Configuring Secure Shell (SSH)

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Prerequisites for Configuring the Switch for Secure Shell (SSH) and Secure Copy Protocol (SCP)

The following are the prerequisites for configuring the switch for secure shell (SSH):

  • For SSH to work, the switch needs an RSA public/private key pair. This is the same with Secure Copy Protocol (SCP), which relies on SSH for its secure transport.

  • Before enabling SCP, you must correctly configure SSH, authentication, and authorization on the switch.

  • Because SCP relies on SSH for its secure transport, the router must have an Rivest, Shamir, and Adelman (RSA) key pair.

  • SCP relies on SSH for security.

  • SCP requires that authentication, authorization, and accounting (AAA) authorization be configured so the router can determine whether the user has the correct privilege level.

  • A user must have appropriate authorization to use SCP.

  • A user who has appropriate authorization can use SCP to copy any file in the Cisco IOS File System (IFS) to and from a switch by using the copy command. An authorized administrator can also do this from a workstation.

Related Concepts
Secure Copy Protocol Concepts

Restrictions for Configuring the Switch for SSH

The following are restrictions for configuring the switch for secure shell.

  • The switch supports Rivest, Shamir, and Adelman (RSA) authentication.

  • SSH supports only the execution-shell application.

  • The SSH server and the SSH client are supported only on DES (56-bit) and 3DES (168-bit) data encryption software.

  • The switch supports the Advanced Encryption Standard (AES) encryption algorithm with a 128-bit key, 192-bit key, or 256-bit key. However, symmetric cipher AES to encrypt the keys is not supported.

  • This software release does not support IP Security (IPSec).

  • When using SCP, you cannot enter the password into the copy command. You must enter the password when prompted.

Related Concepts
Secure Copy Protocol Concepts

Information about SSH

Secure Shell (SSH) is a protocol that provides a secure, remote connection to a device. SSH provides more security for remote connections than Telnet does by providing strong encryption when a device is authenticated. This software release supports SSH Version 1 (SSHv1) and SSH Version 2 (SSHv2).

SSH and Switch Access

For SSH configuration examples, see the “SSH Configuration Examples” section in the “Configuring Secure Shell” section in the “Other Security Features” chapter of the Cisco IOS Security Configuration Guide, Cisco IOS Release 12.4.

SSH functions the same in IPv6 as in IPv4. For IPv6, SSH supports IPv6 addresses and enables secure, encrypted connections with remote IPv6 nodes over an IPv6 transport.


Note

For complete syntax and usage information for the commands used in this section, see the command reference for this release and the “Secure Shell Commands” section of the “Other Security Features” chapter of the Cisco IOS Security Command Reference, Release 12.4 and the Cisco IOS IPv6 Command Reference.

SSH Servers, Integrated Clients, and Supported Versions

The SSH feature has an SSH server and an SSH integrated client, which are applications that run on the switch. You can use an SSH client to connect to a switch running the SSH server. The SSH server works with the SSH client supported in this release and with non-Cisco SSH clients. The SSH client also works with the SSH server supported in this release and with non-Cisco SSH servers.

The switch supports an SSHv1 or an SSHv2 server.

The switch supports an SSHv1 client.

SSH supports the Data Encryption Standard (DES) encryption algorithm, the Triple DES (3DES) encryption algorithm, and password-based user authentication.

SSH also supports these user authentication methods:

  • TACACS+

  • RADIUS

  • Local authentication and authorization

Related Concepts
TACACS+ and Switch Access
RADIUS and Switch Access
Related Tasks
Configuring the Switch for Local Authentication and Authorization

SSH Configuration Guidelines

Follow these guidelines when configuring the switch as an SSH server or SSH client:

  • An RSA key pair generated by a SSHv1 server can be used by an SSHv2 server, and the reverse.

  • If the SSH server is running on a stack master and the stack master fails, the new stack master uses the RSA key pair generated by the previous stack master.

  • If you get CLI error messages after entering the crypto key generate rsa global configuration command, an RSA key pair has not been generated. Reconfigure the hostname and domain, and then enter the crypto key generate rsa command. For more information, see Related Topics below.

  • When generating the RSA key pair, the message No host name specified might appear. If it does, you must configure a hostname by using the hostname global configuration command.

  • When generating the RSA key pair, the message No domain specified might appear. If it does, you must configure an IP domain name by using the ip domain-name global configuration command.

  • When configuring the local authentication and authorization authentication method, make sure that AAA is disabled on the console.

Related Tasks
Setting Up the Switch to Run SSH
Configuring the Switch for Local Authentication and Authorization

Secure Copy Protocol Overview

The Secure Copy Protocol (SCP) feature provides a secure and authenticated method for copying switch configurations or switch image files. SCP relies on Secure Shell (SSH), an application and a protocol that provides a secure replacement for the Berkeley r-tools.

For SSH to work, the switch needs an RSA public/private key pair. This is the same with SCP, which relies on SSH for its secure transport.

Because SSH also relies on AAA authentication, and SCP relies further on AAA authorization, correct configuration is necessary.

  • Before enabling SCP, you must correctly configure SSH, authentication, and authorization on the switch.

  • Because SCP relies on SSH for its secure transport, the router must have an Rivest, Shamir, and Adelman (RSA) key pair.


Note

When using SCP, you cannot enter the password into the copy command. You must enter the password when prompted.

Secure Copy Protocol Concepts

The Secure Copy Protocol (SCP) feature provides a secure and authenticated method for copying switch configurations or switch image files. SCP relies on Secure Shell (SSH), an application and a protocol that provides a secure replacement for the Berkeley r-tools.

To configure the Secure Copy feature, you should understand the SCP concepts.

The behavior of SCP is similar to that of remote copy (rcp), which comes from the Berkeley r-tools suite, except that SCP relies on SSH for security. SCP also requires that authentication, authorization, and accounting (AAA) authorization be configured so the router can determine whether the user has the correct privilege level.

For information about how to configure and verify SCP, see the “Secure Copy Protocol” section in the Cisco IOS Security Configuration Guide: Securing User Services, Release 12.4.

Related References
Prerequisites for Configuring the Switch for Secure Shell (SSH) and Secure Copy Protocol (SCP)
Restrictions for Configuring the Switch for SSH

How to Configure SSH

Setting Up the Switch to Run SSH

Beginning in privileged EXEC mode, follow these steps to set up your switch to run SSH:

Before You Begin

Configure user authentication for local or remote access. This step is required. For more information, see Related Topics below.

SUMMARY STEPS

    1.    configure terminal

    2.    hostname hostname

    3.    ip domain-name domain_name

    4.    crypto key generate rsa

    5.    end


DETAILED STEPS
     Command or ActionPurpose
    Step 1configure terminal


    Example:
    
Switch# configure terminal
     

    Enters the global configuration mode.

     
    Step 2hostname hostname


    Example: 
    Switch(config)# hostname your_hostname
     

    Configures a hostname and IP domain name for your switch.

    Note   

    Follow this procedure only if you are configuring the switch as an SSH server.

     
    Step 3ip domain-name domain_name


    Example: 
    Switch(config)# ip domain-name your_domain
     

    Configures a host domain for your switch.

     
    Step 4crypto key generate rsa


    Example: 
    Switch(config)# crypto key generate rsa
     

    Enables the SSH server for local and remote authentication on the switch and generates an RSA key pair. Generating an RSA key pair for the switch automatically enables SSH.

    We recommend that a minimum modulus size of 1024 bits.

    When you generate RSA keys, you are prompted to enter a modulus length. A longer modulus length might be more secure, but it takes longer to generate and to use.

    Note   

    Follow this procedure only if you are configuring the switch as an SSH server.

     
    Step 5end


    Example:
    
Switch(config)# end
     

    Returns to privileged EXEC mode.

     
    Related Concepts
    SSH Configuration Guidelines
    Related Tasks
    Configuring the Switch for Local Authentication and Authorization

    Configuring the SSH Server

    Beginning in privileged EXEC mode, follow these steps to configure the SSH server:


    Note

    This procedure is only required if you are configuring the switch as an SSH server.

    SUMMARY STEPS

      1.    configure terminal

      2.    ip ssh version [1 | 2]

      3.    ip ssh {timeout seconds | authentication-retries number}

      4.    Use one or both of the following:

      • line vtyline_number [ending_line_number]
      • transport input ssh

      5.    end


    DETAILED STEPS
       Command or ActionPurpose
      Step 1configure terminal


      Example:
      
Switch# configure terminal
       

      Enters the global configuration mode.

       
      Step 2ip ssh version [1 | 2]


      Example: 
      Switch(config)# ip ssh version 1
       

      (Optional) Configures the switch to run SSH Version 1 or SSH Version 2.

      • 1—Configure the switch to run SSH Version 1.

      • 2—Configure the switch to run SSH Version 2.

      If you do not enter this command or do not specify a keyword, the SSH server selects the latest SSH version supported by the SSH client. For example, if the SSH client supports SSHv1 and SSHv2, the SSH server selects SSHv2.

       
      Step 3ip ssh {timeout seconds | authentication-retries number}


      Example: 
      Switch(config)# ip ssh timeout 90 authentication-retries 2
       

      Configures the SSH control parameters:

      • Specify the time-out value in seconds; the default is 120 seconds. The range is 0 to 120 seconds. This parameter applies to the SSH negotiation phase. After the connection is established, the switch uses the default time-out values of the CLI-based sessions.

        By default, up to five simultaneous, encrypted SSH connections for multiple CLI-based sessions over the network are available (session 0 to session 4). After the execution shell starts, the CLI-based session time-out value returns to the default of 10 minutes.

      • Specify the number of times that a client can re-authenticate to the server. The default is 3; the range is 0 to 5.

      Repeat this step when configuring both parameters.

       
      Step 4Use one or both of the following:
      • line vtyline_number [ending_line_number]
      • transport input ssh


      Example:
      Switch(config)# line vty 1 10

      or

      Switch(config-line)# transport input ssh
       

      (Optional) Configures the virtual terminal line settings.

      • Enters line configuration mode to configure the virtual terminal line settings. For line_number and ending_line_number, specify a pair of lines. The range is 0 to 15.

      • Specifies that the switch prevent non-SSH Telnet connections. This limits the router to only SSH connections.

       
      Step 5end


      Example: 
      Switch(config-line)# end
       

      Returns to privileged EXEC mode.

       

      Monitoring the SSH Configuration and Status

      This table displays the SSH server configuration and status.

      Table 1 Commands for Displaying the SSH Server Configuration and Status

      Command

      Purpose

      show ip ssh

      Shows the version and configuration information for the SSH server.

      show ssh

      Shows the status of the SSH server.

      For more information about these commands, see the “Secure Shell Commands” section in the “Other Security Features” chapter of the Cisco IOS Security Command Reference .

      Additional References

      Related Documents

      Related Topic Document Title

      Configuring Identity Control policies and Identity Service templates for Session Aware networking.

      Session Aware Networking Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3850 Switches)

      http:/​/​www.cisco.com/​en/​US/​docs/​ios-xml/​ios/​san/​configuration/​xe-3se/​3850/​san-xe-3se-3850-book.html

      Configuring RADIUS, TACACS+, Secure Shell, 802.1X and AAA.

      Securing User Services Configuration Guide Library, Cisco IOS XE Release 3SE (Catalyst 3850 Switches)

      http:/​/​www.cisco.com/​en/​US/​docs/​ios-xml/​ios/​security/​config_library/​xe-3se/​3850/​secuser-xe-3se-3850-library.html

      Error Message Decoder

      Description Link

      To help you research and resolve system error messages in this release, use the Error Message Decoder tool.

      https:/​/​www.cisco.com/​cgi-bin/​Support/​Errordecoder/​index.cgi

      MIBs

      MIB MIBs Link

      All supported MIBs for this release.

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

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

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