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 either Digital Signature Algorithm (DSA) keys or Rivest, Shamir, and Adelman (RSA) keys. Cisco software supports both SSHv1 and SSHv2.

This module describes how to implement Secure Shell.


Note

Any reference to CiscoSSH in this chapter implies OpenSSH-based implementation of SSH that is available on this platform from Cisco IOS XR Software Release 7.3.2 and later. CiscoSSH replaces Cisco IOS XR SSH, which is the older SSH implementation that existed prior to this release.

For more information, see the CiscoSSH section later in this chapter.

Feature History for Implementing Secure Shell

Release

Modification

Release 6.0

This feature was introduced.

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

The 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).

The SSH client supports the following options:

  • DSCP—DSCP value for SSH client sessions. 
    RP/0/5/CPU0:router#configure 
RP/0/5/CPU0:router(config)#ssh ?
  client   Provide SSH client service
  server   Provide SSH server service
  timeout  Set timeout value for SSH
RP/0/5/CPU0:router(config)#ssh client ?
  • Knownhost—Enable the host pubkey check by local database.
  • Source-interface—Source interface for SSH client sessions. 
    RP/0/5/CPU0:router(config)#ssh client source-interface ?
  ATM                  ATM Network Interface(s)
  BVI                  Bridge-Group Virtual Interface
  Bundle-Ether         Aggregated Ethernet interface(s)
  CEM                  Circuit Emulation interface(s)
  GigabitEthernet      GigabitEthernet/IEEE 802.3 interface(s)
  IMA                  ATM Network Interface(s)
  IMtestmain           IM Test Interface
  Loopback             Loopback interface(s)
  MgmtEth              Ethernet/IEEE 802.3 interface(s)
  Multilink            Multilink network interface(s)
  Null                 Null interface
  PFItestmain          PFI Test Interface
  PFItestnothw         PFI Test Not-HW Interface
  PW-Ether             PWHE Ethernet Interface
  PW-IW                PWHE VC11 IP Interworking Interface
  Serial               Serial network interface(s)
  VASILeft             VASI Left interface(s)
  VASIRight            VASI Right interface(s)
  test-bundle-channel  Aggregated Test Bundle interface(s)
  tunnel-ipsec         IPSec Tunnel interface(s)
  tunnel-mte           MPLS Traffic Engineering P2MP Tunnel interface(s)
  tunnel-te            MPLS Traffic Engineering Tunnel interface(s)
  tunnel-tp            MPLS Transport Protocol Tunnel interface
RP/0/5/CPU0:router(config)#ssh client source-interface 
RP/0/5/CPU0:router(config)#
SSH also supports remote command execution as follows: 
RP/0/5/CPU0:router#ssh ?
  A.B.C.D  IPv4 (A.B.C.D) address
  WORD     Hostname of the remote node
  X:X::X   IPv6 (A:B:C:D...:D) address
  vrf      vrf table for the route lookup
RP/0/5/CPU0:router#ssh 1.1.1.1 ?
  cipher            Accept cipher type
  command           Specify remote command (non-interactive)
  source-interface  Specify source interface
  username          Accept userid for authentication
  <cr>              
RP/0/5/CPU0:router#ssh 12.28.46.6 username admin command "show redundancy sum" 
Password: 


Wed Jan  9 07:05:27.997 PST
    Active Node    Standby Node
    -----------    ------------
       0/4/CPU0        0/5/CPU0 (Node Ready, NSR: Not Configured)

RP/0/5/CPU0:router#

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 XR 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

  • The SFTP transfer is subject to LPTS policer "SSH-Known". Low policer values will affect SFTP transfer speeds


Note

In IOS-XR SW release 4.3.1 onwards the default policer value for SSH-Known has been reset from 2500pps to 300pps. Slower transfers are expected due to this change. You can adjust the lpts policer value for this punt cause to higher values that will allow faster transfers

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 4096 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) formats for it to be imported correctly into the router.


Note

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.


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.

SSHv2 Client Keyboard-Interactive Authentication

An authentication method in which the authentication information is entered using a keyboard is known as keyboard-interactive authentication. This method is an interactive authentication method in the SSH protocol. This type of authentication allows the SSH client to support different methods of authentication without having to be aware of their underlying mechanisms.

Currently, the SSHv2 client supports the keyboard-interactive authentication. This type of authentication works only for interactive applications.


Note

The password authentication is the default authentication method. The keyboard-interactive authentication method is selected if the server is configured to support only the keyboard-interactive authentication.

Prerequisites for Implementing Secure Shell

The following prerequisites are required to implement Secure Shell:

  • 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.

  • Configure user authentication for local or remote access. You can configure authentication with or without authentication, authorization, and accounting (AAA).

  • 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:

  • In order for an outside client to connect to the router, the router needs to have an RSA (for SSHv1 or SSHv2) 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 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 not supported, and the software generates a warning message in this case.

  • Password messages of “none” are unsupported on the SSH client.

  • Files created on the local device lose the original permission information because the router infrastructure does not provide support for UNIX-like file permissions. 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.

Configure SSH

Perform this task to configure SSH.


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

SUMMARY STEPS

  1. configure
  2. hostname hostname
  3. domain name domain-name
  4. Use the commit or end command.
  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 the commit or end command.
  11. show ssh
  12. show ssh session details

DETAILED STEPS

Step 1

configure

Example:


RP/0/RP0/CPU0:router# configure

Enters global configuration mode.

Step 2

hostname hostname

Example:


RP/0/RP0/CPU0:router(config)# hostname router1

Configures a hostname for your router.

Step 3

domain name domain-name

Example:


RP/0/RP0/CPU0:router(config)# domain name cisco.com

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

Step 4

Use the commit or end command.

commit —Saves the configuration changes and remains within the configuration session.

end —Prompts user to take one of these actions:

  • Yes — Saves configuration changes and exits the configuration session.

  • No —Exits the configuration session without committing the configuration changes.

  • Cancel —Remains in the configuration session, without committing the configuration changes.

Step 5

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

Example:


RP/0/RP0/CPU0:router# crypto key generate rsa general-keys

Generates an RSA key pair. The RSA key modulus can be in the range of 512 to 4096 bits.

  • 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/RP0/CPU0:router# crypto key generate dsa

Enables the SSH server for local and remote authentication on the router. The supported key sizes are: 512, 768 and 1024 bits.

  • 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/RP0/CPU0:router# configure

Enters mode.

Step 8

ssh timeout seconds

Example:


RP/0/RP0/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 terminated.

  • If no value is configured, the default value of 30 seconds is used. The range is from 5 to 120.

Step 9

Do one of the following:

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

Example:


RP/0/RP0/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 10

Use the commit or end command.

commit —Saves the configuration changes and remains within the configuration session.

end —Prompts user to take one of these actions:

  • Yes — Saves configuration changes and exits the configuration session.

  • No —Exits the configuration session without committing the configuration changes.

  • Cancel —Remains in the configuration session, without committing the configuration changes.

Step 11

show ssh

Example:


RP/0/RP0/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/RP0/CPU0:router# show ssh session details

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

Configure SSH Client

Perform this task to configure an SSH client.

SUMMARY STEPS

  1. configure
  2. ssh client knownhost device : /filename
  3. Use the commit or end command.
  4. ssh {ipv4-address | ipv6-address | hostname} [ username user- cipher | source-interface type instance]

DETAILED STEPS

Step 1

configure

Example:


RP/0/RP0/CPU0:router# configure

Enters global configuration mode.

Step 2

ssh client knownhost device : /filename

Example:


RP/0/RP0/CPU0:router(config)# ssh client knownhost slot1:/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

Use the commit or end command.

commit —Saves the configuration changes and remains within the configuration session.

end —Prompts user to take one of these actions:

  • Yes — Saves configuration changes and exits the configuration session.

  • No —Exits the configuration session without committing the configuration changes.

  • Cancel —Remains in the configuration session, without committing the configuration changes.

Step 4

ssh {ipv4-address | ipv6-address | hostname} [ username user- cipher | source-interface type instance]

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, the reason could be that the RSA key pair might have been zeroed out or that you have not successfully generated an RSA key pair for your router. Another reason could be that the SSH server to which the user is connecting to using SSHv1 client does not accept SSHv1 connections. Make sure that you have specified a hostname and domain. Then use the crypto key generate rsa command to generate an RSA host-key pair, and then enable the SSH server.

  • If you are using SSHv2 and your SSH connection is being rejected, the reason could be that the DSA, RSA host-key pair might have been zeroed out. Make sure you follow similar steps as mentioned above to generate the required host-key pairs, and then 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 command.

    • No domain specified

    You must configure a host domain for the router using the domain-name 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
    If you are using Putty version 0.63 or higher to connect to the SSH client, set the 'Chokes on PuTTYs SSH2 winadj request' option under SSH > Bugs in your Putty configuration to 'On.' This helps avoid a possible breakdown of the session whenever some long output is sent from IOS XR to the Putty client.

Configuring Secure Shell

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 rsa/dsa
configure
ssh server
end

Multi-channeling in SSH

The multi-channeling (also called multiplexing) feature on the Cisco IOS XR software server allows you to establish multiple channels over the same TCP connection. Thus, rather than opening a new TCP socket for each SSH connection, all the SSH connections are multiplexed into one TCP connection. For example, with multiplexing support on your XR software server, on a single SSH connection you can simultaneously open a pseudo terminal, remotely execute a command and transfer a file using any file transfer protocol. Multiplexing offers the following benefits:

  • You are required to authenticate only once at the time of creating the session. After that, all the SSH clients associated with a particular session use the same TCP socket to communicate to the server.

  • Saves time consumed otherwise wasted in creating a new connection each time.

Multiplexing is enabled by default in the Cisco IOS XR software server. If your client supports multiplexing, you must explicitly set up multiplexing on the client for it to be able to send multi-channel requests to the server. You can use OpenSSH, Putty, Perl, WinSCP, Putty, FileZilla, TTSSH, Cygwin or any other SSH-based tool to set up multiplexing on the client. Configure Client for Multiplexing provides an example of how you can configure the client for multiplexing using OpenSSH.

Restrictions for Multi-channeling Over SSH

  • Do not use client multiplexing for heavy transfer of data as the data transfer speed is limited by the TCP speed limit. Hence, for a heavy data transfer it is advised that you run multiple SSH sessions, as the TCP speed limit is per connection.

  • Client multiplexing must not be used for more than 15 concurrent channels per session simultaneously.

  • You must ensure that the first channel created at the time of establishing the session is always kept alive in order for other channels to remain open.

  • The line template default session-limit command is not supported for SSH.

Client and Server Interaction Over Multichannel Connection

The following figure provides an illustration of a client-server interaction over a SSH multichannel connection.

As depicted in the illustration,

  • The client multiplexes the collection of channels into a single connection. This allows different operations to be performed on different channels simultaneously. The dotted lines indicate the different channels that are open for a single session.

  • After receiving a request from the client to open up a channel, the server processes the request. Each request to open up a channel represents the processing of a single service.


Note
The Cisco IOS XR software supports server-side multiplexing only.

Configure Client for Multiplexing

The SSH client opens up one TCP socket for all the connections. In order to do so, the client multiplexes all the connections into one TCP connection. Authentication happens only once at the time of creating the session. After that, all the SSH clients associated with the particular session uses the same TCP socket to communicate to the server. Use the following steps to configure client multiplexing using OpenSSH:

  1. Edit the ssh_config file.

    Open the ssh_config file with your favorite text editor to configure values for session multiplexing. The system-wide SSH configuration file is located under /etc/ssh/ssh_config. The user configuration file is located under ~/.ssh/config or $HOME/.ssh/config.

  2. Add entries ControlMaster auto and ControlPath

    Add the entry ControlMaster auto and ControlPath to the ssh_config file, save it and exit.

    • ControlMaster determines whether SSH will listen for control connections and what to do about them. Setting the ControlMaster to 'auto' creates a primary session automatically but if there is a primary session already available, subsequent sessions are automatically multiplexed.

    • ControlPath is the location for the control socket used by the multiplexed sessions. Specifying the ControlPath ensures that any time a connection to a particular server uses the same specified primary connection.

      Example:

      Host *
ControlMaster auto
ControlPath ~/.ssh/tmp/%r@%h:%p

  3. Create a temporary folder.

    Create a temporary directory inside the /.ssh folder for storing the control sockets.