Reverse SSH Enhancements

The Reverse SSH Enhancements feature, which is supported for SSH Version 1 and 2, provides an alternative way to configure reverse Secure Shell (SSH) so that separate lines do not need to be configured for every terminal or auxiliary line on which SSH must be enabled. This feature also eliminates the rotary-group limitation.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Prerequisites for Reverse SSH Enhancements

  • SSH must be enabled.

  • The SSH client and server must be running the same version of SSH.

Restrictions for Reverse SSH Enhancements

  • The -l keyword and userid : {number } {ip-address } delimiter and arguments are mandatory when configuring the alternative method of Reverse SSH for console access.

Information About Reverse SSH Enhancements

Reverse Telnet

Reverse telnet allows you to telnet to a certain port range and connect to terminal or auxiliary lines. Reverse telnet has often been used to connect a Cisco device that has many terminal lines to the consoles of other Cisco devices. Telnet makes it easy to reach the device console from anywhere simply by telnet to the terminal server on a specific line. This telnet approach can be used to configure a device even if all network connectivity to that device is disconnected. Reverse telnet also allows modems that are attached to Cisco devices to be used for dial-out (usually with a rotary device).

Reverse SSH

Reverse telnet can be accomplished using SSH. Unlike reverse telnet, SSH provides for secure connections. The Reverse SSH Enhancements feature provides you with a simplified method of configuring SSH. Using this feature, you no longer have to configure a separate line for every terminal or auxiliary line on which you want to enable SSH. The previous method of configuring reverse SSH limited the number of ports that can be accessed to 100. The Reverse SSH Enhancements feature removes the port number limitation. For information on the alternative method of configuring reverse SSH, see How to Configure Reverse SSH Enhancements.

How to Configure Reverse SSH Enhancements

Configuring Reverse SSH for Console Access

To configure reverse SSH console access on the SSH server, perform the following steps.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. line line-number ending-line-number
  4. no exec
  5. login authentication listname
  6. transport input ssh
  7. exit
  8. exit
  9. ssh -l userid : {number } {ip-address }

DETAILED STEPS

  Command or Action Purpose
Step 1

enable

Example:


Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal

Enters global configuration mode.

Step 3

line line-number ending-line-number

Example:


Device# line 1 3

Identifies a line for configuration and enters line configuration mode.

Step 4

no exec

Example:


Device(config-line)# no exec

Disables EXEC processing on a line.

Step 5

login authentication listname

Example:


Device(config-line)# login authentication default

Defines a login authentication mechanism for the lines.

Note 

The authentication method must use a username and password.

Step 6

transport input ssh

Example:


Device(config-line)# transport input ssh

Defines which protocols to use to connect to a specific line of the device.

  • The ssh keyword must be used for the Reverse SSH Enhancements feature.

Step 7

exit

Example:


Device(config-line)# exit

Exits line configuration mode.

Step 8

exit

Example:


Device(config)# exit

Exits global configuration mode.

Step 9

ssh -l userid : {number } {ip-address }

Example:


Device# ssh -l lab:1 router.example.com

Specifies the user ID to use when logging in on the remote networking device that is running the SSH server.

  • userid --User ID.

  • : --Signifies that a port number and terminal IP address will follow the userid argument.

  • number --Terminal or auxiliary line number.

  • ip-address --Terminal server IP address.

Note 

The userid argument and :rotary {number }{ip-address } delimiter and arguments are mandatory when configuring the alternative method of Reverse SSH for modem access.

Configuring Reverse SSH for Modem Access

To configure Reverse SSH for modem access, perform the steps shown in the “SUMMARY STEPS” section below.

In this configuration, reverse SSH is being configured on a modem used for dial-out lines. To get any of the dial-out modems, you can use any SSH client and start a SSH session as shown (in Step 10) to get to the next available modem from the rotary device.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. line line-number ending-line-number
  4. no exec
  5. login authentication listname
  6. rotary group
  7. transport input ssh
  8. exit
  9. exit
  10. ssh -l userid :rotary {number } {ip-address }

DETAILED STEPS

  Command or Action Purpose
Step 1

enable

Example:


Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal

Enters global configuration mode.

Step 3

line line-number ending-line-number

Example:


Device# line 1 200

Identifies a line for configuration and enters line configuration mode.

Step 4

no exec

Example:


Device(config-line)# no exec

Disables EXEC processing on a line.

Step 5

login authentication listname

Example:


Device(config-line)# login authentication default

Defines a login authentication mechanism for the lines.

Note 

The authentication method must use a username and password.

Step 6

rotary group

Example:


Device(config-line)# rotary 1

Defines a group of lines consisting of one or more virtual terminal lines or one auxiliary port line.

Step 7

transport input ssh

Example:


Device(config-line)# transport input ssh

Defines which protocols to use to connect to a specific line of the device.

  • The ssh keyword must be used for the Reverse SSH Enhancements feature.

Step 8

exit

Example:


Device(config-line)# exit

Exits line configuration mode.

Step 9

exit

Example:


Device(config)# exit

Exits global configuration mode.

Step 10

ssh -l userid :rotary {number } {ip-address }

Example:


Device# ssh -l lab:rotary1 router.example.com

Specifies the user ID to use when logging in on the remote networking device that is running the SSH server.

  • userid --User ID.

  • : --Signifies that a port number and terminal IP address will follow the userid argument.

  • number --Terminal or auxiliary line number.

  • ip-address --Terminal server IP address.

Note 

The userid argument and :rotary {number }{ip-address } delimiter and arguments are mandatory when configuring the alternative method of Reverse SSH for modem access.

Troubleshooting Reverse SSH on the Client

To troubleshoot the reverse SSH configuration on the client (remote device), perform the following steps.

SUMMARY STEPS

  1. enable
  2. debug ip ssh client

DETAILED STEPS

  Command or Action Purpose
Step 1

enable

Example:


Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

debug ip ssh client

Example:


Device# debug ip ssh client

Displays debugging messages for the SSH client.

Troubleshooting Reverse SSH on the Server

To troubleshoot the reverse SSH configuration on the terminal server, perform the following steps. The steps may be configured in any order or independent of one another.

SUMMARY STEPS

  1. enable
  2. debug ip ssh
  3. show ssh
  4. show line

DETAILED STEPS

  Command or Action Purpose
Step 1

enable

Example:


Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

debug ip ssh

Example:


Device# debug ip ssh

Displays debugging messages for the SSH server.

Step 3

show ssh

Example:


Device# show ssh

Displays the status of the SSH server connections.

Step 4

show line

Example:


Device# show line

Displays parameters of a terminal line.

Configuration Examples for Reverse SSH Enhancements

Example Reverse SSH Console Access

The following configuration example shows that reverse SSH has been configured for console access for terminal lines 1 through 3:

Terminal Server Configuration


line 1 3
   no exec
   login authentication default
   transport input ssh

Client Configuration

The following commands configured on the SSH client will form the reverse SSH session with lines 1, 2, and 3, respectively: 


ssh -l lab:1 router.example.com
ssh -l lab:2 router.example.com
ssh -l lab:3 router.example.com

Example Reverse SSH Modem Access

The following configuration example shows that dial-out lines 1 through 200 have been grouped under rotary group 1 for modem access: 


line 1 200
   no exec
   login authentication default
   rotary 1
   transport input ssh
   exit

The following command shows that reverse SSH will connect to the first free line in the rotary group: 


ssh -l lab:rotary1 router.example.com

Additional References

Technical Assistance

Description

Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html

Standards

Standards

Title

No new or modified standards are supported by this feature.

--

MIBs

MIBs

MIBs Link

None

To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs

RFCs

RFCs

Title

None

--

Technical Assistance

Description

Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html

Feature Information for Reverse SSH Enhancements

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1. Feature Information for Reverse SSH Enhancements

Feature Name

Releases

Feature Information

Reverse SSH Enhancements

The Reverse SSH Enhancements feature, which is supported for SSH Version 1 and 2, provides an alternative way to configure reverse Secure Shell (SSH) so that separate lines do not need to be configured for every terminal or auxiliary line on which SSH must be enabled. This feature also eliminates the rotary-group limitation.

The following command was introduced: ssh .