Configuring Authentication

Authentication identifies users before they are allowed access to the network and network services. Basically, the Cisco IOS software implementation of authentication is divided into two main categories:

• AAA Authentication Methods

• Non-AAA Authentication Methods

Authentication, for the most part, is implemented through the AAA security services. We recommend that, whenever possible, AAA be used to implement authentication.

This chapter describes both AAA and non-AAA authentication methods. For configuration examples, refer to the "Authentication Configuration Examples" section at the end of this chapter. For a complete description of the commands used in this chapter, refer to the "AAA Authentication Commands" chapter of the Security Command Reference. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.

AAA Authentication Method Lists

To configure AAA authentication, first define a named list of authentication methods, and then apply that list to various interfaces. The method list defines the types of authentication to be performed and the sequence in which they will be performed; it must be applied to a specific interface before any of the defined authentication methods will be performed. The only exception is the default method list (which, by coincidence, is named "default"). The default method list is automatically applied to all interfaces except those that have a named method list explicitly defined. A defined method list overrides the default method list.

A method list is simply a list describing the authentication methods to be queried, in sequence, to authenticate a user. Method lists enable you to designate one or more security protocols to be used for authentication, thus ensuring a backup system for authentication in case the initial method fails. Cisco IOS software uses the first method listed to authenticate users; if that method fails to respond, the Cisco IOS software selects the next authentication method listed in the method list. This process continues until there is successful communication with a listed authentication method, or all methods defined are exhausted.

It is important to note that the Cisco IOS software attempts authentication with the next listed authentication method only when there is no response from the previous method. If authentication fails at any point in this cycle--meaning that the security server or local username database responds by denying the user access--the authentication process stops and no other authentication methods are attempted.

Method List Examples

Figure 3 shows a typical AAA network configuration that includes four security servers: R1 and R2 are RADIUS servers and T1 and T2 are TACACS+ servers. Suppose the system administrator has decided on a security solution where all interfaces will use the same authentication methods to authenticate PPP connections: R1 is contacted first for authentication information, then if there is no response, R2 is contacted. If R2 fails to respond, T1 is contacted; if T1 fails to respond, T2 is contacted. If all designated servers fail to respond, authentication falls over to the local username database on the access server itself. To implement this, the system administrator would create a default method list by entering the following command:

aaa authentication ppp default radius tacacs+ local 

aaa authentication ppp apple radius tacacs+ local
 interface async 3
 ppp authentication chap apple

AAA Authentication General Configuration Procedure

To configure AAA authentication, no matter what method of authentication you select, you need to perform the following tasks:

1. Enable AAA by using the aaa new-model global configuration command. For more information about configuring AAA, refer to the "AAA Overview" chapter.
   
   
2. Configure security protocol parameters, such as RADIUS, TACACS+, or Kerberos, if you are using a security server. For more information about RADIUS, refer to the "Configuring RADIUS" chapter. For more information about TACACS+, refer to the "Configuring TACACS+" chapter. For more information about Kerberos, refer to the "Configuring Kerberos" chapter.
   
   
3. Define the method lists for authentication by using the aaa authentication command.
   
   
4. Apply the method lists to a particular interface or line, if required.
   

AAA Authentication Methods

This section discusses the following AAA authentication methods:

• Configure Login Authentication Using AAA

• Configure PPP Authentication Using AAA

• Configure ARA Authentication Using AAA

• Configure NASI Authentication Using AAA

• Enable Password Protection at the Privileged Level

• Enable an Authentication Override

• Enable Double Authentication

Configure Login Authentication Using AAA

The AAA security services facilitate a variety of login authentication methods. Use the aaa authentication login command to enable AAA authentication no matter which of the supported login authentication methods you decide to use. With the aaa authentication login command, you create one or more lists of authentication methods that are tried at login. These lists are applied using the login authentication line configuration command.

To configure login authentication by using AAA, perform the following tasks, beginning in global configuration mode:

The keyword list-name is any character string used to name the list you are creating. The keyword method refers to the actual method the authentication algorithm tries. The additional methods of authentication are used only if the previous method returns an error, not if it fails. To specify that the authentication should succeed even if all methods return an error, specify none as the final method in the command line.

For example, to specify that authentication should succeed even if (in this example) the TACACS+ server returns an error, enter the following:

aaa authentication login default tacacs+ none

To create a default list that is used if no list is specified in the login authentication command, use the default argument followed by the methods you want used in default situations. The default method list is automatically applied to all interfaces.

For example, to specify RADIUS as the default method for user authentication during login, enter the following:

aaa authentication login default radius

Table 4 lists the supported login authentication methods.

Login Authentication Using Local Password

Use the aaa authentication login command with the local method keyword to specify that the Cisco router or access server will use the local username database for authentication. For example, to specify the local username database as the method of user authentication at login when no other method list has been defined, enter the following:

aaa authentication login default local

For information about adding users into the local username database, refer to the "Establish Username Authentication" section in this chapter.

Login Authentication Using Line Password

Use the aaa authentication login command with the line method keyword to specify the line password as the login authentication method. For example, to specify the line password as the method of user authentication at login when no other method list has been defined, enter the following:

aaa authentication login default line

Before you can use a line password as the login authentication method, you need to define a line password. For more information about defining line passwords, refer to the "Configure Line Password Protection" section in this chapter.

Login Authentication Using Enable Password

Use the aaa authentication login command with the enable method keyword to specify the enable password as the login authentication method. For example, to specify the enable password as the method of user authentication at login when no other method list has been defined, enter:

aaa authentication login default enable

Before you can use the enable password as the login authentication method, you need to define the enable password. For more information about defining enable passwords, refer to the "Configuring Passwords and Privileges" chapter.

Login Authentication Using RADIUS

Use the aaa authentication login command with the radius method keyword to specify RADIUS as the login authentication method. For example, to specify RADIUS as the method of user authentication at login when no other method list has been defined, enter:

aaa authentication login default radius

Before you can use RADIUS as the login authentication method, you need to enable communication with the RADIUS security server. For more information about establishing communication with a RADIUS server, refer to the "Configuring RADIUS" chapter.

Login Authentication Using TACACS+

Use the aaa authentication login command with the tacacs+ method keyword to specify TACACS+ as the login authentication method. For example, to specify TACACS+ as the method of user authentication at login when no other method list has been defined, enter:

aaa authentication login default tacacs+

Before you can use TACACS+ as the login authentication method, you need to enable communication with the TACACS+ security server. For more information about establishing communication with a TACACS+ server, refer to the "Configuring TACACS+" chapter.

Login Authentication Using Kerberos

Authentication via Kerberos is different from most other authentication methods: the user's password is never sent to the remote access server. Remote users logging in to the network are prompted for a username. If the key distribution center (KDC) has an entry for that user, it creates an encrypted ticket granting ticket (TGT) with the password for that user and sends it back to the router. The user is then prompted for a password, and the router attempts to decrypt the TGT with that password. If it succeeds, the user is authenticated and the TGT is stored in the user's credential cache on the router.

A user does not need to run the KINIT program to get a TGT to authenticate to the router. This is because KINIT has been integrated into the login procedure in the Cisco IOS implementation of Kerberos.

Use the aaa authentication login command with the krb5 method keyword to specify Kerberos as the login authentication method. For example, to specify Kerberos as the method of user authentication at login when no other method list has been defined, enter:

aaa authentication login default krb5

Before you can use Kerberos as the login authentication method, you need to enable communication with the Kerberos security server. For more information about establishing communication with a Kerberos server, refer to the "Configuring Kerberos" chapter.

Configure PPP Authentication Using AAA

Many users access network access servers through dialup via async or ISDN. Dialup via async or ISDN bypasses the CLI completely; instead, a network protocol (such as PPP or ARAP) starts as soon as the connection is established.

The AAA security services facilitate a variety of authentication methods for use on serial interfaces running PPP. Use the aaa authentication ppp command to enable AAA authentication no matter which of the supported PPP authentication methods you decide to use.

To configure AAA authentication methods for serial lines using PPP, perform the following tasks in global configuration mode:

With the aaa authentication ppp command, you create one or more lists of authentication methods that are tried when a user tries to authenticate via PPP. These lists are applied using the ppp authentication line configuration command.

To create a default list that is used if no list is specified in the ppp authentication command, use the default argument followed by the methods you want used in default situations.

For example, to specify the local username database as the default method for user authentication, enter the following:

aaa authentication ppp default local

The keyword list-name is any character string used to name the list you are creating. The keyword method refers to the actual method the authentication algorithm tries. The additional methods of authentication are used only if the previous method returns an error, not if it fails. To specify that the authentication should succeed even if all methods return an error, specify none as the final method in the command line.

For example, to specify that authentication should succeed even if (in this example) the TACACS+ server returns an error, enter the following:

aaa authentication ppp default tacacs+ none

Table 5 lists the supported login authentication methods.

PPP Authentication Using Local Password

Use the aaa authentication ppp command with the method keyword local to specify that the Cisco router or access server will use the local username database for authentication. For example, to specify the local username database as the method of authentication for use on lines running PPP when no other method list has been defined, enter:

aaa authentication ppp default local

For information about adding users into the local username database, refer to the "Establish Username Authentication" section in this chapter.

PPP Authentication Using RADIUS

Use the aaa authentication ppp command with the radius method keyword to specify RADIUS as the authentication method for use on interfaces running PPP. For example, to specify RADIUS as the method of user authentication when no other method list has been defined, enter:

aaa authentication ppp default radius

Before you can use RADIUS as the authentication method, you need to enable communication with the RADIUS security server. For more information about establishing communication with a RADIUS server, refer to the "Configuring RADIUS" chapter.

PPP Authentication Using TACACS+

Use the aaa authentication ppp command with the tacacs+ method keyword to specify TACACS+ as the authentication method for use on interfaces running PPP. For example, to specify TACACS+ as the method of user authentication when no other method list has been defined, enter:

aaa authentication ppp default tacacs+

Before you can use TACACS+ as the authentication method, you need to enable communication with the TACACS+ security server. For more information about establishing communication with a TACACS+ server, refer to the "Configuring TACACS+" chapter.

PPP Authentication Using Kerberos

Use the aaa authentication ppp command with the krb5 method keyword to specify Kerberos as the authentication method for use on interfaces running PPP. For example, to specify Kerberos as the method of user authentication when no other method list has been defined, enter:

aaa authentication ppp default krb5

Before you can use Kerberos as the login authentication method, you need to enable communication with the Kerberos security server. For more information about establishing communication with a Kerberos server, refer to the "Configuring Kerberos" chapter.

Configure ARA Authentication Using AAA

With the aaa authentication arap command, you create one or more lists of authentication methods that are tried when AppleTalk Remote Access (ARA) users attempt to log in to the router. These lists are used with the arap authentication line configuration command.

Perform at least the first of the following tasks starting in global configuration mode:

The list-name is any character string used to name the list you are creating. The method refers to the actual list of methods the authentication algorithm tries, in the sequence entered.

To create a default list that is used if no list is specified in the arap authentication command, use the default argument followed by the methods you want to be used in default situations.

The additional methods of authentication are used only if the previous method returns an error, not if it fails. To specify that the authentication should succeed even if all methods return an error, specify none as the final method in the command line.

Table 6 lists the supported login authentication methods.

aaa authentication arap default if-needed none

aaa authentication arap MIS-access if-needed none

ARA Authentication Allowing Guest Logins

Use the aaa authentication arap command with the guest keyword to allow guest logins. This method must be the first listed in the ARA authentication method list but it can be followed by other methods if it does not succeed. For example, to allow all guest logins as the default method of authentication, using RADIUS only if that method fails, enter:

aaa authentication arap default guest radius

For more information about ARA guest logins, refer to the "Configuring AppleTalk" chapter in the Network Protocols Configuration Guide, Part 2.

ARA Authentication Allowing Authorized Guest Logins

Use the aaa authentication arap command with the auth-guest keyword to allow guest logins only if the user has already successfully logged in to the EXEC. This method must first in the ARA authentication method list but it can be followed by other methods if it does not succeed. For example, to allow all authorized guest logins--meaning logins by users who have already successfully logged into the EXEC--as the default method of authentication, using RADIUS only if that method fails, enter:

aaa authentication arap default auth-guest radius

For more information about ARA authorized guest logins, refer to the "Configuring AppleTalk" chapter in the Network Protocols Configuration Guide, Part 2.

ARA Authentication Using Local Password

Use the aaa authentication arap command with the method keyword local to specify that the Cisco router or access server will use the local username database for authentication. For example, to specify the local username database as the method of ARA user authentication when no other method list has been defined, enter:

aaa authentication arap default local

For information about adding users to the local username database, refer to the "Establish Username Authentication" section in this chapter.

ARA Authentication Using Line Password

Use the aaa authentication arap command with the method keyword line to specify the line password as the authentication method. For example, to specify the line password as the method of ARA user authentication when no other method list has been defined, enter:

aaa authentication arap default line

Before you can use a line password as the ARA authentication method, you need to define a line password. For more information about defining line passwords, refer to the "Configure Line Password Protection" section in this chapter.

ARA Authentication Using RADIUS

Use the aaa authentication arap command with the radius method keyword to specify RADIUS as the ARA authentication method. For example, to specify RADIUS as the method of ARA user authentication when no other method list has been defined, enter:

aaa authentication arap default radius

Before you can use RADIUS as the ARA authentication method, you need to enable communication with the RADIUS security server. For more information about establishing communication with a RADIUS server, refer to the "Configuring RADIUS" chapter.

ARA Authentication Using TACACS+

Use the aaa authentication arap command with the tacacs+ method keyword to specify TACACS+ as the ARA authentication method. For example, to specify TACACS+ as the method of ARA user authentication when no other method list has been defined, enter:

aaa authentication arap default tacacs+

Before you can use TACACS+ as the ARA authentication method, you need to enable communication with the TACACS+ security server. For more information about establishing communication with a TACACS+ server, refer to the "Configuring TACACS+" chapter.

Configure NASI Authentication Using AAA

With the aaa authentication nasi command, you create one or more lists of authentication methods that are tried when Netware Asynchronous Services Interface (NASI) users attempt to log in to the router. These lists are used with the nasi authentication line configuration command.

Perform at least the first of the following tasks starting in global configuration mode:

The list-name is any character string used to name the list you are creating. The method refers to the actual list of methods the authentication algorithm tries, in the sequence entered.

To create a default list that is used if no list is specified in the aaa authentication nasi command, use the default argument followed by the methods you want to be used in default situations.

The additional methods of authentication are used only if the previous method returns an error, not if it fails. To specify that the authentication should succeed even if all methods return an error, specify none as the final method in the command line.

Table 7 lists the supported login authentication methods.

NASI Authentication Using Enable Password

Use the aaa authentication nasi command with the argument enable to specify the enable password as the authentication method. For example, to specify the enable password as the method of NASI user authentication when no other method list has been defined, enter:

aaa authentication nasi default enable

Before you can use the enable password as the authentication method, you need to define the enable password. For more information about defining enable passwords, refer to the "Configuring Passwords and Privileges" chapter.

NASI Authentication Using Local Password

Use the aaa authentication nasi command with the method keyword local to specify that the Cisco router or access server will use the local username database for authentication information. For example, to specify the local username database as the method of NASI user authentication when no other method list has been defined, enter:

aaa authentication nasi default local

For information about adding users to the local username database, refer to the "Establish Username Authentication" section in this chapter.

NASI Authentication Using Line Password

Use the aaa authentication nasi command with the method keyword line to specify the line password as the authentication method. For example, to specify the line password as the method of NASI user authentication when no other method list has been defined, enter:

aaa authentication nasi default line

Before you can use a line password as the NASI authentication method, you need to define a line password. For more information about defining line passwords, refer to the "Configure Line Password Protection" section in this chapter.

NASI Authentication Using TACACS+

Use the aaa authentication nasi command with the tacacs+ method keyword to specify TACACS+ as the NASI authentication method. For example, to specify TACACS+ as the method of NASI user authentication when no other method list has been defined, enter:

aaa authentication nasi default tacacs+

Before you can use TACACS+ as the authentication method, you need to enable communication with the TACACS+ security server. For more information about establishing communication with a TACACS+ server, refer to the "Configuring TACACS+" chapter.

Enable Password Protection at the Privileged Level

Use the aaa authentication enable default command to create a series of authentication methods that are used to determine whether a user can access the privileged EXEC command level. You can specify up to four authentication methods. The additional methods of authentication are used only if the previous method returns an error, not if it fails. To specify that the authentication succeed even if all methods return an error, specify none as the final method in the command line.

Perform the following task in global configuration mode:

The method refers to the actual list of methods the authentication algorithm tries, in the sequence entered. Table 8 lists the supported login authentication methods.

Enable an Authentication Override

To configure the Cisco IOS software to check the local user database for authentication before attempting another form of authentication, use the aaa authentication local-override command. This command is useful when you want to configure an override to the normal authentication process for certain personnel (such as system administrators).

Perform the following task in global configuration mode:

Enable Double Authentication

Double Authentication provides additional authentication for Point-to-Point Protocol (PPP) sessions. Previously, PPP sessions could only be authenticated by using a single authentication method: either PAP or CHAP. Double Authentication requires remote users to pass a second stage of authentication--after CHAP or PAP authentication--before gaining network access.

This second ("double") authentication requires a password that is known to the user but not stored on the user's remote host. Therefore, the second authentication is specific to a user, not to a host. This provides an additional level of security that will be effective even if information from the remote host is stolen. In addition, this also provides greater flexibility by allowing customized network privileges for each user.

The second stage authentication can use one-time passwords such as token card passwords, which are not supported by CHAP. If one-time passwords are used, a stolen user password is of no use to the perpetrator.

How Double Authentication Works

With Double Authentication, there are two authentication/authorization stages. These two stages occur after a remote user dials in and a PPP session is initiated.

In the first stage, the user logs in using the remote host name; CHAP (or PAP) authenticates the remote host, and then PPP negotiates with AAA to authorize the remote host. In this process, the network access privileges associated with the remote host are assigned to the user.

In the second stage, the remote user must Telnet to the network access server to be authenticated. When the remote user logs in, the user must be authenticated with AAA login authentication. The user then must enter the access-profile command to be reauthorized using AAA. When this authorization is complete, the user has been double authenticated, and can access the network according to per-user network privileges.

The system administrator determines what network privileges remote users will have after each stage of authentication by configuring appropriate parameters on a security server. To use Double Authentication, the user must activate it by issuing the access-profile command.

Caution Double Authentication can cause certain undesirable events if multiple hosts share a PPP connection to a network access server, as shown in Figure 4. First, if a user, Bob, initiates a PPP session and activates Double Authentication at the NAS (per Figure 4), any other user will automatically have the same network privileges as Bob until Bob's PPP session expires. This happens because Bob's authorization profile is applied to the NAS's interface during the PPP session and any PPP traffic from other users will use the PPP session Bob established. Second, if Bob initiates a PPP session and activates Double Authentication, and then--before Bob's PPP session has expired--another user, Jane, executes the access-profile command (or, if she Telnets to the NAS and autocommand access-profile is executed), a reauthorization will occur and Jane's authorization profile will be applied to the interface--replacing Bob's profile. This can disrupt or halt Bob's PPP traffic, or grant Bob additional authorization privileges he should not have.

Configure Double Authentication

To configure Double Authentication, you must complete the following steps:

1. Enable AAA by using the aaa-new model global configuration command. For more information about enabling AAA, refer to the "AAA Overview" chapter.
   
   
2. Use the aaa authentication command to configure your network access server to use login and PPP authentication method lists, then apply those method lists to the appropriate lines or interfaces.
   
   
3. Use the aaa authorization command to configure AAA network authorization at login. For more information about configuring network authorization, refer to the "Configuring Authorization" chapter.
   
   
4. Configure security protocol parameters (for example, RADIUS or TACACS+). For more information about RADIUS, refer to the "Configuring RADIUS" chapter. For more information about TACACS+, refer to the "Configuring TACACS+" chapter.
   
   
5. Use access control list AV pairs on the security server that the user can connect to the local host only by establishing a Telnet connection.
   
   
6. (Optional) Configure the access-profile command as an autocommand. If you configure the autocommand, remote users will not have to manually enter the access-profile command to access authorized rights associated with their personal user profile. To learn about configuring autocommands, refer to the autocommand command in the Dial Solutions Command Reference.
   

Follow these rules when creating the user-specific authorization statements (These rules relate to the default behavior of the access-profile command):

• Use valid AV pairs when configuring access control list AV pairs on the security server. For a list of valid AV pairs, refer to the "Authentication Commands" chapter in the Security Command Reference.

• If you want remote users to use the interface's existing authorization (that which existed prior to the second stage authentication/authorization), but you want them to have different access control lists (ACLs), you should specify only ACL AV pairs in the user-specific authorization definition. This might be desirable if you set up a default authorization profile to apply to the remote host, but want to apply specific ACLs to specific users.

• When these user-specific authorization statements are later applied to the interface, they can either be added to the existing interface configuration, or replace the existing interface configuration--depending on which form of the access-profile command is used to authorize the user. You should understand how the access-profile command works before configuring the authorization statements.

• If you will be using ISDN or Multilink PPP, you must also configure virtual templates at the local host.

To troubleshoot Double Authentication, use the debug aaa per-user debug command. For more information about this command, refer to the Debug Command Reference.

Access User Profile after Double Authentication

In Double Authentication, when a remote user establishes a PPP link to the local host using the local host name, the remote host is CHAP (or PAP) authenticated. After CHAP (or PAP) authentication, PPP negotiates with AAA to assign network access privileges associated with the remote host to the user. (We suggest that privileges at this stage be restricted to allow the user to connect to the local host only by establishing a Telnet connection.)

When the user needs to initiate the second phase of Double Authentication, establishing a Telnet connection to the local host, the user enters a personal username and password (different from the CHAP or PAP username and password). This action causes AAA reauthentication to occur according to the personal username/password. The initial rights associated with the local host, though, are still in place. By using the access-profile command, the rights associated with the local host are replaced by or merged with those defined for the user in the user's profile.

To access the rights associated with the user after Double Authentication, perform the following tasks in EXEC configuration mode:

If you configured the access-profile command to be executed as an autocommand, it will be executed automatically after the remote user logs in.

Non-AAA Authentication Methods

This section discusses the following non-AAA authentication tasks:

• Configure Line Password Protection

• Establish Username Authentication

• Enable CHAP or PAP Authentication

• Configure TACACS and Extended TACACS Password Protection

Configure Line Password Protection

You can provide access control on a terminal line by entering the password and establishing password checking. To do so, perform the following tasks in line configuration mode:

The password checker is case sensitive and can include spaces; for example, the password "Secret" is different from the password "secret," and "two words" is an acceptable password.

You can disable line password verification by disabling password checking. To do so, perform the following task in line configuration mode:

If you configure line password protection and then configure TACACS or Extended TACACS, the TACACS username and password take precedence over line passwords. If you have not yet implemented a security policy, we recommend that you use AAA.

Establish Username Authentication

You can create a username-based authentication system, which is useful in the following situations:

• To provide a TACACS-like username and encrypted password-authentication system for networks that cannot support TACACS

• To provide special-case logins; for example, access list verification, no password verification, autocommand execution at login, and "no escape" situation

To establish username authentication, perform the following tasks in global configuration mode as needed for your system configuration:

The keyword noescape prevents users from using escape characters on the hosts to which they are connected. The nohangup feature does not disconnect after using the autocommand.

Caution Passwords will be displayed in clear text in your configuration unless you enable the service password-encryption command. For more information about the service password-encryption command, refer to the "Password and Privileges Commands" chapter in the Security Command Reference.

Enable CHAP or PAP Authentication

One of the most common transport protocols used in Internet Service Providers' (ISPs') dial solutions is the Point-to-Point Protocol (PPP). Traditionally, remote users dial in to an access server to initiate a PPP session. After PPP has been negotiated, remote users are connected to the ISP network and to the Internet.

Enable PPP Encapsulation

To enable PPP encapsulation, perform the following task in interface configuration mode:

Enable PAP or CHAP

To enable CHAP or PAP authentication on an interface configured for PPP encapsulation, perform the following task in interface configuration mode:

If you configure ppp authentication chap on an interface, all incoming calls on that interface that initiate a PPP connection will have to be authenticated using CHAP; likewise, if you configure ppp authentication pap, all incoming calls that start a PPP connection will have to be authenticated via PAP. If you configure ppp authentication chap pap, the access server will attempt to authenticate all incoming calls that start a PPP session with CHAP. If the remote device does not support CHAP, the access server will try to authenticate the call using PAP. If the remote device doesn't support either CHAP or PAP, authentication will fail and the call will be dropped. If you configure ppp authentication pap chap, the access server will attempt to authenticate all incoming calls that start a PPP session with PAP. If the remote device does not support PAP, the access server will try to authenticate the call using CHAP. If the remote device doesn't support either protocols, authentication will fail and the call will be dropped. If you configure the ppp authentication command with the callin keyword, the access server will only authenticate the remote device if the remote device initiated the call.

Authentication method lists and the one-time keyword are only available if you have enabled AAA--they will not be available if you are using TACACS or Extended TACACS. If you specify the name of an authentication method list with the ppp authentication command, PPP will attempt to authenticate the connection using the methods defined in the specified method list. If AAA is enabled and no method list is defined by name, PPP will attempt to authenticate the connection using the methods defined as the default. The ppp authentication command with the one-time keyword enables support for one-time passwords during authentication.

The if-needed keyword is only available if you are using TACACS or Extended TACACS. The ppp authentication command with the if-needed keyword means that PPP will only authenticate the remote device via PAP or CHAP if they have not yet authenticated during the life of the current call. If the remote device authenticated via a standard login procedure and initiated PPP from the EXEC prompt, PPP will not authenticate via CHAP if ppp authentication chap if-needed is configured on the interface.

Caution  If you use a list-name that has not been configured with the aaa authentication ppp command, you disable PPP on the line.

For information about adding a username entry for each remote system from which the local router or access server requires authentication, see the "Establish Username Authentication" section.

Inbound and Outbound Authentication

PPP supports two-way authentication. Normally, when a remote device dials in to an access server, the access server requests that the remote device prove that it is allowed access. This is known as inbound authentication. At the same time, the remote device can also request that the access server prove that it is who it says it is. This is known as outbound authentication. An access server also does outbound authentication when it initiates a call to a remote device.

Enabling Outbound PAP Authentication

To enable outbound PAP authentication, perform the following task in interface configuration mode:

The access server uses the username and password specified by the ppp pap sent-username command to authenticate itself whenever it initiates a call to a remote device or when it has to respond to a remote device's request for outbound authentication.

Create a Common CHAP Password

For remote CHAP authentication only, you can configure your router to create a common CHAP secret password to use in response to challenges from an unknown peer; for example, if your router calls a rotary of routers (either from another vendor, or running an older version of the Cisco IOS software) to which a new (that is, unknown) router has been added. The ppp chap password command allows you to replace several username and password configuration commands with a single copy of this command on any dialer interface or asynchronous group interface.

To enable a router calling a collection of routers to configure a common CHAP secret password, perform the following task in interface configuration mode:

Refuse CHAP Authentication Requests

To refuse CHAP authentication from peers requesting it, meaning that CHAP authentication is disabled for all calls, perform the following task in interface configuration mode:

If the callin keyword is used, the router will refuse to answer CHAP authentication challenges received from the peer, but will still require the peer to answer any CHAP challenges the router sends.

If outbound PAP has been enabled (using the ppp pap sent-username command), PAP will be suggested as the authentication method in the refusal packet.

Delay CHAP Authentication Until Peer Authenticates

To specify that the router will not authenticate to a peer requesting CHAP authentication until after the peer has authenticated itself to the router, perform the following task in interface configuration mode:

This command (which is the default) specifies that the router will not authenticate to a peer requesting CHAP authentication until the peer has authenticated itself to the router. The no ppp chap wait command specifies that the router will respond immediately to an authentication challenge.

Configure TACACS and Extended TACACS Password Protection

You can use TACACS or Extended TACACS to control login access to the router. Perform the tasks in the following sections:

• Set TACACS Password Protection at the User Level

• Disable Password Checking at the User Level

Before performing these tasks, you must have enabled communication with a TACACS host on the network. For more information, refer to the "Configuring TACACS and Extended TACACS" chapter in the Security Configuration Guide.

Set TACACS Password Protection at the User Level

You can enable TACACS password checking at login by performing the following task in line configuration mode:

Disable Password Checking at the User Level

If a TACACS server does not respond to a login request, the Cisco IOS software denies the request by default. However, you can prevent login failure in one of two ways:

• Allow a user to access privileged EXEC mode if that user enters the password set by the enable command.

• Ensure a successful login by allowing the user to access the privileged EXEC mode without further question.

To specify one of these features, perform either of the following tasks in global configuration mode:

Authentication Examples

This section contains the following authentication configuration examples:

• RADIUS Authentication Examples

• TACACS+ Authentication Examples

• TACACS and Extended TACACS Authentication Examples

• Kerberos Authentication Examples

• Double Authentication Configuration Example

RADIUS Authentication Examples

This section provides two sample configurations using RADIUS.

The following example shows how to configure the router to authenticate and authorize using RADIUS:

aaa authentication login radius-login RADIUS local
aaa authentication ppp radius-ppp if-needed radius
aaa authorization exec radius if-authenticated
aaa authorization network radius
line 3
login authentication radius-login
interface serial 0
ppp authentication radius-ppp

The lines in this sample RADIUS authentication and authorization configuration are defined as follows:

• The aaa authentication login radius-login RADIUS local command configures the router to use RADIUS for authentication at the login prompt. If RADIUS returns an error, the user is authenticated using the local database.

• The aaa authentication ppp radius-ppp if-needed radius command configures the Cisco IOS software to use PPP authentication using CHAP or PAP if the user has not already logged in. If the EXEC facility has authenticated the user, PPP authentication is not performed.

• The aaa authorization exec radius if-authenticated command queries the RADIUS database for information that is used during EXEC authorization, such as autocommands and privilege levels, but only provides authorization if the user has successfully authenticated.

• The aaa authorization network radius command queries RADIUS for network authorization, address assignment, and other access lists.

• The login authentication radius-login command enables the use-radius method list for line 3.

• The ppp authentication radius-ppp command enables the user-radius method list for serial interface 0.

The following example shows how to configure the router to prompt for and verify a username and password, authorize the user's EXEC level, and specify it as the method of authorization for privilege level 2. In this example, if a local username is entered at the username prompt, that username is used for authentication.

If the user is authenticated using the local database, EXEC authorization using RADIUS will fail because no data is saved from the RADIUS authentication. The method list also uses the local database to find an autocommand. If there is no autocommand, the user becomes the EXEC user. If the user then attempts to issue commands that are set at privilege level 2, TACACS+ is used to attempt to authorize the command.

aaa authentication local-override
aaa authentication login default radius local
aaa authorization exec radius local
aaa authorization command 2 tacacs+ if-authenticated 

The lines in this sample RADIUS authentication and authorization configuration are defined as follows:

• The aaa authentication local-override command specifies that the username prompt appear before authentication starts, and that the authentication always use the local database if the user has a local account.

• The aaa authentication login default radius local command specifies that the username and password are verified by RADIUS or, if RADIUS is not responding, by the router's local user database.

• The aaa authorization exec radius local command specifies that RADIUS authentication information be used to set the user's EXEC level if the user authenticates with RADIUS. If no RADIUS information is used, this command specifies that the local user database be used for EXEC authorization.

• The aaa authorization command 2 tacacs+ if-authenticated command specifies TACACS+ authorization for commands set at privilege level 2, if the user has already successfully authenticated.

TACACS+ Authentication Examples

The following example configures TACACS+ as the security protocol to be used for PPP authentication:

aaa new-model
aaa authentication ppp test tacacs+ local
interface serial 0
ppp authentication chap pap test
tacacs-server host 10.1.2.3
tacacs-server key goaway

The lines in this sample TACACS+ authentication configuration are defined as follows:

• The aaa new-model command enables the AAA security services.

• The aaa authentication command defines a method list, "test," to be used on serial interfaces running PPP. The keyword tacacs+ means that authentication will be done through TACACS+. If TACACS+ returns an ERROR of some sort during authentication, the keyword local indicates that authentication will be attempted using the local database on the network access server.

• The interface command selects the line.

• The ppp authentication command applies the test method list to this line.

• The tacacs-server host command identifies the TACACS+ daemon as having an IP address of 10.1.2.3.

• The tacacs-server key command defines the shared encryption key to be "goaway."

The following example configures AAA authentication for PPP:

aaa authentication ppp default if-needed tacacs+ local

In this example, the keyword default means that PPP authentication is applied by default to all interfaces. The if-needed keyword means that if the user has already authenticated by going through the ASCII login procedure, then PPP is not necessary and can be skipped. If authentication is needed, the keyword tacacs+ means that authentication will be done through TACACS+. If TACACS+ returns an ERROR of some sort during authentication, the keyword local indicates that authentication will be attempted using the local database on the network access server.

The following example creates the same authentication algorithm for PAP but calls the method list "MIS-access" instead of "default":

aaa authentication pap MIS-access if-needed tacacs+ local
interface serial 0
ppp authentication MIS-access

In this example, since the list does not apply to any interfaces (unlike the default list, which applies automatically to all interfaces), the administrator must select interfaces to which this authentication scheme should apply by using the interface command. The administrator must then apply this method list to those interfaces by using the ppp authentication command.

TACACS and Extended TACACS Authentication Examples

The following example shows TACACS enabled for PPP authentication:

int async 1
 ppp authentication chap
 ppp use-tacacs

The following example shows TACACS enabled for ARAP authentication:

line 3
 arap use-tacacs

Kerberos Authentication Examples

To specify Kerberos as the authentication method, use the following command:

aaa authentication login default krb5

Use the following command to specify Kerberos authentication for PPP:

aaa authentication ppp default krb5

Double Authentication Configuration Example

The examples in this section illustrate possible configurations to be used with Double Authentication. Your configurations could differ significantly, depending on your network and security requirements.

These examples are included:

• Configuring the Local Host for AAA with Double Authentication Examples

• Configuring the AAA Server for First-Stage (PPP) Authentication/Authorization Example

• Configuring the AAA Server for Second-Stage (Per-User) Authentication/Authorization Examples

• Complete Sample Configuration with TACACS+

Configuring the Local Host for AAA with Double Authentication Examples

These two examples configure a local host to use AAA for PPP and login authentication, and for network and EXEC authorization. One example is shown for RADIUS and one example for TACACS+.

In both examples, the first three lines configure AAA, with a specific server as the AAA server. The next two lines configure AAA for PPP and login authentication, and the last two lines configure network and EXEC authorization. The last line is necessary only if the access-profile command will be executed as an autocommand.

Example Router Configuration with a RADIUS AAA

aaa new-model
radius-server host secureserver
radius-server key myradiuskey
aaa authentication ppp default radius
aaa authentication login default radius
aaa authorization network radius
aaa authorization exec radius

Example Router Configuration with a TACACS+ Server

aaa new-model
tacacs-server host security
tacacs-server key mytacacskey
aaa authentication ppp default tacacs+
aaa authentication login default tacacs+
aaa authorization network tacacs+
aaa authorization exec tacacs+

Configuring the AAA Server for First-Stage (PPP) Authentication/Authorization Example

This example shows a configuration on the AAA server. A partial sample AAA configuration is shown for RADIUS.

TACACS+ servers can be configured similarly. (See the "Complete Sample Configuration with TACACS+" section later in this document.)

This example defines authentication/authorization for a remote host named "hostx" that will be authenticated by CHAP in the first stage of Double Authentication. Note that the ACL AV pair limits the remote host to Telnet connections to the local host. The local host has the IP address 10.0.0.2.

Example RADIUS AAA Server Configuration

hostx   Password = "welcome"
        User-Service-Type = Framed-User,
        Framed-Protocol = PPP,
        cisco-avpair = "lcp:interface-config=ip unnumbered ethernet 0",
        cisco-avpair = "ip:inacl#3=permit tcp any 172.21.114.0 0.0.0.255 eq telnet",
        cisco-avpair = "ip:inacl#4=deny icmp any any",
        cisco-avpair = "ip:route#5=55.0.0.0 255.0.0.0",
        cisco-avpair = "ip:route#6=66.0.0.0 255.0.0.0",
        cisco-avpair = "ipx:inacl#3=deny any"

Configuring the AAA Server for Second-Stage (Per-User) Authentication/Authorization Examples

This section contains partial sample AAA configurations on a RADIUS server. These configurations define authentication/authorization for a user (Bob) with the username "bobuser," who will be user-authenticated in the second stage of Double Authentication.

TACACS+ servers can be configured similarly. (See the "Complete Sample Configuration with TACACS+" section later in this document.)

Example RADIUS AAA Server Configurations

Three examples show sample RADIUS AAA configurations that could be used with each of the three forms of the access-profile command.

The first example shows a partial sample AAA configuration that works with the default form (no keywords) of the access-profile command. Note that only ACL AV pairs are defined. This example also sets up the access-profile command as an autocommand.

bobuser         Password = "welcome"
        User-Service-Type = Shell-User,
        cisco-avpair = "shell:autocmd=access-profile"
        User-Service-Type = Framed-User,
        Framed-Protocol = PPP,
        cisco-avpair = "ip:inacl#3=permit tcp any host 10.0.0.2 eq telnet",
        cisco-avpair = "ip:inacl#4=deny icmp any any"

The second example shows a partial sample AAA configuration that works with the access-profile merge form of the access-profile command. This example also sets up the access-profile merge command as an autocommand.

bobuser         Password = "welcome"
        User-Service-Type = Shell-User,
        cisco-avpair = "shell:autocmd=access-profile merge"
        User-Service-Type = Framed-User,
        Framed-Protocol = PPP,
        cisco-avpair = "ip:inacl#3=permit tcp any any"
        cisco-avpair = "ip:route=10.0.0.0 255.255.0.0",
        cisco-avpair = "ip:route=10.1.0.0 255.255.0.0",
        cisco-avpair = "ip:route=10.2.0.0 255.255.0.0"

The third example shows a partial sample AAA configuration that works with the access-profile replace form of the access-profile command. This example also sets up the access-profile replace command as an autocommand.

bobuser         Password = "welcome"
        User-Service-Type = Shell-User,
        cisco-avpair = "shell:autocmd=access-profile replace"
        User-Service-Type = Framed-User,
        Framed-Protocol = PPP,
        cisco-avpair = "ip:inacl#3=permit tcp any any",
        cisco-avpair = "ip:inacl#4=permit icmp any any",
        cisco-avpair = "ip:route=10.10.0.0 255.255.0.0",
        cisco-avpair = "ip:route=10.11.0.0 255.255.0.0",
        cisco-avpair = "ip:route=10.12.0.0 255.255.0.0"

Complete Sample Configuration with TACACS+

This example shows TACACS+ authorization profile configurations both for the remote host (used in the first stage of Double Authentication) and for specific users (used in the second stage of Double Authentication). This TACACS+ example contains approximately the same configuration information as shown in the previous RADIUS examples.

This sample configuration shows authentication/authorization profiles on the TACACS+ server for the remote host "hostx" and for three users, with the usernames "bob_default," "bob_merge," and "bob_replace." The configurations for these three usernames illustrate different configurations that correspond to the three different forms of the access-profile command. The three user configurations also illustrate setting up the autocommand for each form of the access-profile command.

Figure 5 shows the topology. The example following the figure shows a TACACS+ configuration file.

TACACS+ Configuration File

This sample configuration shows authentication/authorization profiles on the TACACS+ server for the remote host "hostx" and for three users, with the usernames "bob_default," "bob_merge," and "bob_replace."

key = "mytacacskey"
default authorization = permit
#-----------------------------Remote Host (BRI)-------------------------
#
# This allows the remote host to be authenticated by the local host
# during fist-stage authentication, and provides the remote host
# authorization profile.
#
#-----------------------------------------------------------------------
user = hostx
{
    login = cleartext "welcome"
    chap = cleartext "welcome"
    service = ppp protocol = lcp {
                interface-config="ip unnumbered ethernet 0"
    }
    service = ppp protocol = ip {
            # It is important to have the hash sign and some string after
            # it. This indicates to the NAS that you have a per-user
            # config.
            inacl#3="permit tcp any 172.21.114.0 0.0.0.255 eq telnet"
            inacl#4="deny icmp any any"
            route#5="55.0.0.0 255.0.0.0"
            route#6="66.0.0.0 255.0.0.0"
    }
    service = ppp protocol = ipx {
            # see previous comment about the hash sign and string, in protocol = ip
            inacl#3="deny any"
    }
}
#------------------- "access-profile" default user "only acls" ----------------
#
# - Without arguments, access-profile removes any access-lists it can find
#  in the old configuration (both per-user and per-interface), and makes sure
#  that the new profile contains ONLY access-list definitions.
#
#--------------------------------------------------------------------------------
user = bob_default
{
        login = cleartext "welcome"
        chap = cleartext "welcome"
        service = exec
        {
                # this is the autocommand that executes when bob_default logs in
                autocmd = "access-profile" 
        }
        service = ppp protocol = ip {
                # Put whatever access-lists, static routes, whatever
                # here.
                # If you leave this blank, the user will have NO IP
                # access-lists (not even the ones installed prior to
                # this)!
                inacl#3="permit tcp any host 10.0.0.2 eq telnet"
                inacl#4="deny icmp any any"
        }
        service = ppp protocol = ipx {
                # Put whatever access-lists, static routes, whatever
                # here.
                # If you leave this blank, the user will have NO IPX
                # access-lists (not even the ones installed prior to
                # this)!
        }
}
#--------------------- "access-profile merge" user  ----------------------
#
# With the 'merge' option, first all old access-lists are removed (as before),
#  but then (almost) all AV pairs are uploaded and installed. This
#  will allow for uploading any custom static routes, sap-filters, and so on,
#  that the user may need in his or her profile. This needs to be used with
#  care, as it leaves open the possibility of conflicting configurations.
#
#-----------------------------------------------------------------------------
user = bob_merge
{
        login = cleartext "welcome"
        chap = cleartext "welcome"
        service = exec
        {
                # this is the autocommand that executes when bob_merge logs in
                autocmd = "access-profile merge"
        }
        service = ppp protocol = ip
        {
                # Put whatever access-lists, static routes, whatever
                # here.
                # If you leave this blank, the user will have NO IP
                # access-lists (not even the ones installed prior to
                # this)!
                inacl#3="permit tcp any any"
                route#2="10.0.0.0 255.255.0.0"
                route#3="10.1.0.0 255.255.0.0"
                route#4="10.2.0.0 255.255.0.0"
        }
        service = ppp protocol = ipx
        {
                # Put whatever access-lists, static routes, whatever
                # here.
                # If you leave this blank, the user will have NO IPX
                # access-lists (not even the ones installed prior to
                # this)!
        }
}
#--------------------- "access-profile replace" user  -----------------------
#
#- With the 'replace' option, 
#  ALL old configuration is removed and ALL new configuration is installed.
#
# One caveat: access-profile checks the new configuration for address-pool and
# address AV pairs. As addresses cannot be renegotiated at this point, the
# command will fail (and complain) when it encounters such an AV pair.
# Such AV pairs are considered to be "invalid" for this context.
#-------------------------------------------------------------------------------
user = bob_replace
{
        login = cleartext "welcome"
        chap = cleartext "welcome"
        service = exec
        {
                # this is the autocommand that executes when bob_replace logs in
                autocmd = "access-profile replace"
        }
        service = ppp protocol = ip
        {
                # Put whatever access-lists, static routes, whatever
                # here.
                # If you leave this blank, the user will have NO IP
                # access-lists (not even the ones installed prior to
                # this)!
                inacl#3="permit tcp any any"
                inacl#4="permit icmp any any"
                route#2="10.10.0.0 255.255.0.0"
                route#3="10.11.0.0 255.255.0.0"
                route#4="10.12.0.0 255.255.0.0"
        }
        service = ppp protocol = ipx
        {
                # put whatever access-lists, static routes, whatever
                # here.
                # If you leave this blank, the user will have NO IPX
                # access-lists (not even the ones installed prior to
                # this)!
        }
}
#----------------------------------------------------------------------------