Config Lab: Enabling SSH and Disabling Telnet

Wendell Odom
By Wendell Odom August 27, 2021 13:05

Many of us start out learning about the simplest Cisco security option: One password for all users to reach user mode, with no per-user login. By today’s standards, that security method is archaic. Today’s lab lets you upgrade from that simple beginning point to allow only the more secure SSH login, disallow Telnet, and require each user to use a separate username/password.

All about Config Labs

The blog has a series of lab exercises called “Config Labs.” Each lab presents a topology with the relevant initial configuration for each device. The lab also lists new requirements, after which you should create the additional configuration to meet those requirements. You can do the lab on paper, in a text editor, or use software tools like Cisco Packet Tracer or Cisco Modeling Labs.

Once you have created your answer, you can click various tabs at the bottom of this post to see the lab answers, comments about the lab, and other helpful information.

The Lab Exercise

Requirements

You will configure a Cisco Catalyst switch to support SSH and not support Telnet. From the user perspective, an attempt when using Telnet should be rejected so that the user never sees a prompt for a password or username. SSH users should be allowed in if they supply a correct username and password. For this lab, the switch already has been configured to support IP, and it has been configured with simple passwords; your job is to update the configuration to support SSH only for remote users.

The specific rules for this lab are as follows:

  1. Configure SSH to use an encryption key.
  2. Create the SSH key so that it relies on domain name example.com as input.
  3. Create a username/password pair of Barney/Rubble with the best encryption possible for the password.
  4. Enable support for SSH, but only SSH, using the locally-created usernames/passwords.

Figure 1: Network Used in this Lab

 

Initial Configuration

This lab begins with a switch that has been configured to allow both Telnet and SSH into the switch, both using a simple IP address. The management IP address and the CLI access passwords for user mode and enable mode have been set. Example 1 shows that configuration.

Example 1: SW1 Initial Config

* Note that the no shutdown command is likely unnecessary on real gear, but may be on some virtualization platforms, so we include it here in the initial configuration.

Answer Options - Click Tabs to Reveal

You can learn a lot and strengthen real learning of the topics by creating the configuration – even without a router or switch CLI. In fact, these labs were originally built to be used solely as a paper exercise!

To answer, just think about the lab. Refer to your primary learning material for CCNA, your notes, and create the configuration on paper or in a text editor. Then check your answer versus the answer post, which is linked at the bottom of the lab, just above the comments section.

You can also implement the lab using the Cisco Packet Tracer network simulator. With this option, you use Cisco’s free Packet Tracer simulator. You open a file that begins with the initial configuration already loaded. Then you implement your configuration and test to determine if it met the requirements of the lab.

(Use this link for more information about Cisco Packet Tracer.)

Use this workflow to do the labs in Cisco Packet Tracer:

  1. Download the .pkt file linked below.
  2. Open the .pkt file, creating a working lab with the same topology and interfaces as the lab exercise.
  3. Add your planned configuration to the lab.
  4. Test the configuration using some of the suggestions below.

Download this lab’s Packet Tracer File

You can also implement the lab using Cisco Modeling Labs – Personal (CML-P). CML-P (or simply CML) replaced Cisco Virtual Internet Routing Lab (VIRL) software in 2020, in effect serving as VIRL Version 2.

If you prefer to use CML, use a similar workflow as you would use if using Cisco Packet Tracer, as follows:

  1. Download the CML file (filetype .yaml) linked below.
  2. Import the lab’s CML file into CML and then start the lab.
  3. Compare the lab topology and interface IDs to this lab, as they may differ (more detail below).
  4. Add your planned configuration to the lab.
  5. Test the configuration using some of the suggestions below.

Download this lab’s CML file!

 

Network Device Info:

This table lists the interfaces listed in the lab exercise documentation versus those used in the sample CML file.

Device Lab Port  CML Port
SW1 G0/1 G0/1
SW1 F0/1 G0/2
SW1 F0/2 G0/3

Host device info:

This table lists host information pre-configured in CML, information that might not be required by the lab but may be useful to you.

Device IP Address Mac Address User/password
PC 10.1.1.11 02:00:11:11:11:11 cisco/cisco
S 10.1.1.22 02:00:22:22:22:22 cisco/cisco

Lab Answers Below: Spoiler Alert

Lab Answers: Configuration (Click Tab to Reveal)

Answers

Switch Configuration

Commentary, Issues, and Verification Tips (Click Tabs to Reveal)

Commentary

Telnet transmits information in a non-encrypted form and the information can be intercepted while traversing a network. SSH provides a more secure communications method, encrypting all communications using an encryption key. That communication includes the initial login, which must use a username/password pair.

The initial configuration fully enabled Telnet access. At that point, the switch allowed incoming Telnet and SSH attempts because of a default VTY setting of transport input all (meaning all methods, including both Telnet and SSH).  To disable incoming Telnet sessions, but allow SSH, the answer configuration in this post includes the transport input ssh command.

The combination of the initial configuration plus default settings does not fully configure SSH access. First, the switch needs to generate an encryption key, and to generate that key, the switch needs both a hostname (preconfigured) and domain name (not yet configured). So the first two commands in the solution shown in this post add the domain name and then generate the key.

The command to generate the key, crypto key generate rsa modulus 1024, uses a modulus of 1024. To use SSH version 2, the key must use a key length (modulus) of at least 768. (You can also omit the modulus parameter and IOS will prompt you for the value.)

Finally, SSH requires the use of a username/password pair rather than a shared password. So, the configuration shows the login local command under the VTY ports, enabling the user of local username/password pairs rather than a password without a username. Additionally, the global command username Barney secret Rubble – which creates a username/password with an admittedly weak but fun password – is at least an encrypted password through use of the username secret command instead of the username password command.

Known Issues in this Lab

This section of each Config Lab Answers post hopes to help with those issues by listing any known issues with Packet Tracer related to this lab. In this case, the issues are:

# Summary Detail
1 Cannot paste crypto key command Packet tracer rejects the command crypto key generate rsa modulus 1024 when either typed or pasted into config mode. Instead, use the command crypto key generate rsa ,  press enter, and PT will prompt for the modulus setting.

 

Why Would Cisco Packet Tracer Have Issues?

(Note: The below text is the same in every Config Lab.)

Cisco Packet Tracer (CPT) simulates Cisco routers and switches. However, CPT does not run the same software that runs in real Cisco routers and switches. Instead, developers wrote CPT to predict the output a real router or switch would display given the same topology and configuration – but without performing all the same tasks, an actual device has to do. On a positive note, CPT requires far less CPU and RAM than a lab full of devices so that you can run CPT on your computer as an app. In addition, simulators like CPT help you learn about the Cisco router/switch user interface – the Command Line Interface (CLI) – without having to own real devices.

CPT can have issues compared to real devices because CPT does not run the same software as Cisco devices. CPT does not support all commands or parameters of a command. CPT may supply output from a command that differs in some ways from what an actual device would give. Those differences can be a problem for anyone learning networking technology because you may not have experience with that technology on real gear – so you may not notice the differences. So this section lists differences and issues that we have seen when using CPT to do this lab.

Beyond comparing your answers to this lab’s Answers post, you can test in Cisco Packet Tracer (CPT) or Cisco Modeling Labs (CML). In fact, you can and should explore the lab once configured. For this lab, once you have completed the configuration, try these verification steps. 

  1. Connect to the console, and try to login with the configured password.
  2. Using the PC connected to a LAN interface, attempt to Telnet into the switch, to confirm the Telnet fails (as intended.)
  3. Again from the PC connected to a LAN interface, attempt to SSH into the switch to confirm that SSH works.
  4. Once in user mode after logging in with SSH, use the enable command to then test the enable password.

More Labs with Related Content!

Config Lab: CLI Passwords 2
Config Lab: Switch IP 1
Wendell Odom
By Wendell Odom August 27, 2021 13:05
Write a comment

4 Comments

  1. iciyas December 14, 00:18

    hi,
    First, thank you so much for this blog.
    I’d like to mention that the labs in CPT are no longer compatible with the last version of the software. I can’t open any in CPT because version differences.
    regards.

    Reply to this comment
    • certskills December 17, 16:48

      Hi,
      In the version you’re running in which you have trouble opening the .pkt files you download here, can you check the specific version and post it here? That would be a help in figuring out what’s happening. Normally, older .pkt files can be opened in the newer versions of Packet Tracer. Thanks.
      Wendell

      Reply to this comment
  2. CCNA_Wrecked_Me_Twice February 23, 16:23

    Hi,

    The CLI wouldn’t accept the command:

    crypto key generate rsa modulus 1024

    Had to use:

    crypto key generate rsa general-keys modulus 1024

    Did anyone else encounter this?

    Reply to this comment
    • certskills February 28, 16:21

      Hi,
      Yes. Packet Tracer doesn’t support the command listed in the answer. Check out the “Known Packet Tracer Issues” tab for more detail.

      Reply to this comment
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