I am trying to setup the NTP server on Ubuntu machine. Am breaking my head in this particular issue.

Getting an error ntpdate[5005]: no server suitable for synchronization found when doing the command ntppdate.

Can anyone please help me out in this?

/etc/ntp.conf:

server 0.ubuntu.pool.ntp.org 
server 1.ubuntu.pool.ntp.org 
server 2.ubuntu.pool.ntp.org 
server 3.ubuntu.pool.ntp.org

Also when I gave command netstat -anltp | grep "LISTEN"

tcp    0   0 127.0.0.1:53            0.0.0.0:*         LISTEN   1816/dnsmasq
tcp    0   0 0.0.0.0:22              0.0.0.0:*         LISTEN   939/sshd
tcp    0   0 127.0.0.1:631           0.0.0.0:*         LISTEN   1013/cupsd
tcp    0   0 127.0.0.1:39558         0.0.0.0:*         LISTEN   5529/rsession
tcp    0   0 0.0.0.0:902             0.0.0.0:*         LISTEN   1275/vmware-authdla
tcp    0   0 127.0.0.1:47304         0.0.0.0:*         LISTEN   5822/rsession
tcp6   0   0 :::80                   :::*              LISTEN   1400/apache2
tcp6   0   0 :::22                   :::*              LISTEN   939/sshd
tcp6   0   0 ::1:631                 :::*              LISTEN   1013/cupsd

So what should I do so that it listens on 123?

If I get output as

PORT    STATE SERVICE
123/udp open  ntp

If I give command nmap -p 123 -sU -P0 192.168.36.198, it means UDP is open right? Then why doesn't it show in the command to to show listening ports?