Configuring SSH and Telnet

This chapter contains the following sections:

Configuring SSH and Telnet

Information About SSH and Telnet

SSH Server

The Secure Shell Protocol (SSH) server feature enables a SSH client to make a secure, encrypted connection to a Cisco Nexus device. SSH uses strong encryption for authentication. The SSH server in the Cisco Nexus device switch interoperates with publicly and commercially available SSH clients.

The user authentication mechanisms supported for SSH are RADIUS, TACACS+, and the use of locally stored user names and passwords.

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 switch to make a secure, encrypted connection to another Cisco Nexus device or to any other device running an SSH server. This connection provides an outbound connection that is encrypted. With authentication and encryption, the SSH client allows for a secure communication over an insecure network.

The SSH client in the Cisco Nexus device works with publicly and commercially available SSH servers.

SSH Server Keys

SSH requires server keys for secure communications to the Cisco Nexus device. You can use SSH keys for the following SSH options:

  • SSH version 2 using Rivest, Shamir, and Adelman (RSA) public-key cryptography

  • SSH version 2 using the Digital System Algrorithm (DSA)

Be sure to have an SSH server key-pair with the appropriate version before enabling the SSH service. You can generate the SSH server key-pair according to the SSH client version used. The SSH service accepts three types of key-pairs for use by SSH version 2:

  • The dsa option generates the DSA key-pair for the SSH version 2 protocol.

  • The rsa option generates the RSA key-pair for the SSH version 2 protocol.

By default, the Cisco Nexus device generates an RSA key using 1024 bits.

SSH supports the following public key formats:

  • OpenSSH

  • IETF Secure Shell (SECSH)


Caution

If you delete all of the SSH keys, you cannot start the SSH services.

Telnet Server

The Telnet protocol enables TCP/IP connections to a host. Telnet allows a user at one site to establish a TCP connection to a login server at another site, and then passes the keystrokes from one system to the other. Telnet can accept either an IP address or a domain name as the remote system address.

The Telnet server is enabled by default on the Cisco Nexus device.

Guidelines and Limitations for SSH

SSH has the following configuration guidelines and limitations:

  • The Cisco Nexus device supports only SSH version 2 (SSHv2).

  • SSH public and private keys imported into user accounts that are remotely authenticated through a AAA protocol (such as RADIUS or TACACS+) for the purpose of SSH Passwordless File Copy will not persist when the Nexus device is reloaded unless a local user account with the same name as the remote user account is configured on the device before the SSH keys are imported.

Configuring SSH

Generating SSH Server Keys

You can generate an SSH server key based on your security requirements. The default SSH server key is an RSA key that is generated using 1024 bits.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# ssh key {dsa [force] | rsa [bits [force]]}
  3. switch(config)# exit
  4. (Optional) switch# show ssh key
  5. (Optional) switch# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# ssh key {dsa [force] | rsa [bits [force]]}

Generates the SSH server key.

The bits argument is the number of bits used to generate the key. The range is from 768 to 4096 and the default value is 1024.

Use the force keyword to replace an existing key.

Step 3

switch(config)# exit

Exits global configuration mode.

Step 4

(Optional) switch# show ssh key

(Optional)

Displays the SSH server keys.

Step 5

(Optional) switch# copy running-config startup-config

(Optional)

Copies the running configuration to the startup configuration.

Example

The following example shows how to generate an SSH server key: 

switch# configure terminal
switch(config)# ssh key rsa 2048
switch(config)# exit
switch# show ssh key
switch# copy running-config startup-config

Specifying the SSH Public Keys for User Accounts

You can configure an SSH public key to log in using an SSH client without being prompted for a password. You can specify the SSH public key in one of three different formats:

  • Open SSH format

  • IETF SECSH format

  • Public Key Certificate in PEM format

Specifying the SSH Public Keys in Open SSH Format

You can specify the SSH public keys in SSH format for user accounts.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# username username sshkey ssh-key
  3. switch(config)# exit
  4. (Optional) switch# show user-account
  5. (Optional) switch# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# username username sshkey ssh-key

Configures the SSH public key in SSH format.

Step 3

switch(config)# exit

Exits global configuration mode.

Step 4

(Optional) switch# show user-account

(Optional)

Displays the user account configuration.

Step 5

(Optional) switch# copy running-config startup-config

(Optional)

Copies the running configuration to the startup configuration.

Example

The following example shows how to specify an SSH public key in open SSH format: 

switch# configure terminal
switch(config)# username User1 sshkey ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAIEAri3mQy4W1AV9Y2t2hrEWgbUEYz
CfTPO5B8LRkedn56BEy2N9ZcdpqE6aqJLZwfZcTFEzaAAZp9AS86dgBAjsKGs7UxnhGySr8ZELv+DQBsDQH6rZt0KR+2Da8hJD4Z
XIeccWk0gS1DQUNZ300xstQsYZUtqnx1bvm5Ninn0McNinn0Mc=
switch(config)# exit
switch# show user-account
switch# copy running-config startup-config

Note

The username command in the example above is a single line that has been broken for legibility.

Specifying the SSH Public Keys in IETF SECSH Format

You can specify the SSH public keys in IETF SECSH format for user accounts.

SUMMARY STEPS

  1. switch# copy server-file bootflash: filename
  2. switch# configure terminal
  3. switch(config)# username username sshkey file filename
  4. switch(config)# exit
  5. (Optional) switch# show user-account
  6. (Optional) switch# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# copy server-file bootflash: filename

Downloads the file that contains the SSH key in IETF SECSH format from a server. The server can be FTP, SCP, SFTP, or TFTP.

Step 2

switch# configure terminal

Enters global configuration mode.

Step 3

switch(config)# username username sshkey file filename

Configures the SSH public key in SSH format.

Step 4

switch(config)# exit

Exits global configuration mode.

Step 5

(Optional) switch# show user-account

(Optional)

Displays the user account configuration.

Step 6

(Optional) switch# copy running-config startup-config

(Optional)

Copies the running configuration to the startup configuration.

Example

The following example shows how to specify the SSH public key in the IETF SECSH format: 

switch#copy tftp://10.10.1.1/secsh_file.pub bootflash:secsh_file.pub
switch# configure terminal
switch(config)# username User1 sshkey file bootflash:secsh_file.pub
switch(config)# exit
switch# show user-account
switch# copy running-config startup-config

Specifying the SSH Public Keys in PEM-Formatted Public Key Certificate Form

You can specify the SSH public keys in PEM-formatted Public Key Certificate form for user accounts.

SUMMARY STEPS

  1. switch# copy server-file bootflash: filename
  2. switch# configure terminal
  3. (Optional) switch# show user-account
  4. (Optional) switch# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# copy server-file bootflash: filename

Downloads the file that contains the SSH key in PEM-formatted Public Key Certificate form from a server. The server can be FTP, SCP, SFTP, or TFTP

Step 2

switch# configure terminal

Enters global configuration mode.

Step 3

(Optional) switch# show user-account

(Optional)

Displays the user account configuration.

Step 4

(Optional) switch# copy running-config startup-config

(Optional)

Copies the running configuration to the startup configuration.

Example

The following example shows how to specify the SSH public keys in PEM-formatted public key certificate form: 

switch# copy tftp://10.10.1.1/cert.pem bootflash:cert.pem
switch# configure terminal
switch# show user-account
switch# copy running-config startup-config

Starting SSH Sessions to Remote Devices

You can start SSH sessions to connect to remote devices from your Cisco Nexus device.

SUMMARY STEPS

  1. switch# ssh {hostname | username@hostname} [vrf vrf-name]

DETAILED STEPS

Command or Action Purpose

switch# ssh {hostname | username@hostname} [vrf vrf-name]

Creates an SSH session to a remote device. The hostname argument can be an IPv4 address or a hostname.

Clearing SSH Hosts

When you download a file from a server using SCP or SFTP, you establish a trusted SSH relationship with that server.

SUMMARY STEPS

  1. switch# clear ssh hosts

DETAILED STEPS

Command or Action Purpose

switch# clear ssh hosts

Clears the SSH host sessions.

Disabling the SSH Server

By default, the SSH server is enabled on the Cisco Nexus device.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# [no] feature ssh
  3. switch(config)# exit
  4. (Optional) switch# show ssh server
  5. (Optional) switch# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# [no] feature ssh

Enables/disables the SSH server. The default is enabled.

Step 3

switch(config)# exit

Exits global configuration mode.

Step 4

(Optional) switch# show ssh server

(Optional)

Displays the SSH server configuration.

Step 5

(Optional) switch# copy running-config startup-config

(Optional)

Copies the running configuration to the startup configuration.

Deleting SSH Server Keys

You can delete SSH server keys after you disable the SSH server.


Note

To reenable SSH, you must first generate an SSH server key.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# no feature ssh
  3. switch(config)# no ssh key [dsa | rsa]
  4. switch(config)# exit
  5. (Optional) switch# show ssh key
  6. (Optional) switch# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# no feature ssh

Disables the SSH server.

Step 3

switch(config)# no ssh key [dsa | rsa]

Deletes the SSH server key.

The default is to delete all the SSH keys.

Step 4

switch(config)# exit

Exits global configuration mode.

Step 5

(Optional) switch# show ssh key

(Optional)

Displays the SSH server configuration.

Step 6

(Optional) switch# copy running-config startup-config

(Optional)

Copies the running configuration to the startup configuration.

Clearing SSH Sessions

You can clear SSH sessions from the Cisco Nexus device.

SUMMARY STEPS

  1. switch# show users
  2. switch# clear line vty-line

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# show users

Displays user session information.

Step 2

switch# clear line vty-line

Clears a user SSH session.

Configuration Examples for SSH

The following example shows how to configure SSH:

SUMMARY STEPS

  1. Generate an SSH server key.
  2. Enable the SSH server.
  3. Display the SSH server key.
  4. Specify the SSH public key in Open SSH format.
  5. Save the configuration.

DETAILED STEPS

Step 1

Generate an SSH server key. 

switch(config)# ssh key rsa
generating rsa key(1024 bits).....
.
generated rsa key

Step 2

Enable the SSH server. 

switch# configure terminal
switch(config)# feature ssh

Note

 

This step should not be required because the SSH server is enabled by default.

Step 3

Display the SSH server key. 

switch(config)# show ssh key
rsa Keys generated:Fri May  8 22:09:47 2009
 
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAIEAri3mQy4W1AV9Y2t2hrEWgbUEYzCfTPO5B8LRkedn56BEy2N9ZcdpqE6aqJLZwfZ/
cTFEzaAAZp9AS86dgBAjsKGs7UxnhGySr8ZELv+DQBsDQH6rZt0KR+2Da8hJD4ZXIeccWk0gS1DQUNZ300xstQsYZUtqnx1bvm5/
Ninn0Mc=
 
bitcount:1024 
fingerprint:
4b:4d:f6:b9:42:e9:d9:71:3c:bd:09:94:4a:93:ac:ca
**************************************
could not retrieve dsa key information
**************************************

Step 4

Specify the SSH public key in Open SSH format. 

switch(config)# username User1 sshkey ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAIEAri3mQy4W1AV9Y2t2hrEWgbUEYz
CfTPO5B8LRkedn56BEy2N9ZcdpqE6aqJLZwfZcTFEzaAAZp9AS86dgBAjsKGs7UxnhGySr8ZELv+DQBsDQH6rZt0KR+2Da8hJD4Z
XIeccWk0gS1DQUNZ300xstQsYZUtqnx1bvm5Ninn0McNinn0Mc=

Step 5

Save the configuration. 

switch(config)# copy running-config startup-config

Configuring Telnet

Enabling the Telnet Server

By default, the Telnet server is enabled. You can disable the Telnet server on your Cisco Nexus device.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# [no] feature telnet

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# [no] feature telnet

Enables/disables the Telnet server. The default is enabled.

Reenabling the Telnet Server

If the Telnet server on your Cisco Nexus device has been disabled, you can reenable it.

SUMMARY STEPS

  1. switch(config)# [no] feature telnet

DETAILED STEPS

Command or Action Purpose

switch(config)# [no] feature telnet

Reenables the Telnet server.

Starting Telnet Sessions to Remote Devices

Before you start a Telnet session to connect to remote devices, you should do the following:

  • Obtain the hostname for the remote device and, if needed, obtain the username on the remote device.

  • Enable the Telnet server on the Cisco Nexus device.

  • Enable the Telnet server on the remote device.

SUMMARY STEPS

  1. switch# telnet hostname

DETAILED STEPS

Command or Action Purpose

switch# telnet hostname

Creates a Telnet session to a remote device. The hostname argument can be an IPv4 address or a device name.

Example

The following example shows how to start a Telnet session to connect to a remote device: 


switch# telnet 10.10.1.1
Trying 10.10.1.1...
Connected to 10.10.1.1.
Escape character is '^]'.
switch login:

Clearing Telnet Sessions

You can clear Telnet sessions from the Cisco Nexus device.

SUMMARY STEPS

  1. switch# show users
  2. switch# clear line vty-line

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# show users

Displays user session information.

Step 2

switch# clear line vty-line

Clears a user Telnet session.

Verifying the SSH and Telnet Configuration

To display the SSH configuration information, perform one of the following tasks:

Procedure

  • switch# show ssh key [dsa | rsa]

    Command or Action Purpose
    switch# show running-config security[all]

    Displays the SSH and user account configuration in the running configuration. The all keyword displays the default values for the SSH and user accounts.

    switch# show ssh server

    Displays the SSH server configuration.
    switch# show user-account

    Displays user account information

Default Settings for SSH

The following table lists the default settings for SSH parameters.

Table 1. Default SSH Parameters

Parameters

Default

SSH server

Enabled

SSH server key

RSA key generated with 1024 bits

RSA key bits for generation

1024

Telnet server

Enabled