Secure Shell Configuration Guide, Cisco IOS XE Release 3S (Cisco ASR 1000)
Reverse SSH Enhancements
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Reverse SSH Enhancements

Reverse SSH Enhancements

Last Updated: July 24, 2012

The Reverse SSH Enhancements feature provides an alternative method of configuring reverse Secure Shell (SSH). Using this feature, you can configure reverse SSH without having to list separate lines for every terminal or auxiliary line on which SSH has to be enabled. This feature also eliminates the rotary-group limitation. This feature is supported for SSH Version 1 and SSH Version 2.

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 at the end of this module.

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

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

Reverse SSH

Reverse telnetting can be accomplished using SSH. Unlike reverse telnetting, 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 the section "How to Configure Reverse SSH Enhancements."

How to Configure Reverse SSH Enhancements

Configuring Reverse SSH for Console Access

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 useridargument 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

Reverse SSH is 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 useridargument 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

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

Security commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples

Cisco IOS Security Command Reference

Configuring Secure Shell

Configuring Secure Shell and Secure Shell Version 2 Support feature modules.

Standards

Standards

Title

None.

--

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 Configuration Information

Reverse SSH Enhancements

Cisco IOS XE Release 2.1

The Reverse SSH Enhancements feature provides an alternative method of configuring reverse Secure Shell (SSH). Using this feature, you can configure reverse SSH without having to list separate lines for every terminal or auxiliary line on which SSH has to be enabled. This feature also eliminates the rotary-group limitation. This feature is supported for SSH Version 1 and SSH Version 2.

In Cisco IOS XE Release 2.1, this feature was introduced on Cisco ASR 1000 Series Aggregation Services Routers.

The following command was introduced or modified: ssh.

Cisco and the Cisco logo are trademarks or registered trademarks of Cisco and/or its affiliates in the U.S. and other countries. To view a list of Cisco trademarks, go to this URL: www.cisco.com/go/trademarks. Third-party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1110R)

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.

© 2012 Cisco Systems, Inc. All rights reserved.