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Security Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)

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Book Title

Security Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)

Chapter Title

Configuring Secure Shell (SSH)

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Updated:
January 31, 2018

Chapter: Configuring Secure Shell (SSH)

Configuring Secure Shell (SSH)

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 http:/​/​www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

Prerequisites for Configuring Secure Shell

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

  • For SSH to work, the switch needs an Rivest, Shamir, and Adleman (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.

  • The Secure Shell (SSH) server requires an IPsec (Data Encryption Standard [DES] or 3DES) encryption software image; the SSH client requires an IPsec (DES or 3DES) encryption software image.)
  • Configure a hostname and host domain for your device by using the hostname and ip domain-name commands in global configuration mode.
Related Concepts
Secure Copy Protocol

Restrictions for Configuring Secure Shell

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 Data Encryption Standard (DES) (56-bit) and 3DES (168-bit) data encryption software. In DES software images, DES is the only encryption algorithm available. In 3DES software images, both DES and 3DES encryption algorithms are available.

  • 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.

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

  • The login banner is not supported in Secure Shell Version 1. It is supported in Secure Shell Version 2.

  • The -l keyword and userid :{number} {ip-address} delimiter and arguments are mandatory when configuring the alternative method of Reverse SSH for console access.

Related Concepts
Secure Copy Protocol

Information About Configuring Secure Shell

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

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 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.

SSH Servers, Integrated Clients, and Supported Versions

The Secure Shell (SSH) Integrated Client feature is an application that runs over the SSH protocol to provide device authentication and encryption. The SSH client enables a Cisco device to make a secure, encrypted connection to another Cisco device or to any other device running the SSH server. This connection provides functionality similar to that of an outbound Telnet connection except that the connection is encrypted. With authentication and encryption, the SSH client allows for secure communication over an unsecured network.

The SSH server and SSH integrated client are applications that run on the switch. The SSH server works with the SSH client supported in this release and with non-Cisco SSH clients. The SSH client works with publicly and commercially available SSH servers. The SSH client supports the ciphers of Data Encryption Standard (DES), 3DES, and password authentication.

The switch supports an SSHv1 or an SSHv2 server.

The switch supports an SSHv1 client.


Note
The SSH client functionality is available only when the SSH server is enabled.

User authentication is performed like that in the Telnet session to the device. SSH also supports the following 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

The Secure Copy Protocol (SCP) feature provides a secure and authenticated method for copying switch configurations or switch image files. 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 switch can determine whether the user has the correct privilege level. To configure the Secure Copy feature, you should understand the SCP concepts.

Related References
Prerequisites for Configuring Secure Shell
Restrictions for Configuring Secure Shell

How to Configure SSH

Setting Up the Switch to Run SSH

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.    enable

    2.    configure terminal

    3.    hostname hostname

    4.    ip domain-name domain_name

    5.    crypto key generate rsa

    6.    end

    7.    show running-config

    8.    copy running-config startup-config


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example: 
    Switch> enable
     

    Enables privileged EXEC mode. Enter your password if prompted.

     
    Step 2configure terminal


    Example: 
    
Switch# configure terminal
     

    Enters the global configuration mode.

     
    Step 3hostname 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 4ip domain-name domain_name


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

    Configures a host domain for your Switch.

     
    Step 5crypto 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 6end


    Example: 
    
Switch(config)# end
     

    Returns to privileged EXEC mode.

     
    Step 7show running-config


    Example: 
    
Switch# show running-config
     

    Verifies your entries.

     
    Step 8copy running-config startup-config


    Example: 
    Switch# copy running-config startup-config
     

    (Optional) Saves your entries in the configuration file.

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

    Configuring the SSH Server

    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.    enable

      2.    configure terminal

      3.    ip ssh version [1 | 2]

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

      5.    Use one or both of the following:

      • line vtyline_number [ending_line_number]
      • transport input ssh

      6.    end

      7.    show running-config

      8.    copy running-config startup-config


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 enable


      Example: 
      Switch> enable
       

      Enables privileged EXEC mode. Enter your password if prompted.

       
      Step 2configure terminal


      Example: 
      
Switch# configure terminal
       

      Enters the global configuration mode.

       
      Step 3ip 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 4ip 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 5Use 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 6end


      Example: 
      Switch(config-line)# end
       

      Returns to privileged EXEC mode.

       
      Step 7show running-config


      Example: 
      
Switch# show running-config
       

      Verifies your entries.

       
      Step 8copy running-config startup-config


      Example: 
      Switch# copy running-config startup-config
       

      (Optional) Saves your entries in the configuration file.

       

      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.

      Additional References for Configuring Secure Shell

      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)

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

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

      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

      Standards and RFCs

      Standard/RFC Title

      None

      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

      Technical Assistance

      Description Link

      The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

      To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

      Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

      http:/​/​www.cisco.com/​support

      Feature Information for Configuring Secure Shell

      Release Feature Information
      Cisco IOS XE 3.2SE

      This feature was introduced.

      Cisco IOS 15.2(1)E

      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 supported on CAT4500-X, CAT4500E-SUP6E, CAT4500E-SUP6L-E, CAT4500E-SUP7E, CAT4500E-SUP7L-E.

      The following command was introduced: ssh.

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