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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.
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 Table at the end of this document.
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.
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 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."
To configure reverse SSH console access on the SSH server, perform the following steps.
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.
Command or Action | Purpose | |||
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Example: Router> enable |
Enables privileged EXEC mode.
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Example: Router# configure terminal |
Enters global configuration mode. |
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Example: Router# line 1 200 |
Identifies a line for configuration and enters line configuration mode. |
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Example: Router (config-line)# no exec |
Disables EXEC processing on a line. |
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Example: Router (config-line)# login authentication default |
Defines a login authentication mechanism for the lines.
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Example: Router (config-line)# rotary 1 |
Defines a group of lines consisting of one or more virtual terminal lines or one auxiliary port line. |
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Example: Router (config-line)# transport input ssh |
Defines which protocols to use to connect to a specific line of the router.
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Example: Router (config-line)# exit |
Exits line configuration mode. |
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Example: Router (config)# exit |
Exits global configuration mode. |
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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.
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To troubleshoot the reverse SSH configuration on the client (remote device), perform the following steps.
Command or Action | Purpose | |
---|---|---|
|
Example: Router> enable |
Enables privileged EXEC mode.
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Example: Router# debug ip ssh client |
Displays debugging messages for the SSH client. |
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.
Command or Action | Purpose | |
---|---|---|
|
Example: Router> enable |
Enables privileged EXEC mode.
|
|
Example: Router# debug ip ssh |
Displays debugging messages for the SSH server. |
|
Example: Router# show ssh |
Displays the status of the SSH server connections. |
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Example: Router# show line |
Displays parameters of a terminal line. |
The following configuration example shows that reverse SSH has been configured for console access for terminal lines 1 through 3:
line 1 3 no exec login authentication default transport input ssh
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
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
Related Topic |
Document Title |
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Cisco IOS commands |
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Configuring Secure Shell |
See the following modules:
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Security commands |
Standards |
Title |
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No new or modified standards are supported by this feature. |
-- |
MIBs |
MIBs Link |
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None |
To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL: |
RFCs |
Title |
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None |
-- |
Description |
Link |
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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. |
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 |
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.