[Git][NTPsec/ntpsec][master] Decribe DHCP munging of ntp.conf.

Eric S. Raymond gitlab at mg.gitlab.com
Sat Aug 27 10:08:06 UTC 2016

Eric S. Raymond pushed to branch master at NTPsec / ntpsec

2261b9c3 by Eric S. Raymond at 2016-08-27T06:07:58-04:00
Decribe DHCP munging of ntp.conf.

- - - - -

1 changed file:

- docs/clientstart.txt


--- a/docs/clientstart.txt
+++ b/docs/clientstart.txt
@@ -38,6 +38,11 @@ If your NTP configuration was installed from a binary package (such as
 a deb or RPM file under Linux) you can use this introduction as a
 guide to reading it, but may not have to modify it at all.
+If you are using a typical residential setup, in which your machine
+performs DHCP to your ISP's servers and recieves a dynamic address,
+your ntp.conf may be altered or generated by DHCP at
+address-allocation time.
 == Configuration basics ==
@@ -211,6 +216,42 @@ For details on setting up the GPSD end, see the
 http://catb.org/gpsd/gpsd-time-service-howto.html[GPSD Time Service
+== Special considerations when using DHCP ==
+If your machine uses DHCP to get a dynamic IP address from your ISP, 
+the ISPs provide you with a list of NTP servers to use during that
+handshake.  Suspect this if, when you look at your ntp.conf, you
+see server domain names obviously belonging to your ISP.
+The way this works is that your DHCP client requests the list,
+then then restarts your +{ntpd}+ with a custom configuration file
+generated from that list.
+A good thing about this is that your ISP is likely to hand you servers
+that are close to you on its network and will thus have fairly steady
+ping times.  A bad thing is that you may have difficulty making
+configuration of a local reference clock stick.
+One family of systems with this behavior is Debian Linux, including
+Ubuntu.  On these systems the DHCP client is NetworkManager.  If you
+look in your /etc/init.d/ntp file, you may see something like this:
+if [ -e /var/lib/ntp/ntp.conf.dhcp ]; then
+        NTPD_OPTS="$NTPD_OPTS -c /var/lib/ntp/ntp.conf.dhcp"
+The -c option tells +{ntpd} that the path to a generated configuration
+file follows.  The generation process might pick up your local changes
+to ntp.conf or it might not; this depends on your OS supplier (Debian
+derivatives normally 'do' base on your local ntp.conf). If it does,
+all is well.  If it does not, you may have to modify the hook scripts
+that generate that file, or disable the generation process.
+// end

View it on GitLab: https://gitlab.com/NTPsec/ntpsec/commit/2261b9c36bcd5744994352441485500f0993a939
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