Cisco Nexus 7000 Series NX-OS Security Configuration Guide, Release 4.2
Configuring SSH and Telnet
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Configuring SSH and Telnet

Contents

Configuring SSH and Telnet

This chapter describes how to configure Secure Shell Protocol (SSH) and Telnet on Cisco NX-OS devices.

This chapter includes the following sections:

Information About SSH and Telnet

This section includes information about SSH and Telnet.

SSH Server

You can use the SSH server to enable an SSH client to make a secure, encrypted connection to a Cisco NX-OS device. SSH uses strong encryption for authentication. The SSH server in the Cisco NX-OS software can interoperate with publicly and commercially available SSH clients.

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

SSH Client

The SSH client feature is an application that runs over the SSH protocol to provide device authentication and encryption. The SSH client enables a Cisco NX-OS device to make a secure, encrypted connection to another Cisco NX-OS device or to any other device that runs the 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 NX-OS software works with publicly and commercially available SSH servers.

SSH Server Keys

SSH requires server keys for secure communications to the Cisco NX-OS device. You can use SSH server 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 two 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 NX-OS software generates an RSA key using 1024 bits.

SSH supports the following public key formats:


  • OpenSSH

  • IETF Secure Shell (SECSH)

  • Public Key Certificate in Privacy-Enhanced Mail (PEM)


Caution


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


SSH Authentication Using Digital Certificates

SSH authentication on Cisco NX-OS devices provide X.509 digital certificate support for host authentication. An X.509 digital certificate is a data item that ensures the origin and integrity of a message. It contains encryption keys for secured communications and is signed by a trusted certification authority (CA) to verify the identity of the presenter. The X.509 digital certificate support provides either DSA or RSA algorithms for authentication.

The certificate infrastructure uses the first certificate that supports the Secure Socket Layer (SSL) and is returned by the security infrastructure, either through a query or a notification. Verification of certificates is successful if the certificates are from any of the trusted CAs.

You can configure your device for either SSH authentication using an X.509 certificate or SSH authentication using a Public Key Certificate, but not both. If either of them is configured and the authentication fails, you are prompted for a password.

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 device to the other. Telnet can accept either an IP address or a domain name as the remote device address.

The Telnet server is disabled by default on the Cisco NX-OS device.

Virtualization Support for SSH and Telnet

SSH and Telnet configuration and operation are local to the virtual device context (VDC). For more information on VDCs, see the Cisco Nexus 7000 Series NX-OS Virtual Device Context Configuration Guide, Release 4.2.

Licensing Requirements for SSH and Telnet

The following table shows the licensing requirements for this feature:

Product

License Requirement

Cisco NX-OS

SSH and Telnet require no license. Any feature not included in a license package is bundled with the Cisco NX-OS system images and is provided at no extra charge to you. For an explanation of the Cisco NX-OS licensing scheme, see the Cisco Nexus 7000 Series NX-OS Licensing Guide, Release 4.2.

Prerequisites for SSH and Telnet

SSH and Telnet have the following prerequisites:


  • You have configured IP on a Layer 3 interface, out-of-band on the mgmt 0 interface, or inband on an Ethernet interface.

Guidelines and Limitations for SSH and Telnet

SSH and Telnet have the following configuration guidelines and limitations:


  • The Cisco NX-OS software supports only SSH version 2 (SSHv2).

  • You can configure your device for either SSH authentication using an X.509 certificate or SSH authentication using a public key certificate but not both. If either of them is configured and the authentication fails, you are prompted for a password.


Note


If you are familiar with the Cisco IOS CLI, be aware that the Cisco NX-OS commands for this feature might differ from the Cisco IOS commands that you would use.


Default Settings for SSH and Telnet

This table lists the default settings for SSH and Telnet parameters.
Table 1 Default SSH and Telnet Parameters

Parameters

Default

SSH server

Enabled

SSH server key

RSA key generated with 1024 bits

RSA key bits for generation

1024

Telnet server

Disabled

Telnet port number

23

Configuring SSH

This section describes how to configure 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.    configure terminal

2.    no feature ssh

3.    ssh key {dsa [force] | rsa [bits [force]]}

4.    feature ssh

5.    exit

6.    (Optional) show ssh key

7.    (Optional) copy running-config startup-config


DETAILED STEPS
  Command or Action Purpose
Step 1 configure terminal


Example:
switch# configure terminal
switch(config)#
 

Enters global configuration mode.

 
Step 2 no feature ssh


Example:
switch(config)# no feature ssh
 

Disables SSH.

 
Step 3 ssh key {dsa [force] | rsa [bits [force]]}


Example:
switch(config)# ssh key rsa 2048
 

Generates the SSH server key.

The bits argument is the number of bits used to generate the RSA key. In Cisco NX-OS Release 4.2, the range is from 768 to 2048. The default value is 1024.

You cannot specify the size of the DSA key. It is always set to 1024 bits.

Use the force keyword to replace an existing key.

 
Step 4 feature ssh


Example:
switch(config)# feature ssh
 

Enables SSH.

 
Step 5 exit


Example:
switch(config)# exit
switch#
 

Exits global configuration mode.

 
Step 6 show ssh key


Example:
switch# show ssh key
 
(Optional)

Displays the SSH server keys.

 
Step 7 copy running-config startup-config


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

Copies the running configuration to the startup configuration.

 

Specifying the SSH Public Keys for User Accounts

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


  • OpenSSH format

  • IETF SECSH format

  • Public Key Certificate in PEM format

Specifying the SSH Public Keys in IETF SECSH Format

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

Before You Begin

Generate an SSH public key in IETF SCHSH format.


SUMMARY STEPS

1.    copy server-file bootflash:filename

2.    configure terminal

3.    username username sshkey file bootflash:filename

4.    exit

5.    (Optional) show user-account

6.    (Optional) copy running-config startup-config


DETAILED STEPS
  Command or Action Purpose
Step 1 copy server-file bootflash:filename


Example:
switch# copy tftp://10.10.1.1/secsh_file.pub bootflash:secsh_file.pub
 

Downloads the file containing the SSH key in IETF SECSH format from a server. The server can be FTP, secure copy (SCP), secure FTP (SFTP), or TFTP.

 
Step 2 configure terminal


Example:
switch# configure terminal
switch(config)#
 

Enters global configuration mode.

 
Step 3 username username sshkey file bootflash:filename


Example:
switch(config)# username User1 sshkey file bootflash:secsh_file.pub
 

Configures the SSH public key in IETF SECSH format.

 
Step 4 exit


Example:
switch(config)# exit
switch#
 

Exits global configuration mode.

 
Step 5 show user-account


Example:
switch# show user-account
 
(Optional)

Displays the user account configuration.

 
Step 6 copy running-config startup-config


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

Copies the running configuration to the startup configuration.

 

Specifying the SSH Public Keys in OpenSSH Format

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

Before You Begin

Generate an SSH public key in OpenSSH format.


SUMMARY STEPS

1.    configure terminal

2.    username username sshkey ssh-key

3.    exit

4.    (Optional) show user-account

5.    (Optional) copy running-config startup-config


DETAILED STEPS
  Command or Action Purpose
Step 1 configure terminal


Example:
switch# configure terminal
switch(config)#
 

Enters global configuration mode.

 
Step 2 username username sshkey ssh-key


Example:
switch(config)# username User1 sshkey ssh-rsa 
AAAAB3NzaC1yc2EAAAABIwAAAIEAy19oF6QaZl9G+3f1XswK3OiW4H7YyUyuA50rv7gsEPj
hOBYmsi6PAVKui1nIf/DQhum+lJNqJP/eLowb7ubO+lVKRXFY/G+lJNIQW3g9igG30c6k6+
XVn+NjnI1B7ihvpVh7dLddMOXwOnXHYshXmSiH3UD/vKyziEh5S4Tplx8=

 

Configures the SSH public key in OpenSSH format.

 
Step 3 exit


Example:
switch(config)# exit
switch#

 

Exits global configuration mode.

 
Step 4 show user-account


Example:
switch# show user-account

 
(Optional)

Displays the user account configuration.

 
Step 5 copy running-config startup-config


Example:
switch# copy running-config startup-config

 
(Optional)

Copies the running configuration to the startup configuration.

 

Starting SSH Sessions

You can start SSH sessions using IPv4 or IPv6 to connect to remote devices from the Cisco NX-OS device.

Before You Begin

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

Enable the SSH server on the remote device.


SUMMARY STEPS

1.    ssh [username@]{ipv4-address | hostname} [vrf vrf-name]

2.    ssh6 [username@]{ipv6-address | hostname} [vrf vrf-name]


DETAILED STEPS
  Command or Action Purpose
Step 1 ssh [username@]{ipv4-address | hostname} [vrf vrf-name]


Example:
switch# ssh 10.10.1.1
 

Creates an SSH IPv4 session to a remote device using IPv4. The default VRF is the default VRF.

 
Step 2 ssh6 [username@]{ipv6-address | hostname} [vrf vrf-name]


Example:
switch# ssh6 HostA
 

Creates an SSH IPv6 session to a remote device using IPv6.

 

Clearing SSH Hosts

When you download a file from a server using SCP or SFTP, or when you start an SSH session from this device to a remote host, you establish a trusted SSH relationship with that server. You can clear the list of trusted SSH servers for your user account.

SUMMARY STEPS

1.    clear ssh hosts


DETAILED STEPS
  Command or Action Purpose
Step 1 clear ssh hosts


Example:
switch# clear ssh hosts
 

Clears the SSH host sessions and the known host file.

 

Disabling the SSH Server

By default, the SSH server is enabled on the Cisco NX-OS device. You can disable the SSH server to prevent SSH access to the switch.

SUMMARY STEPS

1.    configure terminal

2.    no feature ssh

3.    exit

4.    (Optional) show ssh server

5.    (Optional) copy running-config startup-config


DETAILED STEPS
  Command or Action Purpose
Step 1 configure terminal


Example:
switch# configure terminal
switch(config)#
 

Enters global configuration mode.

 
Step 2 no feature ssh


Example:
switch(config)# no feature ssh
 

Disables SSH.

 
Step 3 exit


Example:
switch(config)# exit
switch#
 

Exits global configuration mode.

 
Step 4 show ssh server


Example:
switch# show ssh server
 
(Optional)

Displays the SSH server configuration.

 
Step 5 copy running-config startup-config


Example:
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 on the Cisco NX-OS device after you disable the SSH server.


Note


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


SUMMARY STEPS

1.    configure terminal

2.    no feature ssh

3.    no ssh key [dsa | rsa]

4.    exit

5.    (Optional) show ssh key

6.    (Optional) copy running-config startup-config


DETAILED STEPS
  Command or Action Purpose
Step 1 configure terminal


Example:
switch# configure terminal
switch(config)#
 

Enters global configuration mode.

 
Step 2 no feature ssh


Example:
switch(config)# no feature ssh
 

Disables SSH.

 
Step 3 no ssh key [dsa | rsa]


Example:
switch(config)# no ssh key rsa 
 

Deletes the SSH server key.

The default is to delete all the SSH keys.

 
Step 4 exit


Example:
switch(config)# exit
switch#
 

Exits global configuration mode.

 
Step 5 show ssh key


Example:
switch# show ssh key
 
(Optional)

Displays the SSH server key configuration.

 
Step 6 copy running-config startup-config


Example:
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 NX-OS device.

SUMMARY STEPS

1.    show users

2.    clear line vty-line


DETAILED STEPS
  Command or Action Purpose
Step 1 show users


Example:
switch# show users
 

Displays user session information.

 
Step 2 clear line vty-line


Example:
switch(config)# clear line pts/12
 

Clears a user SSH session.

 

Configuring Telnet

This section describes how to configure Telnet on the Cisco NX-OS device.

Enabling the Telnet Server

You can enable the Telnet server on the Cisco NX-OS device. By default, the Telnet server is disabled.

SUMMARY STEPS

1.    configure terminal

2.    feature telnet

3.    exit

4.    (Optional) show telnet server

5.    (Optional) copy running-config startup-config


DETAILED STEPS
  Command or Action Purpose
Step 1 configure terminal


Example:
switch# configure terminal
switch(config)#
 

Enters global configuration mode.

 
Step 2 feature telnet


Example:
switch(config)# feature telnet
 

Enables the Telnet server. The default is disabled.

 
Step 3 exit


Example:
switch(config)# exit
switch#
 

Exits global configuration mode.

 
Step 4 show telnet server


Example:
switch# show telnet server
 
(Optional)

Displays the Telnet server configuration.

 
Step 5 copy running-config startup-config


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

Copies the running configuration to the startup configuration.

 

Starting Telnet Sessions to Remote Devices

You can start Telnet sessions to connect to remote devices from the Cisco NX-OS device. You can start Telnet sessions using either IPv4 or IPv6.

Before You Begin

Obtain the hostname or IP address for the remote device and, if needed, the username on the remote device.

Enable the Telnet server on the Cisco NX-OS device.

Enable the Telnet server on the remote device.


SUMMARY STEPS

1.    telnet {ipv4-address | host-name} [port-number] [vrf vrf-name]

2.    telnet6 {ipv6-address | host-name} [port-number] [vrf vrf-name]


DETAILED STEPS
  Command or Action Purpose
Step 1 telnet {ipv4-address | host-name} [port-number] [vrf vrf-name]


Example:
switch# telnet 10.10.1.1
 

Starts a Telnet session to a remote device using IPv4. The default port number is 23. The range is from 1 to 65535. The default VRF is the default VRF.

 
Step 2 telnet6 {ipv6-address | host-name} [port-number] [vrf vrf-name]


Example:
switch# telnet6 2001:0DB8::ABCD:1 vrf management
 

Starts a Telnet session to a remote device using IPv6. The default port number is 23. The range is from 1 to 65535. The default VRF is the default VRF.

 

Clearing Telnet Sessions

You can clear Telnet sessions from the Cisco NX-OS device.

Before You Begin

Enable the Telnet server on the Cisco NX-OS device.


SUMMARY STEPS

1.    show users

2.    clear line vty-line


DETAILED STEPS
  Command or Action Purpose
Step 1 show users


Example:
switch# show users
 

Displays user session information.

 
Step 2 clear line vty-line


Example:
switch(config)# clear line pts/12
 

Clears a user Telnet session.

 

Verifying the SSH and Telnet Configuration

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

Command

Purpose

show ssh key [dsa | rsa]

Displays SSH server key-pair information.

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.

show ssh server

Displays the SSH server configuration.

show telnet server

Displays the Telnet server configuration.

For detailed information about the fields in the output from these commands, see the Cisco Nexus 7000 Series NX-OS Security Command Reference, Release 4.2.

Configuration Example for SSH

The following example shows how to configure SSH with an OpenSSH key:

SUMMARY STEPS

1.    Disable the SSH server.

2.    Generate an SSH server key.

3.    Enable the SSH server.

4.    Display the SSH server key.

5.    Specify the SSH public key in OpenSSH format.

6.    Save the configuration.


DETAILED STEPS
Step 1   Disable the SSH server.

Example:
switch# configure terminal      
switch(config)# no feature ssh      

Step 2   Generate an SSH server key.

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

Step 3   Enable the SSH server.

Example:
switch(config)# feature ssh      

Step 4   Display the SSH server key.

Example:
switch(config)# show ssh key      
rsa Keys generated:Sat Sep 29 00:10:39 2007

ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAIEAvWhEBsF55oaPHNDBnpXOTw6+/OdHoLJZKr
+MZm99n2U0ChzZG4svRWmHuJY4PeDWl0e5yE3g3EO3pjDDmt923siNiv5aSga60K36lr39
HmXL6VgpRVn1XQFiBwn4na+H1d3Q0hDt+uWEA0tka2uOtXlDhliEmn4HVXOjGhFhoNE=

bitcount:1024
fingerprint:
51:6d:de:1c:c3:29:50:88:df:cc:95:f0:15:5d:9a:df
**************************************
could not retrieve dsa key information
**************************************

Step 5   Specify the SSH public key in OpenSSH format.

Example:
switch(config)# username User1 sshkey ssh-rsa      
AAAAB3NzaC1yc2EAAAABIwAAAIEAy19oF6QaZl9G+3f1XswK3OiW4H7YyUyuA50r
v7gsEPjhOBYmsi6PAVKui1nIf/DQhum+lJNqJP/eLowb7ubO+lVKRXFY/G+lJNIQ
W3g9igG30c6k6+XVn+NjnI1B7ihvpVh7dLddMOXwOnXHYshXmSiH3UD/vKyziEh5
4Tplx8=

Step 6   Save the configuration.

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


Additional References for SSH and Telnet

This section describes additional information related to implementing SSH and Telent.

Related Documents

Related Topic

Document Title

Cisco NX-OS licensing

Cisco Nexus 7000 Series NX-OS Licensing Guide, Release 4.2

Command reference

Cisco Nexus 7000 Series NX-OS Security Command Reference, Release 4.2

VRF configuration

Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.2

Standards

Standards

Title

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

MIBs

MIBs

MIBs Link


  • CISCO-SECURE-SHELL-MIB

To locate and download MIBs, go to the following URL:

http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml

Feature History for SSH and Telnet

This table lists the release history for these features.
Table 2 Feature History for SSH and Telnet

Feature Name

Releases

Feature Information

PKI

4.2(1)

Added support for digital certificates.