Last updated: 2026-05-15

Chrony NTS Configuration Guide

This guide provides recommended Network Time Security (NTS) settings for chrony. NTS is a cryptographic extension to NTP that authenticates time synchronization using TLS, preventing man-in-the-middle attacks on time data. Chrony has supported NTS since version 4.0.

Prerequisites#

• Chrony 4.0 or later (4.1+ recommended; see version notes below). RHEL 8.0--8.3 ships chrony 3.5 (no NTS); upgrade to RHEL 8.4+ for chrony 4.x. SLES 15 SP1--SP3 also ships chrony 3.x.
• GnuTLS or OpenSSL library (chrony's NTS-KE uses TLS 1.3)
• A valid TLS certificate and private key (for server mode)
• Network access to port 4460 (NTS-KE) and port 123 (NTP)

Path note: This guide uses RHEL/CentOS paths (/etc/chrony.conf). On Debian/Ubuntu, the config file is /etc/chrony/chrony.conf and the chrony user/group is _chrony (with underscore) instead of chrony.

NTS Overview#

NTS works in two phases:

1. NTS-KE (Key Establishment) -- A TLS 1.3 handshake on port 4460 authenticates the server and establishes shared keys
2. NTP with cookies -- Subsequent NTP packets on port 123 carry encrypted cookies and authentication extensions derived from the NTS-KE session

Client Configuration#

Basic NTS Client#

Enable NTS for upstream time servers by adding the nts keyword:

# /etc/chrony.conf

server time.cloudflare.com iburst nts
server nts.netnod.se iburst nts
server ptbtime1.ptb.de iburst nts
server ntppool1.time.nl iburst nts

NTS-KE Port#

If a server uses a non-standard NTS-KE port:

server nts.example.com iburst nts ntsport 4461

Certificate Trust Store#

Chrony uses the system trust store by default. To specify a custom CA bundle:

# RHEL/CentOS:
ntstrustedcerts /etc/pki/tls/certs/ca-bundle.crt
# Debian/Ubuntu: /etc/ssl/certs/ca-certificates.crt
# SLES: /etc/ssl/ca-bundle.pem

NTS Cookie Storage#

Persist NTS cookies across restarts to avoid repeated NTS-KE handshakes:

ntsdumpdir /var/lib/chrony

Rate Limiting NTS-KE#

Limit how often chrony retries NTS-KE on failure:

ntsrefresh 86400

Version note: ntsrefresh requires chrony 4.1+. On chrony 4.0 (Debian 11), omit this directive.

This sets the maximum interval (in seconds) between NTS-KE requests to a server.

Server Configuration#

NTS-KE Server#

Configure chrony as an NTS-capable NTP server:

# /etc/chrony.conf

# NTS-KE server settings
ntsserverkey /etc/chrony/ssl/privkey.pem
ntsservercert /etc/chrony/ssl/fullchain.pem

The certificate must be valid for the server's hostname. Let's Encrypt certificates work well.

NTS-KE Port#

By default, chrony listens on port 4460 for NTS-KE. To change it:

ntsport 4460

Key Rotation#

NTS uses server cookies encrypted with a key that chrony rotates automatically. Persist the keys across restarts:

ntsdumpdir /var/lib/chrony

Chrony generates new cookie keys automatically and retains old keys long enough for clients to transition.

NTS Process Separation#

For security, run the NTS-KE helper as a separate unprivileged process:

ntsprocesses 1

Version note: ntsprocesses requires chrony 4.1+. On chrony 4.0 (Debian 11), omit this directive — NTS-KE will run in the main process instead.

Complete Client Configuration#

# /etc/chrony.conf

# NTS-authenticated time sources
server time.cloudflare.com iburst nts
server nts.netnod.se iburst nts
server ptbtime1.ptb.de iburst nts
server ntppool1.time.nl iburst nts

# Fallback (unauthenticated) sources
pool pool.ntp.org iburst maxsources 2

# NTS cookie storage
ntsdumpdir /var/lib/chrony

# Allow system clock to be stepped on first sync
makestep 1.0 3

# Record tracking data
driftfile /var/lib/chrony/drift

# RTC synchronization
rtcsync

# Logging
logdir /var/log/chrony
log measurements statistics tracking

Complete Server Configuration#

# /etc/chrony.conf

# Upstream NTS sources
server time.cloudflare.com iburst nts
server nts.netnod.se iburst nts

# NTS server settings
ntsserverkey /etc/chrony/ssl/privkey.pem
ntsservercert /etc/chrony/ssl/fullchain.pem
ntsprocesses 1
ntsdumpdir /var/lib/chrony

# Serve time to clients
allow 10.0.0.0/8
allow 172.16.0.0/12
allow 192.168.0.0/16

# Serve as stratum 2 if upstream sources are lost
local stratum 2

# System clock
makestep 1.0 3
driftfile /var/lib/chrony/drift
rtcsync

# Rate limiting
ratelimit interval 1 burst 16

# Logging
logdir /var/log/chrony
log measurements statistics tracking

Certificate Management#

Using Let's Encrypt with Certbot#

certbot certonly --standalone -d ntp.example.com

Configure chrony to use the certificates:

ntsserverkey /etc/letsencrypt/live/ntp.example.com/privkey.pem
ntsservercert /etc/letsencrypt/live/ntp.example.com/fullchain.pem

Chrony automatically detects certificate file changes and reloads them without restart, which works well with certbot's automatic renewal.

File Permissions#

Ensure chrony can read the private key:

chown root:chrony /etc/chrony/ssl/privkey.pem   # Debian/Ubuntu: chown root:_chrony
chmod 640 /etc/chrony/ssl/privkey.pem

Firewall#

Open the required ports:

# NTS-KE (TLS 1.3 key establishment)
firewall-cmd --permanent --add-port=4460/tcp

# NTP (time synchronization)
firewall-cmd --permanent --add-service=ntp

firewall-cmd --reload

Security Notes#

NTS (Network Time Security, RFC 8915) uses TLS 1.3 exclusively for the NTS-KE (Key Establishment) phase. Chrony links against GnuTLS or OpenSSL for this; check your distribution's build. Because NTS mandates TLS 1.3, many older SSL/TLS attacks are simply not in scope:

• POODLE / BEAST / FREAK / LOGJAM / Sweet32: Not applicable. These attacks target SSL 3.0 and TLS 1.0/1.1 weaknesses; NTS-KE requires TLS 1.3 and rejects older protocol versions.
• Forward secrecy: Mandatory in TLS 1.3; every NTS-KE session uses an ephemeral key exchange.
• CRIME (CVE-2012-4929, 2012): TLS compression is not supported in TLS 1.3.
• Lucky13 / CBC padding oracles: Not applicable. TLS 1.3 uses AEAD-only ciphers and does not support CBC mode.
• Downgrade attacks: TLS 1.3 includes built-in downgrade protection via the ServerHello random value.

The following are not addressable through NTS/TLS configuration alone:

• Heartbleed (CVE-2014-0160, 2014): Only relevant if chrony is built against OpenSSL 1.0.1 through 1.0.1f. Fixed in OpenSSL 1.0.1g (April 7, 2014). Not applicable if linked against GnuTLS. Addressed by keeping the TLS library patched.
• BREACH / CRIME (HTTP-layer): Not applicable. NTS is not an HTTP-based protocol.
• DROWN (CVE-2016-0800, 2016): Not applicable. NTS-KE only supports TLS 1.3; SSLv2 is not used.

Verification#

Check NTS status for all sources:

chronyc -N authdata

You should see NTS in the Mode column and a non-zero cookie count:

Name/IP address  Mode KeyID Type KLen Last Atmp  NAK Cook CLen
================================================================
time.cloudflare>  NTS     1   15  256  33m    0    0    8  100
nts.netnod.se    NTS     1   15  256  33m    0    0    8  100

Check NTS-KE connection details:

chronyc ntpdata time.cloudflare.com

Verify the NTS-KE handshake uses TLS 1.3:

gnutls-cli --port 4460 time.cloudflare.com < /dev/null

Check overall time synchronization:

chronyc tracking
chronyc sources -v