How to check Port TCP/UDP connectivity by telnet/netcat


I: Install telnet netcat

I.a: Install telnet netcat on Linux:

yum install nc -y
yum install telnet -y

I.b: Install telnet netcat on Windows:
How to install netcat on Windows
How to install telnet client on Windows Server 2008

II: Checking TCP port connectivity

II.a: Checking TCP port connectivity with telnet

telnet hostname/IP-address port-number

Example successful connection:

C:\Users\Administrator>telnet 127.0.0.1 11211

Example unsuccessful connection:

C:\Users\Administrator>telnet 127.0.0.1 22
Connecting To 127.0.0.1...Could not open connection to the host, on port 22: Con
nect failed

II.b: Checking TCP port connectivity with netcat

nc -z -v hostname/IP-address port-number

Example successful connection:

$ nc -z -v 127.0.0.1 11211
Connection to 127.0.0.1 11211 port [tcp/*] succeeded!

Example unsuccessful connection:

$ nc -z -v 127.0.0.1 81
nc: connect to 127.0.0.1 port 81 (tcp) failed: Connection refused

III: Checking UDP port connectivity (by netcat)

nc -z -v -u hostname/IP-address port-number

Example successful connection:

$ nc -z -v -u 192.168.1.27 4444
Connection to 192.168.1.27 4444 port [udp/*] succeeded!

Check UDP server we see:

Data received from client : X
Received 1 bytes from 192.168.1.27:50394

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