Configuring Secure Shell
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Configuring Secure Shell

Last Updated: July 25, 2011

The Secure Shell (SSH) feature is an application and a protocol that provides a secure replacement to the Berkeley r-tools. The protocol secures the 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. This document describes SSH Version 1. For information about SSH Version 2, see the Secure Shell Version 2 Support feature module.


Note
Hereafter, unless otherwise noted, the term “SSH” denotes “SSH Version 1” only.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Prerequisites for Configuring SSH

Perform the following tasks before configuring SSH:

  • Download the required image on the router. The SSH server requires an IPsec (Data Encryption Standard [DES] or 3DES) encryption software image from Cisco IOS Release 12.1(1)T or a later release; the SSH client requires an IPsec (DES or 3DES) encryption software image from Cisco IOS Release 12.1(3)T or a later release.) See the Cisco IOS Configuration Fundamentals Configuration Guide for more information on downloading a software image.
  • Configure a hostname and host domain for your router by using the hostname and ip domain-name commands in global configuration mode.
  • Generate a Rivest, Shamir and Adleman (RSA) key pair for your router. This key pair automatically enables SSH and remote authentication when the crypto key generate rsa command is entered in global configuration mode.

Note
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.
  • Configure user authentication for local or remote access. You can configure authentication with or without authentication, authorization, and accounting (AAA). For more information, see the Configuring Authentication Configuring Authorization and Configuring Accounting feature modules for more information.

Restrictions for Configuring SSH

SSH has the following restrictions:

  • The SSH server and SSH client are supported on 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.

Information About Secure Shell


Note
Hereafter, unless otherwise noted, the term “SSH” denotes “SSH Version 1” only.

SSH Server

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 software authentication. The SSH server in Cisco IOS software works with publicly and commercially available SSH clients.

SSH Integrated Client

The SSH Integrated Client feature is an application thats runs 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 that is 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 secure communication over an insecure network.

The SSH client in Cisco IOS software works with publicly and commercially available SSH servers. The SSH client supports the ciphers of DES, 3DES, and password authentication. User authentication is performed like that in the Telnet session to the router. The user authentication mechanisms supported for SSH are RADIUS, TACACS+, and the use of locally stored usernames and passwords.


Note
The SSH client functionality is available only when the SSH server is enabled.

RSA Authentication Support

RSA authentication available in SSH clients is not supported on the SSH server for Cisco IOS software by default. See the “ Configuring a Router for SSH Version 2 Using Private Public Key Pairs” section of the “Secure Shell Version 2 Support” chapter for the procedure to configure RSA authentication support.

How to Configure SSH


Note
Hereafter, unless otherwise noted, the term “SSH” denotes “SSH Version 1” only.

Configuring an SSH Server

Perform the following steps to configure an SSH server. This task helps you to enable the Cisco router for SSH.


Note
The SSH client feature runs in user EXEC mode and has no specific configuration on the router.

Note
The SSH commands are optional and are disabled when the SSH server is disabled. If SSH parameters are not configured, then the default values are used.
SUMMARY STEPS

1.    enable

2.    configure terminal

3.    ip ssh {timeout seconds | authentication-retries integer}

4.   


DETAILED STEPS
 Command or ActionPurpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Router# configure terminal

 

Enters global configuration mode.

 
Step 3
ip ssh {timeout seconds | authentication-retries integer}


Example:

Router(config) # ip ssh timeout 30

 

Configures SSH control parameters on your router.

  • Select one of the SSH control variables.
  • The seconds argument specifies the timeout in seconds, not to exceed 120 seconds. The default is 120. This setting applies to the SSH negotiation phase. Once the EXEC session starts, the standard timeouts configured for the vty apply.
  • By default, five vtys are defined (0-4); therefore five terminal sessions are possible. After the SSH executes a shell, the vty timeout starts. The vty timeout defaults to 10 minutes.
 
Step 4

 
  • The integer argument specifies the number of authentication retries, not to exceed five authentication retries. The default is three.
Note    This command can also be used to establish the number of password prompts provided to the user. The number is the lower of the following two values:
    • Value proposed by the client using the ssh -o numberofpasswordprompt command.
    • Value configured on the router using the ip ssh authentication-retries integercommand, plus one.
 

Invoking an SSH Client

Perform this task to invoke an SSH client.

SUMMARY STEPS

1.    enable

2.    ssh -l username -vrf vrf-name ip-address


DETAILED STEPS
 Command or ActionPurpose
Step 1
enable


Example:

Router> enable

 

(Optional) Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
ssh -l username -vrf vrf-name ip-address


Example:

Router# ssh -l user1 -vrf vrf1 192.0.2.1

 

(Optional) Invokes the Cisco IOS SSH client to connect to an IP host or address in the specified virtual routing and forwarding (VRF) instance.

 

Troubleshooting Tips

  • If your SSH configuration commands are rejected as illegal commands, you have not successfully generated an RSA key pair for your router. Make sure that you have specified a hostname and domain. Then use the crypto key generate rsacommand to generate an RSA key pair and enable the SSH server.
  • When configuring the RSA key pair, you might encounter the following error messages:
    • No hostname specified

You must configure a hostname for the router using the hostname global configuration command. See the IPsec and Quality of Service feature module for more information.

    • No domain specified

You must configure a host domain for the router using the ip domain-name global configuration command. See the IPsec and Quality of Service feature module for more information.

  • The number of allowable SSH connections is limited to the maximum number of vtys configured for the router. Each SSH connection uses a vty resource.
  • SSH uses either local security or the security protocol that is configured through AAA on your router for user authentication. When configuring AAA, you must ensure that AAA is disabled on the console for user authentication. AAA authorization is disabled on the console by default. If AAA authorization is enabled on the console, disable it by configuring the no aaa authorization console command during the AAA configuration stage.

Configuration Examples for SSH

This section provides the following configuration examples, which are output from the show running-config EXEC command on a Cisco 7200, Cisco 7500, and Cisco 12000 routers.


Note
Hereafter, unless otherwise noted, the term “SSH” denotes “SSH Version 1” only.

Note
The crypto key generate rsa command is not displayed in the show running-config output.

Example SSH on a Cisco 7200 Series Router

In the following example, SSH is configured on a Cisco 7200 with a timeout that is not to exceed 60 seconds and no more than 2 authentication retries. Before the SSH server feature is configured on the router, TACACS+ is specified as the method of authentication.

hostname Router72K
aaa new-model
aaa authentication login default tacacs+
aaa authentication login aaa7200kw none
enable password password
username username1 password 0 password1
username username2 password 0 password2
ip subnet-zero
no ip domain-lookup
ip domain-name cisco.com
! Enter the ssh commands.
ip ssh timeout 60
ip ssh authentication-retries 2
controller E1 2/0
controller E1 2/1
interface Ethernet1/0
ip address 192.168.110.2 255.255.255.0 secondary
ip address 192.168.109.2 255.255.255.0
no ip directed-broadcast
no ip route-cache
no ip mroute-cache
no keepalive
no cdp enable
interface Ethernet1/1
no ip address
no ip directed-broadcast
no ip route-cache
no ip mroute-cache
shutdown
no cdp enable
interface Ethernet1/2
no ip address
no ip directed-broadcast
no ip route-cache
no ip mroute-cache
shutdown
no cdp enable
no ip classless
ip route 192.168.1.0 255.255.255.0 10.1.10.1
ip route 192.168.9.0 255.255.255.0 10.1.1.1
ip route 192.168.10.0 255.255.255.0 10.1.1.1
map-list atm
ip 10.1.10.1 atm-vc 7 broadcast
no cdp run
tacacs-server host 192.168.109.216 port 9000
tacacs-server key cisco
radius-server host 192.168.109.216 auth-port 1650 acct-port 1651
radius-server key cisco
line con 0
exec-timeout 0 0
login authentication aaa7200kw
transport input none
line aux 0
line vty 0 4
password password
end

Example SSH on a Cisco 7500 Series Router

In the following example, SSH is configured on a Cisco 7500 with a timeout that is not to exceed 60 seconds and no more than 5 authentication retries. Before the SSH server feature is configured on the router, RADIUS is specified as the method of authentication.

hostname Router75K
aaa new-model
aaa authentication login default radius
aaa authentication login aaa7500kw none
enable password password
 
username username1 password 0 password1
username username2 password 0 password2
ip subnet-zero
no ip cef
no ip domain-lookup
ip domain-name cisco.com
! Enter ssh commands.
ip ssh timeout 60
ip ssh authentication-retries 5
controller E1 3/0
channel-group 0 timeslots 1
 
controller E1 3/1
channel-group 0 timeslots 1
channel-group 1 timeslots 2
interface Ethernet0/0/0
no ip address
no ip directed-broadcast
no ip route-cache distributed
shutdown
 
interface Ethernet0/0/1
no ip address
no ip directed-broadcast
no ip route-cache distributed
shutdown
interface Ethernet0/0/2
no ip address
no ip directed-broadcast
no ip route-cache distributed
shutdown
 
interface Ethernet0/0/3
no ip address
no ip directed-broadcast
no ip route-cache distributed
shutdown
 
interface Ethernet1/0
ip address 192.168.110.2 255.255.255.0 secondary
ip address 192.168.109.2 255.255.255.0
no ip directed-broadcast
no ip route-cache
no ip mroute-cache
 
interface Ethernet1/1
ip address 192.168.109.2 255.255.255.0
no ip directed-broadcast
no ip route-cache
no ip mroute-cache
shutdown
 
interface Ethernet1/2
no ip address
no ip directed-broadcast
no ip route-cache
no ip mroute-cache
 
interface Ethernet1/3
no ip address
no ip directed-broadcast
no ip route-cache
no ip mroute-cache
shutdown
 
interface Ethernet1/4
no ip address
no ip directed-broadcast
no ip route-cache
no ip mroute-cache
shutdown
interface Ethernet1/5
no ip address
no ip directed-broadcast
no ip route-cache
no ip mroute-cache
shutdown
 
interface Serial2/0
ip address 10.1.1.2 255.0.0.0
no ip directed-broadcast
encapsulation ppp
no ip route-cache
no ip mroute-cache
ip classless
ip route 192.168.9.0 255.255.255.0 10.1.1.1
ip route 192.168.10.0 255.255.255.0 10.1.1.1
tacacs-server host 192.168.109.216 port 9000
tacacs-server key cisco
radius-server host 192.168.109.216 auth-port 1650 acct-port 1651
radius-server key cisco
line con 0
exec-timeout 0 0
login authentication aaa7500kw
transport input none
line aux 0
transport input all
line vty 0 4
 
end

Example SSH on a Cisco 12000 Series Router

In the following example, SSH is configured on a Cisco 12000 with a timeout that is not to exceed 60 seconds and no more than two authentication retries. Before the SSH server feature is configured on the router, TACACS+ is specified as the method of authentication.

hostname Router12K
aaa new-model
aaa authentication login default tacacs+ local
aaa authentication login aaa12000kw local
enable password password
 
username username1 password 0 password1
username username2 password 0 password2
redundancy
main-cpu
auto-sync startup-config
ip subnet-zero
no ip domain-lookup
ip domain-name cisco.com
! Enter ssh commands.
ip ssh timeout 60
ip ssh authentication-retries 2
interface ATM0/0
no ip address
no ip directed-broadcast
no ip route-cache cef
shutdown
interface POS1/0
ip address 10.100.100.2 255.255.255.0
no ip directed-broadcast
encapsulation ppp
no ip route-cache cef
no keepalive
crc 16
no cdp enable
 
interface POS1/1
no ip address
no ip directed-broadcast
no ip route-cache cef
shutdown
crc 32
 
interface POS1/2
no ip address
no ip directed-broadcast
no ip route-cache cef
shutdown
crc 32
 
interface POS1/3
no ip address
no ip directed-broadcast
no ip route-cache cef
shutdown
crc 32
 
interface POS2/0
ip address 10.1.1.1 255.255.255.0
no ip directed-broadcast
encapsulation ppp
no ip route-cache cef
crc 16
 
interface Ethernet0
ip address 172.17.110.91 255.255.255.224
no ip directed-broadcast
 
router ospf 1
network 0.0.0.0 255.255.255.255 area 0.0.0.0
ip classless
ip route 0.0.0.0 0.0.0.0 172.17.110.65
 
logging trap debugging
tacacs-server host 172.17.116.138
tacacs-server key cisco
 
radius-server host 172.17.116.138 auth-port 1650 acct-port 1651
radius-server key cisco
 
line con 0
exec-timeout 0 0
login authentication aaa12000kw
transport input none
line aux 0
line vty 0 4
 
no scheduler max-task-time
no exception linecard slot 0 sqe-registers
no exception linecard slot 1 sqe-registers
no exception linecard slot 2 sqe-registers
no exception linecard slot 3 sqe-registers
no exception linecard slot 4 sqe-registers
no exception linecard slot 5 sqe-registers
no exception linecard slot 6 sqe-registers
end

Example Verifying SSH

To verify that the SSH server is enabled and to display the version and configuration data for your SSH connection, use the show ip sshcommand. The following example shows that SSH is enabled:

Router# show ip ssh
SSH Enabled - version 1.5
Authentication timeout: 120 secs; Authentication retries: 3

The following example shows that SSH is disabled:

Router# show ip ssh
%SSH has not been enabled

To verify the status of your SSH server connections, use the show ssh command. The following example shows the SSH server connections on the router when SSH is enabled:

Router# show ssh
Connection      Version     Encryption	State	Username
	0	1.5	3DES	Session Started		guest

The following example shows that SSH is disabled:

Router# show ssh
%No SSH server connections running.

Additional References

Related Documents

Related Topic

Document Title

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

Authentication, authorization, and accounting (AAA)

  • Configuring Accounting feature module
  • Configuring Authentication feature module
  • Configuring Authorization feature module

IPsec

IPsec and Quality of Service feature module

SSH Version 2

Secure Shell Version 2 Support feature module

Downloading a software image

Cisco IOS Configuration Fundamentals Configuration Guide

Standards

Standard

Title

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

--

MIBs

MIB

MIBs Link

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

To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs

RFCs

RFC

Title

No new or modified RFCs are supported by this feature.

--

Technical Assistance

Description

Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html

Feature Information for Configuring Secure Shell

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Table 1Feature Information for Configuring Secure Shell

Feature Name

Releases

Feature Information

Secure Shell

12.0(5)S

The Secure Shell (SSH) feature is an application and a protocol that provides a secure replacement to the Berkeley r-tools. The protocol secures the 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. This document describes SSH Version 1.

Cisco and the Cisco Logo are trademarks of Cisco Systems, Inc. and/or its affiliates in the U.S. and other countries. A listing of Cisco's trademarks can be found at www.cisco.com/go/trademarks. Third party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1005R)

Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.