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.

14.1. Using the Chrony suite to configure NTP

Accurate timekeeping is important for several reasons in IT. In networking for example, accurate time stamps in packets and logs are required. In Linux systems, the NTP protocol is implemented by a daemon running in user space.

The user space daemon updates the system clock running in the kernel. The system clock can keep time by using various clock sources. Usually, the Time Stamp Counter (TSC) is used. The TSC is a CPU register which counts the number of cycles since it was last reset. It is very fast, has a high resolution, and there are no interruptions.

Starting with Red Hat Enterprise Linux 8, the NTP protocol is implemented by the chronyd daemon, available from the repositories in the chrony package.

The following sections describe how to use the chrony suite to configure NTP.

14.1.1. Introduction to chrony suite

chrony is an implementation of the Network Time Protocol (NTP). 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), and 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 chrony daemon, chronyd, 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.

14.1.2. 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
```
chronyc must run as root if some of the restricted commands are to be used.
The chronyc command prompt will be displayed as follows:
```
chronyc>
```
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:
```
chronyc command
```

Note

Changes made using chronyc are not permanent, they will be lost after a chronyd restart. For permanent changes, modify /etc/chrony.conf.

14.1.3. 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:

# 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

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.

14.1.3.1. 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:

# 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

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.

14.2. Using Chrony

The following sections describe how to install, start, and stop chronyd, and how to check if chrony is synchronized. Sections also describe how to manually adjust System Clock.

14.2.1. Managing chrony

The following procedure describes how to install, start, stop, and check the status of chronyd.

Procedure

The chrony suite is installed by default on Red Hat Enterprise Linux. To ensure that it is, run the following command as root:
```
# yum install chrony
```
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:

$  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

To start chronyd, issue the following command as root:
```
# systemctl start chronyd
```
To ensure chronyd starts automatically at system start, issue the following command as root:
```
# systemctl enable chronyd
```
To stop chronyd, issue the following command as root:
```
# systemctl stop chronyd
```
To prevent chronyd from starting automatically at system start, issue the following command as root:
```
# systemctl disable chronyd
```

14.2.2. Checking if chrony is synchronized

The following procedure describes how to check if chrony is synchronized with the use of the tracking, sources, and sourcestats commands.

Procedure

To check chrony tracking, issue the following command:

$  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

The sources command displays information about the current time sources that chronyd is accessing. To check chrony sources, issue the following command:

$ 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

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:
```
$  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
```
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

14.2.3. Manually adjusting the System Clock

The following procedure describes how to manually adjust the System Clock.

Procedure

To step the system clock immediately, bypassing any adjustments in progress by slewing, issue the following command as root:
```
# chronyc makestep
```

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.

14.2.4. 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.

If you enable NetworkManager on your system to manage networking configuration, 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.

Prerequisites

You installed NetworkManager on your system and enabled it.
Root access

Procedure

To disable the chrony dispatcher script, edit the /etc/NetworkManager/dispatcher.d/20-chrony-onoffline file as follows:
```
#!/bin/sh
exit 0
```
Note
When you upgrade or reinstall the chrony package, the packaged version of the dispatcher script replaces your modified dispatcher script.

14.2.5. Setting up chrony for a system 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.

The following procedure describes how to set up chrony for a system in an isolated network.

Procedure

On the system selected to be the server, using a text editor running as root, edit /etc/chrony.conf as follows:
```
driftfile /var/lib/chrony/drift
commandkey 1
keyfile /etc/chrony.keys
initstepslew 10 client1 client3 client6
local stratum 8
manual
allow 192.0.2.0/24
```
Where 192.0.2.0/24 is the network or subnet address from which the clients are allowed to connect. For more details, see chrony.conf(7) man page on your system
On the systems selected to be direct clients of the server, using a text editor running as root, edit the /etc/chrony.conf as follows:
```
server ntp1.example.net
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 192.0.2.123
```
Where 192.0.2.123 is the address of the server, and ntp1.example.net is the host name 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.

14.2.6. Configuring remote monitoring access

chronyc can access chronyd in two ways:

Internet Protocol, IPv4 or IPv6.
Unix domain socket, which is accessible locally by the root or chrony user.

By default, chronyc connects to the Unix domain socket. The default path is /var/run/chrony/chronyd.sock. If this connection fails, which can happen for example when chronyc is running under a non-root user, 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 with the cmdallow directive in the configuration file of chronyd, or 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

Allow access from both IPv4 and IPv6 addresses by adding the following to the /etc/chrony.conf file:
```
bindcmdaddress 0.0.0.0
```
or
```
bindcmdaddress ::
```
Allow commands from the remote IP address, network, or subnet by using the cmdallow directive.
Add the following content to the /etc/chrony.conf file:
```
cmdallow 192.168.1.0/24
```
Open port 323 in the firewall to connect from a remote system:
```
# firewall-cmd --zone=public --add-port=323/udp
```
Optionally, you can open port 323 permanently using the --permanent option:
```
# firewall-cmd --permanent --zone=public --add-port=323/udp
```
If you opened port 323 permanently, reload the firewall configuration:
```
# firewall-cmd --reload
```

Additional resources

chrony.conf(5) man page on your system

14.2.7. 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. Previous 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

---
- hosts: timesync-test
  vars:
    timesync_ntp_servers:
      - hostname: 2.rhel.pool.ntp.org
        pool: yes
        iburst: yes
  roles:
    - rhel-system-roles.timesync

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

14.2.8. Additional resources

chronyc(1) and chronyd(8) man pages on your system
Frequently Asked Questions

14.3. Chrony with HW timestamping

Hardware timestamping is a feature supported in some Network Interface Controller (NICs) which 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 reach 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.

The following sections describe how to:

Verify support for hardware timestamping
Enable hardware timestamping
Configure client polling interval
Enable interleaved mode
Configure server for large number of clients
Verify hardware timestamping
Configure PTP-NTP bridge

14.3.1. 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.

Example 14.2. Verifying support for hardware timestamping on a specific interface

# ethtool -T eth0

Output:

Timestamping parameters for eth0:
Capabilities:
        hardware-transmit     (SOF_TIMESTAMPING_TX_HARDWARE)
        software-transmit     (SOF_TIMESTAMPING_TX_SOFTWARE)
        hardware-receive      (SOF_TIMESTAMPING_RX_HARDWARE)
        software-receive      (SOF_TIMESTAMPING_RX_SOFTWARE)
        software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
        hardware-raw-clock    (SOF_TIMESTAMPING_RAW_HARDWARE)
PTP Hardware Clock: 0
Hardware Transmit Timestamp Modes:
        off                   (HWTSTAMP_TX_OFF)
        on                    (HWTSTAMP_TX_ON)
Hardware Receive Filter Modes:
        none                  (HWTSTAMP_FILTER_NONE)
        all                   (HWTSTAMP_FILTER_ALL)
        ptpv1-l4-sync         (HWTSTAMP_FILTER_PTP_V1_L4_SYNC)
        ptpv1-l4-delay-req    (HWTSTAMP_FILTER_PTP_V1_L4_DELAY_REQ)
        ptpv2-l4-sync         (HWTSTAMP_FILTER_PTP_V2_L4_SYNC)
        ptpv2-l4-delay-req    (HWTSTAMP_FILTER_PTP_V2_L4_DELAY_REQ)
        ptpv2-l2-sync         (HWTSTAMP_FILTER_PTP_V2_L2_SYNC)
        ptpv2-l2-delay-req    (HWTSTAMP_FILTER_PTP_V2_L2_DELAY_REQ)
        ptpv2-event           (HWTSTAMP_FILTER_PTP_V2_EVENT)
        ptpv2-sync            (HWTSTAMP_FILTER_PTP_V2_SYNC)
        ptpv2-delay-req       (HWTSTAMP_FILTER_PTP_V2_DELAY_REQ)

14.3.2. Enabling hardware timestamping

To enable hardware timestamping, use 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. Use the wildcard specification in case that no other application, like ptp4l from the linuxptp package, is using hardware timestamping on an interface. Multiple hwtimestamp directives are allowed in the chrony configuration file.

Example 14.3. Enabling hardware timestamping by using the hwtimestamp directive

hwtimestamp eth0
hwtimestamp eth1
hwtimestamp *

14.3.3. Configuring client polling interval

The default range of a polling interval (64-1024 seconds) is recommended for servers on the Internet. For local servers and hardware timestamping, a shorter polling interval needs to be configured in order to minimize offset of the system clock.

The following directive in /etc/chrony.conf specifies a local NTP server using one second polling interval:

server ntp.local minpoll 0 maxpoll 0

14.3.4. Enabling interleaved mode

NTP servers that are not hardware NTP appliances, but rather general purpose computers running a software NTP implementation, like chrony, will get a hardware transmit timestamp only after sending a packet. This behavior prevents the server from saving the timestamp in the packet to which it corresponds. In order to enable NTP clients receiving transmit timestamps that were generated after the transmission, configure the clients to use the NTP interleaved mode by adding the xleave option to the server directive in /etc/chrony.conf:

server ntp.local minpoll 0 maxpoll 0 xleave

14.3.5. Configuring server for large number of clients

The default server configuration allows a few thousands of clients at most to use the interleaved mode concurrently. To configure the server for a larger number of clients, increase the clientloglimit directive in /etc/chrony.conf. This directive specifies the maximum size of memory allocated for logging of clients' access on the server:

clientloglimit 100000000

14.3.6. Verifying hardware timestamping

To verify that the interface has successfully enabled hardware timestamping, check the system log. The log should contain a message from chronyd for each interface with successfully enabled hardware timestamping.

Example 14.4. Log messages for interfaces with enabled hardware timestamping

chronyd[4081]: Enabled HW timestamping on eth0
chronyd[4081]: Enabled HW timestamping on eth1

When chronyd is configured as an NTP client or peer, you can have the transmit and receive timestamping modes and the interleaved mode reported for each NTP source by the chronyc ntpdata command:

Example 14.5. Reporting the transmit, receive timestamping and interleaved mode for each NTP source

# chronyc ntpdata

Output:

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

Example 14.6. Reporting the stability of NTP measurements

# chronyc sourcestats

With hardware timestamping enabled, stability of NTP measurements should be in tens or hundreds of nanoseconds, under normal load. This stability is reported in the Std Dev column of the output of the chronyc sourcestats command:

Output:

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

14.3.7. 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.

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:

Example 14.7. Configuring chronyd to provide the system time using the other interface

bindaddress 203.0.113.74
hwtimestamp eth1
local stratum 1

14.4. Achieving some settings previously supported by NTP in chrony

Some settings that were in previous 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.

14.4.1. 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 14.8. Using the tracking command

$ 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

Example 14.9. Using the ntpstat utility

$ ntpstat
synchronised to NTP server (10.5.27.10) at stratum 2
   time correct to within 80 ms
   polling server every 64 s

14.4.2. 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.

14.4.3. 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.

14.4.4. 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.

14.5. 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 to the /etc/sysconfig/chronyd file.

14.5.1. Enabling Network Time Security (NTS) in the client configuration file

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

Server with the NTS support

Procedure

In the client configuration file:

Specify the server with the nts option in addition to the recommended iburst option.

For example:
server time.example.com iburst nts
server nts.netnod.se iburst nts
server ptbtime1.ptb.de iburst nts

To avoid repeating the Network Time Security-Key Establishment (NTS-KE) session during system boot, add the following line to chrony.conf, if it is not present:
```
ntsdumpdir /var/lib/chrony
```
Add the following line to /etc/sysconfig/network to disable synchronization with Network Time Protocol (NTP) servers provided by DHCP:
```
PEERNTP=no
```
Save your changes.
Restart the chronyd service:
```
systemctl restart chronyd
```

Verification

Verify if the NTS keys were successfully established:

# 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.sth1.ntp.se   NTS     1   15  256  33m    0    0    8  100
nts.sth2.ntp.se   NTS     1   15  256  33m    0    0    8  100

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:

# chronyc -N sources

MS Name/IP address Stratum Poll Reach LastRx Last sample
=========================================================
time.example.com   3        6   377    45   +355us[ +375us] +/-   11ms
nts.sth1.ntp.se    1        6   377    44   +237us[ +237us] +/-   23ms
nts.sth2.ntp.se    1        6   377    44   -170us[ -170us] +/-   22ms

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

14.5.2. Enabling Network Time Security (NTS) on the 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

Specify the private key and the certificate file in chrony.conf. For example:

ntsserverkey /etc/pki/tls/private/<ntp-server.example.net>.key
ntsservercert /etc/pki/tls/certs/<ntp-server.example.net>.crt

Ensure that both the key and certificate files are readable by the chrony system user, by setting the group ownership. For example:
```
# chown :chrony /etc/pki/tls//<ntp-server.example.net>.
```
Ensure the ntsdumpdir /var/lib/chrony directive is present in the chrony.conf.
Restart the chronyd service:
```
# systemctl restart chronyd
```
Important
If the server has a firewall, it needs to allow both the UDP 123 and TCP 4460 ports for NTP and Network Time Security-Key Establishment (NTS-KE).

Verification

Perform a quick test from a client machine with the following command:

$ 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

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:
```
# 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
```
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.

Chapter 14. Configuring time synchronization

14.1. Using the Chrony suite to configure NTP

14.1.1. Introduction to chrony suite

14.1.2. Using chronyc to control chronyd

14.1.3. Migrating to chrony

14.1.3.1. Migration script

14.2. Using Chrony

14.2.1. Managing chrony

14.2.2. Checking if chrony is synchronized

14.2.3. Manually adjusting the System Clock

14.2.4. Disabling a chrony dispatcher script

14.2.5. Setting up chrony for a system in an isolated network

14.2.6. Configuring remote monitoring access

14.2.7. Managing time synchronization using RHEL system roles

14.2.8. Additional resources

14.3. Chrony with HW timestamping

14.3.1. Verifying support for hardware timestamping

14.3.2. Enabling hardware timestamping

14.3.3. Configuring client polling interval

14.3.4. Enabling interleaved mode

14.3.5. Configuring server for large number of clients

14.3.6. Verifying hardware timestamping

14.3.7. Configuring PTP-NTP bridge

14.4. Achieving some settings previously supported by NTP in chrony

14.4.1. Monitoring by ntpq and ntpdc

14.4.2. Using authentication mechanism based on public key cryptography

14.4.3. Using ephemeral symmetric associations

14.4.4. multicast/broadcast client

14.5. Overview of Network Time Security (NTS) in chrony

14.5.1. Enabling Network Time Security (NTS) in the client configuration file

14.5.2. Enabling Network Time Security (NTS) on the server

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