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Cisco IOS Security Configuration Guide: Securing User Services, Release 12.2SR
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Securing User Services Overview
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Autosecure
- Authentication, Authorization, and Accounting (AAA)
- Security Server Protocols
- RADIUS and TACACS+ Attributes
- Secure Shell (SSH)
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Cisco IOS Login Enhancements (Login Block)
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Cisco IOS Resilient Configuration
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Image Verification
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IP Source Tracker
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Role-Based CLI Access
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Table Of Contents
Cisco IOS Login Enhancements (Login Block)
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
Configuration Examples for Login Parameters
Setting Login Parameters: Example
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
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Information About Cisco IOS Login Enhancements
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Information About Cisco IOS Login Enhancements
To use login enhancements, you should understand the following concepts:
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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:
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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:
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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.
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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 2003The 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 2003System 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 2003The 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 2003How to Configure Cisco IOS Login Enhancements
This section contains the following procedures:
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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:
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SUMMARY STEPS
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DETAILED STEPS
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
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DETAILED STEPS
Examples
The following sample output from the show login command verifies that no login parameters have been specified:
Router# show loginNo 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 trapsRouter NOT enabled to watch for login AttacksThe 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 loginA 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 loginA 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 failuresInformation about login failure's with the deviceUsername Source IPAddr lPort Count TimeStamptry1 10.1.1.1 23 1 21:52:49 UTC Sun Mar 9 2003try2 10.1.1.2 23 1 21:52:52 UTC Sun Mar 9 2003The 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:
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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 100Router(config)# login quiet-mode access-class myaclRouter(config)# login on-failure log every 10Router(config)# login on-success log every 15Additional References
The following sections provide references related to Cisco IOS Login Enhancements.
Related Documents
AutoSecure
"AutoSecure" feature module.
Secure Management/Administrative Access
"Role-Based CLI Access" feature module.
Standards
MIBs
None
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:
RFCs
Technical Assistance
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
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