Ubuntu configures its NTP service to pull time updates from the NTP Pool Project and from ntp.ubuntu.com. From /etc/ntp.conf:

# Use servers from the NTP Pool Project. Approved by Ubuntu Technical Board
# on 2011-02-08 (LP: #104525). See http://www.pool.ntp.org/join.html for
# more information.
server 0.ubuntu.pool.ntp.org
server 1.ubuntu.pool.ntp.org
server 2.ubuntu.pool.ntp.org
server 3.ubuntu.pool.ntp.org

# Use Ubuntu's ntp server as a fallback.
server ntp.ubuntu.com


You may want to change this default NTP config to use your preferred NTP provider. For more information, please see https://help.ubuntu.com/lts/serverguide/NTP.html.

IDS Alerts

Anybody can join the NTP Pool Project and provide NTP service. Occasionally, somebody provides NTP service from a residential DHCP address that at some point in time may have also been used for Tor. This results in IDS alerts for Tor nodes where the port is 123 (NTP). This is another good reason to modify the NTP configuration to pull time updates from your preferred NTP provider.