Cisco IOS Security Configuration Guide: Securing User Services, Release 12.4T - Cisco IOS Login Enhancements (Login Block) [Cisco IOS Software Releases 12.4 T]

Table Of Contents

Cisco IOS Login Enhancements (Login Block)

Finding Feature Information

Contents

Information About Cisco IOS Login Enhancements

Protecting Against Denial of Service and Dictionary Login Attacks

Login Enhancements Functionality Overview

Delays Between Successive Login Attempts

Login Shutdown If DoS Attacks Are Suspected

Generation of System Logging Messages for Login Detection

How to Configure Cisco IOS Login Enhancements

Configuring Login Parameters

Login Parameter Defaults

What to Do Next

Verifying Login Parameters

Examples

Configuration Examples for Login Parameters

Setting Login Parameters: Example

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Feature Information for Cisco IOS Login Enhancements (Login Block)

Cisco IOS Login Enhancements (Login Block)

First Published: August 2005
Last Updated: August 26, 2009

The Cisco IOS Login Enhancements (Login Block) feature allows users to enhance the security of a router by configuring options to automatically block further login attempts when a possible denial-of-service (DoS) attack is detected.

The login block and login delay options introduced by this feature can be configured for Telnet or SSH virtual connections. By enabling this feature, you can slow down "dictionary attacks" by enforcing a "quiet period" if multiple failed connection attempts are detected, thereby protecting the routing device from a type of denial-of-service attack.

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 for Cisco IOS Login Enhancements (Login Block)" section.

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://tools.cisco.com/ITDIT/CFN/jsp/index.jsp. An account on Cisco.com is not required.

Contents

•Information About Cisco IOS Login Enhancements

•How to Configure Cisco IOS Login Enhancements

•Configuration Examples for Login Parameters

•Additional References

•Feature Information for Cisco IOS Login Enhancements (Login Block)

Information About Cisco IOS Login Enhancements

To use login enhancements, you should understand the following concepts:

•Protecting Against Denial of Service and Dictionary Login Attacks

•Login Enhancements Functionality Overview

Protecting Against Denial of Service and Dictionary Login Attacks

Connecting to a routing device for the purposes of administering (managing) the device, at either the User or Executive level, is most frequently performed using Telnet or SSH (secure shell) from a remote console (such as a PC). SSH provides a more secure connection option because communication traffic between the user's device and the managed device are encrypted. The Login Block capability, when enabled, applies to both Telnet connections and SSH connections. Beginning in Release versions 12.3(33)SRB2, 12.2(33)SXH2, and 12.4(15)T1, the Login Block capability also applies to HTTP connections."

The automated activation and logging of the Login Block and Quiet Period capabilities introduced by this feature are designed to further enhance the security of your devices by specifically addressing two well known methods that individuals use to attempt to disrupt or compromise network devices.

If the connection address of a device is discovered and is reachable, a malicious user may attempt to interfere with the normal operations of the device by flooding it with connection requests. This type of attack is referred to as an attempted Denial-of-Service, because it is possible that the device may become too busy trying to process the repeated login connection attempts to properly handle normal routing services or are not able to provide the normal login service to legitimate system administrators.

The primary intention of a dictionary attack, unlike a typical DoS attack, is to actually gain administrative access to the device. A dictionary attack is an automated process to attempt to login by attempting thousands, or even millions, of username/password combinations. (This type of attack is called a "dictionary attack" because it typically uses, as a start, every word found in a typical dictionary as a possible password.) As scripts or programs are used to attempt this access, the profile for such attempts is typically the same as for DoS attempts; multiple login attempts in a short period of time.

By enabling a detection profile, the routing device can be configured to react to repeated failed login attempts by refusing further connection request (login blocking). This block can be configured for a period of time, called a "quiet period". Legitimate connection attempts can still be permitted during a quiet period by configuring an access-list (ACL) with the addresses that you know to be associated with system administrators.

Login Enhancements Functionality Overview

To better configure security for virtual login connections, the following requirements have been added to the login process:

•Delays Between Successive Login Attempts

•Login Shutdown If DoS Attacks Are Suspected

•Generation of System Logging Messages for Login Detection

Delays Between Successive Login Attempts

A Cisco IOS device can accept virtual connections as fast as they can be processed. Introducing a delay between login attempts helps to protect the Cisco IOS software-based device against malicious login connections such as dictionary attacks and DoS attacks. Delays can be enabled in one of the following ways:

•Through the auto secure command. If you enable the AutoSecure feature, the default login delay time of one second is automatically enforced.

•Through the login block-for command. You must enter this command before issuing the login delay command. If you enter only the login block-for command, the default login delay time of one second is automatically enforced.

•Through the new global configuration mode command, login delay, which allows you to specify a the login delay time to be enforced, in seconds.

Login Shutdown If DoS Attacks Are Suspected

If the configured number of connection attempts fail within a specified time period, the Cisco IOS device does not accept any additional connections for a "quiet period." (Hosts that are permitted by a predefined access-control list [ACL] are excluded from the quiet period.)

The number of failed connection attempts that trigger the quiet period can be specified through the new global configuration mode command login block-for. The predefined ACL that is excluded from the quiet period can be specified through the new global configuration mode command login quiet-mode access-class.

This functionality is disabled by default, and it is not enabled if autosecure if enabled.

Generation of System Logging Messages for Login Detection

After the router switches to and from quiet mode, logging messages are generated. Also, if configured, logging messages are generated upon every successful or failed login request.

Logging messages can be generated for successful login requests through the new global configuration command login on-success; the login on-failure command generates logs for failed login requests.

Logging messages for failed login attempts are automatically enabled when the auto secure command is issued; they are not automatically enabled for successful login attempts through autosecure.

Note Currently, only system logging (syslog) messages can be generated for login-related events. Support for SNMP notifications (traps) are added in a later release.

System Logging Messages for a Quiet Period

The following logging message is generated after the router switches to quiet-mode:
00:04:07:%SEC_LOGIN-1-QUIET_MODE_ON:Still timeleft for watching failures is 158 seconds, 
[user:sfd] [Source:10.4.2.11] [localport:23] [Reason:Invalid login], [ACL:22] at 16:17:23 
UTC Wed Feb 26 2003
The following logging message is generated after the router switches from quiet mode back to normal mode:
00:09:07:%SEC_LOGIN-5-QUIET_MODE_OFF:Quiet Mode is OFF, because block period timed out at 
16:22:23 UTC Wed Feb 26 2003
System Logging Messages for Successful and Failed Login Requests

The following logging message is generated upon a successful login request:
00:04:32:%SEC_LOGIN-5-LOGIN_SUCCESS:Login Success [user:test] [Source:10.4.2.11] 
[localport:23] at 20:55:40 UTC Fri Feb 28 2003
The following logging message is generated upon a failed login request:
00:03:34:%SEC_LOGIN-4-LOGIN_FAILED:Login failed [user:sdfs] [Source:10.4.2.11] 
[localport:23] [Reason:Invalid login] at 20:54:42 UTC Fri Feb 28 2003
How to Configure Cisco IOS Login Enhancements

This section contains the following procedures:

•Configuring Login Parameters (Required)

•Verifying Login Parameters (Optional)

Configuring Login Parameters

Use this task to configure your Cisco IOS device for login parameters that help detect suspected DoS attacks and slow down dictionary attacks.

Login Parameter Defaults

All login parameters are disabled by default. You must issue the login block-for command, which enables default login functionality, before using any other login commands. After the login block-for command is enabled, the following defaults are enforced:

•A default login delay of one second

•All login attempts made through Telnet or SSH are denied during the quiet period; that is, no ACLs are exempt from the login period until the login quiet-mode access-class command is issued.

SUMMARY STEPS

1. enable

2. configure terminal

3. login block-for seconds attempts tries within seconds

4. login quiet-mode access-class {acl-name | acl-number}

5. login delay seconds

6. login on-failure log [every login]

7. login on-success log [every login]

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

login block-for seconds attempts tries within seconds
Example:

Router(config)# login block-for 100 attempts 2 within 100

Configures your Cisco IOS device for login parameters that help provide DoS detection.

Note This command must be issued before any other login command can be used.

Step 4

login quiet-mode access-class {acl-name | acl-number}
Example:

Router(config)# login quiet-mode access-class myacl

(Optional) Specifies an ACL that is to be applied to the router when it switches to quiet mode.

If this command is not enabled, all login requests are denied during quiet mode.

Step 5

login delay seconds
Example:

Router(config)# login delay 10

(Optional) Configures a delay between successive login attempts.

Step 6

login on-failure log [every login]
Example:

Router(config)# login on-failure log

(Optional) Generates logging messages for failed login attempts.

Step 7

login on-success log [every login]
Example:

Router(config)# login on-success log every 5

(Optional) Generates logging messages for successful login attempts.

What to Do Next

After you have configured login parameters on your router, you may wish to verify the settings. To complete this task, see the following section ""Verifying Login Parameters" section."

Verifying Login Parameters

Use this task to verify the applied login configuration and present login status on your router.

SUMMARY STEPS

1. enable

2. show login [failures]

DETAILED STEPS

Command or Action

Purpose

Step 1

enable
Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2

show login [failures]
Example:

Router# show login

Displays login parameters.

•failures—Displays information related only to failed login attempts.

Examples

The following sample output from the show login command verifies that no login parameters have been specified:
Router# show login
No login delay has been applied.
No Quiet-Mode access list has been configured.
All successful login is logged and generate SNMP traps.
All failed login is logged and generate SNMP traps
Router NOT enabled to watch for login Attacks
The following sample output from the show login command verifies that the login block-for command is issued. In this example, the command is configured to block login hosts for 100 seconds if 16 or more login requests fail within 100 seconds; five login requests have already failed.
Router# show login
A default login delay of 1 seconds is applied.
No Quiet-Mode access list has been configured.
All successful login is logged and generate SNMP traps.
All failed login is logged and generate SNMP traps.
Router enabled to watch for login Attacks.
If more than 15 login failures occur in 100 seconds or less, logins will be disabled for 
100 seconds.
Router presently in Watch-Mode, will remain in Watch-Mode for 95 seconds.
Present login failure count 5.
The following sample output from the show login command verifies that the router is in quiet mode. In this example, the login block-for command was configured to block login hosts for 100 seconds if 3 or more login requests fail within 100 seconds.
Router# show login
A default login delay of 1 seconds is applied.
No Quiet-Mode access list has been configured.
All successful login is logged and generate SNMP traps.
All failed login is logged and generate SNMP traps.
Router enabled to watch for login Attacks.
If more than 2 login failures occur in 100 seconds or less, logins will be disabled for 
100 seconds.
Router presently in Quiet-Mode, will remain in Quiet-Mode for 93 seconds.
Denying logins from all sources.
The following sample output from show login failures command shows all failed login attempts on the router:
Router# show login failures
Information about login failure's with the device
Username      Source IPAddr  lPort Count  TimeStamp
try1          10.1.1.1        23    1     21:52:49 UTC Sun Mar 9 2003
try2          10.1.1.2        23    1     21:52:52 UTC Sun Mar 9 2003
The following sample output from show login failures command verifies that no information is presently logged:
Router# show login failures
*** No logged failed login attempts with the device.***
Configuration Examples for Login Parameters

This section includes the following example:

•Setting Login Parameters: Example

Setting Login Parameters: Example

The following example shows how to configure your router to enter a 100 second quiet period if 15 failed login attempts is exceeded within 100 seconds; all login requests are denied during the quiet period except hosts from the ACL "myacl." Also, logging messages are be generated for every 10th failed login and every 15th successful login.
Router(config)# login block-for 100 attempts 15 within 100
Router(config)# login quiet-mode access-class myacl
Router(config)# login on-failure log every 10
Router(config)# login on-success log every 15
Additional References

The following sections provide references related to Cisco IOS Login Enhancements.

Related Documents

Related Topic

Document Title

AutoSecure

"AutoSecure" feature module.

Secure Management/Administrative Access

"Role-Based CLI Access" feature module.

Standards

Standards

Title

None

—

MIBs

MIBs

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

RFCs

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 Cisco IOS Login Enhancements (Login Block)

Table 1 lists the release history for this feature.

Not all commands may be available in your Cisco IOS software release. For release information about a specific command, 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://tools.cisco.com/ITDIT/CFN/jsp/index.jsp. 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 Cisco IOS Login Enhancements (Login Block)

Feature Name

Releases

Feature Information

Cisco IOS Login Enhancements (Login Block)

12.3(4)T
12.2(25)S
12.2(33)SRA
12.2(33)SRB 12.2(33)SXH
12.4(15)T1
Cisco IOS XE Release 2.1

The Cisco IOS Login Enhancements (Login Block) feature allows users to enhance the security of a router by configuring options to automatically block further login attempts when a possible DoS attack is detected.

This feature was introduced in Cisco IOS Release 12.3(4)T.

This feature was integrated into Cisco IOS Release 12.2(25)S.

This feature was integrated into Cisco IOS Release 12.2(33)SRA.

Support for HTTP login blocking was integrated into Cisco IOS Release 12.2(33)SRB, 12.2(33)SXH, 12.4(15)T1.

This feature was introduced in Cisco IOS XE Release 2.1

The following commands were introduced or modified: login block-for, login delay, login on-failure, login on-success, login quiet-mode access-class, show login.

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