Securing User Services Overview
First Published: June 5, 2009
Last Updated: June 5, 2009
The Securing User Services Overview document covers the topics of identifying users through the
authentication, authorization, and accounting (AAA) protocol, controlling user access to remote devices
and using security server information to track services on Cisco IOS networking devices.
Finding Feature Information
Your software release may not support all the features documented in this overveiw module. For the
latest feature information and caveats, see the release notes for your platform and software release.
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 An
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AutoSecure, page 2
Authentication, Authorization, and Accounting, page 2
Security Server Protocols, page 4
RADIUS and TACACS+ Attributes, page 5
Secure Shell, page 5
Cisco IOS Login Enhancements, page 6
Cisco IOS Resilient Configuration, page 6
Image Verification, page 6
IP Source Tracker, page 6
Role-Based CLI Access, page 6
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Securing User Services Overview
Security with Passwords, Privileges, and Login Usernames for CLI Sessions on Networking
Devices, page 7
Kerberos, page 7
Lawful Intercept, page 7
The AutoSecure feature simplifies the security configuration of a router and hardens the router
configuration by disabling common IP services that can be exploited for network attacks and enable IP
services and features that can aid in the defense of a network when under attack.
AutoSecure secures both the management and forwarding planes in the following ways:
Securing the management plane is accomplished by turning off certain global and interface services
that can be potentially exploited for security attacks and turning on global services that help mitigate
the threat of attacks. Secure access and secure logging are also configured for the router.
Securing the forwarding plane is accomplished by enabling Cisco Express Forwarding (CEF) or
distributed CEF (dCEF) on the router whenever possible. Because there is no need to build cache
entries when traffic starts arriving for new destinations, CEF behaves more predictably than other
modes when presented with large volumes of traffic addressed to many destinations. Thus, routers
configured for CEF perform better under SYN attacks than routers using the traditional cache.
Authentication, Authorization, and Accounting
Cisco’s authentication, authorization, and accounting (AAA) paradigm is an architectural framework for
configuring a set of three independent security functions in a consistent, modular manner. AAA provides
a primary method for authenticating users (for example, a username/password database stored on a
TACACS+ server) and then specify backup methods (for example, a locally stored username/password
database).The backup method is used if the primary method’s database cannot be accessed by the
networking device. To configure AAA, refer to the Authentication, Authorization, and Accounting
chapters. You can configure up to four sequential backup methods.
If backup methods are not configured, access is denied to the device if the
username/password database cannot be accessed for any reason.
The following sections discuss the AAA security functions in greater detail:
Authentication, page 3
Authorization, page 3
Accounting, page 3
Authentication Proxy, page 3
802.1x Authentication Services, page 4
Network Admission Control, page 4
Securing User Services Overview
Authentication, Authorization, and Accounting
Authentication provides the method of identifying users, including login and password dialog, challenge
and response, messaging support, and, depending on the security protocol you select, encryption.
Authentication is the way a user is identified prior to being allowed access to the network and network
services. AAA authentication is configured by defining a named list of authentication methods and then
applying that list to various interfaces.
Authorization provides the method for remote access control, including one-time authorization or
authorization for each service, per-user account list and profile, user group support, and support of IP,
Internetwork Packet Exchange (IPX), AppleTalk Remote Access (ARA), and Telnet.
Remote security servers, such as RADIUS and TACACS+, authorize users for specific rights by
associating attribute-value (AV) pairs, which define those rights, with the appropriate user. AAA
authorization works by assembling a set of attributes that describe what the user is authorized to perform.
These attributes are compared with the information contained in a database for a given user, and the
result is returned to AAA to determine the user’s actual capabilities and restrictions.
Accounting provides the method for collecting and sending security server information used for billing,
auditing, and reporting, such as user identities, start and stop times, executed commands (such as PPP),
number of packets, and number of bytes. Accounting enables you to track the services users are
accessing, as well as the amount of network resources they are consuming.
You can configure authentication outside of AAA. However, you must configure AAA if you want to use
RADIUS, TACACS+, or Kerberos or if you want to configure a backup authentication method.
Authentication Proxy
The Cisco IOS Firewall Authentication Proxy feature is used by network administrators to apply
dynamic, per-user authentication and authorization security policies, which authenticates users in
addition to industry standard TACACS+ and RADIUS authentication protocols. Authenticating and
authorizing connections by users provides more robust protection against network attacks because users
can be identified and authorized on the basis of their per-user policy.
Once the authentication proxy feature is implemented, users can log into the network or access the
Internet through HTTP, and their specific access profiles are automatically retrieved and applied from a
CiscoSecure ACS, or other RADIUS or TACACS+ authentication server. The user profiles are active
only when there is active traffic from the authenticated users.
Authentication proxy is compatible with other Cisco IOS security features such as Network Address
Translation (NAT), Context-Based Access Control (CBAC), IP security (IPsec) encryption, and Cisco
Secure VPN Client (VPN client) software.
Securing User Services Overview
Security Server Protocols
802.1x Authentication Services
802.1x Authentication Services feature is used to configure local 802.1x port-based authentication and
Virtual Private Network (VPN) access on Cisco integrated services routers (ISRs) through the
IEEE 802.1X protocol framework. IEEE 802.1x authentication prevents unauthorized devices
(supplicants) from gaining access to the network.
Cisco ISRs can combine the functions of a router, a switch, and an access point, depending on the fixed
configuration or installed modules. The switch functions are provided by either built-in switch ports or
a plug-in module with switch ports.
The IEEE 802.1x standard defines a client-server-based access control and authentication protocol that
prevents unauthorized clients from connecting to a LAN through publicly accessible ports unless they
are properly authenticated. The authentication server authenticates each client connected to a port before
making available any services offered by the device or the network.
Until the client is authenticated, IEEE 802.1x access control allows only Extensible Authentication
Protocol over LAN (EAPOL), Cisco Discovery Protocol (CDP), and Spanning Tree Protocol (STP)
traffic through the port to which the client is connected. After authentication is successful, normal traffic
can pass through the port.
Network Admission Control
The Cisco Network Admission Control (NAC) feature addresses the increased threat and impact of
worms and viruses have on business networks. This feature is part of the Cisco Self-Defending Network
Initiative that helps customers identify, prevent, and adapt to security threats.
NAC enables Cisco routers to enforce access privileges when an endpoint attempts to connect to a
network. This access decision can be made on the basis of information about the endpoint device, such
as its current antivirus state, which includes information such as version of antivirus software, virus
definitions, and version of scan engine.
NAC allows noncompliant devices to be denied access, placed in a quarantined area, or given restricted
access to computing resources, thus keeping insecure nodes from infecting the network. The key
component of NAC is the Cisco Trust Agent (CTA), which resides on an endpoint system and
communicates with Cisco routers on the network. The CTA collects security state information, such as
what antivirus software is being used, and communicates this information to Cisco routers. The
information is then relayed to a Cisco Secure Access Control Server (ACS) where access control
decisions are made. The ACS directs the Cisco router to perform enforcement against the endpoint.
Security Server Protocols
AAA security protocols are used on a router or network access server administers its security functions.
AAA is the means through which communication is established between the network access server and
Cisco supported RADIUS and TACACS+ security server protocols.
If the database on a security server is used to store login username/password pairs, the router or access
server must be configured to support the applicable protocol; in addition, because most supported
security protocols must be administered through the AAA security services, AAA must be enabled.
The following sections discuss the RADIUS and TACACS+ security server protocols in greater detail:
RADIUS, page 5
TACACS+, page 5
Securing User Services Overview
RADIUS and TACACS+ Attributes
The RADIUS distributed client/server system is implemented through the AAA protocol. RADIUS
secures networks against unauthorized access. In the Cisco implementation, RADIUS clients run on
Cisco routers and send authentication requests to a central RADIUS server that contains all user
authentication and network service access information.
The TACACS+ security application is implemented through AAA and provides centralized validation of
users attempting to gain access to a router or network access server. TACACS+ services are maintained
in a database on a TACACS+ daemon running, typically, on a UNIX or Windows NT workstation.
TACACS+ provides for separate and modular authentication, authorization, and accounting facilities.
The protocol was designed to scale as networks grow and to adapt to new security technology. The
underlying architecture of the TACACS+ protocol complements the independent AAA architecture.
RADIUS and TACACS+ Attributes
There are various vendor interpretations of the RADIUS and TACACS+ RFCs. Although different
vendors can be in compliance with any RFC does not guarantee interoperability. Interoperability is
guaranteed only if standard RFCs are used for the RADIUS and TACACS+ protocols.
When nonstandard RADIUS and TACACS+ RFCs are used, attributes must be developed and
implemented by vendors so that their respective devices can interoperate with each other.
The following sections discuss the RADIUS and TACACS+ attributes in greater detail:
RADIUS Attributes, page 5
TACACS+ Attributes, page 5
RADIUS Attributes
RADIUS attributes are used to define specific AAA elements in a user profile, which is stored on the
RADIUS daemon.
TACACS+ Attributes
TACACS+ attribute-value pairs are used to define specific AAA elements in a user profile, which is
stored on the TACACS+ daemon.
Secure Shell
The Secure Shell (SSH) feature is an application and a protocol that provides a secure replacement to a
suite of UNIX r-commands such as rsh, rlogin and rcp. (Cisco IOS supports rlogin.) The protocol secures
the sessions using standard cryptographic mechanisms, and the application can be used similarly to the
Berkeley rexec and rsh tools. There are currently two versions of SSH available: SSH Version 1 and SSH
Version 2.
Securing User Services Overview
Cisco IOS Login Enhancements
Cisco IOS Login Enhancements
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.
Cisco IOS Resilient Configuration
The Cisco IOS Resilient Configuration feature enables a router to secure and maintain a working copy
of the running image and configuration so that those files can withstand malicious attempts to erase the
contents of persistent storage (NVRAM and flash).
Image Verification
Image Verification feature allows users to automatically verify the integrity of Cisco IOS images. Thus,
users can be sure that the image is protected from accidental corruption, which can occur at any time
during transit, starting from the moment the files are generated by Cisco until they reach the user.
IP Source Tracker
The IP Source Tracker feature allows information to be gathered about the traffic to a host that is
suspected of being under attack. This feature also allows you to easily trace an attack to its entry point
into the network.
Role-Based CLI Access
The Role-Based CLI Access feature allows the network administrator to define “views,” which are a set
of operational commands and configuration capabilities that provide selective or partial access to
Cisco IOS EXEC and configuration (config) mode commands. Views restrict user access to Cisco IOS
command-line interface (CLI) and configuration information; that is, a view can define what commands
are accepted and what configuration information is visible. Thus, network administrators can exercise
better control over access to Cisco networking devices.
Securing User Services Overview
Security with Passwords, Privileges, and Login Usernames for CLI Sessions on Networking Devices
Security with Passwords, Privileges, and Login Usernames for
CLI Sessions on Networking Devices
There are conditions where networking devices are installed on the network with no security options
configured, or a networking device is installed and help is needed to understand how baseline of security
is implemented on the Cisco IOS CLI operating system session running on the networking device.
In this document, the following basic security topics are discussed:
Different levels of authorization for CLI sessions can be differentiated to control access to
commands that can modify the status of the networking device versus commands that are used to
monitor the device
Passwords can be assigned to CLI sessions
Users can be required to log in to a networking device with a username
Privilege levels of commands can be changed to create new authorization levels for CLI sessions
The Kerberos feature is a secret-key network authentication protocol implemented through AAA that
uses the Data Encryption Standard (DES) cryptographic algorithm for encryption and authentication.
Kerberos was designed to authenticate requests for network resources and is based on the concept of a
trusted third-party that performs secure verification of users and services. It is primarily used to verify
that users and the network services they use are really who and what they claim to be. To accomplish
this verification, a trusted Kerberos server issues tickets that have a limited lifespan, are stored in a user’s
credential cache, and can be used in place of the standard username-and-password authentication
Lawful Intercept
The Lawful Intercept (LI) feature supports service providers in meeting the requirements of law
enforcement agencies to provide the ability to intercept Voice over IP (VoIP) or data traffic going through
the edge routers. The Lawful Intercept (LI) architecture includes the Cisco Service Independent
Intercept architecture and PacketCable Lawful Intercept architecture.
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Securing User Services Overview
Lawful Intercept
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figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses in illustrative content is unintentional and
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