Cisco IOS XE Everest: Secure Shell (SSH) Configuration Guide (EN)

Prerequisites for Configuring Secure Shell

Configuring Secure Shell The Secure Shell (SSH) feature 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 SSH are available: SSH Version 1 and SSH Version 2. Unless otherwise noted, the term “SSH” denotes “SSH Version 1” only. For information about SSH Version 2, see the “ Secure Shell Version 2 Support” feature module.

Prerequisites for Configuring Secure Shell, on page 1 Restrictions for Configuring Secure Shell, on page 2 Information About Configuring Secure Shell , on page 3 How to Configure Secure Shell, on page 5 Configuration Examples for Secure Shell, on page 7 Additional References for Secure Shell, on page 9 Feature History for Configuring Secure Shell, on page 9 Prerequisites for Configuring Secure Shell Unless otherwise noted, the term “SSH” denotes “SSH Version 1” only. Note • 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.

• Download the required image on the device. 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. • Generate a Rivest, Shamir, and Adleman (RSA) key pair for your device. This key pair automatically enables SSH and remote authentication when the crypto key generate rsa command is entered in global configuration mode. Configuring Secure Shell 1

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Restrictions for Configuring Secure Shell

To delete the RSA key pair, use the crypto key zeroize rsa global configuration command. Once you delete the RSA key pair, you automatically disable the SSH server. Note • Configure user authentication for local or remote access. You can configure authentication with or without authentication, authorization, and accounting (AAA). • 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.) Restrictions for Configuring Secure Shell Unless otherwise noted, the term “SSH” denotes “SSH Version 1” only.

Note • The Secure Shell (SSH) server and SSH client are supported on Data Encryption Standard (DES) (56-bit) and 3DES (168-bit) data encryption software images only. In DES software images, DES is the only encryption algorithm available. In 3DES software images, both DES and 3DES encryption algorithms are available. • Execution shell is the only application supported. • The login banner is not supported in Secure Shell Version 1. It is supported in Secure Shell Version 2. • The SFTP server is not supported. • Starting from Cisco IOS XE Release 17.10, the following Key Exchange and MAC algorithms are removed from the default list: Key Exchange algorithm:

• diffie-hellman-group14-sha1 MAC algorithms: • hmac-sha1 • hmac-sha2-256 • hmac-sha2-512 Note You can use the ip ssh server algorithm kex command to configure the Key Exchange algorithm and the ip ssh server algorithm mac command to configure the MAC algorithms. Configuring Secure Shell Restrictions for Configuring Secure Shell Configuring Secure Shell 2

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RSA Authentication Support

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 2 (SSHv2). SSH Server Unless otherwise noted, the term “SSH” denotes “SSH Version 1” only. Note The Secure Shell (SSH) Server feature enables an SSH client to make a secure, encrypted connection to a Cisco device. 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 Integrated Client Unless otherwise noted, the term “SSH” denotes “SSH Version 1” only. Note 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 client in Cisco software works with publicly and commercially available SSH servers. The SSH client supports the ciphers of Data Encryption Standard (DES), 3DES, and password authentication. User authentication is performed like that in the Telnet session to the device. The user authentication mechanisms supported for SSH are RADIUS, TACACS+, and the use of locally stored usernames and passwords. The SSH client functionality is available only when the SSH server is enabled.

Note RSA Authentication Support Rivest, Shamir, and Adleman (RSA) authentication available in Secure Shell (SSH) clients is not supported on the SSH server for Cisco software by default. For more information about RSA authentication support, see the “Configuring a Device for SSH Version 2 Using RSA Pairs” section of the “Secure Shell Version 2 Support” module. Configuring Secure Shell Information About Configuring Secure Shell Configuring Secure Shell 3

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SSH Configuration Guidelines

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 SSH client functionality is available only when the SSH server is enabled. Note 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 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 an active switch and the active switch fails, the new active switch uses the RSA key pair generated by the previous active switch. • 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. • 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 command in global configuration mode.

• 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 command in global configuration mode. • When configuring the local authentication and authorization authentication method, make sure that AAA is disabled on the console. Configuring Secure Shell SSH Servers, Integrated Clients, and Supported Versions Configuring Secure Shell 4

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