A Raspberry Pi is behind a firewall which is configured to only allow it to access a single external IP (with all ports) which belongs to a Centos7 server.How can the two devices be configured to ensure that the Raspberry Pi's UTC time is always accurate.Steps that I have taken...Configured ntp on server, pointed to server 0.us.pool.ntp.org, configured firewall, etc.On Pi client, comment out default servers and point to my server, restart.However, the time is about 1 hour off.  Do I still need to configure my server to allow the client to access?  If so, can/should I allow all clients, or just mine?Raspberry Pimichael@pi:~ $ ntpq -c lpeer     remote           refid      st t when poll reach   delay   offset  jitter============================================================================== 12.345.67.890   .INIT.          16 u    -   64    0    0.000    0.000   0.000michael@pi:~ $ date -RTue, 20 Jun 2017 10:37:17 +0000About 24 hours latermichael@pi:~ $ ntpq -c lpeer     remote           refid      st t when poll reach   delay   offset  jitter============================================================================== 74.208.80.161   .INIT.          16 u    - 1024    0    0.000    0.000   0.000michael@pi:~ $ date -RWed, 21 Jun 2017 11:29:08 +0000michael@pi:~ $ grep '^[^ #]' /etc/ntp.confdriftfile /var/lib/ntp/ntp.driftstatistics loopstats peerstats clockstatsfilegen loopstats file loopstats type day enablefilegen peerstats file peerstats type day enablefilegen clockstats file clockstats type day enableserver 12.345.67.890    #This is the Centos serverrestrict -4 default kod notrap nomodify nopeer noqueryrestrict -6 default kod notrap nomodify nopeer noqueryrestrict 127.0.0.1restrict ::1CentOS[michael@centos7 ~]$ ntpq -np     remote           refid      st t when poll reach   delay   offset  jitter==============================================================================+104.156.99.226  192.12.19.20     2 u   21 1024  377   68.864    0.518   0.930+108.59.2.24     130.133.1.10     2 u   68 1024  377   56.934   -0.234   0.881+66.7.96.1       216.218.254.202  2 u  480 1024  367   71.228    0.149   1.752*97.107.128.58   209.51.161.238   2 u   87 1024  377   52.881    1.430   1.507[michael@centos7 ~]$About 12 hours later[michael@centos7 ~]$ ntpq -np     remote           refid      st t when poll reach   delay   offset  jitter==============================================================================+104.156.99.226  204.123.2.72     2 u  432 1024  377   68.857    0.466   0.128+108.59.2.24     130.133.1.10     2 u  838 1024  377   56.550   -0.114   0.083*66.7.96.1       216.218.192.202  2 u  911 1024  377   73.391    0.920   1.763-97.107.128.58   209.51.161.238   2 u  655 1024  377   56.110    2.394   0.125[michael@centos7 ~]$ date -RWed, 21 Jun 2017 12:30:44 +0000[michael@centos7 ~]$ grep '^[^ #]' /etc/ntp.confdriftfile /var/lib/ntp/driftrestrict default nomodify notrap nopeer noqueryrestrict 127.0.0.1restrict ::1server 0.us.pool.ntp.orgserver 1.us.pool.ntp.orgserver 2.us.pool.ntp.orgserver 3.us.pool.ntp.orgincludefile /etc/ntp/crypto/pwkeys /etc/ntp/keysdisable monitorlogfile /var/log/ntp.log
How to keep clock accurate with limited Internet access
time;ntp