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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.
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 for Configuring Secure Shell" section.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
•Prerequisites for Configuring SSH
•Restrictions for Configuring SSH
•Information About Secure Shell
•Configuration Examples for SSH
•Feature Information for Configuring Secure Shell
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.
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.
Note Hereafter, unless otherwise noted, the term "SSH" denotes "SSH Version 1" only.
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.
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 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.
•Configuring an SSH Server (required)
•Invoking an SSH Client (optional)
Note Hereafter, unless otherwise noted, the term "SSH" denotes "SSH Version 1" only.
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.
1. enable
2. configure terminal
3. ip ssh {timeout seconds | authentication-retries integer}
Perform this task to invoke an SSH client.
1. enable
2. ssh -l username -vrf vrf-name ip-address
•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 rsa command 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.
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.
•Example: SSH on a Cisco 7200 Series Router
•Example: SSH on a Cisco 7500 Series Router
•Example: SSH on a Cisco 12000 Series Router
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.
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
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
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
To verify that the SSH server is enabled and to display the version and configuration data for your SSH connection, use the show ip ssh command. 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.
|
|
---|---|
Cisco IOS commands |
|
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 |
|
|
---|---|
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature. |
— |
|
|
---|---|
No new or modified RFCs are supported by this feature. |
— |
Table 1 lists the release history for this feature.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Note Table 1 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.