[RndTbl] More about network time...
Gilbert E. Detillieux
gedetil at cs.umanitoba.ca
Wed Sep 13 14:18:41 CDT 2017
Wyatt's presentation on NTPD last night prompted some discussion about
other NTP implementations. (Chrony was mentioned in the presentation,
and systemd-timesyncd was mentioned in discussion.)
The latest Raspbian boot image for the Raspberry Pi (based on Debian
stretch) no longer installs ntpd by default, but relies on
systemd-timesyncd instead. (Presumably Debian stretch does something
similar, though that may not be the case...) Of course, ntpd, chrony,
and openntpd are all available as alternatives.
The good news is that systemd-timesyncd seems to work for the very
simple case of a client-only setup, where you just want to get the time
reasonably close to some official time source (it only implements an NTP
client, not a server), requires no special configuration, and seems to
get out of the way if you install any of the more-capable NTP server
The bad news is that it doesn't do as good a job of keeping time in
sync. It only contacts one of the available NTP servers, and sticks to
that one until it's not reachable, in which case it tries the next
available one, etc. It will use as time sources one of the available
pool servers for the distribution, or (if provided) one of those
indicated by the DHCP server. So, for the typical client, it may be fine.
Also, it doesn't give you much by way of stats. You can use the
"timedatectl" command, to find out if NTP synchronization is enabled or
not, but that's about it. You can also use the "systemctl -l status
systemd-tymesyncd" command, and look at the log entries showing which
time server was used...
Sep 09 07:02:29 raspberrypi systemd-timesyncd: Synchronized to time
server 184.108.40.206:123 (2.debian.pool.ntp.org).
Sep 09 23:33:45 raspberrypi systemd-timesyncd: Timed out waiting
for reply from 220.127.116.11:123 (2.debian.pool.ntp.org).
Sep 09 23:33:45 raspberrypi systemd-timesyncd: Synchronized to time
server 18.104.22.168:123 (2.debian.pool.ntp.org).
Sep 10 00:43:48 raspberrypi systemd-timesyncd: Timed out waiting
for reply from 22.214.171.124:123 (2.debian.pool.ntp.org).
Sep 10 00:43:48 raspberrypi systemd-timesyncd: Synchronized to time
server 126.96.36.199:123 (2.debian.pool.ntp.org).
No way to check for offset and other such stats, though. (It may not
even track that.)
Chrony seems to be a quite capable replacement for NTPD, with a
configuration file format that looks quite similar (at least for the
basic config). There are a couple different commands to get stats...
[root at host ~]# chronyc sources
210 Number of sources = 7
MS Name/IP address Stratum Poll Reach LastRx Last sample
^* ntp1.mydomain.net 3 10 377 280 -274us[ -216us] +/-
^+ ntp2.mydomain.net 3 10 377 585 -348us[ -290us] +/-
^+ ntp3.mydomain.net 3 10 377 510 -696us[ -638us] +/-
^+ james.kirk.hungrycats.org 2 10 377 229 -3138us[-3138us] +/-
^+ ns541050.ip-144-217-252.n 3 9 377 390 -2763us[-2705us] +/-
^+ sanction.treborlogic.com 2 10 377 983 +573us[ +630us] +/-
^+ 188.8.131.52 3 10 377 455 +13ms[ +13ms] +/-
[root at host ~]# chronyc sourcestats
210 Number of sources = 7
Name/IP Address NP NR Span Frequency Freq Skew Offset
ntp1.mydomain.net 39 19 273m -0.000 0.007 -814us
ntp2.mydomain.net 16 9 90m -0.002 0.066 -619us
ntp3.mydomain.net 27 17 226m -0.010 0.013 -784us
james.kirk.hungrycats.org 6 4 86m +0.160 1.290 -3291us
ns541050.ip-144-217-252.n 8 6 60m -0.143 0.280 -3050us
sanction.treborlogic.com 6 3 86m +0.287 0.623 +614us
184.108.40.206 12 9 189m +0.129 0.121 +13ms
[root at hedge ~]#
The default config is set up to query servers in the pool for the distro
(rhel in the above example, with the last 4 servers shown), but will
also include any NTP servers indicated by DHCP (the first 3 servers
shown, with bogus host names in this example).
I haven't tried OpenNTPD (The OpenBSD NTP daemon), so I don't know how
that one looks compared to the classic NTPD.
Manitoba UNIX User Group E-mail: <gedetil at muug.ca>
c/o Gilbert E. Detillieux Web: http://muug.ca/
University of Manitoba Phone: (204)474-8161
Winnipeg MB CANADA R3T 2N2 Fax: (204)474-7609
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