Cisco IOS Security Configuration Guide: Securing the Data Plane, Release 12.2SR - Controlling Access to a Virtual Terminal Line [Cisco IOS Software Releases 12.2 SR]

Table Of Contents

Controlling Access to a Virtual Terminal Line

Contents

Restrictions for Controlling Access to a Virtual Terminal Line

Information About Controlling Access to a Virtual Terminal Line

Benefits of Controlling Access to a Virtual Terminal Line

How to Control Access to a Virtual Terminal Line

Controlling Inbound Access to a vty

Controlling Outbound Access to a vty

Outbound Access List on a Line Specifies a Destination Address

Configuration Examples for Controlling Access to a Virtual Terminal Line

Controlling Inbound Access on vtys: Example

Controlling Outbound Access on vtys: Example

Where to Go Next

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Feature Information for Controlling Access to a Virtual Terminal Line

Controlling Access to a Virtual Terminal Line

First Published: August 18, 2006

Last Updated: August 18, 2006

You can control who can access the virtual terminal lines (vtys) to a router by applying an access list to inbound vtys. You can also control the destinations that the vtys from a router can reach by applying an access list to outbound vtys.

Finding Feature Information in This Module

Your Cisco IOS software release may not support all of the features documented in this module. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for Controlling Access to a Virtual Terminal Line" section.

Finding Support Information for Platforms and Cisco IOS and Catalyst OS Software Images

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

Contents

•Restrictions for Controlling Access to a Virtual Terminal Line

•Information About Controlling Access to a Virtual Terminal Line

•How to Control Access to a Virtual Terminal Line

•Configuration Examples for Controlling Access to a Virtual Terminal Line

•Where to Go Next

•Additional References

Restrictions for Controlling Access to a Virtual Terminal Line

When you apply an access list to a vty (by using the access-class command), the access list must be a numbered access list, not a named access list.

Information About Controlling Access to a Virtual Terminal Line

Before you control access to a vty, you should understand the following concepts:

•Benefits of Controlling Access to a Virtual Terminal Line

Benefits of Controlling Access to a Virtual Terminal Line

By applying an access list to an inbound vty, you can control who can access the lines to a router. By applying an access list to an outbound vty, you can control the destinations that the lines from a router can reach.

How to Control Access to a Virtual Terminal Line

This section contains the following procedures:

•Controlling Inbound Access to a vty

•Controlling Outbound Access to a vty

Controlling Inbound Access to a vty

Perform this task when you want to control access to a vty coming into the router by using an access list. Access lists are very flexible; this task illustrates one access-list deny command and one access-list permit command. You will decide how many of each command you should use and their order to achieve the restrictions you want.

SUMMARY STEPS

1. enable

2. configure terminal

3. access-list access-list-number deny {source [source-wildcard] | any} [log]

4. access-list access-list-number permit {source [source-wildcard] | any} [log]

5. line vty line-number [ending-line-number]

6. access-class access-list-number in [vrf-also]

7. exit

8. Repeat Steps 5 and 6 for each line to set identical restrictions on all the virtual terminal lines because a user can connect to any of them.

9. end

10. show line [line-number | summary]

DETAILED STEPS

Command or Action

Purpose

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

access-list access-list-number deny {source [source-wildcard] | any} [log]
Example:

Router(config)# access-list 1 deny 172.16.7.34

(Optional) Denies the specified source based on a source address and wildcard mask.

•If the source-wildcard is omitted, a wildcard mask of 0.0.0.0 is assumed, meaning match on all bits of the source address.

•Optionally use the keyword any as a substitute for the source source-wildcard to specify the source and source wildcard of 0.0.0.0 255.255.255.255.

•In this example, host 172.16.7.34 is denied passing the access list.

Step 4

access-list access-list-number permit {source [source-wildcard] | any}[log]
Example:

Router(config)# access-list 1 permit 172.16.0.0 0.0.255.255

Permits the specified source based on a source address and wildcard mask.

•If the source-wildcard is omitted, a wildcard mask of 0.0.0.0 is assumed, meaning match on all bits of the source address.

•Optionally use the keyword any as a substitute for the source source-wildcard to specify the source and source wildcard of 0.0.0.0 255.255.255.255.

•In this example, hosts on network 172.16.0.0 (other than the host denied in the prior step) pass the access list, meaning they can access the vtys identified in the line command.

Step 5

line vty line-number [ending-line-number]
Example:

Router(config)# line vty 5 10

Identifies a specific line for configuration and enters line configuration mode.

•Entering the line command with the optional line type vty designates the line number as a relative line number.

•You also can use the line command without specifying a line type. In this case, the line number is treated as an absolute line number.

Step 6

access-class access-list-number in [vrf-also]
Example:

Router(config-line)# access-class 1 in vrf-also

Restricts incoming connections between a particular vty (into a Cisco device) and the networking devices associated with addresses in the access list.

•If you do not specify the vrf-also keyword, incoming Telnet connections from interfaces that are part of a VPN routing and forwarding (VRF) instance are rejected.

Step 7

exit
Example:

Router(config-line)# exit

Returns the user to the next highest configuration mode.

Step 8

Repeat Steps 5 and 6 for each line to set identical restrictions on all the vtys because a user can connect to any of them.

If you indicated the full range of vty lines in Step 5 with the line command, you do not need to repeat Steps 5 and 6.

Step 9

end
Example:

Router(config-line)# end

Returns the user to privileged EXEC mode.

Step 10

show line [line-number | summary]
Example:

Router# show line 5

Displays parameters of a terminal line.

Controlling Outbound Access to a vty

Perform this task when you want to control access from a vty to a destination. Access lists are very flexible; this task illustrates one access-list deny command and one access-list permit command. You will decide how many of each command you should use and their order to achieve the restrictions you want.

Outbound Access List on a Line Specifies a Destination Address

When a standard access list is applied to a line with the access-class out command, the address specified in the access list is not a source address (as it is in an access list applied to an interface), but a destination address.

SUMMARY STEPS

1. enable

2. configure terminal

3. access-list access-list-number deny {destination [destination-wildcard] | any}[log]

4. access-list access-list-number permit {destination [destination-wildcard] | any}[log]

5. line vty line-number [ending-line-number]

6. access-class access-list-number out

7. exit

8. Repeat Steps 5 and 6 for each line to set identical restrictions on all the virtual terminal lines because a user can connect to any of them.

9. end

10. show line [line-number | summary]

DETAILED STEPS

Command or Action

Purpose

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

access-list access-list-number deny {destination [destination-wildcard] | any} [log]
Example:

Router(config)# access-list 2 deny 172.16.7.34

Denies line access to the specified destination based on a destination address and wildcard mask.

•If the destination-wildcard is omitted, a wildcard mask of 0.0.0.0 is assumed, meaning match on all bits of the source address.

•Optionally use the keyword any as a substitute for the destination destination-wildcard to specify the source and source wildcard of 0.0.0.0 255.255.255.255.

•In this example, host 172.16.7.34 is denied passing the access list, meaning the line cannot connect to it.

Step 4

access-list access-list-number permit {source [source-wildcard] | any} [log]
Example:

Router(config)# access-list 2 permit 172.16.0.0 0.0.255.255

Permits the specified source based on a source address and wildcard mask.

•If the source-wildcard is omitted, a wildcard mask of 0.0.0.0 is assumed, meaning match on all bits of the source address.

•Optionally use the keyword any as a substitute for the source source-wildcard to specify the source and source wildcard of 0.0.0.0 255.255.255.255.

•In this example, hosts on network 172.16.0.0 (other than the host denied in the prior step) pass the access list, meaning they can be connected to by the vtys identified in the line command.

Step 5

line vty line-number [ending-line-number]
Example:

Router(config)# line vty 5 10

Identifies a specific line for configuration and enter line configuration mode.

•Entering the line command with the optional line type vty designates the line number as a relative line number.

•You also can use the line command without specifying a line type. In this case, the line number is treated as an absolute line number.

Step 6

access-class access-list-number out
Example:

Router(config-line)# access-class 2 out

Restricts connections between a particular vty (into a Cisco device) out to the networking devices associated with addresses in the access list.

Step 7

exit
Example:

Router(config-line)# exit

Returns the user to the next highest configuration mode.

Step 8

Repeat Steps 5 and 6 for each line to set identical restrictions on all the vtys because a user can connect to any of them.

If you indicated the full range of vtys in Step 5 with the line command, you do not need to repeat Steps 5 and 6.

Step 9

end
Example:

Router(config-line)# end

Returns the user to privileged EXEC mode.

Step 10

show line [line-number | summary]
Example:

Router# show line 5

Displays parameters of a terminal line.

Configuration Examples for Controlling Access to a Virtual Terminal Line

This section provides the following configuration examples:

•Controlling Inbound Access on vtys: Example

•Controlling Outbound Access on vtys: Example

Controlling Inbound Access on vtys: Example

The following example defines an access list that permits only hosts on network 172.19.5.0 to connect to the virtual terminal lines 1 through 5 on the router. Because the vty keyword is omitted from the line command, the line numbers 1 through 5 are absolute line numbers.
access-list 12 permit 172.19.5.0 0.0.0.255
line 1 5
 access-class 12 in
Controlling Outbound Access on vtys: Example

The following example defines an access list that denies connections to networks other than network 171.20.0.0 on terminal lines 1 through 5. Because the vty keyword is omitted from the line command, the line numbers 1 through 5 are absolute line numbers.
access-list 10 permit 172.20.0.0 0.0.255.255
line 1 5
 access-class 10 out
Where to Go Next

You can further secure a vty by configuring a password with the password line configuration command. See the password (line configuration) command in the Cisco IOS Security Command Reference.

Additional References

The following sections provide references related to Controlling Access to a Virtual Terminal Line.

Related Documents

Related Topic

Document Title

Configuring a password on a line

Cisco IOS Security Command Reference

Standards

Standard

Title

None

—

MIBs

MIB

MIBs Link

None

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

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

RFCs

RFC

Title

None

—

Technical Assistance

Description

Link

The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

http://www.cisco.com/techsupport

Feature Information for Controlling Access to a Virtual Terminal Line

Table 1 lists the features in this module and provides links to specific configuration information. Only features that were introduced or modified in Cisco IOS Release 12.2(1) or a later release appear in the table.

Not all commands may be available in your Cisco IOS software release. For details on when support for a specific command was introduced, see the command reference documentation.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS 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 Cisco IOS software release that introduced support for a given feature in a given Cisco IOS software release train. Unless noted otherwise, subsequent releases of that Cisco IOS software release train also support that feature.

Table 1 Feature Information for Controlling Access to a Virtual Terminal Line

Feature Name

Releases

Feature Configuration Information

This table is intentionally left blank because no features were introduced or modified in Cisco IOS Release 12.2(1) or a later release. This table will be updated when feature information is added to this module.

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