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The access service security paradigm presented in this guide uses the authentication, authorization, and accounting (AAA) facility. Authentication requires dialin users to prove they are who they say they are. When you require authentication before users can access your network, you are preventing users from either accessing lines on the access server or connecting through the lines directly to network resources. You need to secure every access point.
Authorization prevents each user from gaining access to services and devices on the network that they do not need to or are not supposed to access. Accounting provides records of who is connected and how long they have been connected for billing and other recording purposes. This chapter does not describe how to configure accounting.
This chapter describes how to configure security using a local database resident on the access server or using a remote security database for TACACS+ and RADIUS. To understand the concept of local versus remote authentication, refer to the section "Local Versus Remote Server Authentication" later in this chapter.
 | Caution This chapter does not provide an exhaustive security overview. For example, it does not describe how to configure TACACS, Extended TACACS, Kerberos, or access lists. It presents the most commonly used security mechanisms to prevent unauthenticated and unauthorized access to network resources through Cisco access servers. For a comprehensive overview of Cisco security mechanisms, refer to the Security Configuration Guide. |
Specifically, this chapter describes the following:
This chapter assumes the following:
This section describes the differences between local and remote security databases and the basic authentication process for each. Remote security databases described in this chapter include Terminal Access Controller Access Control System with Cisco proprietary enhancements (TACACS+) and Remote Authentication Dial-In User Service (RADIUS).
Generally the size of the network and type of corporate security policies and control determines whether you use a local or remote security database.
If you have one or two access servers providing access to your network, you probably want to store username and password security information on the Cisco access server. This is referred to as local authentication. (See Figure 3-1.)
A local security database is useful if you have very few access servers providing network access. A local security database does not require a separate (and costly) security server.
As your network grows, you need a centralized security database that provides username and password information to each of the access servers on the network. This centralized security database resides in a security server. (See Figure 3-2.)
An example of a remote security database server is the CiscoSecure product from Cisco Systems, Inc. CiscoSecure is a UNIX security daemon solution, with which the administrator creates a database that defines the network users and their privileges. CiscoSecure uses a central database that stores user and group profiles with authentication and authorization information.
The Cisco access server exchanges user authentication information with a TACACS+ or RADIUS database on the security server by transmitting encrypted TACACS+ or RADIUS packets across the network.
For specific information about the interaction between the security server and the access server, refer to the Security Configuration Guide.
A remote, centralized security database is useful when you have a large number of access servers providing network access. It prevents having to update each access server with new or changed authentication and authorization information for potentially hundreds of thousands of dialin network users. A centralized security database also helps establish consistent remote access policies throughout a corporation.
Using the AAA facility, you can authenticate users with either a local or a remote security database. For more information about what a local and remote security database are, refer to the previous section "Local Versus Remote Server Authentication."
Whether you maintain a local or remote security database, or use TACACS+ or RADIUS authentication and authorization, the process of configuring the access server for these different databases and protocols is similar. The basic process of configuring the Cisco IOS software for authentication requires the following tasks:
The first thing you secure is access to privileged EXEC (enable) mode. Enable mode provides access to configuration mode, which enables any type of configuration change to the access server. To secure Privileged EXEC mode, use one of the commands listed in Table 3-1:
| Command | Purpose |
|---|---|
| enable password password | Requires that network administrators enter a password to access privileged EXEC mode. Do not provide access to non administrators. |
| enable secret password | Specifies a secret password that is encrypted, so that the password cannot be read when crossing a network. After you issue this command, the encryption cannot be reversed. The encrypted version of the password appears in output of the show running-config and show startup-config commands. The enable secret password has precedence over the enable password. Do not enter the same password as the enable password. If the two passwords are the same, the enable secret password is not a secret, because the enable password appears in output of show running-config and show startup-config commands. |
| Caution If you use the enable secret command and specify an encryption type, you must enter the encrypted version of a specific password. Do not enter the cleartext version of the password after specifying an encryption type. You must comply with the following procedure when you specify an encryption type or you will be locked irretrievably out of privileged EXEC (enable) mode. The only way to regain access to privileged EXEC mode will be to erase the contents of NVRAM, erase your entire configuration, and reconfigure the router again. |
To enter an encryption type with the enable secret command, perform the following steps:
Step 1 From within global configuration mode, enter the enable secret command, followed by the cleartext password that you will use to gain access to privileged EXEC mode. Do not specify an encryption type.
Step 2 Exit from global configuration mode and enter the command show running-config to view the encrypted version of the password. The following example illustrates these first two steps:
enable secret mypassword
exit
show running-config
Step 3 At this point, select and copy the encrypted password following enable secret 5 in the configuration output ($1$h7dd$VTNs4.BAfQMUU0Lrvw6570).
Step 4 Enter global configuration mode and enter the enable secret command, followed by the encryption type (5 is the only valid encryption type for enable secret), then paste in the encrypted version of the password, as shown in the following example:
enable secret 5 $1$h7dd$VTNs4.BAfQMUU0Lrvw6570
Step 5 Exit from global configuration mode and copy the running configuration to NVRAM.
exit
copy running-config startup-config
You can also specify additional protection for privileged EXEC mode, including the following:
For more information about these security tools, refer to the Security Configuration Guide in the Cisco IOS configuration guides and command references documentation.
This section describes the Cisco IOS software commands that enable the access server to communicate with a security server. This process is similar for communicating with TACACS+ and RADIUS servers, and the following sections describe the process.
If you are using local authentication, you can refer to the section "Enabling AAA Globally on the Access Server."
If you are using a remote security server for authentication and authorization, you must configure the security server before performing the tasks described in this chapter. The section "Security Examples" at the end of this chapter shows some typical TACACS+ and RADIUS server entries corresponding to the access server security configurations.
To enable communication between the TACACS+ security (database) server and the access server, issue the commands listed in Table 3-2 in global configuration mode.
| Command | Purpose |
|---|---|
| tacacs-server host {hostname | ip-address} | Specifies the IP address or the host name of the remote TACACS+ server host. This host is typically a UNIX system running TACACS+ software. |
| tacacs-server key shared-secret-text-string | Specifies a shared secret text string used between the access server and the TACACS+ server. The access server and TACACS+ server use this text string to encrypt passwords and exchange responses. |
For example, to enable the remote TACACS+ server to communicate with the access server, enter the commands as follows:
2511#configure terminal2511(config)#tacacs-server host alcatraz2511(config)#tacacs-server key abra2cad
The host name of the TACACS+ server in the previous example is alcatraz. The key (abra2cad) in the previous example is the encryption key shared between the TACACS+ server and the access server.
For more information about these commands, refer to the Security Command Reference, which is part of the Cisco IOS configuration guides and command references documentation.
To enable communication between the RADIUS security (database) server and the access server, issue the commands listed in Table 3-3 in global configuration mode.
| Command | Purpose |
|---|---|
| radius-server host {hostname | ip-address} | Specifies the IP address or the host name of the remote RADIUS server host. This host is normally a UNIX system running RADIUS software. |
| radius-server key shared-secret-text-string | Specifies a shared secret text string used between the router and the RADIUS server. The router and RADIUS server use this text string to encrypt passwords and exchange responses. |
For example, to enable the remote RADIUS server to communicate with the access server, enter the commands as follows:
2511#configure terminal2511(config)#radius-server host alcatraz2511(config)#radius-server key abra2cad
The host name of the RADIUS server in the previous example is alcatraz. The key (abra2cad) in the previous example is the encryption key shared between the RADIUS server and the access server.
You can use any of the following optional commands to interact with the RADIUS server host:
For more information about these commands, refer to the Security Command Reference, which is part of the Cisco IOS configuration guides and command references documentation.
On most TACACS+ security servers, there are three ways to authenticate a user for login:
The following is the configuration for global authentication:
user = mswartz {
global = cleartext "mswartz global password"
}
To assign different passwords for ARAP, CHAP, and a normal login, you must enter a string for each user that specifies the security protocols, whether the password is cleartext, and if it authentication is performed via a DES card. The following example shows a user carol, who has authentication configured for ARAP, CHAP, and login. Her ARAP and CHAP passwords, "arap password" and "chap password", are shown in cleartext. Her login password has been encrypted.
user = carol {
arap = cleartext "arap password"
chap = cleartext "chap password"
login = des XQj4892fjk
}
The default authentication is to deny authentication. You can change this at the top level of the configuration file to have the default use passwd(5) file, by issuing the following command:
default authentication = /etc/passwd
user= fred {
login = skey
}
On the access server, you configure authentication on all lines including the VTY and Console lines by entering the following commands, beginning in privileged EXEC mode:
2511#configure terminal2511(config)#aaa new-model2511(config)#aaa authentication login default tacacs+enable
| Caution When you issue the aaa authentication login default tacacs+ enable command, you are specifying that if your TACACS+ server fails to respond (because it is set up incorrectly), you can log in to the access server by using your enable password. If you do not have an enable password set on the router, you will not be able to log in to it until you have a functioning TACACS+ daemon configured with usernames and passwords. The enable password in this case is a last-resort authentication method. You also can specify none as the last-resort method, which means that no authentication is required if all other methods failed. |
To use the AAA security facility in the Cisco IOS software, you must issue the aaa new-model command from global configuration mode.
When you issue the aaa new-model command, all lines on the access server receive the implicit login authentication default method list, and all interfaces with PPP enabled have an implicit ppp authentication pap default method list applied.
| Caution If you intend to authenticate users via a security server, make sure you do not inadvertently lock yourself out of the access server ports after you issue the aaa new-model command. Enter line configuration mode and issue the aaa authentication login default tacacs+ enable global configuration command. This command specifies that if your TACACS+ (or RADIUS) server is not functioning properly, you can enter your enable password to log in to the access server. In general, make sure you have a last-resort access method before you are certain that your security server is set up and functioning properly. For more information about the aaa authentication command, refer to the "Defining Authentication Method Lists" section. |
For example, enter the following commands to enable AAA in the Cisco IOS software:
2511#configure terminal2511(config)#aaa new-model
After you enable AAA globally on the access server, you need to define authentication method lists, which you then apply to lines and interfaces. These authentication method lists are security profiles that indicate the protocol (ARAP or PPP) or login and authentication method (TACACS+, RADIUS, or local authentication).
To define an authentication method list, perform the following steps, which are described in this section:
After you define these authentication method lists, you apply them to one of the following:
The section "Applying Authentication Method Lists" describes how to apply these lists.
To define an authentication method list, start by issuing the aaa authentication global configuration command, as shown in the following example:
2511#configure terminal2511(config)#aaa authentication
After you issue aaa authentication, you must specify one of the following dialin protocols as applicable for your network:
You can specify only one dialin protocol per authentication method list. However, you can create multiple authentication method lists with each of these options. You must give each list a different name, as described in the next section "3. Identify a List Name."
If you specify the ppp option, the default authentication method for PPP is PAP. For greater security, specify CHAP. The full command is aaa authentication ppp chap. If you specify the arap option, the authentication method built into ARA is used. The full command is aaa authentication arap.
For example, if you specify PPP authentication, the configuration thus far looks like this:
2511#configure terminal2511(config)#aaa authentication ppp
A list name identifies each authentication list. You can choose either to use the keyword default, or choose any other name that describes the authentication list. For example, you might give it the name isdn-radius if you intend to apply it to interfaces configured for ISDN and RADIUS authentication. The list name can be any alphanumeric string. Use default as the list name for most lines and interfaces, and use different names on an exception basis.
You can create different authentication method lists and apply them to lines and interfaces selectively. You can even create a named authentication method list that you do not apply to a line or interface, but which you intend to apply at some later point, such as when you deploy a new login method for users.
After you define a list name, you must identify additional security attributes (such as local authentication versus TACACS+ or RADIUS).
In the following example, the default authentication method list for PPP dialin clients uses the local security database.
2511#configure terminal2511(config)#aaa authentication ppp default
In the following example, the PPP authentication method list name is insecure.
2511#configure terminal2511(config)#aaa authentication ppp insecure
In the following example, the ARA authentication method list name is callback (because asynchronous callback is used on the access server).
2511#configure terminal2511(config)#aaa authentication arap callback
In the following example, the login authentication method list name is deveng.
2511#configure terminal2511(config)#aaa authentication login deveng
After you identify a list name, you must specify an authentication method. An authentication method identifies how users are authenticated. For example, will users be authenticated by a local security database resident on the access server (local method)? Will they be authenticated by a remote security database, such as by a TACACS+ or RADIUS daemon? Will guest access to an AppleTalk network be permitted?
Authentication methods are defined with optional keywords in the aaa authentication command. The available authentication methods for PPP are described in Table 3-4. The available authentication methods for ARA are described in Table 3-5.
| Authentication Methods for PPP | Purpose |
|---|---|
| if-needed | Authenticates only if not already authenticated. No duplicate authentication. |
| krb5 | Specifies Kerberos 5 authentication. |
| local | Uses the local username database in the access server. This is defined with the username global configuration command. |
| none | No authentication is required. Do not prompt for a username or password. |
| radius | Use RADIUS authentication as defined on a RADIUS security server. |
| tacacs+ | Use TACACS+ authentication as defined on a TACACS+ security server. |
 | Time Saver If you are not sure whether you should use TACACS+ or RADIUS, here are some comparisons: TACACS+ encrypts the entire payload of packets passed across the network, whereas RADIUS only encrypts the password when it crosses the network. TACACS+ can query the security server multiple times, whereas a RADIUS server gives one response only and is therefore not as flexible regarding per-user authentication and authorization attempts. Moreover, RADIUS does not support authentication of ARA. |
| Authentication Methods for ARA | Purpose |
|---|---|
| auth-guest | Allows guests to log in only if they have already been authenticated at the EXEC. |
| guest | Allows guests to log in. |
| line | Uses the line (login) password for authentication. |
| local | Uses the local username database in the access server for authentication. This database is defined with the username global configuration command. |
| tacacs+ | Use TACACS+ authentication as defined on a TACACS+ security server. |
You can specify multiple authentication methods for each authentication list. The following example authentication method list for PPP first queries a TACACS+ server, then a RADIUS server, then the local security database. Multiple authentication methods can be useful if you have multiple types of security servers on the network and one or more types of security server do not respond:
2511(config)# aaa authentication ppp testbed tacacs+ radius local
If you specify more than one authentication method and the first method (TACACS+ in the previous example) is not available, the Cisco IOS software attempts to authenticate using the next method (such as RADIUS). If in the previous example the RADIUS server has no information about the user, or if no RADIUS server can be found, the user is authenticated using the local username database that was populated with the username command.
However, if authentication fails using the first method listed, the Cisco IOS software does not permit access. It does not attempt to authenticate using the subsequent security methods if the user entered the incorrect password.
If you specify local as the security method, you must specify username profiles for each user who might log in. An example of specifying local authentication is as follows:
2511(config)# aaa authentication login deveng local
This command specifies that any time a user attempts to log in to a line on an access server, the Cisco IOS software checks the username database. To create a local username database, define username profiles using the username global configuration command.
The following example shows how to use the username command for a user jnieters with password n1vriti:
2511(config)# username jnieters password n1vriti
The show running-config command shows the encrypted version of the password, as follows:
2511# show running-config
Building configuration...
Current configuration:
!
version 11.1
! most of config omitted
username jnieters password 7 0215055500070C294D
This section shows some examples of authentication lists.
The following example creates a local authentication list for users logging in to any line on the access server.
2511(config)# aaa authentication login default local
The following example specifies login authentication using RADIUS (the RADIUS daemon is polled for authentication profiles):
2511(config)# aaa authentication login default radius
The following example specifies login authentication using TACACS+ (the TACACS+ daemon is polled for authentication profiles):
2511(config)# aaa authentication login default tacacs+
The following example creates a local authentication list for Macintosh users dialing in to an AppleTalk network through the access server.
2511(config)# aaa authentication arap default local
The following example specifies that Macintosh users dialing into an AppleTalk network through the access server be authenticated by a TACACS+ daemon:
2511(config)# aaa authentication arap default tacacs+
The following example creates an authentication method list that does the following:
2511(config)# aaa authentication arap default auth-guest tacacs+ line local
The following example creates a TACACS+ authentication list for users connecting to interfaces (such as ISDN BRI or asynchronous interfaces) configured for dialin using PPP. The name of the list is marketing. This example specifies that a remote TACACS+ daemon be used as the security database. If this security database is not available, the Cisco IOS software then polls the RADIUS daemon. Users are not authenticated if they are already authenticated on a TTY line.
2511(config)# aaa authentication ppp marketing if-needed tacacs+ radius
In this example, default can be substituted for marketing if the administrator wants this list to be the default list.
As described in the "Defining Authentication Method Lists" section, the aaa authentication global configuration command creates authentication method lists or profiles. You apply these authentication method lists to lines or interfaces by issuing the login authentication, arap authentication, or ppp authentication command, as described in Table 3-6.
You can create more than one authentication list or profile for login and protocol authentication and apply them to different lines or interfaces. The following examples show the line or interface authentication commands that correspond to the aaa authentication global configuration command.
The following example shows the default login authentication list applied to the console port and the default virtual terminal (VTY) lines on the access server:
2511(config)#aaa authentication login default local2511(config)#line console 02511(config-line)#login authentication default2511(config-line)#line vty 0 42511(config-line)#login authentication default
In the following example, the login authentication list named rtp2-office, which uses RADIUS authentication, is created. It is applied to all 40 lines on a Cisco 2509 access server, including the console (CTY) port, the 8 physical asynchronous (TTY) lines, the auxiliary (AUX) port, and 30 virtual terminal (VTY) lines:
2509(config)#aaa authentication login rtp2-office radius2509(config)#line 0 392509(config-line)#login authentication rtp2-office
The following sample output shows lines and their status on the access server:
2509#sho line
Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns
* 0 CTY - - - - - 0 0 0/0
* 1 TTY 57600/57600 - inout - - - 0 0 0/0
...
I 8 TTY 115200/115200 - inout - - - 0 0 0/0
9 AUX 38400/38400 - - - - - 0 0 0/0
10 VTY - - - - - 0 0 0/0
...
39 VTY - - - - - 0 0 0/0
In the following example, the ARA authentication list bldg-d-list is created, then applied to lines 1 through 16 (the physical asynchronous lines) on a Cisco 2511 access server:
2511(config)#aaa authentication arap bldg-d-list auth-guest tacacs+2511(config)#line 1 162511(config-line)#arap authentication bldg-d-list
The following example creates the PPP authentication list marketing, which uses TACACS+, then RADIUS authentication. The list marketing requires authentication only if the user has not already been authenticated on another line. It is then applied to asynchronous lines 1 through 48 on a Cisco AS5200 access server and uses CHAP authentication, instead of the default of PAP:
AS5200(config)#aaa authentication ppp marketing if-needed tacacs+ radiusAS5200(config)#line 1 48AS5200(config-line)#ppp authentication chap marketing
You can configure the access server to restrict user access to the network so that users can only perform certain functions after successful authentication. As with authentication, authorization can be used with either a local or remote security database. This guide describes only remote security server authorization.
A typical configuration most likely uses the EXEC facility and network authorization. EXEC authorization restricts access to the EXEC, and network authorization restricts access to network services, including PPP and ARA.
Authorization must be configured on both the access server and the security daemon. The default authorization is different on the access server and the security server:
| Time Saver If authentication has not been set up for a user, per-user authorization attributes are not enabled for that user. That is, if you want a user to authorize himself before he has access to network resources, you must first require that the user authenticate himself. For example, if you want to specify the aaa authorization network tacacs+ (or radius) command, you must first specify the aaa authentication {ppp | arap} default if-needed tacacs+ (or radius) command. |
You typically have three methods for configuring default authorization on the security server:
default authorization = permit
default service = permit
(a) If the AV pair from the access server is mandatory, look for an exact match in the daemon's mandatory list. If found, add the AV pair to the output.
(b) If an exact match doesn't exist. look in the daemon's optional list for the first attribute match. If found, add the access server AV pair to the output.
(c) If no attribute match exists, deny the command if the default is to deny, or if the default is permit, add the access server AV pair to the output.
(d) If the AV pair from the access server is optional, look for an exact attribute, value match in the mandatory list. If found, add the daemon's AV pair to output.
(e) If not found, look for the first attribute match in the mandatory list. If found, add daemon's AV pair to output.
(f) If no mandatory match exists, look for an exact attribute, value pair match among the daemon's optional AV pairs. If found add the daemon's matching AV pair to the output.
(g) If no exact match exists, locate the first attribute match among the daemon's optional AV pairs. If found add the daemon's matching AV pair to the output.
(h) If no match is found, delete the AV pair if default is deny, or if the default is permit, add the access server AV pair to the output.
(i) If there is no attribute match already in the output list after all AV pairs have been processed for each mandatory daemon AV pair, add the AV pair (add only one AV pair for each mandatory attribute).
To specify network authorization, which means that you are preventing unauthorized users from accessing network resources, issue the aaa authorization network command. To restrict users from logging into the EXEC facility, issue the aaa authorization exec command. See the following example:
2511(config)#aaa authorization network2511(config)#aaa authorization exec
Authorization methods are defined as optional keywords in the aaa authorization command. You can specify any of the authorization methods listed in Table 3-7 for both network and EXEC authorization.
| Authorization Methods | Purpose |
|---|---|
| if-authenticated | User is authorized if already authenticated. |
| local | Uses the local database for authorization. The local database is created using the username privilege command to assign users to a privilege level from 0 to 15 and the privilege level command to assign commands to these different levels. |
| none | Authorization always succeeds. |
| radius | Uses RADIUS authorization as defined on a RADIUS server. |
| tacacs+ | Uses TACACS+ authorization as defined on a TACACS+ server. |
When you configure authorization, you must ensure that the parameters established on the access server correspond with those set on the TACACS+ server.
The following example uses a TACACS+ server to authorize the use of network services, including PPP and ARA. If the TACACS+ server is not available or has no information about a user, no authorization is performed, and the user can use all network services:
2511(config)# aaa authorization network tacacs+ none
The following example permits the user to run the EXEC process if the user is already authenticated. If the user is not already authenticated, the Cisco IOS software defers to a RADIUS server for authorization information.
2511(config)# aaa authorization exec if-authenticated radius
The following example configures network authorization. If the TACACS+ server does not respond or has no information about the username being authorized, the RADIUS server is polled for authorization information for the user. If the RADIUS server does not respond, the user still can access all network resources without authorization requirements.
2511(config)# aaa authorization network tacacs+ radius none
This series of examples shows complete security configuration components of a configuration file on an access server. Each of these examples shows authentication and authorization.
This sample configuration uses AAA to configure default authentication using a local security database on the access server. All lines and interfaces have the default authentication lists applied. Users judithn, jnieters, and enieters have been assigned privilege level 7, which prevents them from issuing the ppp arap, and slip commands, because these commands have been assigned to privilege level 8.
aaa new-model aaa authentication login default local aaa authentication arap default local aaa authentication ppp default local aaa authorization exec local aaa authorization network local aaa authorization ! username judithn privilege exec level 7 privilege network level 8 password 7 095E470B1110 username jnieters privilege network level 7 password 7 0215055500070C294D username enieters privilege network level 7 password 7 095E4F10140A1916 ! privilege exec level 8 ppp privilege exec level 8 arap privilege exec level 8 slip line console 0 login authentication default ! line 1 16 arap authentication default ! interface Group-Async1 ppp authentication chap default group-range 1 16
With this configuration, the sign-on dialog from a remote PC appears as follows:
atdt5551234 CONNECT 14400/ARQ/V32/LAPM/V42BIS User Access Verification Username:judithnPassword: Router>enablePassword: Router#
The following example shows how to create and apply the following authentication lists:
hostname 2511 ! tacacs-server host dog-house tacacs-server key shepard4 ! aaa authentication login rtp2-office tacacs+ aaa authentication ppp marketing if-needed tacacs+ aaa authentication arap los-banos-office tacacs+ ! line console0 login authentication rtp2-office ! interface group-async0 ppp authentication chap marketing group-range 1 16 ! line 1 16 arap authentication los-banos-office !
The following example shows how to create the following authentication lists:
radius-server host pig-pen radius-server key BaBe218 ! privilege exec level 14 configure privilege exec level 14 reload privilege exec level 8 arap privilege exec level 8 ppp ! aaa authentication login fly radius aaa authentication ppp maaaa if-needed radius aaa authorization network radius aaa authorization exec radius ! line 1 39 login authentication fly ! interface group-async658 ppp authentication chap maaaa group-range 1 16 !
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