# Differences from NTP Classic

| Accept no imitations.

## 1. Related Links

- A list of all links is on the Site Map page.

## 2. Table of Contents

- Introduction

- Incompatible Changes

- Security Improvements

- Time Synchronization Improvements

- Configuration Improvements

- Other user-visible changes

## 3. Introduction

The design objectives of this distribution, NTPsec, are in many ways a break with NTP’s past. We have deliberately jettisoned support for ancient legacy hardware and operating systems in order to ship code that is security-hardened, simpler, drastically less bulky, easier to understand, and easier to maintain.

We retain, however, almost full compatibility and interoperation with NTP Classic. The qualification "almost" is required mainly because we do not support the Autokey (RFC 5906) public-key encryption scheme. It had interoperability and exploitable vulnerability issues too severe to be patched. We have also dropped broadcast and anycast modes because they cannot be secured.

A major new feature is that we implement IETF’s Network Time Security standard for strong cryptographic authentication of time service.

This project began as an effort to address serious security issues with NTP Classic, and we intend to keep a particularly strong focus on code security and code verifiability.

Most of the changes are under the hood, internal to the codebase. A few will be user-visible.

## 4. Incompatible Changes

Normally NTPsec is a drop-in replacement for legacy versions. We have tried to hold incompatible changes to a minimum, but there are a few. Some can be reverted by building the software in strict compatibility mode with --enable-classic-mode (note that this is a build-time switch, not a run-time one).

- The sntp program has been renamed ntpdig in order to make NTP installables have a uniform name prefix and take up less namespace. Also, ntp-keygen is now ntpkeygen, ntp-wait is ntpwait, and update-leap is now ntpleapfetch.

- Log timestamps look a little different; they are now in ISO 8601 format. Reverted in the --enable-classic-mode build.

- Clock identifiers in log files are normally the driver shortname followed by the unit number in parentheses, rather than the magic IP addresses formerly used. This change affects the peerstats, rawstats, and clockstats files. Reverted in the --enable-classic-mode build.

- The -!m, -\>, and -< options of some Classic commands are not supported. (The argument-parsing framework code that implemented them in Classic was overcomplicated and buggy and had to be removed.)

- The shortname of --help options is now -h, not -?

- If you had a refclock on a path of the form /dev/palisadeNNN, that link needs to change to /dev/trimbleNNN. Reverted in the --enable-classic-mode build.

- If you had a refclock on a path of the form /dev/actsNNN, that link needs to change to /dev/modemNNN. Reverted in the --enable-classic-mode build.

- An instance of ntpq built from the NTPsec code querying a legacy NTP daemon will not automatically display peers with 127.127.t.u addresses as refclocks; that assumption has been removed from the NTPsec code as part of getting it fully IPv6-ready.

- ntpq no longer has the -i/--interactive option, as there was no situation in which it was meaningful. The orders in which the default set of system, peer, and clock variables are dumped have changed.

- Interleave mode has been removed. It was buggy and the root cause of at least two CVEs.

## 5. Security Improvements

We have spent more effort than anything else on reducing attack surface and hardening code. In toto, more than 74% of the NTP Classic codebase has been outright removed, with less than 5% new code added to the security-critical core.

- Network Time Security is implemented. See our page on Standards Conformance

- NTPsec conforms to the NTP Client Data Minimization draft RFC, which changes the client-side generation of some packet headers to prevent client fingerprinting.

- The deprecated ntpdc utility, long a chronic locus of security vulnerabilities, has been removed. Its function has been merged into ntpq.

- Autokey is not supported; that code has been removed, as it was chronically prone to security vulnerabilities.

- Peer mode has been removed. The keyword peer in ntp.conf is now just an alias for keyword server.

- Broadcast- and multicast modes, which are impossible to secure, have been removed.

- The authentication requirement for remote configuration commands (e.g., via ntpq) can no longer be disabled.

- The deprecated and vulnerability-prone ntpdate program has been replaced with a shell wrapper around ntpdig. Its -e and -p options are not implemented. It is no longer documented, but can be found in the attic/ directory of the source distribution.

- A large number of obsolete refclocks have been removed in order to reduce attack surface, code bulk, and documentation complexity.

- Various features related to runtime dumping of the configuration state have been removed for security reasons. These include the saveconfig command in ntpq, the --saveconfigquit option of ntpd, and the implementation of related config declarations in ntp.conf.

- Likewise, the poorly-documented ntpdsim code has also been removed to gain a significant reduction in code complexity.

- The ntpsnmpd daemon, incomplete and not conformant with RFC 5907, has been removed.

- The trap feature has been removed. It was broken by bit-rot in recent versions of NTP Classic, and if not broken would have been at high risk for bugs that would enable DoS vulnerabilities.

- Interleave mode has been removed. It didn’t work correctly (there was an implementation error in the timestamp handling), so no point in allowing it to increase attack surface.

- The code has been systematically hardened, with unsafe string copy and formatting functions replaced by safe (bounded) ones.

## 6. Time-synchronization improvements

- Internally, there is more consistent use of nanosecond precision. A visible effect of this is that time stepping with sufficiently high-precision time sources could be accurate down to nanoseconds rather than microseconds; this might actually matter for GPSDOs and high-quality radio clocks.

- For a driver that ships 4-digit year stamps (notably including the NMEA driver) that data is no longer ignored in favor of deducing the year from the system clock. This means that (a) it is now possible for ntpd using a local clock source to recover from a trashed or zeroed system clock (e.g. at boot time on a system with missing or damaged battery back up) without requiring sync to a remote peer.

- We’ve fixed a bug inherited from Classic that could cause the jitter of a bad peer to be incorrectly zeroed, possibly causing that peer to be selected. This probably accounts for some flakiness within 8 polling intervals of startup on older versions.

- Flagging a PPS peer with "prefer" allows its pulse-per-second input to be used with any source. This capability had been claimed in the NTP Classic documentation but if it was ever actually implemented that code was lost without trace by the time NTPsec forked.

## 7. Client Tool Improvements

- A new tool, ntpmon, performs real-time monitoring of your peer and MRU status with efficient (least-cost) querying.

- Both ntpq and ntpmon can now optionally do display with time units shown and scaled, in the manner of "chrony sources".

- There is a new data-visualization tool, ntpviz, which can produce various useful and interesting plots from the NTP statistics logs. These should assist in monitoring a time-server’s performance, fixing configuration problems, and identifying noise sources in network weather and elsewhere.

- Because ntpviz exists, a number of ancient and poorly-documented scripts in awk, Perl, and S, formerly used for making statistical summaries, have been removed from the distribution in order to reduce overall maintenance burden and complexity. If you miss any of this cruft, the project team will (a) be quite surprised, and (b) work with you on better analytics using ntpviz and modern tools.

- The ntpq utility resizes its display to take advantage of wide terminal windows, allowing more space for long peer addresses.

- When running as root, the ntpq utility looks in /etc/ntp.conf and /usr/local/etc/ntp.keys to find credentials for control requests that require authentication. Thus it will often not be necessary to enter them by hand. Note that your installation’s locations for config and key files may differ from these; in that case this convenience feature will fail, as we have elected not to chase it down a complexity rathole.

- A new utility, ntpfrob, collects several small diagnostic functions for reading and tweaking the local clock hardware, including reading the clock tick rate, precision, and jitter. Part of it formerly traveled as tickadj.

- Mode 6 now fully supports RFC 5907: "Definitions of Managed Objects for Network Time Protocol Version 4 (NTPv4)" by reporting a sys_rootdist variable that is the root distance of the selected peer. This can fill in the MIB’s ntpEntTimeDistance entry.

- ntpq displays the root distance (aka. synchronization distance) in the sysinfo command.

## 8. Configuration Improvements

- The notorious collision between pool and nopeer in older implementations has been fixed; the pool keyword is now fully usable.

- There is a new, simpler syntax for declaring refclocks. The old syntax with the magic 127.127.t.u addresses and fudge command is still supported, but no longer documented. It may be removed in a future release. Relevant examples of the new syntax are included on each refclock page. One major feature of the new syntax is that refclock drivers are referred to by names, not numbers.

- Entries for GPS devices can be fudged for era wraparound using the time1 offset. Each g on the end of the offset adds the number of seconds in a 10-bit GPS era (1024 weeks); each G adds the number of seconds in a 13-bit GPS era (8192 weeks).

- The unpeer command now takes a type-unit specification when unpeering a clock.

- For the generic (parse) driver only: Using the new refclock syntax, the maximum number of units that can be set up changes from 4 (numbers 0-3) to unlimited. However, the old magic-address syntax will not work correctly - you must use the new syntax to declare generic-driver refclocks. If the software was compiled with the --enable-classic-mode switch, the foregoing is reversed.

- In server entries, its now possible to specify a time offset to be applied to incoming timestamps (analogues to the fudge on certain refclocks). This may be useful for client systems communicating over ADSL lines, which have large but relatively fixed asymmetric delays.

- The restrict statement can now take an address range in CIDR notation rather than as an address/mask pair.

- You can now turn off restriction flags with an unrestrict statement that takes arguments exactly like a restrict. With no arguments it removes any rule matching the address mask entirely. This is expected to be useful mainly with the "ntpq :config" command.

- The includefile directive now evaluates relative pathnames not with respect to the current working directory but with respect to the directory name of the last pushed file in the stack. This means that you can run ntpd from any directory with "includefile foo" in /etc/ntp.conf finding /etc/foo rather than looking for foo in your current directory.

- If there is an /etc/ntp.d directory, its subfiles are scanned for more configuration declarations. Only files with the extension ".conf" are interpreted; others are ignored. This feature is intended to make assembling configuration easier for administration and package-configuration scripts. See ntpd(8) for details.

- It is now possible to set the peer maximum dispersion with "tos maxdisp". See RFC 5905 for discussion of this synchronization parameter.

- The default bitrate of the NMEA driver has been changed to 9600 to match the default speed of almost all modern GPSes. The code can be built in a strict NTP Classic compatibility mode that restores the old 4800 bps default.

- Most refclock drivers now support configuration options to override the default device path, the default PPS device path (if any) and the serial baud rate.

- NIST lockclock mode is now a runtime option set by the (previously unused) flag1 mode bit of the local-clock driver, rather than a compile-time option.

## 9. Other user-visible changes

- The documentation has been extensively updated and revised. One important change is that manual pages are now generated from the same masters as this web documentation, so the two will no longer drift out of synchronization.

## 10. Future directions

- Now that we have full Network Time Security, a near-future direction is to remove older insecure authentication methods (MAC and MS-SNTP).

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