Cisco ASR 9000 Series Aggregation Services Router System Security Configuration Guide, Release 4.1
Implementing Secure Shell
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Implementing Secure Shell

Secure Shell (SSH) is an application and a protocol that provides a secure replacement to the Berkeley r-tools. The protocol secures sessions using standard cryptographic mechanisms, and the application can be used similarly to the Berkeley rexec and rsh tools.

Two versions of the SSH server are available: SSH Version 1 (SSHv1) and SSH Version 2 (SSHv2). SSHv1 uses Rivest, Shamir, and Adelman (RSA) keys and SSHv2 uses Digital Signature Algorithm (DSA) keys. Cisco IOS XR software supports both SSHv1 and SSHv2.

This module describes how to implement Secure Shell on the the Cisco ASR 9000 Series Router.


Note

For a complete description of the Secure Shell commands used in this module , see the Secure Shell Commands module of the Cisco ASR 9000 Series Aggregation Services Router System Security Command Reference publication. To locate documentation of other commands that appear in this module , use the command reference master index, or search online.

Feature History for Implementing Secure Shell

Release

Modification

Release 3.7.2

This feature was introduced.

Release 3.9.0

Support was added for the following enhancements:

  • RSA based authentication on the SSH server
  • SFTP client in interactive mode
  • SFTP server implementation

Prerequisites to Implementing Secure Shell

The following prerequisites are required to implement Secure Shell:

  • You must be in a user group associated with a task group that includes the proper task IDs. The command reference guides include the task IDs required for each command. If you suspect user group assignment is preventing you from using a command, contact your AAA administrator for assistance.
  • Download the required image on your router. The SSH server and SSH client require you to have a a crypto package (data encryption standard [DES], 3DES and AES) from Cisco downloaded on your router.
  • To run an SSHv2 server, you must have a VRF. This may be the default VRF or a specific VRF. VRF changes are applicable only to the SSH v2 server.
  • Configure user authentication for local or remote access. You can configure authentication with or without authentication, authorization, and accounting (AAA). For more information, see the Authentication, Authorization, and Accounting Commands on Cisco IOS XR Software module in the Cisco ASR 9000 Series Aggregation Services Router System Security Command Reference publication and Configuring AAA Services on Cisco IOS XR Software module in the Cisco ASR 9000 Series Aggregation Services Router System Security Configuration Guide publication.
  • AAA authentication and authorization must be configured correctly for Secure Shell File Transfer Protocol (SFTP) to work.

Restrictions for Implementing Secure Shell

The following are some basic SSH restrictions and limitations of the SFTP feature:

  • A VRF is not accepted as inband if that VRF is already set as an out-of-band VRF. SSH v1 continues to bind only to the default VRF.
  • In order for an outside client to connect to the router, the router needs to have an RSA (for SSHv1) or DSA (for SSHv2) key pair configured. DSA and RSA keys are not required if you are initiating an SSH client connection from the router to an outside routing device. The same is true for SFTP: DSA and RSA keys are not required because SFTP operates only in client mode.
  • In order for SFTP to work properly, the remote SSH server must enable the SFTP server functionality. For example, the SSHv2 server is configured to handle the SFTP subsystem with a line such as /etc/ssh2/sshd2_config:
  • subsystem-sftp /usr/local/sbin/sftp-server
  • The SFTP server is usually included as part of SSH packages from public domain and is turned on by default configuration.
  • SFTP is compatible with sftp server version OpenSSH_2.9.9p2 or higher.
  • RSA-based user authentication is supported in the SSH and SFTP servers. The support however, is not extended to the SSH client.
  • Execution shell and SFTP are the only applications supported.
  • The AES encryption algorithm is supported on the SSHv2 server and client, but not on the SSHv1 server and client. Any requests for an AES cipher sent by an SSHv2 client to an SSHv1 server are ignored, with the server using 3DES instead.
  • The SFTP client does not support remote filenames containing wildcards (*, ?, []). The user must issue the sftp command multiple times or list all of the source files from the remote host to download them on to the router. For uploading, the router SFTP client can support multiple files specified using a wildcard provided that the issues mentioned in the first through third bullets in this section are resolved.
  • The cipher preference for the SSH server follows the order AES128, AES192, AES256, and, finally, 3DES. The server rejects any requests by the client for an unsupported cipher, and the SSH session does not proceed.
  • Use of a terminal type other than vt100 is unsupported, and the software generates a warning message in this case.
  • Password messages of “none” are unsupported on the SSH client.
  • Because the router infrastructure does not provide support for UNIX-like file permissions, files created on the local device lose the original permission information. For files created on the remote file system, the file permission adheres to the umask on the destination host and the modification and last access times are the time of the copy.

Information About Implementing Secure Shell

To implement SSH, you should understand the following concepts:

SSH Server

The SSH server feature enables an SSH client to make a secure, encrypted connection to a Cisco router. This connection provides functionality that is similar to that of an inbound Telnet connection. Before SSH, security was limited to Telnet security. SSH allows a strong encryption to be used with the Cisco IOS XR software authentication. The SSH server in Cisco IOS XR software works with publicly and commercially available SSH clients.

SSH Client

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

The SSH client in the Cisco IOS XR software worked with publicly and commercially available SSH servers. The SSH client supported the ciphers of AES, 3DES, message digest algorithm 5 (MD5), SHA1, and password authentication. User authentication was performed in the Telnet session to the router. The user authentication mechanisms supported for SSH were RADIUS, TACACS+, and the use of locally stored usernames and passwords.

SFTP Feature Overview

SSH includes support for standard file transfer protocol (SFTP) , a new standard file transfer protocol introduced in SSHv2. This feature provides a secure and authenticated method for copying router configuration or router image files.

The SFTP client functionality is provided as part of the SSH component and is always enabled on the router. Therefore, a user with the appropriate level can copy files to and from the router. Like the copy command, the sftp command can be used only in EXEC mode.

The SFTP client is VRF-aware, and you may configure the secure FTP client to use the VRF associated with a particular source interface during connections attempts. The SFTP client also supports interactive mode, where the user can log on to the server to perform specific tasks via the Unix server.

The SFTP Server is a sub-system of the SSH server. In other words, when an SSH server receives an SFTP server request, the SFTP API creates the SFTP server as a child process to the SSH server. A new SFTP server instance is created with each new request.

The SFTP requests for a new SFTP server in the following steps:

  • The user runs the sftp command with the required arguments
  • The SFTP API internally creates a child session that interacts with the SSH server
  • The SSH server creates the SFTP server child process
  • The SFTP server and client interact with each other in an encrypted format

When the SSH server establishes a new connection with the SSH client, the server daemon creates a new SSH server child process. The child server process builds a secure communications channel between the SSH client and server via key exchange and user authentication processes. If the SSH server receives a request for the sub-system to be an SFTP server, the SSH server daemon creates the SFTP server child process. For each incoming SFTP server subsystem request, a new SSH server child and a SFTP server instance is created. The SFTP server authenticates the user session and initiates a connection. It sets the environment for the client and the default directory for the user.

Once the initialization occurs, the SFTP server waits for the SSH_FXP_INIT message from the client, which is essential to start the file communication session. This message may then be followed by any message based on the client request. Here, the protocol adopts a 'request-response' model, where the client sends a request to the server; the server processes this request and sends a response.

The SFTP server displays the following responses:

  • Status Response
  • Handle Response
  • Data Response
  • Name Response

Note

The server must be running in order to accept incoming SFTP connections.

RSA Based Host Authentication

Verifying the authenticity of a server is the first step to a secure SSH connection. This process is called the host authentication, and is conducted to ensure that a client connects to a valid server.

The host authentication is performed using the public key of a server. The server, during the key-exchange phase, provides its public key to the client. The client checks its database for known hosts of this server and the corresponding public-key. If the client fails to find the server's IP address, it displays a warning message to the user, offering an option to either save the public key or discard it. If the server’s IP address is found, but the public-key does not match, the client closes the connection. If the public key is valid, the server is verified and a secure SSH connection is established.

The IOS XR SSH server and client had support for DSA based host authentication. But for compatibility with other products, like IOS, RSA based host authentication support is also added.

RSA Based User Authentication

One of the method for authenticating the user in SSH protocol is RSA public-key based user authentication. The possession of a private key serves as the authentication of the user. This method works by sending a signature created with a private key of the user. Each user has a RSA keypair on the client machine. The private key of the RSA keypair remains on the client machine.

The user generates an RSA public-private key pair on a unix client using a standard key generation mechanism such as ssh-keygen. The max length of the keys supported is 2048 bits, and the minimum length is 512 bits. The following example displays a typical key generation activity:

bash-2.05b$ ssh-keygen –b 1024 –t rsa
Generating RSA private key, 1024 bit long modulus

The public key must be in base64 encoded (binary) format for it to be imported correctly into the box. You can use third party tools available on the Internet to convert the key to the binary format.

Once the public key is imported to the router, the SSH client can choose to use the public key authentication method by specifying the request using the “-o” option in the SSH client. For example:

client$ ssh -o PreferredAuthentications=publickey 1.2.3.4

If a public key is not imported to a router using the RSA method, the SSH server initiates the password based authentication. If a public key is imported, the server proposes the use of both the methods. The SSH client then chooses to use either method to establish the connection. The system allows only 10 outgoing SSH client connections.

Currently, only SSH version 2 and SFTP server support the RSA based authentication. For more information on how to import the public key to the router, see the Implementing Certification Authority Interoperability on the Cisco ASR 9000 Series Router chapter in this guide.


Note

The preferred method of authentication would be as stated in the SSH RFC. The RSA based authentication support is only for local authentication, and not for TACACS/RADIUS servers.

Authentication, Authorization, and Accounting (AAA) is a suite of network security services that provide the primary framework through which access control can be set up on your Cisco router or access server. For more information on AAA, see the Authentication, Authorization, and Accounting Commands on the Cisco ASR 9000 Series RouterSoftware module in the Cisco ASR 9000 Series Aggregation Services Router System Security Command Reference publication and the Configuring AAA Services on the Cisco ASR 9000 Series Router module in the Cisco ASR 9000 Series Aggregation Services Router System Security Configuration Guide publication.

How to Implement Secure Shell

To configure SSH, perform the tasks described in the following sections:

Configuring SSH

Perform this task to configure SSH.


Note

For SSHv1 configuration, Step 1 to Step 4 are required. For SSHv2 configuration, Step 1 to Step 4 are optional.

SUMMARY STEPS

    1.    configure

    2.    hostname hostname

    3.    domain name domain-name

    4.    exit

    5.    crypto key generate rsa [usage keys | general-keys] [keypair-label]

    6.    crypto key generate dsa

    7.    configure

    8.    ssh timeout seconds

    9.    Do one of the following:

    • ssh server [vrf vrf-name]
    • ssh server v2

    10.    Use one of these commands:

    • end
    • commit

    11.    show ssh

    12.    show ssh session details


DETAILED STEPS
     Command or ActionPurpose
    Step 1 configure


    Example: 
    RP/0/RSP0/CPU0:router# configure
     

    Enters global configuration mode.

     
    Step 2hostname hostname


    Example: 
    RP/0/RSP0/CPU0:router(config)# hostname router1
     

    Configures a hostname for your router.

     
    Step 3domain name domain-name


    Example: 
    RP/0/RSP0/CPU0:router(config)# domain name cisco.com
     

    Defines a default domain name that the software uses to complete unqualified host names.

     
    Step 4 exit


    Example: 
    RP/0/RSP0/CPU0:router(config)# exit
     

    Exits global configuration mode, and returns the router to EXEC mode.

     
    Step 5crypto key generate rsa [usage keys | general-keys] [keypair-label]


    Example: 
    RP/0/RSP0/CPU0:router# crypto key generate rsa general-keys
     

    Generates an RSA key pair.

    • To delete the RSA key pair, use the crypto key zeroize rsa command. This command is used for SSHv1 only.
     
    Step 6 crypto key generate dsa


    Example: 
    RP/0/RSP0/CPU0:router# crypto key generate dsa
     

    Enables the SSH server for local and remote authentication on the router.

    • The recommended minimum modulus size is 1024 bits.
    • Generates a DSA key pair. To delete the DSA key pair, use the crypto key zeroize dsa command. This command is used only for SSHv2.
     
    Step 7 configure


    Example: 
    RP/0/RSP0/CPU0:router# configure
     

    Enters global configuration mode.

     
    Step 8ssh timeout seconds


    Example: 
    RP/0/RSP0/CPU0:router(config)# ssh timeout 60
     

    (Optional) Configures the timeout value for user authentication to AAA.

    • If the user fails to authenticate itself to AAA within the configured time, the connection is aborted.
    • If no value is configured, the default value of 30 seconds is used. The range is from 5 to 120.
     
    Step 9Do one of the following:
    • ssh server [vrf vrf-name]
    • ssh server v2


    Example: 
    RP/0/RSP0/CPU0:router(config)# ssh server

    or

    RP/0/RSP0/CPU0:router(config)# ssh server v2
     
    • (Optional) Brings up an SSH server using a specified VRF of up to 32 characters. If no VRF is specified, the default VRF is used. To stop the SSH server from receiving any further connections for the specified VRF, use the no form of this command. If no VRF is specified, the default is assumed.
    Note   

    The SSH server can be configured for multiple VRF usage.

    • (Optional) Forces the SSH server to accept only SSHv2 clients if you configure the SSHv2 option by using the ssh server v2 command. If you choose the ssh server v2 command, only the SSH v2 client connections are accepted.
     
    Step 10Use one of these commands:
    • end
    • commit


    Example: 
    RP/0/RSP0/CPU0:router(config)# end

    or

    RP/0/RSP0/CPU0:router(config)# commit
     

    Saves configuration changes.

    • When you issue the end command, the system prompts you to commit changes: 
      Uncommitted changes found, commit them
before exiting(yes/no/cancel)? [cancel]:
      • Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.
      • Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.
      • Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.
    • Use the commit command to save the configuration changes to the running configuration file, and remain within the configuration session.
     
    Step 11 show ssh


    Example: 
    RP/0/RSP0/CPU0:router# show ssh
     

    (Optional) Displays all of the incoming and outgoing SSHv1 and SSHv2 connections to the router.

     
    Step 12 show ssh session details


    Example: 
    RP/0/RSP0/CPU0:router# show ssh session details
     

    (Optional) Displays a detailed report of the SSHv2 connections to and from the router.

     


    Note

    SSH server supports setting DSCP value in the outgoing packets.

    ssh server dscp <value from 0 – 63>

    If not configured, the default DSCP value set in packets is 16 (for both client and server).

    Configuring the SSH Client

    Perform this task to configure an SSH client.

    SUMMARY STEPS

      1.    configure

      2.    ssh client knownhost device : /filename

      3.    exit

      4.    ssh {ipv4-address | hostname} [ username user- id | cipher des | source-interface type instance]


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 configure


      Example: 
      RP/0/RSP0/CPU0:router# configure
       

      Enters global configuration mode.

       
      Step 2ssh client knownhost device : /filename


      Example: 
      RP/0/RSP0/CPU0:router(config)# ssh client knownhost slot0:/server_pubkey
       

      (Optional) Enables the feature to authenticate and check the server public key (pubkey) at the client end.

      Note   

      The complete path of the filename is required. The colon (:) and slash mark (/) are also required.

       
      Step 3 exit


      Example: 
      RP/0/RSP0/CPU0:router(config)# exit
       

      Exits global configuration mode, and returns the router to EXEC mode.

       
      Step 4ssh {ipv4-address | hostname} [ username user- id | cipher des | source-interface type instance]

      Example: 
      RP/0/RSP0/CPU0:router# ssh remotehost username user1234
       

      Enables an outbound SSH connection.

      • To run an SSHv2 server, you must have a VRF. This may be the default or a specific VRF. VRF changes are applicable only to the SSH v2 server.
      • The SSH client tries to make an SSHv2 connection to the remote peer. If the remote peer supports only the SSHv1 server, the peer internally spawns an SSHv1 connection to the remote server.
      • The cipher des option can be used only with an SSHv1 client.
      • The SSHv1 client supports only the 3DES encryption algorithm option, which is still available by default for those SSH clients only.
      • If the hostname argument is used and the host has both IPv4 and IPv6 addresses, the IPv6 address is used.
       

      • If you are using SSHv1 and your SSH connection is being rejected, you have not successfully generated an RSA key pair for your router. Make sure that you have specified a hostname and domain. Then use the crypto key generate rsa command to generate an RSA key pair and enable the SSH server.
      • If you are using SSHv2 and your SSH connection is being rejected, you have not successfully generated a DSA key pair for your router. Make sure that you have specified a hostname and domain. Then use the crypto key generate dsa command to generate a DSA key pair and enable the SSH server.
      • When configuring the RSA or DSA key pair, you might encounter the following error messages:
        • No hostname specified
        You must configure a hostname for the router using the hostname global configuration command.
        • No domain specified
        You must configure a host domain for the router using the domain-name global configuration command.
      • The number of allowable SSH connections is limited to the maximum number of virtual terminal lines configured for the router. Each SSH connection uses a vty resource.
      • SSH uses either local security or the security protocol that is configured through AAA on your router for user authentication. When configuring AAA, you must ensure that the console is not running under AAA by applying a keyword in the global configuration mode to disable AAA on the console.

      Note

      SSH Client supports setting DSCP value in the outgoing packets.

      ssh client dscp <value from 0 – 63>

      If not configured, the default DSCP value set in packets is 16 (for both client and server).

      Configuration Examples for Implementing Secure Shell

      This section provides the following configuration example:

      Configuring Secure Shell: Example

      The following example shows how to configure SSHv2 by creating a hostname, defining a domain name, enabling the SSH server for local and remote authentication on the router by generating a DSA key pair, bringing up the SSH server, and saving the configuration commands to the running configuration file.

      After SSH has been configured, the SFTP feature is available on the router.

      configure
hostname router1
domain name cisco.com
exit
crypto key generate dsa
configure
ssh server
end

      Additional References

      The following sections provide references related to implementing secure shell.

      Related Documents

      Related Topic

      Document Title

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

      Authentication, Authorization, and Accounting Commands on the Cisco ASR 9000 Series Router Software module in Cisco ASR 9000 Series Aggregation Services Router System Security Command Reference.

      AAA configuration tasks

      Configuring AAA Services on the Cisco ASR 9000 Series RouterSoftware module in Cisco ASR 9000 Series Aggregation Services Router System Security Configuration Guide.

      Host services and applications commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples

      Host Services and Applications Commands on the Cisco ASR 9000 Series Router module in Cisco ASR 9000 Series Aggregation Services Router IP Addresses and Services Command Reference.

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

      IPSec Network Security Commands on the Cisco ASR 9000 Series RouterSoftware module in Cisco ASR 9000 Series Aggregation Services Router System Security Command Reference

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

      Secure Shell Commands on the Cisco ASR 9000 Series Router Software module in Cisco ASR 9000 Series Aggregation Services Router System Security Command Reference

      Standards

      Standards

      Title

      Draft-ietf-secsh-userauth-17.txt

      SSH Authentication Protocol, July 2003

      Draft-ietf-secsh-connect-17.txt

      SSH Connection Protocol, July 2003

      Draft-ietf-secsh-architecture-14.txt

      SSH Protocol Architecture, July 2003

      Draft-ietf-secsh-transport-16.txt

      SSH Transport Layer Protocol, July 2003

      MIBs

      MIBs

      MIBs Link
      —

      To locate and download MIBs using Cisco IOS XR software, use the Cisco MIB Locator found at the following URL and choose a platform under the Cisco Access Products menu: http:/​/​cisco.com/​public/​sw-center/​netmgmt/​cmtk/​mibs.shtml

      RFCs

      RFCs

      Title

      No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.

      		 —

      Technical Assistance

      Description

      Link

      The Cisco Technical Support website contains thousands of pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.

      http:/​/​www.cisco.com/​techsupport