December 2018
Beginner
826 pages
22h 54m
English
Content preview from Linux Administration Cookbook
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Checking if NTP traffic is flowing
To confirm that NTP traffic is actually flowing, the catchall method is to check the port with tcpdump. NTP uses port 123, which is very easy to remember (there may be a quiz on this).
We know that default traffic should use eth0 for communication, so that's what we'll specify:
$ sudo tcpdump port 123 -i eth0
Soon, you should see your client trying to talk to upstream servers, asking for time information:
10:07:33.229507 IP centos1.37284 > ntp3.wirehive.net.ntp: NTPv4, Client, length 4810:07:33.266188 IP ntp3.wirehive.net.ntp > centos1.37284: NTPv4, Server, length 4810:07:39.411433 IP centos1.49376 > 5751b502.skybroadband.com.ntp: NTPv4, Client, length 4810:07:39.453834 IP 5751b502.skybroadband.com.ntp > ...