Cisco Nexus 1000V Security Configuration Guide, 4.2(1)SV2(1.1) - Configuring SSH [Cisco Nexus 1000V Series Switches]

Contents
Configuring SSH
Information About SSH
SSH Server
SSH Client
SSH Server Keys
Prerequisites for SSH
Guidelines and Limitations for SSH
Default Settings
Configuring SSH
Generating SSH Server Keys
Configuring a User Account with a Public Key
Configuring an OpenSSH Key
Configuring IETF or PEM Keys
Starting SSH Sessions
Clearing SSH Hosts
Disabling the SSH Server
Deleting SSH Server Keys
Clearing SSH Sessions
Verifying the SSH Configuration
Configuration Example for SSH
Feature History for SSH
Configuring SSH

This chapter contains the following sections:

Information About SSH
Prerequisites for SSH
Guidelines and Limitations for SSH
Default Settings
Configuring SSH
Verifying the SSH Configuration
Configuration Example for SSH
Feature History for SSH

Information About SSH
SSH Server

You can use the SSH server to enable an SSH client to make a secure, encrypted connection. SSH uses strong encryption for authentication. The SSH server can operate with publicly and commercially available SSH clients.

TACACS+ user authentication and locally stored usernames and passwords are supported for SSH.

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 secure, encrypted connection to any device that runs the SSH server. This connection provides an encrypted outbound connection. With authentication and encryption, the SSH client produces secure communication over an insecure network.

The SSH client works with publicly and commercially available SSH servers.

SSH Server Keys

SSH requires server keys for secure communication. 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 correct version before enabling the SSH service. 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, an RSA key using 1024 bits is generated.

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.

Prerequisites for SSH

SSH has the following prerequisites:

Configure IP on a Layer 3 interface, out-of-band on the mgmt 0 interface or inband on an Ethernet interface.

Before enabling the SSH server, obtain the SSH key.

Guidelines and Limitations for SSH

SSH has the following guidelines and limitations

Only SSH version 2 (SSHv2) is supported.

SSH is enabled by default.

Cisco NX-OS commands might differ from the Cisco IOS commands.

Default Settings

Parameters

Default

SSH server

Enabled

SSH server key

RSA key generated with 1024 bits

RSA key bits for generation

1024

Configuring SSH
Generating SSH Server Keys
Use this procedure to 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

Before You Begin
Before beginning this procedure, you must be logged in to the CLI in EXEC mode.

Procedure
Command or Action Purpose
Step 1 switch# configure terminal
Places you into global configuration mode.

Step 2 switch(config)# no feature ssh
Disables SSH.

Step 3 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 2048 and the default value is 1024.

Use the force keyword to replace an existing key.

Step 4 switch(config)# feature ssh
Enables SSH.

Step 5 switch# show ssh key (Optional)
Displays the SSH server keys.

Step 6 switch# copy running-config startup-config (Optional)
Copies the running configuration to the startup configuration.
switch# configure terminal
switch(config)# no feature ssh 
XML interface to system may become unavailable since ssh is disabled
switch(config)# ssh key dsa force
generating dsa key(1024 bits).....
.
generated dsa key
n1000v(config)# feature ssh
n1000v(config)# show ssh key
**************************************
rsa Keys generated:Sun Jul 27 15:18:46 2008

ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAyKcb7Nv9Ki1OOId9/tdHHa/ngQujlvK5mXyL/n+DeOXK
fVhHbX2a+V0cm7CCLUkBh+BvZRmpmOVTmU/5awfVhVxMKXMiPOPBc+A6/n3FVroyRwupMki6mWoM6Uwa
GID5gsVPqFjFNSgMWtbhjo97XVKhgjFW+wOVt8QoAcrEtnwEfsnQk1EIr/0XIP1mqTsrqTsmjZ2vLk+f
FzTGYAxMvYZI+BrN47aoH2ywS7CpnODjCDXJuDYSPbc3PA8t0ghU/60m9R+s6AZPuljVQbGfxPrahEu4
GVc6sMJNU1JxmqDJkodhMArObB4Umzj7E3Rdby/ZWx/clTYiXQR1X1VfhQ==

bitcount:2048
fingerprint:
fd:ca:48:73:b9:ee:e7:86:9e:1e:40:46:f1:50:1d:44
**************************************
dsa Keys generated:Sun Jul 27 15:20:12 2008

ssh-dss AAAAB3NzaC1kc3MAAACBALpdxLjXNS/jcCNY+F1QZV9HegxBEb0DMUmq9bSq2N+KAcvHllEh
GnaiHhqarOlcEKqhLbIbuqtKTCvfa+Y1hBIAhWVjg1UR3/M22jqxnfhnxL5YRc1Q7fcesFax0myayAIU
nXrkO5iwv9XHTu+EInRc4kJ0XrG9SxtLmDe/fi2ZAAAAFQDbRabAjZa6GfDpwjXw5smRhrElJwAAAIEA
r50yi3hHawNnb5qgYLXhN+KA8XJF753eCWHtMw7NR8fz6fjQ1R2J97UjjGuQ8DvwpGeNQ5S+AuIo0rGq
svdg7TTecBcbgBOnR7Fs2+W5HiSVEGbvj1xaeK8fkNE6kaJumBB343b8Rgj0G97MP/os1GfkEqmX9glB
0IOM2mgHHyoAAACAfRir27hHy+fw8CxPlsK0R6cFhxYyd/qYYogXFKYIOPxpLoYrjqODeOFThU7TJuBz
aS97eXiruzbffHwzUGfXgmQT5o9IMZRTClWPA/5Ju4O9YABYHccUghf0W+QtgGOT6FOSvBh8uOV0kcHC
GMJAP8omphauZJlc+wgFxhnkyh4=

bitcount:1024
fingerprint:
44:91:32:1f:7a:d1:83:3c:f3:5e:db:53:0a:2d:ce:69
**************************************
Configuring a User Account with a Public Key

You 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

Configuring an OpenSSH Key
Configuring IETF or PEM Keys
Configuring an OpenSSH Key
Use this procedure to specify the SSH public keys in OpenSSH format for user accounts.

Use this procedure to 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

Before You Begin
Before beginning this procedure, be sure you have:

Logged in to the CLI in EXEC mode

Generated an SSH public key in OpenSSH format

An existing user account

Procedure
Command or Action Purpose
Step 1 switch# configure terminal
Places you into global configuration mode.

Step 2 switch(config)# username username sshkey ssh-key
Configures the SSH public key in OpenSSH format with an exiting user account.

To create a user account use the username name password pwd command

Step 3 switch(config)# exit
Exits global configuration mode and returns you to EXEC mode.

Step 4 switch# show user-account (Optional)
Displays the user account configuration.

Step 5 switch# copy running-config startup-config (Optional)
Copies the running configuration to the startup configuration.
switch# configure terminal
switch(config)# username user1 sshkey ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAyK
cb7Nv9Ki1OOId9/tdHHa/ngQujlvK5mXyL/n+DeOXKfVhHbX2a+V0cm7CCLUkBh+BvZRmpmOVTmU/5aw
fVhVxMKXMiPOPBc+A6/n3FVroyRwupMki6mWoM6UwaGID5gsVPqFjFNSgMWtbhjo97XVKhgjFW+wOVt8
QoAcrEtnwEfsnQk1EIr/0XIP1mqTsrqTsmjZ2vLk+fFzTGYAxMvYZI+BrN47aoH2ywS7CpnODjCDXJuD
YSPbc3PA8t0ghU/60m9R+s6AZPuljVQbGfxPrahEu4GVc6sMJNU1JxmqDJkodhMArObB4Umzj7E3Rdby
/ZWx/clTYiXQR1X1VfhQ==
switch(config)# exit
switch# show user-account
user:admin
        this user account has no expiry date
        roles:network-admin
user:user1
        this user account has no expiry date
        roles:network-operator
        ssh public key: ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAyKcb7Nv9Ki1OOId9/tdH
Ha/ngQujlvK5mXyL/n+DeOXKfVhHbX2a+V0cm7CCLUkBh+BvZRmpmOVTmU/5awfVhVxMKXMiPOPBc+A6
/n3FVroyRwupMki6mWoM6UwaGID5gsVPqFjFNSgMWtbhjo97XVKhgjFW+wOVt8QoAcrEtnwEfsnQk1EI
r/0XIP1mqTsrqTsmjZ2vLk+fFzTGYAxMvYZI+BrN47aoH2ywS7CpnODjCDXJuDYSPbc3PA8t0ghU/60m
9R+s6AZPuljVQbGfxPrahEu4GVc6sMJNU1JxmqDJkodhMArObB4Umzj7E3Rdby/ZWx/clTYiXQR1X1Vf
hQ==
switch# copy running-config startup-config
Configuring IETF or PEM Keys
Use this procedure to specify the SSH public keys in IETF SECSH or PEM format for user accounts.

Use this procedure to 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

Before You Begin
Before beginning this procedure, you must have done the following:

Logged in to the CLI in EXEC mode

Generated an SSH public key in one of the following formats:
IETF SECSH format

Public Key Certificate in PEM format

Procedure
Command or Action Purpose
Step 1 switch# copy server-file bootflash: filename
Downloads the file containing the SSH key from a server. The server can be FTP, secure copy (SCP), secure FTP (SFTP), or TFTP.

Step 2 switch# configure terminal
Places you into global configuration mode.

Step 3 switch(config)# username username sshkey file bootflash:filename
Configures the SSH public key.

Step 4 switch(config)# exit
Exits global configuration mode and returns you to EXEC mode.

Step 5 switch# show user-account (Optional)
Displays the user account configuration.

Step 6 switch# copy running-config startup-config (Optional)
Copies the running configuration to the startup configuration.
switch# copy tftp://10.78.1.10/secsh_file.pub bootflash:secsh_file.pub vrf management
Trying to connect to tftp server......
Connection to server Established.
|
TFTP get operation was successful
switch# configure terminal
switch(config)# username User1 sshkey file bootflash:secsh_file.pub
switch(config)# exit
switch# show user-account 
user:admin
        this user account has no expiry date
        roles:network-admin 
user:user2
        this user account has no expiry date
        roles:network-operator 
        ssh public key: ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAyKcb7Nv9Ki1OOId9/tdHHa/
ngQujlvK5mXyL/n+DeOXKfVhHbX2a+V0cm7CCLUkBh+BvZRmpmOVTmU/5awfVhVxMKXMiPOPBc+A6/n3FVroyRwupMki6
mWoM6UwaGID5gsVPqFjFNSgMWtbhjo97XVKhgjFW+wOVt8QoAcrEtnwEfsnQk1EIr/0XIP1mqTsrqTsmjZ2vLk+
fFzTGYAxMvYZI+BrN47aoH2ywS7CpnODjCDXJuDYSPbc3PA8t0ghU/60m9R+s6AZPuljVQbGfxPrahEu4GVc6sMJN
U1JxmqDJkodhMArObB4Umzj7E3Rdby/ZWx/clTYiXQR1X1VfhQ==
switch# copy running-config startup-config
Starting SSH Sessions
Use this procedure to start SSH sessions using IP to connect to remote devices.

Before You Begin
Before beginning this procedure, be sure you have done the following:

Logged in to the CLI in EXEC mode.

Obtained the hostname and, if needed, the username, for the remote device.

Enabled the SSH server on the remote device

Procedure
Command or Action Purpose
Step 1 switch# ssh [root@] {ip address | hostname } [vrf vrf-name]
Creates an SSH IP session to a remote device using IP. The default virtual routing and forwarding (VRF) instance is the default VRF.
switch# ssh root@172.28.30.77
root@172.28.30.77's password: 
Last login: Sat Jul 26 11:07:23 2008 from 171.70.209.64
Clearing SSH Hosts
Use this procedure to clear from your account the list of trusted SSH servers that were added when you downloaded a file from a server using SCP or SFTP, or when you started an SSH session to a remote host.

Procedure
Command or Action Purpose
Step 1 switch# clear ssh hosts
Clears the SSH host sessions.
switch# clear ssh hosts
Disabling the SSH Server
Use this procedure to disable the SSH server to prevent SSH access to the switch. By default, the SSH server is enabled.

If you disable SSH, to enable it again you must first generate an SSH server key

Before You Begin
Before beginning this procedure, you must be logged in to the CLI in EXEC mode.

Procedure
Command or Action Purpose
Step 1 switch# configure terminal
Places you into global configuration mode.

Step 2 switch(config)# no feature ssh
Disables the SSH server. The default is enabled.

Step 3 switch(config)# show ssh server (Optional)
Displays the SSH server configuration.

Step 4 switch(config)# copy running-config startup-config (Optional)
Copies the running configuration to the startup configuration.
switch# configure terminal
switch(config)# no feature ssh 
XML interface to system may become unavailable since ssh is disabled
switch(config)#  show ssh server
ssh is not enabled
switch(config)# copy running-config startup-config
Deleting SSH Server Keys
Use this procedure to delete SSH server keys after you disable the SSH server.

If you disable SSH, to enable it again you must first generate an SSH server key.

Before You Begin
Before beginning this procedure, you must be logged in to the CLI in EXEC mode.

Procedure
Command or Action Purpose
Step 1 switch# configure terminal
Places you into 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)# show ssh key (Optional)
Displays the SSH server key configuration.

Step 5 switch(config)# copy running-config startup-config (Optional)
Copies the running configuration to the startup configuration.
switch# configure terminal
switch(config)# no feature ssh 
switch(config)# no ssh key rsa
switch(config)# show ssh key
**************************************
rsa Keys generated:Sun Jul 27 15:18:46 2008

ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAyKcb7Nv9Ki1OOId9/tdHHa/ngQujlvK5mXyL/n+DeOXK
fVhHbX2a+V0cm7CCLUkBh+BvZRmpmOVTmU/5awfVhVxMKXMiPOPBc+A6/n3FVroyRwupMki6mWoM6Uwa
GID5gsVPqFjFNSgMWtbhjo97XVKhgjFW+wOVt8QoAcrEtnwEfsnQk1EIr/0XIP1mqTsrqTsmjZ2vLk+f
FzTGYAxMvYZI+BrN47aoH2ywS7CpnODjCDXJuDYSPbc3PA8t0ghU/60m9R+s6AZPuljVQbGfxPrahEu4
GVc6sMJNU1JxmqDJkodhMArObB4Umzj7E3Rdby/ZWx/clTYiXQR1X1VfhQ==

bitcount:2048
fingerprint:
fd:ca:48:73:b9:ee:e7:86:9e:1e:40:46:f1:50:1d:44
**************************************
dsa Keys generated:Sun Jul 27 15:20:12 2008

ssh-dss AAAAB3NzaC1kc3MAAACBALpdxLjXNS/jcCNY+F1QZV9HegxBEb0DMUmq9bSq2N+KAcvHllEh
GnaiHhqarOlcEKqhLbIbuqtKTCvfa+Y1hBIAhWVjg1UR3/M22jqxnfhnxL5YRc1Q7fcesFax0myayAIU
nXrkO5iwv9XHTu+EInRc4kJ0XrG9SxtLmDe/fi2ZAAAAFQDbRabAjZa6GfDpwjXw5smRhrElJwAAAIEA
r50yi3hHawNnb5qgYLXhN+KA8XJF753eCWHtMw7NR8fz6fjQ1R2J97UjjGuQ8DvwpGeNQ5S+AuIo0rGq
svdg7TTecBcbgBOnR7Fs2+W5HiSVEGbvj1xaeK8fkNE6kaJumBB343b8Rgj0G97MP/os1GfkEqmX9glB
0IOM2mgHHyoAAACAfRir27hHy+fw8CxPlsK0R6cFhxYyd/qYYogXFKYIOPxpLoYrjqODeOFThU7TJuBz
aS97eXiruzbffHwzUGfXgmQT5o9IMZRTClWPA/5Ju4O9YABYHccUghf0W+QtgGOT6FOSvBh8uOV0kcHC
GMJAP8omphauZJlc+wgFxhnkyh4=

bitcount:1024
fingerprint:
44:91:32:1f:7a:d1:83:3c:f3:5e:db:53:0a:2d:ce:69
**************************************
mcs-srvr43(config)# no ssh key rsa
mcs-srvr43(config)# show ssh key
**************************************
could not retrieve rsa key information
**************************************
dsa Keys generated:Sun Jul 27 15:20:12 2008

ssh-dss AAAAB3NzaC1kc3MAAACBALpdxLjXNS/jcCNY+F1QZV9HegxBEb0DMUmq9bSq2N+KAcvHllEh
GnaiHhqarOlcEKqhLbIbuqtKTCvfa+Y1hBIAhWVjg1UR3/M22jqxnfhnxL5YRc1Q7fcesFax0myayAIU
nXrkO5iwv9XHTu+EInRc4kJ0XrG9SxtLmDe/fi2ZAAAAFQDbRabAjZa6GfDpwjXw5smRhrElJwAAAIEA
r50yi3hHawNnb5qgYLXhN+KA8XJF753eCWHtMw7NR8fz6fjQ1R2J97UjjGuQ8DvwpGeNQ5S+AuIo0rGq
svdg7TTecBcbgBOnR7Fs2+W5HiSVEGbvj1xaeK8fkNE6kaJumBB343b8Rgj0G97MP/os1GfkEqmX9glB
0IOM2mgHHyoAAACAfRir27hHy+fw8CxPlsK0R6cFhxYyd/qYYogXFKYIOPxpLoYrjqODeOFThU7TJuBz
aS97eXiruzbffHwzUGfXgmQT5o9IMZRTClWPA/5Ju4O9YABYHccUghf0W+QtgGOT6FOSvBh8uOV0kcHC
GMJAP8omphauZJlc+wgFxhnkyh4=

bitcount:1024
fingerprint:
44:91:32:1f:7a:d1:83:3c:f3:5e:db:53:0a:2d:ce:69
**************************************
mcs-srvr43(config)# no ssh key dsa
mcs-srvr43(config)# show ssh key
**************************************
could not retrieve rsa key information
**************************************
could not retrieve dsa key information
**************************************
no ssh keys present. you will have to generate them
**************************************
Clearing SSH Sessions
Use this procedure to clear SSH sessions from the device.

Before You Begin
Before beginning this procedure, you must be logged in to the CLI in EXEC mode.

Procedure
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.

Step 3 switch# show users (Optional)
Displays user session information.
switch# show users
NAME     LINE         TIME         IDLE          PID COMMENT
admin    tty1         Jul 25 19:13  old         2867
admin    pts/0        Jul 28 09:49 00:02       28556 (10.21.148.122)
admin    pts/1        Jul 28 09:46   .         28437 (::ffff:10.21.148.122)*
switch# clear line 0
switch# show users
NAME     LINE         TIME         IDLE          PID COMMENT
admin    tty1         Jul 25 19:13  old         2867
admin    pts/1        Jul 28 09:46   .         28437 (::ffff:10.21.148.122)*
mcs-srvr43(config)#
Verifying the SSH Configuration

Use one of the following commands to verify the configuration.

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
Configuration Example for SSH
This example shows the steps you use to configure SSH with an OpenSSH key.
Disable the SSH server.
switch# configure terminal
switch(config)# no feature ssh 
Generate an SSH server key.
switch(config)# ssh key rsa
generating rsa key(1024 bits).....
.generated rsa key
Enable the SSH server.
switch(config)# feature ssh 
Display the SSH server key.
switch(config)# show ssh key
rsa Keys generated:Sat Sep 29 00:10:39 2007

ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAIEAvWhEBsF55oaPHNDBnpXOTw6+/OdHoLJZKr+MZm99n2U0
ChzZG4svRWmHuJY4PeDWl0e5yE3g3EO3pjDDmt923siNiv5aSga60K36lr39HmXL6VgpRVn1XQFiBwn4
na+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
**************************************
Specify the SSH public key in OpenSSH format.
switch(config)# username User1 sshkey ssh-rsa 
AAAAB3NzaC1yc2EAAAABIwAAAIEAy19oF6QaZl9G+3f1XswK3OiW4H7YyUyuA50rv7gsEPjhOBYmsi6PAVKui1nIf/
DQhum+lJNqJP/eLowb7ubO+lVKRXFY/G+lJNIQW3g9igG30c6k6+XVn+NjnI1B7ihvpVh7dLddMOXwOnXHYshXmSiH
3UD/vKyziEh5S4Tplx8=
Save the configuration.
switch(config)# copy running-config startup-config
Feature History for SSH

This table only includes updates for those releases that have resulted in additions to the feature.

Feature Name

Releases

Feature Information

SSH

4.0(4)SV1(1)

This feature was introduced.