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.

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 works with publicly and commercially available SSH servers. The SSH client supports the ciphers of AES, 3DES, the hash algorithm SHA1, and password authentication. 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 using this command:

ssh client dscp dscp-value

The dscp-value ranges from 0 to 63. If not configured, 16 is set as the default DSCP value in the packets (for both client and server).

You can use the ssh client command in the XR Config mode to configure various SSH client options.

Router#ssh 192.0.2.1 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)
Router#

SSH includes support for secure 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	The default policer value for SSH-Known is set to 300pps. Slower transfers are expected due to this. You can adjust the lpts policer value for this punt cause to higher values that allows faster transfers.

You can increase the throughput of SCP or SFTP over inband using the ssh server tcp-window-scale command.

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 SFTP server instances are created. The SSH 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.

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.

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 key pair on the client machine. The private key of the RSA key pair 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) 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 supports the RSA based authentication. For more information on how to import the public key to the router, see the Implementing Certification Authority Interoperability 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 provides the primary framework through which access control can be set up on your Cisco router or access server. For more information on AAA, the Configuring AAA Services chapter in this guide.

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.

The SSH and SFTP components are present in the baseline Cisco IOS XR software image. The management and control plane components (such as the IPSec control plane) are also present in the base package. However, the data plane components (such as the MACSec and the IPSec data plane) are part of the security package as per the export compliance regulations. This segregation of package components makes the software more modular. It also gives you the flexibility of including or excluding the security package as per your requirements.

The base package and the security package allow FIPS, so that the control plane can negotiate FIPS-approved algorithms.

This feature brings in the functionality of automatically generating the SSH host-key pairs for the DSA, ECDSA (such as ecdsa-nistp256 , ecdsa-nistp384 , and ecdsa-nistp521 ) and RSA algorithms. This in turn eliminates the need for explicitly generating each SSH host-key pair after the router boots up. Because the keys are already present in the system, the SSH client can establish connection with the SSH server soon after the router boots up with the basic SSH configuration. This is useful especially during zero touch provisioning (ZTP) and Golden ISO boot up scenarios.

Although the host keys are auto-generated with the introduction of this feature, you still have the flexibility to select only the required algorithms on the SSH server. You can use the ssh server algorithms host-key command in XR Config mode to achieve the same. Alternatively, you can also use the crypto key zeroize command in XR EXEC mode to remove the algorithms that are not required.

Note	In a system upgrade scenario from version 1 to version 2, the system does not generate the SSH host-key pairs automatically if they were already generated in version 1. The host-key pairs are generated automatically only if they were not generated in version 1.

If the SSH host-key pairs are not present in some scenarios, you can execute the crypto key generate command in XR EXEC mode to generate the required host-key pairs.

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 from the SSH clients originating from the same host. Thus, rather than opening a new TCP socket for each SSH connection, all the SSH connections are multiplexed into one TCP connection and a single SSH session. 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, Perl, WinSCP, FileZilla, TTSSH, Cygwin or any other SSH-based tool to set up multiplexing on the client. See Configure Client for Multiplexing provides an example of how you can configure the client for multiplexing using OpenSSH.

The Cisco IOS XR software provides a new configuration option to control the key algorithms to be negotiated with the peer while establishing an SSH connection with the router. With this feature, you can enable the insecure SSH algorithms on the SSH server, which are otherwise disabled by default. A new configuration option is also available to restrict the SSH client from choosing the HMAC, or hash-based message authentication codes algorithm while trying to connect to the SSH server on the router.

You can also configure a list of ciphers as the default cipher list, thereby having the flexibility to enable or disable any particular cipher.

Caution

Use caution in enabling the insecure SSH algorithms to avoid any possible security attack.

To disable the HMAC algorithm, use the ssh client disable hmac command or ssh server disable hmac command in XR Config mode.

To enable the required cipher, use the ssh client enable cipher command or the ssh server enable cipher command in XR Config mode.

The supported encryption algorithms (in the order of preference) are:

1. aes128-ctr

2. aes192-ctr

3. aes256-ctr

4. aes128-gcm@openssh.com

5. aes256-gcm@openssh.com

6. aes128-cbc

7. aes192-cbc

8. aes256-cbc

9. 3des-cbc

In SSH, the CBC-based ciphers are disabled by default. To enable these, you can use the ssh client enable cipher command or the ssh server enable cipher command with the respective CBC options (aes-cbc or 3des-cbc). All CTR-based and GCM-based ciphers are enabled by default.