SSH Terminal-Line Access

The SSH Terminal-Line Access feature provides users secure access to tty (text telephone) lines. tty allows the hearing- and speech-impaired to communicate by using a telephone to type messages.

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 SSH Terminal-Line Access

Download the required image to your router. The secure shell (SSH) server requires the router to have an IPSec (Data Encryption Standard (DES) or 3DES) encryption software image from Cisco IOS Release 12.1(1)T or a later release. The SSH client requires the router to have an IPSec (DES or 3DES) encryption software image from Cisco IOS Release 12.1(3)T or a later release. See the Cisco IOS Configuration Fundamentals Configuration Guide , Release 12.4T for more information on downloading a software image.

The SSH server requires the use of a username and password, which must be defined through the use of a local username and password, TACACS+, or RADIUS.


Note

The SSH Terminal-Line Access feature is available on any image that contains SSH.

Restrictions for SSH Terminal-Line Access

Console Server Requirement

To configure secure console server access, you must define each line in its own rotary and configure SSH to use SSH over the network when user want to access each of those devices.

Memory and Performance Impact

Replacing reverse Telnet with SSH may reduce the performance of available tty lines due to the addition of encryption and decryption processing above the vty processing. (Any cryptographic mechanism uses more memory than a regular access.)

Information About SSH Terminal-Line Access

Overview of SSH Terminal-Line Access

Cisco IOS supports reverse Telnet, which allows users to Telnet through the router--via a certain port range--to connect them to tty (asynchronous) lines. Reverse Telnet has allowed users to connect to the console ports of remote devices that do not natively support Telnet. However, this method has provided very little security because all Telnet traffic goes over the network in the clear. The SSH Terminal-Line Access feature replaces reverse Telnet with SSH. This feature may be configured to use encryption to access devices on the tty lines, which provide users with connections that support strong privacy and session integrity.

SSH is an application and a protocol that provides secure replacement for the suite of Berkeley r-tools such as rsh, rlogin, and rcp. (Cisco IOS supports rlogin.) The protocol secures the sessions using standard cryptographic mechanisms, and the application can be used similarly to the Berkeley rexec and rsh tools. Currently two versions of SSH are available: SSH Version 1 and SSH Version 2. Only SSH Version 1 is implemented in the Cisco IOS software.

The SSH Terminal-Line Access feature enables users to configure their router with secure access and perform the following tasks:

  • Connect to a router that has multiple terminal lines connected to consoles or serial ports of other routers, switches, or devices.

  • Simplify connectivity to a router from anywhere by securely connecting to the terminal server on a specific line.

  • Allow modems attached to routers to be used for dial-out securely.

  • Require authentication of each of the lines through a locally defined username and password, TACACS+, or RADIUS.


Note

The session slot command that is used to start a session with a module requires Telnet to be accepted on the virtual tty (vty) lines. When you restrict vty lines only to SSH, you cannot use the command to communicate with the modules. This applies to any Cisco IOS device where the user can telnet to a module on the device.

How to Configure SSH Terminal-Line Access

Configuring SSH Terminal-Line Access

Perform this task to configure a Cisco router to support reverse secure Telnet.


Note

SSH must already be configured on the router.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    line line-number [ending-line-number]

    4.    no exec

    5.    login {local | authentication listname}

    6.    rotary group

    7.    transport input {all | ssh}

    8.    exit

    9.    ip ssh port portnum rotary group


DETAILED STEPS
     Command or ActionPurpose
    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(config)# line 1 200
     

    Identifies a line for configuration and enters line configuration mode.

    Note   

    For router console configurations, each line must be defined in its own rotary, and SSH must be configured to listen in on each rotary.

    Note   

    An authentication method requiring a username and password must be configured for each line. This may be done through the use of a local username and password stored on the router, through the use of TACACS+, or through the use of RADIUS. Neither Line passwords nor the enable password are sufficient to be used with SSH.

     
    Step 4 no exec


    Example:
    Router(config-line)# no exec
     

    Disables exec processing on each of the lines.

     
    Step 5 login {local | authentication listname}


    Example:
    Router(config-line)# login authentication default
     
    Defines a login authentication mechanism for the lines.
    Note   

    The authentication method must utilize a username and password.

     
    Step 6 rotary group


    Example:
    Router(config-line)# rotary 1
     

    Defines a group of lines consisting of one or more lines.

    Note   

    All rotaries used must be defined, and each defined rotary must be used when SSH is enabled.

     
    Step 7 transport input {all | ssh}


    Example:
    Router(config-line)# transport input ssh
     

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

     
    Step 8 exit


    Example:
    Router(config-line)# exit
     

    Exits line configuration mode.

     
    Step 9 ip ssh port portnum rotary group


    Example:
    Router(config)# ip ssh port 2000 rotary 1
     

    Enables secure network access to the tty lines.

    • Use this command to connect the portnum argument with the rotary groupargument, which is associated with a line or group of lines.

    Note   

    The group argument must correspond with the rotary group number chosen in Step 6.

     

    Verifying SSH Terminal-Line Access

    To verify that this functionality is working, you can connect to a router using an SSH client.

    Configuration Examples for SSH Terminal-Line Access

    Example SSH Terminal-Line Access Configuration

    The following example shows how to configure the SSH Terminal-Line Access feature on a modem used for dial-out on lines 1 through 200. To get any of the dial-out modems, use any SSH client and start an SSH session to port 2000 of the router to get to the next available modem from the rotary.

    line 1 200
 no exec
 login authentication default
 rotary 1
 transport input ssh
 exit
ip ssh port 2000 rotary 1

    Example SSH Terminal-Line Access for a Console Serial Line Ports Configuration

    The following example shows how to configure the SSH Terminal-Line Access feature to access the console or serial line interface of various devices. For this type of access, each line is put into its own rotary, and each rotary is used for a single port. In this example, lines 1 through 3 are used; the port (line) mappings of the configuration are shown in the table below.

    Table 1 Port (line) Configuration Mappings

    Line Number

    SSH Port Number

    1

    2001

    2

    2002

    3

    2003 

    line 1
 no exec
 login authentication default
 rotary 1
 transport input ssh
line 2
 no exec
 login authentication default
 rotary 2
 transport input ssh
line 3
 no exec
 login authentication default
 rotary 3
 transport input ssh
ip ssh port 2001 rotary 1 3

    Additional References

    Related Documents

    Related Topic

    Document Title

    Cisco IOS commands

    Cisco IOS Master Commands List, All Releases

    SSH

    Cisco IOS Security Configuration Guide: Securing User Services

    SSH commands

    Cisco IOS Security Command Reference

    Dial Technologies

    Cisco IOS Dial Technologies Configuration Guide

    Dial commands

    Cisco IOS Dial Technologies Command Reference

    Downloading a software image

    Cisco IOS Configuration Fundamentals Configuration Guide

    Standards

    Standard

    Title

    --

    MIBs

    MIB

    MIBs Link

    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

    RFC

    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 SSH Terminal-Line Access

    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 2 Feature Information for SSH Terminal-Line Access

    Feature Name

    Releases

    Feature Information

    SSH Terminal-Line Access

    Cisco IOS 12.2(4)JA

    Cisco IOS 12.2(15)T

    Cisco IOS 12.2(6th)S

    The SSH Terminal-Line Access feature provides users secure access to tty (text telephone) lines. tty allows the hearing- and speech-impaired to communicate by using a telephone to type messages.

    This feature was introduced in Cisco IOS Release 12.2(4)JA.

    This feature was integrated into Cisco IOS Release 12.2(15)T.

    This feature was integrated into Cisco IOS Release 12.2(6th)S.

    The following command was introduced or modified: ip ssh port.