When UDP is used for NTP service and when TCP is used for NTP service?

I check from /etc/services and I find that there are 2 lines for ntp services:
ntp             123/tcp
ntp             123/udp

I got an answer from EE expert that I cannot telnet to NTP server using port 123 as it is UDP. I wonder when TCP is used for NTP service.
Who is Participating?
Pétur Ingi EgilssonConnect With a Mentor Software Engineer -- ConsultantCommented:
use `netstat -na|grep LISTEN` to see if anything is listening on the 123/TCP

As i stated.. 123/TCP is only listed in /etc/services as a reservation...
Pétur Ingi EgilssonSoftware Engineer -- ConsultantCommented:
ntp/tcp is listed as a reserved port for ntp.
Does not mean anything is listening on it.

It would confuse people if you would have different reserved ports for both the TCP and UDP protocols.
Pétur Ingi EgilssonSoftware Engineer -- ConsultantCommented:
port 80 TCP is reserved for HTTPd
It would be confusing if you had port 80 TCP for HTTPd and port 80 UDP running something completely diffrent.
tballahAuthor Commented:
I know a bit from DNS service that it uses port 53/UDP for query and 53/TCP for zone transfer between primary and secondary DNS server. I was thinking the same way for NTP service as defined in /etc/services. By the way who could I know the nature of the service whether it is pure UDP or TCP?
Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

All Courses

From novice to tech pro — start learning today.