How to configure SSH on Cisco IOS

In a previous lesson, I explained how you can use telnet for remote access to your Cisco IOS devices. The problem with telnet is that everything is sent in plaintext, for that reason you shouldn’t use it.

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.

SSH Client

The most common SSH client is probably putty. The only thing you have to do is to select the SSH protocol, enter the IP address and leave the default port at 22:

Putty SSH

You will see this on the putty console:

login as: admin
Using keyboard-interactive authentication.
Password:
R1>

You can also use another Cisco IOS device as a SSH client. Here’s how:

R2#ssh ?
  -c    Select encryption algorithm
  -l    Log in using this user name
  -m    Select HMAC algorithm
  -o    Specify options
  -p    Connect to this port
  -v    Specify SSH Protocol Version
  -vrf  Specify vrf name
  WORD  IP address or hostname of a remote system

There are quite some options but as a minimum, we should specify a username and IP address:

R2#ssh -l admin 192.168.12.1
Password: 

R1>

We are now connected to R1 through SSH.

Configurations

Want to take a look for yourself? Here you will find the configuration of each device.

R1

hostname R1
!
ip domain name NETWORKLESSONS.LOCAL
ip cef
!
username admin password 0 my_password
!
interface GigabitEthernet0/1
 ip address 192.168.12.1 255.255.255.0
!
ip ssh version 2
!
line vty 0 4
 login local
 transport input ssh
!
end

R2

hostname R2
!
ip cef
!
interface GigabitEthernet0/1
 ip address 192.168.12.2 255.255.255.0
!
end

SSH Server and Client

Conclusion

You have now learned how to configure the SSH server on your Cisco IOS router or switch and how to use the SSH client.

  • SSH is a secure method for remote access to your router or switch, unlike telnet.
  • SSH requires a RSA public/private key pair.
  • SSH version 2 is more secure than version 1.
  • Make sure you have an IOS image that supports crypto features, otherwise you can’t use SSH.

Forum Replies

  1. Hi Rene,

    I have always done this using the command:

    ip http secure-server

    And then:

    control-plane host
      management-interface FastEthernet0/0 allow ftp https ssh tftp snmp
    

    And:

    transport input ssh

    I tried it the way you show by generating the crypto key as you have shown above and using the control-plane host lines and it seems to achieve the same results without specifying transport input ssh on the VTY lines. You can only SSH into the router. Is this achieving the same end? The only difference I can see by using your method and issuing a sh run is you don’t

    ... Continue reading in our forum

  2. Hello Matt!

    The way that you implement your configuration achieves something similar, but not exactly the same as that which Rene has done in his example.

    Rene’s example applies SSH on the VTY line. This means that you can connect to the device via SSH from any of its interfaces to the VTY connections. In your configuration, you are binding the ssh configuration only to the management interface. This of course is a legitamite configuration assuming you only apply out of band management, and if it works for you that’s great.

    Also, in your configuration the ip

    ... Continue reading in our forum

  3. Hello Petr

    That’s a great idea. I will convey it to Rene to see if that can be added.

    Thanks!

    Laz

  4. Hello Muhammad

    The command initiated on the VTY line configuration is transport input ssh. This indicates that only the SSH protocol will be used for incoming CLI management requests. If we use transport output ssh, then we are specifying the protocol that will be used when this VTY line is used as a client to connect to another SSH server. Such a configuration is possible, cut is not often implemented.

    I hope this has been helpful!

    Laz

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