Stratum-3 NTP Server Configuration File - read more at http://supratim-sanyal.blogspot.com/2017/03/free-public-stratum-2-and-stratum-3-ntp.html

ntp.conf

# --
# This is Supratim Sanyal's NTP Server configuration file
# To test, use "ntpdate -q <this host>" from another host
# This server will not respond to peer queries ("ntpdc -q") etc. from
# external hosts.
#
# The stratum 1 servers used by this server are mostly educational and
# military servers from:
# http://support.ntp.org/bin/view/Servers/StratumOneTimeServers?redirectedfrom=Servers.StratumOne
# Comment out the stratum 2 servers to make this server stratum 2 with
# only stratum 1 peers. All configured peer servers in this file need to
# be stratum 1 servers for this server to be stratum 2.
# Do not use burst, but iburst is ok with most stratum 1 servers.
#
# Supratim Sanyal, supratim at riseup dot net
# ++

driftfile /var/lib/ntp/drift
restrict default kod nomodify notrap nopeer noquery
restrict -6 default kod nomodify notrap nopeer noquery
restrict 127.0.0.1
restrict -6 ::1

# ---------------
# Supratim Sanyal's Free Public NTP Servers
# Stratum 2 Peers
# ---------------
server sanyalnet-cloud-vps.freeddns.org burst iburst prefer
server sanyalnet-cloud-vps2.freeddns.org burst iburst prefer
server sanyalnet-ntp.freeddns.org burst iburst prefer

# ---------------
# Stratum 1 Peers
# ---------------

# Preferred
server time.nist.gov iburst
server tick.usno.navy.mil iburst
server tock.usno.navy.mil iburst
server ntp2.usno.navy.mil iburst

# Other
server clock.isc.org iburst
server clock.uregina.ca iburst
server cronos.cenam.mx iburst
server gnomon.cc.columbia.edu iburst
server hora.roa.es iburst
server minuto.roa.es iburst
server navobs1.gatech.edu iburst
server navobs1.oar.net iburst
server navobs1.wustl.edu iburst
server now.okstate.edu iburst
server ntp1.fau.de iburst
server ntp1.inrim.it iburst
server ntp1.nl.uu.net iburst
server ntp1.oma.be iburst
server ntp1.usv.ro iburst
server ntp2.fau.de iburst
server ntp2.inrim.it iburst
server ntp2.oma.be iburst
server ntp.atomki.mta.hu iburst
server ntp.colby.edu iburst
server ntp.fizyka.umk.pl iburst
server ntp-galway.hea.net iburst
server ntp.i2t.ehu.eus iburst
server ntp.lcf.mx iburst
server ntp.neel.ch iburst
server ntp.neu.edu.cn iburst
server ntp.nic.cz iburst
server ntp.nict.jp iburst
server ntps1-0.cs.tu-berlin.de iburst
server ntps1-0.uni-erlangen.de
server ntps1-1.cs.tu-berlin.de iburst
server ntps1-1.uni-erlangen.de
server ntp-s1.cise.ufl.edu iburst
server ntps1.pads.ufrj.br iburst
server ntp.shoa.cl iburst
server ptbtime1.ptb.de iburst
server ptbtime2.ptb.de iburst
server rustime01.rus.uni-stuttgart.de iburst
server rustime02.rus.uni-stuttgart.de iburst
server sesku.planeacion.net iburst
server tick.usask.ca iburst
server time1.stupi.se iburst
server time.apple.com iburst
server time.esa.int iburst
server time.ufe.cz iburst
server tock.usask.ca iburst
server zeit.fu-berlin.de iburst

# Local fall back if no external peers are reachable
# Stratum 16 means this server is unsynchronized
server 127.127.1.0
fudge 127.127.1.0 stratum 16