nc command
To test TCP connection with nc command, add -v and -z option.
$ nc -vz hostname port
-v: verbose output-z: do not send data
http connection check
$ nc -vz blog.toshima.ru 80
Connection to blog.toshima.ru port 80 [tcp/http] succeeded!
SSl/TLS connection check
$ nc -vz blog.toshima.ru 443
Connection to blog.toshima.ru port 443 [tcp/https] succeeded!
UDP connection check
$ nc -vzu 1.1.1.1 53
Connection to 1.1.1.1 port 53 [udp/domain] succeeded!