Synchronizing Linux/BSD systems with PublicNTP time servers

1. Install NTP server software

This step varies based on what OS/distribution you may be using. Full details are beyond the scope of this page, but here’s a starting point for some of the more common systems:

Ubuntu 16.04 LTS

• sudo apt-get update
• sudo apt-get -y install ntp

Red Hat Enterprise Linux/CentOS 7.x

• yum install -y ntp
• systemctl enable ntpd
• systemctl start ntpd

FreeBSD 11

• Add ntpd_enable="YES" to /etc/rc.conf
• service ntpd start

2. Find best/closest PublicNTP servers

Consulting the map of servers, identify the PublicNTP server deployment that’s closest to the client system geographically.

To be more accurate, the UNIX traceroute utility to confirm which site has the least number of network “hops” and lowest network latency from your location.

Below are two example outputs from traceroutes originating in Chicago, Illinois, USA.

First, from Chicago, Illinois to Washington, DC (~700 miles/1,122 km):

$ traceroute stratum2.iad01.publicntp.org
traceroute to stratum2.iad01.publicntp.org (52.206.70.54), 30 hops max, 60 byte packets
5 equinix01-chi2.amazon.com (206.223.119.98) 0.469 ms 0.577 ms 0.548 ms
6 52.95.62.100 (52.95.62.100) 3.612 ms 3.379 ms 3.327 ms
7 52.95.62.45 (52.95.62.45) 0.753 ms 52.95.62.31 (52.95.62.31) 0.724 ms 52.95.62.61 (52.95.62.61) 0.622 ms
8 54.239.43.209 (54.239.43.209) 18.708 ms 54.239.43.211 (54.239.43.211) 18.756 ms 54.239.43.209 (54.239.43.209) 18.733 ms
9 54.239.42.59 (54.239.42.59) 18.740 ms 54.239.43.221 (54.239.43.221) 19.363 ms *
10 52.95.3.152 (52.95.3.152) 19.401 ms * 54.239.42.69 (54.239.42.69) 20.770 ms
11 54.239.110.147 (54.239.110.147) 45.775 ms 54.239.110.171 (54.239.110.171) 96.660 ms 54.239.110.187 (54.239.110.187) 66.894 ms
12 54.239.109.245 (54.239.109.245) 19.492 ms 54.239.111.115 (54.239.111.115) 20.147 ms 54.239.43.158 (54.239.43.158) 20.877 ms
13 205.251.244.216 (205.251.244.216) 19.427 ms 205.251.244.210 (205.251.244.210) 19.374 ms 205.251.244.216 (205.251.244.216) 19.306 ms
14 * * *

Now from Chicago, Illinois, to Mumbai, India (~8,000 miles/13,000 km):

$ traceroute stratum2.bom01.publicntp.org
traceroute to stratum2.bom01.publicntp.org (52.66.76.135), 30 hops max, 60 byte packets
4 te0-0-1-1.rcr12.b002281-5.ord03.atlas.cogentco.com (38.104.103.237) 1.235 ms 1.492 ms 1.566 ms
5 be2461.ccr41.ord03.atlas.cogentco.com (154.54.29.237) 2.245 ms 1.002 ms 2.102 ms
6 btn.ord03.atlas.cogentco.com (154.54.13.254) 0.860 ms 0.996 ms 0.969 ms
7 TenGE0-1-0-6.br02.sin03.pccwbtn.net (63.218.248.118) 222.310 ms TenGE0-1-0-17.br02.sin03.pccwbtn.net (63.218.248.234) 225.022 ms TenGE0-0-0-1.br02.sin03.pccwbtn.net (63.218.248.70) 237.199 ms
8 TenGE0-1-0-23.br02.sin03.pccwbtn.net (63.218.248.230) 222.085 ms TenGE0-0-0-3.br02.sin03.pccwbtn.net (63.218.248.78) 245.578 ms TenGE0-0-0-11.br02.sin03.pccwbtn.net (63.218.248.122) 219.868 ms
9 63-218-213-206.static.pccwglobal.net (63.218.213.206) 236.023 ms 235.952 ms 244.604 ms
10 52.93.9.144 (52.93.9.144) 237.116 ms 52.93.9.122 (52.93.9.122) 238.968 ms 52.93.9.56 (52.93.9.56) 249.836 ms
11 52.93.9.157 (52.93.9.157) 238.631 ms 52.93.9.133 (52.93.9.133) 233.322 ms 52.93.9.113 (52.93.9.113) 246.186 ms
12 52.93.10.120 (52.93.10.120) 235.264 ms 240.839 ms 52.93.10.116 (52.93.10.116) 247.237 ms
13 54.239.44.64 (54.239.44.64) 253.196 ms 54.239.44.5 (54.239.44.5) 238.891 ms 54.239.44.64 (54.239.44.64) 240.044 ms
14 52.93.19.159 (52.93.19.159) 242.164 ms 52.95.67.14 (52.95.67.14) 253.358 ms 52.95.67.36 (52.95.67.36) 242.211 ms
15 52.95.67.71 (52.95.67.71) 240.354 ms 52.95.67.49 (52.95.67.49) 253.576 ms 54.239.44.62 (54.239.44.62) 238.309 ms
16 52.95.66.184 (52.95.66.184) 249.127 ms 52.95.66.96 (52.95.66.96) 240.691 ms 52.95.66.184 (52.95.66.184) 249.043 ms
17 52.95.67.65 (52.95.67.65) 236.027 ms 52.95.67.21 (52.95.67.21) 238.863 ms 52.95.66.181 (52.95.66.181) 236.032 ms
18 52.95.66.180 (52.95.66.180) 235.396 ms 52.95.66.198 (52.95.66.198) 243.373 ms 52.95.66.154 (52.95.66.154) 246.503 ms
19 52.95.66.175 (52.95.66.175) 231.917 ms 231.707 ms *
20 * 52.95.67.212 (52.95.67.212) 230.906 ms *
21 * * *

3. Configure NTP software to connect to closest server

Edit the /etc/ntp.conf to contain the following:

# File to store local drift data
driftfile /var/lib/ntp/ntp.drift

# Specify PublicNTP server
server stratum2.iad01.publicntp.org

# Include pool.ntp.org servers as well
pool 0.pool.ntp.org
pool 1.pool.ntp.org
pool 2.pool.ntp.org

# Setting permissions for both IPv4 and IPv6
restrict -4 default notrap nomodify nopeer noquery limited
restrict -6 default notrap nomodify nopeer noquery limited

# Needed for adding pool entries
restrict source notrap nomodify noquery

4. Restart NTP Server

Doing this while the system is running depends on operating system/distribution as well, but the easiest way to have the service read the new configuration is to reboot the operating system.