How to configure SSH on Cisco ?

SSH (Secure Shell) is a secure method for remote access as is includes authentication and encryption. To do this, it uses a RSA public/private keypair.

There are two versions: version 1 and 2. Version 2 is more secure and commonly used.

Last but not least, to configure SSH you require an IOS image that supports crypto features. Otherwise you won’t be able to configure SSH.

Configuration

To demonstrate SSH, I will use the following topology:

R1 R2 Gigabit Links

We will configure SSH on R1 so that we can access it from any other device. R2 will be used as a SSH client.

SSH Server

The name of the RSA keypair will be the hostname and domain name of the router. Let’s configure a hostname:

Router(config)#hostname R1

And a domain name:

R1(config)#ip domain-name NETWORKLESSONS.LOCAL

Now we can generate the RSA keypair:

R1(config)#crypto key generate rsa
The name for the keys will be: R1.NETWORKLESSONS.LOCAL
Choose the size of the key modulus in the range of 360 to 4096 for your
General Purpose Keys. Choosing a key modulus greater than 512 may take
a few minutes.

How many bits in the modulus [512]: 2048
% Generating 2048 bit RSA keys, keys will be non-exportable…
[OK] (elapsed time was 3 seconds)

When you use the crypto key generate rsa command, it will ask you how many bits you want to use for the key size. How much should you pick?

It’s best to check the next generation encryption article from Cisco for this. At this moment, a key size of 2048 bits is acceptable. Key sizes of 1024 or smaller should be avoided. Larger key sizes also take longer to calculate.

Once the keypair has been generated, the following message will appear:

R1#
%SSH-5-ENABLED: SSH 1.99 has been enabled

As you can see above, SSH version 1 is the default version. Let’s switch to version 2:

R1(config)#ip ssh version 2

SSH is enabled but we also have to configure the VTY lines:

R1(config)#line vty 0 4
R1(config-line)#transport input ssh
R1(config-line)#login local

This ensures that we only want to use SSH (not telnet or anything else) and that we want to check the local database for usernames. Let’s create a user:

R1(config)#username admin password my_password

Everything is now in place. We should be able to connect to R1 through SSH now.

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