Telnet 并不支持 UDP 端口的测试，可以使用 nc 命令来进行测试。nc 命令两种都支持：

TCP

# nc -z -v -u [hostname/IP address] [port number] 

 

# nc -z -v 192.168.10.12 22 

Connection to 192.118.20.95 22 port [tcp/ssh] succeeded!

UDP

# nc -z -v [hostname/IP address] [port number] 

 

# nc -z -v -u 192.168.10.12 123 

Connection to 192.118.20.95 123 port [udp/ntp] succeeded!

下面是一些 nc 的简单用例：

  - Start a listener on the specified TCP port and send a file into it:
    nc -l -p port < filename

  - Connect to a target listener on the specified port and receive a file from it:
    nc host port > received_filename

  - Scan the open TCP ports of a specified host:
    nc -v -z -w timeout_in_seconds host start_port-end_port

  - Start a listener on the specified TCP port and provide your local shell access to the connected party (this is dangerous and can be abused):
    nc -l -p port -e shell_executable

  - Connect to a target listener and provide your local shell access to the remote party (this is dangerous and can be abused):
    nc host port -e shell_executable

  - Act as a proxy and forward data from a local TCP port to the given remote host:
    nc -l -p local_port | nc host remote_port

  - Send an HTTP GET request:
    echo -e "GET / HTTP/1.1\nHost: host\n\n" | nc host 80