CASE STUDY_Change the SSH port of a Cisco Router :: SSH Rotary

 Let’s imagine we are asked to configure Router1 to run SSH access using the port 2009.

As we already know, SSH uses the tcp port 22 by default. So, how can we change the port that R1(Router) uses to listen for SSH connections? There is a way to do it straightforward, with a configuration only in R1. This feature is called SSH rotary.

Let’s see how to configure it, the first thing we have to do is to generate the RSA key pair for the router with the command crypto key generate rsa. But first, we need to configure the hostname and the domain-name server in the router:

router(config)# hostname R1

R1(config)# ip domain name cisco.com

R1(config)# crypto key generate rsa

The name for the keys will be: R1.cisco.com

Choose the size of the key modulus in the range of 360 to 2048 for your

  General Purpose Keys. Choosing a key modulus greater than 512 may take

  a few minutes.

How many bits in the modulus [512]: 1024

% Generating 1024 bit RSA keys, keys will be non-exportable...[OK]

Once we have created the RSA key pair, it’s necessary to configure the user and password to access the router, and to apply it into the VTY line settings:

R1(config)# username cisco password CISCO

R1(config)# line vty 0 4

R1(config-line)# login local

Now, we will configure the router to expect ssh connections on port 2009. This is done with the command ip ssh port and applying a rotary group. Then, that rotary group is configured on the VTY lines:

R1(config)# ip ssh port 2009 rotary 1

R1(config)# line vty 0 4

R1(config-line)# rotary 1

Let’s verify that the configuration its actually working:

R1#ssh -l cisco -p 2009 2.2.2.1

Password:

R1>sh tcp brief

TCB       Local Address               Foreign Address             (state)

66CCF71C  2.2.2.1.18922               2.2.2.1.2009                ESTAB

66CCFC78  2.2.2.1.2009                2.2.2.1.18922               ESTAB

So here you have it, how to configure a router to be accessed via SSH with a non-by-default port.