Notes by Lex Toumbourou

Client side

• /etc/ntp.conf
  • server <ntp-server> iburst mode allows for quick synchronisation to a server
• ntpdate -q instructor.example.com
• ntpd -c pe

> ntpq -c pe
remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*instructor.exam LOCAL(0)        11 u  483  512  377    0.241   -0.129   0.026

• jitter - 0.000 would mean problem talking to the server
• st - strata
• poll - need to research this

Server side

• Peers/clients
• Each server has a stratum classification - defines how close to reliable time it is (hardware clock is 0) - lower is more accurate
• Clients have higher stratum value
• restrict option is used to allow access to NTP server
• server and peer option takes an NTP server and options argument

> vi /etc/ntp.conf
+ server  instructor.example.com  iburst
+ peer    desktop1.example.com
+ peer    desktop2.example.com

• Common flags include:
  • ignore - deny packets of all kinds, including ntpd and ntpdc queries
  • kod - Kiss-of-death packets are sent which notifies the client it hit an access violation
  • nomodify - deny ntpq and ntpdc queries whcih attempt to modify the state of the server
  • noquery - deny ntpq and ntpd queries. Time service is not affected
  • nopeer - self-explainatory
  • notrap - decline to provide mode 6 control message

> vi /etc/ntp.conf
# Hosts on local network are less restricted.
restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap

Example ntp.conf

#/etc/ntp.conf

restrict default kod nomodify notrap nopeer noquery # Anyone with ipv4
address can query ntp (noquery != ignore)
restrict -6 default ignore # no one with ipv6 can query NTP

restrict 192.168.0.0 mask 255.255.255.0 nomodify notrap nopeer
restrict 192.168.0.101 kod nomodify notrap
restrict 192.168.0.200

server 192.168.0.2
server 192.168.0.3
peer 192.168.0.101