From Fedora Project Wiki

Network Time Security

Summary

Support for the Network Time Security (NTS) authentication mechanism in the NTP client/server (chrony) and installer (anaconda).

Owner

Current status

Detailed Description

NTP is a widely used protocol for synchronization of clocks over network. Authentication of NTP packets is important to prevent a Man-in-the-middle (MITM) attacker from taking full control over the client's clock (e.g. force it to jump to a distant future or past). Several different authentication mechanisms have been specified for NTP. The oldest and simplest one uses secret keys, where each client has its own key which needs to be securely distributed to the server and client. This means it is mostly limited to local networks. Autokey is a newer mechanism based on public-key cryptography, but it was shown to be insecure and it is rarely supported on public servers.

NTS is a new authentication mechanism specified by the IETF for NTP. NTS has an NTS-KE protocol using Transport Layer Security (TLS) to establish the keys and provide the client with cookies which allow the NTP server to not keep any client-specific state. NTP packets are authenticated using Authenticated Encryption with Associated Data (AEAD). NTS is expected to scale well to a large numbers of clients. There are already some public NTP servers with NTS support.

The default NTP client and server on Fedora is chrony. Support for NTS is added in version 4.0. It uses the GnuTLS library for TLS and the Nettle library for AEAD.

NTS is not enabled by default. It can be enabled on clients by adding the nts option to the server or pool directive in /etc/chrony.conf. Until a standard NTS-KE port is assigned for NTS by IANA and used by NTP servers, the port needs to be specified with the ntsport option. For example

server time.example.com iburst nts ntsport 12123

There are several issues that may prevent NTS from working correctly:

  • Firewalls may block the NTS-KE port.
  • ISPs may block or rate limit longer NTP packets as a mitigation for amplification attacks using NTP mode 6 and 7. NTS-KE supports port negotiation and an alternative port could be used to avoid this issue.
  • Computers with no RTC (e.g. some ARM boards), or RTC that is too far from the real time, will fail to verify TLS certificates. It is possible to disable the time checks before the first update of the clock by adding nocerttimecheck 1 to /etc/chrony.conf, but it has an impact on security.

Benefit to Fedora

This change enables Fedora users to securely synchronize the system clock to local or public NTP servers.

Scope

  • Proposal owners:
  1. Update chrony to 4.0 and enable the NTS support (adding dependency on GnuTLS)
  2. Add NTS support to the NTP settings in anaconda
  • Other developers: N/A (not a System Wide Change)
  • Release engineering: N/A (not needed for this Change)
  • Policies and guidelines: N/A (not a System Wide Change)
  • Trademark approval: N/A (not needed for this Change)

Upgrade/compatibility impact

There is no impact as NTS is not enabled by default.

How To Test

Client NTS is enabled in /etc/chrony.conf by adding the nts option to the server or pool directive, e.g.

   server time.cloudflare.com nts ntsport 1234 iburst
   server nts.ntp.se nts ntsport 3443

The installer needs to be tested that it enables NTS in /etc/chrony.conf as expected.

The chronyc -N sources command can be used to verify that NTP sources are responding. The chronyc ntpdata command can be used to verify that the NTP sources are authenticated. For example:

   # chronyc -N sources
   MS Name/IP address         Stratum Poll Reach LastRx Last sample               
   ===============================================================================
   ^* time.cloudflare.com           3   6   377    28   -115us[ -111us] +/-   13ms
   ^+ nts.ntp.se                    2   6   377    27   +212us[ +212us] +/-   22ms
   # chronyc ntpdata | grep Auth
   Authenticated   : Yes
   Authenticated   : Yes

User Experience

Client NTS can be enabled in the NTP settings in the installer.

Client and server NTS can be enabled by editing /etc/chrony.conf as documented in the chrony.conf man page.

Dependencies

N/A (not a System Wide Change)

Contingency Plan

  • Contingency mechanism: N/A (not a System Wide Change)
  • Contingency deadline: N/A (not a System Wide Change)
  • Blocks release? N/A (not a System Wide Change)
  • Blocks product?

Documentation

N/A (not a System Wide Change)

Release Notes

TBD