User Security Configuration Guide Cisco IOS Release 15MT
Cisco IOS Login Enhancements-Login Block
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Cisco IOS Login Enhancements-Login Block

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 caveats and feature information, see Bug Search Tool and 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 module.

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.

Information About Cisco IOS Login Enhancements

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

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 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.

How to Configure Cisco IOS Login Enhancements

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.

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.    exit

    7.    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 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) Although this command is optional, it is recommended that it be configured to specify an ACL that is to be applied to the router when the router switches to quiet mode. When the router is in quiet mode, all login requests are denied and the only available connection is through the console.

    If this command is not configured, then the default ACL sl_def_acl is created on the router. This ACL is hidden in the running configuration. Use the show access-list sl_def_acl to view the parameters for the default ACL.

    For example:

    Router#show access-lists sl_def_acl
     
    Extended IP access list sl_def_acl
        10 deny tcp any any eq telnet
        20 deny tcp any any eq www
        30 deny tcp any any eq 22
        40 permit ip any any
     
    Step 5 login delay seconds


    Example:
    Router(config)# login delay 10
     

    (Optional) Configures a delay between successive login attempts.

     
    Step 6 exit


    Example:
    Router# exit
     

    Exits to privileged EXEC mode.

     
    Step 7 show login failures


    Example:
    Router# show login
     

    Displays login parameters.

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

    Configuration Examples for Login Parameters

    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.”

    Router(config)# login block-for 100 attempts 15 within 100
    Router(config)# login quiet-mode access-class myacl
    

    Showing login Parameters Example

    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-forcommand 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.***

    Additional References

    Related Documents

    Related Topic

    Document Title

    AutoSecure

    AutoSecure feature module.

    Secure Management/Administrative Access

    Role-Based CLI Access feature module.

    Technical Assistance

    Description

    Link

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    http:/​/​www.cisco.com/​cisco/​web/​support/​index.html

    Feature Information for Cisco IOS Login Enhancements-Login Block

    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 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

    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.