Reverse SSH Enhancements

First Published: September 18, 2004
Last Updated: October 7, 2009

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 feature information and caveats, see 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 for Reverse SSH Enhancements" section.

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

Contents

•Prerequisites for Reverse SSH Enhancements

•Restrictions for Reverse SSH Enhancements

•Information About Reverse SSH Enhancements

•How to Configure Reverse SSH Enhancements

•Configuration Examples for Reverse SSH Enhancements

•Additional References

•Feature Information for Reverse SSH Enhancements

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 SSH

Reverse Telnet

Cisco IOS software has for quite some time included a feature called Reverse telnet, whereby you can telnet to a certain port range and connect to terminal or auxiliary lines. Reverse telnet has often been used to connect a Cisco IOS router that has many terminal lines to the consoles of other Cisco IOS routers or to other devices. Telnet makes it easy to reach the router console from anywhere simply by telnet to the terminal server on a specific line. This telnet approach can be used to configure a router even if all network connectivity to that router is disconnected. Reverse telnet also allows modems that are attached to Cisco IOS routers 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" section."

How to Configure Reverse SSH Enhancements

•Configuring Reverse SSH for Console Access

•Configuring Reverse SSH for Modem Access

•Troubleshooting Reverse SSH on the Client

•Troubleshooting Reverse SSH on the Server

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:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

line line-number [ending-line-number]

Example:

Router# line 1 3

Identifies a line for configuration and enters line configuration mode.

Step 4 

no exec

Example:

Router (config-line)# no exec

Disables EXEC processing on a line.

Step 5 

login authentication listname

Example:

Router (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:

Router (config-line)# transport input ssh

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

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

Step 7 

exit

Example:

Router (config-line)# exit

Exits line configuration mode.

Step 8 

exit

Example:

Router (config)# exit

Exits global configuration mode.

Step 9 

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

Example:

Router# 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:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

line line-number [ending-line-number]

Example:

Router# line 1 200

Identifies a line for configuration and enters line configuration mode.

Step 4 

no exec

Example:

Router (config-line)# no exec

Disables EXEC processing on a line.

Step 5 

login authentication listname

Example:

Router (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:

Router (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:

Router (config-line)# transport input ssh

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

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

Step 8 

exit

Example:

Router (config-line)# exit

Exits line configuration mode.

Step 9 

exit

Example:

Router (config)# exit

Exits global configuration mode.

Step 10 

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

Example:

Router# 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:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

debug ip ssh client

Example:

Router# 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:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

debug ip ssh

Example:

Router# debug ip ssh

Displays debugging messages for the SSH server.

Step 3 

show ssh

Example:

Router# show ssh

Displays the status of the SSH server connections.

Step 4 

show line

Example:

Router# show line

Displays parameters of a terminal line.

Configuration Examples for Reverse SSH Enhancements

•Example: Reverse SSH Console Access

•Example: Reverse SSH Modem Access

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

Related Documents

Related Topic
Document Title

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

Configuring Secure Shell

See the following modules:

•"Configuring Secure Shell"

•"Secure Shell Version 2 Support"

•"SSH Terminal-Line Access"

Security commands

Cisco IOS Security Command Reference


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

Table 1 lists the release history for this feature.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Note Table 1 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.

Table 1 Feature Information for Reverse SSH Enhancements

Feature Name
Releases
Feature Information

Reverse SSH Enhancements

12.3(11)T

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.

This feature was introduced in Cisco IOS Release 12.3(11)T.

The following command was introduced: ssh.


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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

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