I am currently setting up a new Linux server, and as always, I am struggling with the NTP daemon (Debian buster, package ntp, so it is the BSD variant), especially with /etc/ntp.conf and the restrict clause.

I have read the man page (man ntp.conf), and I have visited several tutorials and online man pages, for example https://docs.ntpsec.org/latest/ntp_conf.html and the "orignal" https://www.freebsd.org/cgi/man.cgi?query=ntp.conf&sektion=5&manpath=freebsd-release-ports.

However, these pages seem to be misleading or wrong. Two examples (taken from the latter link which actually should be the authoritative reference):

  • The -4 and -6 tokens are explained in the section Configuration Commands which does not include the restrict command. Instead, the restrict command is considered to belong to the Access Control Commands and is explained in a separate section of exactly this name. Given that, if you don't have much experience with the NTP daemon, you could come to the conclusion that you can't use -4 as an address in restrict commands. This obviously is wrong.

  • In the explanation of the restrict command (in section Access Control Commands), there is the following passage at the end: "Default restriction list entries with the flags ignore, interface, ntpport, [...]". So there should be a flag named interface, but no such flag is explained or even mentioned in the list of flags.

Since I am not the sort of guy who wants to copy-and-paste some examples from dubious sources and hope that it will work somehow, I'd be highly interested in a complete manual for ntp.conf. Does anybody know about such a thing?

My last resort would be reading the source code, but obviously, I'd like to avoid that.

  • Given that that is the ntp.conf manual, I'd be visiting bugs.ntp.org rather than Stack Exchange, myself.
    – JdeBP
    Mar 24, 2020 at 12:02
  • 2
    I would expect the authoritative reference to be what is linked under "Official Documentation" on the NTP site: ntp.org/documentation.html which is neither of the links you have provided. interface is not mentioned as a default flag for restrict there. The official documentation does explicitly support your claim that man pages might be misleading or wrong
    – Fox
    Mar 24, 2020 at 12:02
  • I see. Thank you very much. When searching for the manual, I didn't come across the link you gave. You are right, this is the official documentation / reference. But now there's a new problem: What to do / believe if the man page differs from the official reference? How can I know whether or not my Linux distribution has altered the software so that it works according to its man page? However, this was not part of my question, and you have answered my question. Hence, if you make your comment an answer, I'll upvote and accept it.
    – Binarus
    Mar 24, 2020 at 16:13

1 Answer 1


The NTP site lists several resources including "Official NTP Documentation". This official documentation states that

[This documentation] has been widely copied, cached and morphed to other formats, including man pages, with varying loss of fidelity. However, these HTML pages are the ONLY authoritative and definitive reference.

which implies that the NTP project does not publish man pages for its own software.

On that, the “interface” flag mentioned in the man pages regarding restrict is simply not mentioned at all in this official documentation. (However, it was mentioned in the older version 4.2.4; its inclusion may have been a mistake.) Your comment regarding the -4 and -6 tokens still holds.

In any case, the issue isn't a lack of a complete reference, it is a case of buggy documentation. The solution to buggy documentation is filing a bug report to its source. Perhaps the man pages have simply not been updated to reflect the current documentation, since they are not a part of the NTP software distribution.

  • 1
    Your comment regarding the -4 and -6 tokens still holds. But at least, it is now stated (additionally?) in the release notes, as a general statement, i.e. not related to a certain class of commands / directives. I saw that when I studied the link you gave and followed some of its references. It is here: eecis.udel.edu/~mills/ntp/html/release.html (first item in section "Changes and Upgrades Since the NTPv3 Version (xntp3-5)").
    – Binarus
    Mar 24, 2020 at 16:36
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    I call shenanigans. The manual page pointed to in the question comes from the NTP project's own source, in the ntpd/ntp.conf.def file, precompiled into ntp.conf.5man, ntp.conf.5mdoc, ntp.conf.html, ntp.conf.mdoc.in, and ntp.conf.man.in files right there next to it.
    – JdeBP
    Mar 24, 2020 at 18:31
  • @JdeBP From the NTP site: "The NTP documentation is distributed as HTML only. There are no manpages or other forms of documentation for the project." Can't help it if they lie about their own project. Someone could file a bug report informing them that they're lying though
    – Fox
    Mar 24, 2020 at 20:04

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