Configuring Secure Shell
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. |
DETAILED STEPS
Invoking an SSH Client
Perform this task to invoke an SSH client.
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
|
|
Example: Router> enable |
(Optional) Enables privileged EXEC mode.
|
|
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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.
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
- Example SSH on a Cisco 7500 Series Router
- Example SSH on a Cisco 12000 Series Router
- Example Verifying SSH
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 |
|
| Authentication, authorization, and accounting (AAA) |
|
| 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. |
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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: |
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. |
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 1 | Feature 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.
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