Question & Answer

Question

Using netcat to ascertain if a particular port is open between sender and receiver

Answer

The utility netcat can be used to ascertain if a particular TCP or UDP port is open between two endpoints. This can be extremely useful for determining whether a connection can be made between an Aspera client and server.

Obtaining netcat

Windows: Download from http://joncraton.org/media/files/nc111nt.zip (password for the archive is "nc")
Linux/Unix: Install netcat from your usual package manager for example with CentOS: yum install netcat

Instructions

Ensure netcat is available on both computers you are testing with.

First, set up one computer to be the "listener" listening on a specific port. Then try to connect to the listening computer on the same port from the other computer.

STEP 1

Connect to both local and remote computer. Make sure the netcat files are present on both computers.

STEP 2

On your Aspera server open up a command prompt and navigate to the directory with netcat. Then type in:

nc -l -u -p 33001

-l means it should listen -u that it should use UDP and -p specifies the port.

STEP 3

Now try and connect from the remote machine. Open up a command prompt and navigate to the directory with netcat. Use the following command where ip_address is the IP address of your Aspera server:

nc -u ip_address 33001

STEP 4

If the connection was successful you can type something on the remote machine and will see the same echoed on the listening machine.
If the port is blocked you will be able to type something on the remote machine but you will not see anything appear on the listening machine.

STEP 5 (optional)

Try netcat with a port you know works and is open just for verification. For example to test TCP/33001:

nc -l -p 33001 # on the listening servernc ip_address # 33001 on the remote machine

Related Information

UDP connectivity testing

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