Here is a short post to check port [TCP/UDP] connectivity from a Linux server. A TCP/IP network connection may be either blocked, dropped, open, or filtered. These actions are generally controlled by the IPtables firewall the system uses and is independent of any process or program that may be listening on a network port.
Telnet and nc are common tools used to test port connectivity from Linux server. Telnet can be used to test tcp port connections, where as nc can be used to test both tcp/udp ports connectivity. Make sure telnet and nc tools are installed on the Linux server you are trying to test connectivity.
# yum install nc # yum install telnet
Testing TCP port connectivity with telnet
Lets see how we can use telnet command to test the TCP port connectivity. The syntax to use the telnet command is as follows:
# telnet [hostname/IP address] [port number]
Example of successful connection:
# telnet 192.168.12.10 22 Trying 192.168.12.10... Connected to 192.168.12.10. Escape character is '^]'. SSH-2.0-OpenSSH_6.6.1 Protocol mismatch. Connection closed by foreign host.
Example of unsuccessful connection:
# telnet 192.168.12.10 22 Trying 192.168.12.10... telnet: connect to address 192.168.12.10: No route to host
Using nc command to test TCP port connectivity
The syntax to use nc command for testing TCP post connectivity is as follows:
# nc -z -v [hostname/IP address] [port number]
Example of successful connection:
# nc -z -v 192.168.10.12 22 Connection to 192.118.20.95 22 port [tcp/ssh] succeeded!
Example of unsuccessful connection:
# nc -z -v 192.168.10.12 22 nc: connect to 192.118.20.95 port 22 (tcp) failed: No route to host
Testing UDP port connectivity wit nc command
The syntax to test UDP port connectivity with nc command is as follows:
# nc -z -v -u [hostname/IP address] [port number]
Example of successful connection:
# nc -z -v -u 192.168.10.12 123 Connection to 192.118.20.95 123 port [udp/ntp] succeeded!