to keep chronyd polling the servers constantly. The NetworkManager executes the chrony dispatcher script during interface reconfiguration, stop or start operations. However, if you configure certain interfaces or routes outside of NetworkManager, you can encounter the following situation: The dispatcher script might run when no route to the NTP servers exists, causing the NTP servers to switch to the offline state. If you establish the route later, the script does not run again by default, and the NTP servers remain in the offline state. To ensure that chronyd can synchronize with your NTP servers, which have separately managed interfaces, disable the dispatcher script. Procedure To disable the chrony dispatcher script, edit the /etc/NetworkManager/dispatcher.d/20-chrony-onoffline file as follows: Note When you upgrade or reinstall the chrony package, the packaged version of the dispatcher script replaces your modified dispatcher script. 14.4.5. Setting up chrony in an isolated network For a network that is never connected to the Internet, one computer is selected to be the primary timeserver. The other computers are either direct clients of the server, or clients of clients. On the server, the drift file must be manually set with the average rate of drift of the system clock. If the server is rebooted, it will obtain the time from surrounding systems and calculate an average to set its system clock. Thereafter it resumes applying adjustments based on the drift file. The drift file will be updated automatically when the settime command is used. To set up chrony for a system in an isolated network, follow the steps mentioned below: Procedure On the system selected to be the server, edit /etc/chrony.conf as follows: Where <subnet> is the network from which the clients are allowed to connect. Use Classless Inter-Domain Routing (CIDR) notation to specify the subnet. On the systems selected to be direct clients of the server, edit the /etc/chrony.conf as follows: Where <server_fqdn> is the host name of the server, and <server_ip_address> is the address of the server . Clients with this configuration will resynchronize with the server if it restarts. On the client systems which are not to be direct clients of the server, the /etc/chrony.conf file should be the same except that the local and allow directives should be omitted. In an isolated network, you can also use the local directive that enables a local reference mode, which allows chronyd operating as an NTP server to appear synchronized to real time, even when it was never synchronized or the last update of the clock happened a long time ago. To allow multiple servers in the network to use the same local configuration and to be synchronized to one another, without confusing clients that poll more than one server, use the orphan option of the local directive which enables the orphan mode. Each server needs to be configured to poll all other servers with local . This ensures that only the server with the smallest reference ID has the local reference active and other servers are synchronized to it. When the server fails, another one will take over. 14.4.6. Configuring remote monitoring access The chronyc utility can access chronyd by using the following methods: IPv4 or IPv6. A domain socket, which is accessible locally by the root and chrony user. By default, chronyc connects to the Unix domain socket. The default path is /var/run/chrony/chronyd.sock . If this connection fails, chronyc tries to connect to 127.0.0.1 and then ::1. Only the following monitoring commands, which do not affect the behavior of chronyd , are allowed from the network: activity manual list rtcdata smoothing sources sourcestats tracking waitsync The set of hosts from which chronyd accepts these commands can be configured by using the following methods: You can use the cmdallow directive in the configuration file of chronyd . Run the cmdallow command in chronyc . By default, the commands are accepted only from localhost (127.0.0.1 or ::1). All other commands are allowed only through the Unix domain socket. When sent over the network, chronyd responds with a Not authorised error, even if it is from localhost. The following procedure describes how to access chronyd remotely with chronyc . Procedure Configure chrony to listen on local interface by adding the following to the /etc/chrony.conf file: and Allow commands from remote IP addresses, networks, and subnet: Add the following content to the /etc/chrony.conf file: Open port 323 in the firewall to allow connections from remote systems: Reload the firewall configuration: Additional resources chrony.conf(5) man page on your system 14.4.7. Managing time synchronization using RHEL system roles You can manage time synchronization on multiple target machines using the timesync role. The timesync role installs and configures an NTP or PTP implementation to operate as an NTP or PTP client to synchronize the system clock. Note that using the timesync role also facilitates migration to chrony , because you can use the same playbook on all versions of Red Hat Enterprise Linux starting with RHEL 6 regardless of whether the system uses ntp or chrony to implement the NTP protocol. Warning The timesync role replaces the configuration of the given or detected provider service on the managed host. settings are lost, even if they are not specified in the role variables. The only preserved setting is the choice of provider if the timesync_ntp_provider variable is not defined. The following example shows how to apply the timesync role in a situation with just one pool of servers. Example 14.1. An example playbook applying the timesync role for a single pool of servers For a detailed reference on timesync role variables, install the rhel-system-roles package, and see the README.md or README.html files in the /usr/share/doc/rhel-system-roles/timesync directory. Additional resources Preparing a control node and managed nodes to use RHEL system roles 14.4.8. Additional resources chronyc(1) and chronyd(8) man pages on your system Frequently Asked Questions 14.5. Chrony with HW timestamping Hardware timestamping (HW) in some Network Interface Controller (NICs) provides accurate timestamping of incoming and outgoing packets. NTP timestamps are usually created by the kernel and chronyd with the use of the system clock. However, when HW timestamping is enabled, the NIC uses its own clock to generate the timestamps when packets are entering or leaving the link layer or the physical layer. When used with NTP , hardware timestamping can significantly improve the accuracy of synchronization. For best accuracy, both NTP servers and NTP clients need to use hardware timestamping. Under ideal conditions, a sub-microsecond accuracy may be possible. Another protocol for time synchronization that uses hardware timestamping is PTP . Unlike NTP , PTP relies on assistance in network switches and routers. If you want to achieve the best accuracy of synchronization, use PTP on networks that have switches and routers with PTP support, and prefer NTP on networks that do not have such switches and routers. 14.5.1. Verifying support for hardware timestamping To verify that hardware timestamping with NTP is supported by an interface, use the ethtool -T command. An interface can be used for hardware timestamping with NTP if ethtool lists the SOF_TIMESTAMPING_TX_HARDWARE and SOF_TIMESTAMPING_TX_SOFTWARE capabilities and also the HWTSTAMP_FILTER_ALL filter mode. Procedure Display a device's time stamping capabilities and associated PTP hardware clock: 14.5.2. Enabling hardware timestamping You can enable hardware timestamping on one or multiple interfaces by using the hwtimestamp directive in the /etc/chrony.conf file. The directive can either specify a single interface, or a wildcard character can be used to enable hardware timestamping on all interfaces that support it. Procedure Edit the /etc/chrony.conf file and make the following changes: Add the hwtimestamp setting for interfaces which support hardware timestamping. For example: You can use the * wildcard if no other application, such as ptp4l uses hardware timestamping. Configure a short client polling interval by appending the minpoll and maxpoll options to the server setting, for example: For hardware timestamping, you must configure a shorter polling interval than the default range (64-1024 seconds) to minimize the offset of the system clock. Enable the NTP interleaved mode by appending the xleave option to the server setting: With this setting, chrony gets the hardware transmit timestamp only after sending a packet. This behavior prevents the serever from saving the timestamp in packets to which it responds. With the xleave option, chrony can receive transmit timestamps that were generated after the transmission. Optional: Increase the maximum size of memory allocated for logging of client's access on the server, for example: The default server configuration allows a few thousands of clients to use the interleaved mode concurrently. By increasing the value of the clientloglimit setting, you can configure the server for a large number of clients. Restart the chronyd service: Verification Optional: Verify in the /var/log/messages log file that hardware timesamping is enabled: If chronyd is configured as an NTP client or peer, display the transmit and receive timestamping modes and the interleaved mode: Report the stability of NTP measurements: This stability is reported in the Std Dev column. With hardware timestamping enabled, stability of NTP measurements should be in tens or hundreds of nanoseconds, under normal load. 14.5.3. Configuring PTP-NTP bridge If a highly accurate Precision Time Protocol ( PTP ) primary timeserver is available in a network that does not have switches or routers with PTP support, a computer may be dedicated to operate as a PTP client and a stratum-1 NTP server. Such a computer needs to have two or more network interfaces, and be close to the primary timeserver or have a direct connection to it. This will ensure highly accurate synchronization in the network. Procedure Configure the ptp4l and phc2sys programs from the linuxptp packages to use one interface to synchronize the system clock using PTP . Configure chronyd to provide the system time using the other interface: Restart the chronyd service: 14.6. Achieving some settings previously supported by NTP in chrony Some settings that were in major version of Red Hat Enterprise Linux supported by ntp , are not supported by chrony . The following sections list such settings, and describe ways to achieve them on a system with chrony . 14.6.1. Monitoring by ntpq and ntpdc chronyd cannot be monitored by the ntpq and ntpdc utilities from the ntp distribution, because chrony does not support the NTP modes 6 and 7. It supports a different protocol and chronyc is the client implementation. For more information, see the chronyc(1) man page on your system. To monitor the status of the system clock sychronized by chronyd , you can: Use the tracking command Use the ntpstat utility, which supports chrony and provides a similar output as it used to with ntpd Example 14.2. Using the tracking command Example 14.3. Using the ntpstat utility 14.6.2. Using authentication mechanism based on public key cryptography In Red Hat Enterprise Linux 7, ntp supported Autokey , which is an authentication mechanism based on public key cryptography. In Red Hat Enterprise Linux 8, chronyd supports Network Time Security (NTS), a modern secure authentication mechanism, instead of Autokey . For more information, see Overview of Network Time Security (NTS) in chrony . 14.6.3. Using ephemeral symmetric associations In Red Hat Enterprise Linux 7, ntpd supported ephemeral symmetric associations, which can be mobilized by packets from peers which are not specified in the ntp.conf configuration file. In Red Hat Enterprise Linux 8, chronyd needs all peers to be specified in chrony.conf . Ephemeral symmetric associations are not supported. Note that using the client/server mode enabled by the server or pool directive is more secure compared to the symmetric mode enabled by the peer directive. 14.6.4. multicast/broadcast client Red Hat Enterprise Linux 7 supported the broadcast/multicast NTP mode, which simplifies configuration of clients. With this mode, clients can be configured to just listen for packets sent to a multicast/broadcast address instead of listening for specific names or addresses of individual servers, which may change over time. In Red Hat Enterprise Linux 8, chronyd does not support the broadcast/multicast mode. The main reason is that it is less accurate and less secure than the ordinary client/server and symmetric modes. There are several options of migration from an NTP broadcast/multicast setup: Configure DNS to translate a single name, such as ntp.example.com, to multiple addresses of different servers Clients can have a static configuration using only a single pool directive to synchronize with multiple servers. If a server from the pool becomes unreacheable, or otherwise unsuitable for synchronization, the clients automatically replace it with another server from the pool. Distribute the list of NTP servers over DHCP When NetworkManager gets a list of NTP servers from the DHCP server, chronyd is automatically configured to use them. This feature can be disabled by adding PEERNTP=no to the /etc/sysconfig/network file. Use the Precision Time Protocol (PTP) This option is suitable mainly for environments where servers change frequently, or if a larger group of clients needs to be able to synchronize to each other without having a designated server. PTP was designed for multicast messaging and works similarly to the NTP broadcast mode. A PTP implementation is available in the linuxptp package. PTP normally requires hardware timestamping and support in network switches to perform well. However, PTP is expected to work better than NTP in the broadcast mode even with software timestamping and no support in network switches. In networks with very large number of PTP clients in one communication path, it is recommended to configure the PTP clients with the hybrid_e2e option to reduce the amount of network traffic generated by the clients. You can configure a computer running chronyd as an NTP client, and possibly NTP server, to operate also as a primary PTP timeserver to distribute synchronized time to a large number of computers using multicast messaging. 14.7. Overview of Network Time Security (NTS) in chrony Network Time Security (NTS) is an authentication mechanism for Network Time Protocol (NTP), designed to scale substantial clients. It verifies that the packets received from the server machines are unaltered while moving to the client machine. Network Time Security (NTS) includes a Key Establishment (NTS-KE) protocol that automatically creates the encryption keys used between the server and its clients. Warning NTS is not compatible with the FIPS and OSPP profile. When you enable the FIPS and OSPP profile, chronyd that is configured with NTS can abort with a fatal message. You can disable the OSPP profile and FIPS mode for chronyd service by adding the GNUTLS_FORCE_FIPS_MODE=0 setting to the /etc/sysconfig/chronyd file. 14.7.1. Enabling Network Time Security (NTS) on a client By default, Network Time Security (NTS) is not enabled. You can enable NTS in the /etc/chrony.conf . For that, perform the following steps: Prerequisites The time server supports NTS. Procedure Edit the /etc/crony.conf file, and make the following changes: Specify the server with the nts option in addition to the recommended iburst option. Add the following setting to avoid repeating the Network Time Security-Key Establishment (NTS-KE) session during system boot: Add the following line to /etc/sysconfig/network to disable synchronization with Network Time Protocol (NTP) servers provided by DHCP : Restart the chronyd service: Verification Verify if the NTS keys were successfully established: The KeyID , Type , and KLen should have non-zero values. If the value is zero, check the system log for error messages from chronyd . Verify the client is making NTP measurements: The Reach column should have a non-zero value; ideally 377. If the value rarely gets 377 or never gets to 377, it indicates that NTP requests or responses are getting lost in the network. Additional resources chrony.conf(5) man page on your system 14.7.2. Enabling Network Time Security (NTS) on a time server If you run your own Network Time Protocol (NTP) server, you can enable the server Network Time Security (NTS) support to facilitate its clients to synchronize securely. If the NTP server is a client of other servers, that is, it is not a Stratum 1 server, it should use NTS or symmetric key for its synchronization. Prerequisites Server private key in PEM format Server certificate with required intermediate certificates in PEM format Procedure Edit the /etc/chrony.conf file, and make the following changes: Set permissions on both the private key and the certificate file that allow the chrony user to read the files, for example Ensure that the ntsdumpdir /var/lib/chrony setting is present. Open the required ports in firewalld: Restart the chronyd service: Verification Perform a test from a client machine: The System clock wrong message indicates the NTP server is accepting NTS-KE connections and responding with NTS-protected NTP messages. Verify the NTS-KE connections and authenticated NTP packets observed on the server: If the value of the NTS-KE connections accepted and Authenticated NTP packets field is a non-zero value, it means that at least one client was able to connect to the NTS-KE port and send an authenticated NTP request.
chronyc chronyc> chronyc command chronyd -q 'server ntp.example.com iburst' 2018-05-18T12:37:43Z chronyd version 3.3 starting (+CMDMON +NTP +REFCLOCK +RTC +PRIVDROP +SCFILTER +SIGND +ASYNCDNS +SECHASH +IPV6 +DEBUG) 2018-05-18T12:37:43Z Initial frequency -2.630 ppm 2018-05-18T12:37:48Z System clock wrong by 0.003159 seconds (step) 2018-05-18T12:37:48Z chronyd exiting python3 /usr/share/doc/chrony/ntp2chrony.py -b -v Reading /etc/ntp.conf Reading /etc/ntp/crypto/pw Reading /etc/ntp/keys Writing /etc/chrony.conf Writing /etc/chrony.keys yum install chrony systemctl status chronyd chronyd.service - NTP client/server Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled) Active: active (running) since Wed 2013-06-12 22:23:16 CEST; 11h ago systemctl start chronyd systemctl enable chronyd systemctl stop chronyd systemctl disable chronyd chronyc tracking Reference ID : CB00710F (ntp-server.example.net) Stratum : 3 Ref time (UTC) : Fri Jan 27 09:49:17 2017 System time : 0.000006523 seconds slow of NTP time Last offset : -0.000006747 seconds RMS offset : 0.000035822 seconds Frequency : 3.225 ppm slow Residual freq : 0.000 ppm Skew : 0.129 ppm Root delay : 0.013639022 seconds Root dispersion : 0.001100737 seconds Update interval : 64.2 seconds Leap status : Normal chronyc sources 210 Number of sources = 3 MS Name/IP address Stratum Poll Reach LastRx Last sample =============================================================================== #* GPS0 0 4 377 11 -479ns[ -621ns] /- 134ns ^? a.b.c 2 6 377 23 -923us[ -924us] +/- 43ms ^ d.e.f 1 6 377 21 -2629us[-2619us] +/- 86ms chronyc sourcestats 210 Number of sources = 1 Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev =============================================================================== abc.def.ghi 11 5 46m -0.001 0.045 1us 25us chronyc makestep #!/bin/sh exit 0 driftfile /var/lib/chrony/drift commandkey 1 keyfile /etc/chrony.keys initstepslew 10 client1 client3 client6 local stratum 8 manual allow <subnet> server <server_fqdn> driftfile /var/lib/chrony/drift logdir /var/log/chrony log measurements statistics tracking keyfile /etc/chrony.keys commandkey 24 local stratum 10 initstepslew 20 ntp1.example.net allow <server_ip_address> bindcmdaddress 0.0.0.0 bindcmdaddress :: cmdallow 192.168.1.0/24 cmdallow 2001:db8::/64 firewall-cmd --permanent --add-port=323/udp firewall-cmd --reload --- - hosts: timesync-test vars: timesync_ntp_servers: - hostname: 2.rhel.pool.ntp.org pool: yes iburst: yes roles: - rhel-system-roles.timesync ethtool -T enp1s0 hwtimestamp enp1s0 hwtimestamp eno* server ntp.example.comlocal minpoll 0 maxpoll 0 server ntp.example.comlocal minpoll 0 maxpoll 0 xleave clientloglimit 100000000 systemctrl restart chronyd chronyd[4081]: Enabled HW timestamping on enp1s0 chronyd[4081]: Enabled HW timestamping on eno1 chronyc ntpdata Output: [literal,subs="+quotes,verbatim,normal"] Remote address : 203.0.113.15 (CB00710F) Remote port : 123 Local address : 203.0.113.74 (CB00714A) Leap status : Normal Version : 4 Mode : Server Stratum : 1 Poll interval : 0 (1 seconds) Precision : -24 (0.000000060 seconds) Root delay : 0.000015 seconds Root dispersion : 0.000015 seconds Reference ID : 47505300 (GPS) Reference time : Wed May 03 13:47:45 2017 Offset : -0.000000134 seconds Peer delay : 0.000005396 seconds Peer dispersion : 0.000002329 seconds Response time : 0.000152073 seconds Jitter asymmetry: +0.00 NTP tests : 111 111 1111 Interleaved : Yes Authenticated : No TX timestamping : Hardware RX timestamping : Hardware Total TX : 27 Total RX : 27 Total valid RX : 27 chronyc sourcestats Output: [literal,subs="+quotes,verbatim,normal"] . 210 Number of sources = 1 Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev ntp.local 12 7 11 +0.000 0.019 +0ns 49ns . bindaddress 203.0.113.74 hwtimestamp enp1s0 local stratum 1 systemctrl restart chronyd chronyc -n tracking Reference ID : 0A051B0A (10.5.27.10) Stratum : 2 Ref time (UTC) : Thu Mar 08 15:46:20 2018 System time : 0.000000338 seconds slow of NTP time Last offset : +0.000339408 seconds RMS offset : 0.000339408 seconds Frequency : 2.968 ppm slow Residual freq : +0.001 ppm Skew : 3.336 ppm Root delay : 0.157559142 seconds Root dispersion : 0.001339232 seconds Update interval : 64.5 seconds Leap status : Normal ntpstat synchronised to NTP server (10.5.27.10) at stratum 2 time correct to within 80 ms polling server every 64 s For example: server time.example.com iburst nts server nts.netnod.se iburst nts server ptbtime1.ptb.de iburst nts ntsdumpdir /var/lib/chrony PEERNTP=no systemctl restart chronyd chronyc -N authdata Name/IP address Mode KeyID Type KLen Last Atmp NAK Cook CLen ================================================================ time.example.com NTS 1 15 256 33m 0 0 8 100 nts.netnod.se NTS 1 15 256 33m 0 0 8 100 ptbtime1.ptb.de NTS 1 15 256 33m 0 0 8 100 chronyc -N sources MS Name/IP address Stratum Poll Reach LastRx Last sample ========================================================= time.example.com 3 6 377 45 +355us[ +375us] +/- 11ms nts.netnod.se 1 6 377 44 +237us[ +237us] +/- 23ms ptbtime1.ptb.de 1 6 377 44 -170us[ -170us] +/- 22ms ntsserverkey /etc/pki/tls/private/ <ntp-server.example.net> .key ntsservercert /etc/pki/tls/certs/ <ntp-server.example.net> .crt chown root:chrony /etc/pki/tls/private/<ntp-server.example.net>.key /etc/pki/tls/certs/<ntp-server.example.net>.crt chmod 644 /etc/pki/tls/private/<ntp-server.example.net>.key /etc/pki/tls/certs/<ntp-server.example.net>.crt firewall-cmd -permannent --add-port={323/udp,4460/tcp} firewall-cmd --reload systemctl restart chronyd chronyd -Q -t 3 'server ntp-server.example.net iburst nts maxsamples 1' 2021-09-15T13:45:26Z chronyd version 4.1 starting (+CMDMON +NTP +REFCLOCK +RTC +PRIVDROP +SCFILTER +SIGND +ASYNCDNS +NTS +SECHASH +IPV6 +DEBUG) 2021-09-15T13:45:26Z Disabled control of system clock 2021-09-15T13:45:28Z System clock wrong by 0.002205 seconds (ignored) 2021-09-15T13:45:28Z chronyd exiting chronyc serverstats NTP packets received : 7 NTP packets dropped : 0 Command packets received : 22 Command packets dropped : 0 Client log records dropped : 0 NTS-KE connections accepted: 1 NTS-KE connections dropped : 0 Authenticated NTP packets: 7