NTPd listens on UDP port 123 (all IP addresses), by default. Is this necessary for just updating the server time?
If it's necessary to listen on an address, what single address should be used? The possible addresses it can listen on are:
- server IP
- ::1 (IPv6)
- server IP (IPv6)
Why does it even have to listen on localhost (127.0.0.1 and ::1)? Nobody can reach the server on that address.
To bind NTPd to a single address, the
-I option can be used. For my Debian box, I had to edit
/etc/default/ntp, and replace
-g -I 220.127.116.11 -I 127.0.0.1
This causes NTPd to listen on the public address 18.104.22.168 (replace it by your own) and 127.0.0.1.