Cisco ASDM User Guide, 6.1
Applying AAA for Network Access
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Applying AAA for Network Access

Table Of Contents

Applying AAA for Network Access

AAA Performance

Configuring Authentication for Network Access

Information About Authentication

One-Time Authentication

Applications Required to Receive an Authentication Challenge

Security Appliance Authentication Prompts

Static PAT and HTTP

Configuring Network Access Authentication

Enabling the Redirection Method of Authentication for HTTP and HTTPS

Enabling Secure Authentication of Web Clients

Authenticating Directly with the Security Appliance

Authenticating Telnet Connections with a Virtual Server

Authenticating HTTP(S) Connections with a Virtual Server

Configuring the Authentication Proxy Limit

Configuring Authorization for Network Access

Configuring TACACS+ Authorization

Configuring RADIUS Authorization

Configuring a RADIUS Server to Send Downloadable Access Control Lists

Configuring a RADIUS Server to Download Per-User Access Control List Names

Configuring Accounting for Network Access

Using MAC Addresses to Exempt Traffic from Authentication and Authorization


Applying AAA for Network Access


This chapter describes how to enable AAA (pronounced "triple A") for network access.

For information about AAA for management access, see the "Configuring AAA for System Administrators" section on page 16-20.

This chapter includes the following sections:

AAA Performance

Configuring Authentication for Network Access

Configuring Authorization for Network Access

Configuring Accounting for Network Access

Using MAC Addresses to Exempt Traffic from Authentication and Authorization

AAA Performance

The security appliance uses "cut-through proxy" to significantly improve performance compared to a traditional proxy server. The performance of a traditional proxy server suffers because it analyzes every packet at the application layer of the OSI model. The security appliance cut-through proxy challenges a user initially at the application layer and then authenticates against standard AAA servers or the local database. After the security appliance authenticates the user, it shifts the session flow, and all traffic flows directly and quickly between the source and destination while maintaining session state information.

Configuring Authentication for Network Access

This section includes the following topics:

Information About Authentication

Configuring Network Access Authentication

Enabling the Redirection Method of Authentication for HTTP and HTTPS

Enabling Secure Authentication of Web Clients

Authenticating Directly with the Security Appliance

Configuring the Authentication Proxy Limit

Information About Authentication

The security appliance lets you configure network access authentication using AAA servers. This section includes the following topics:

One-Time Authentication

Applications Required to Receive an Authentication Challenge

Security Appliance Authentication Prompts

Static PAT and HTTP

Configuring Network Access Authentication

One-Time Authentication

A user at a given IP address only needs to authenticate one time for all rules and types, until the authentication session expires. (See the Configuration > Firewall > Advanced > Global Timeouts pane for timeout values.) For example, if you configure the security appliance to authenticate Telnet and FTP, and a user first successfully authenticates for Telnet, then as long as the authentication session exists, the user does not also have to authenticate for FTP.

Applications Required to Receive an Authentication Challenge

Although you can configure the security appliance to require authentication for network access to any protocol or service, users can authenticate directly with HTTP, HTTPS, Telnet, or FTP only. A user must first authenticate with one of these services before the security appliance allows other traffic requiring authentication.

The authentication ports that the security appliance supports for AAA are fixed:

Port 21 for FTP

Port 23 for Telnet

Port 80 for HTTP

Port 443 for HTTPS

Security Appliance Authentication Prompts

For Telnet and FTP, the security appliance generates an authentication prompt.

For HTTP, the security appliance uses basic HTTP authentication by default, and provides an authentication prompt. You can optionally configure the security appliance to redirect users to an internal web page where they can enter their username and password (configured on the Configuration > Firewall > AAA Rules > Advanced > AAA Rules Advanced Options dialog box; see the "Enabling the Redirection Method of Authentication for HTTP and HTTPS" section).

For HTTPS, the security appliance generates a custom login screen. You can optionally configure the security appliance to redirect users to an internal web page where they can enter their username and password (configured on the Configuration > Firewall > AAA Rules > Advanced > AAA Rules Advanced Options dialog box; see the "Enabling the Redirection Method of Authentication for HTTP and HTTPS" section).

Redirection is an improvement over the basic method because it provides an improved user experience when authenticating, and an identical user experience for HTTP and HTTPS in both Easy VPN and firewall modes. It also supports authenticating directly with the security appliance.

You might want to continue to use basic HTTP authentication if: you do not want the security appliance to open listening ports; if you use NAT on a router and you do not want to create a translation rule for the web page served by the security appliance; basic HTTP authentication might work better with your network. For example non-browser applications, like when a URL is embedded in email, might be more compatible with basic authentication.

After you authenticate correctly, the security appliance redirects you to your original destination. If the destination server also has its own authentication, the user enters another username and password. If you use basic HTTP authentication and need to enter another username and password for the destination server, then you need to configure virtual HTTP (see the Configuration >Firewall > Advanced Options > Virtual Access pane).


Note If you use HTTP authentication, by default the username and password are sent from the client to the security appliance in clear text; in addition, the username and password are sent on to the destination web server as well. See the "Enabling Secure Authentication of Web Clients" section for information to secure your credentials.


For FTP, a user has the option of entering the security appliance username followed by an at sign (@) and then the FTP username (name1@name2). For the password, the user enters the security appliance password followed by an at sign (@) and then the FTP password (password1@password2). For example, enter the following text.

name> jamiec@patm
password> letmein@he110

This feature is useful when you have cascaded firewalls that require multiple logins. You can separate several names and passwords by multiple at signs (@).

Static PAT and HTTP

For HTTP authentication, the security appliance checks real ports when static PAT is configured. If it detects traffic destined for real port 80, regardless of the mapped port, the security appliance intercepts the HTTP connection and enforces authentication.

For example, assume that outside TCP port 889 is translated to port 80 (www) and that any relevant access lists permit the traffic:

static (inside,outside) tcp 10.48.66.155 889 192.168.123.10 www netmask 255.255.255.255

Then when users try to access 10.48.66.155 on port 889, the security appliance intercepts the traffic and enforces HTTP authentication. Users see the HTTP authentication page in their web browsers before the security appliance allows HTTP connection to complete.

If the local port is different than port 80, as in the following example:

static (inside,outside) tcp 10.48.66.155 889 192.168.123.10 111 netmask 255.255.255.255

Then users do not see the authentication page. Instead, the security appliance sends to the web browser an error message indicating that the user must be authenticated prior using the requested service.

Configuring Network Access Authentication

To enable network access authentication, perform the following steps. For more information about authentication, see the "Information About Authentication" section.


Step 1 From the Configuration > Firewall > AAA Rules pane, choose Add > Add Authentication Rule.

The Add Authentication Rule dialog box appears.

Step 2 From the Interface drop-down list, choose the interface for applying the rule.

Step 3 In the Action field, click one of the following, depending on the implementation:

Authenticate

Do not Authenticate.

Step 4 From the AAA Server Group drop-down list, choose a server group. To add a AAA server to the server group, click Add Server. See the "Configuring AAA Server Groups" section on page 14-9 for more information.

If you chose LOCAL for the AAA server group, you can optionally add a new user by clicking Add User. See the "Adding a User Account" section on page 14-18 for more information.

Step 5 In the Source field, add the source IP address, or click the ellipsis (...) to choose an IP address already defined in ASDM.

Step 6 In the Destination field, enter the destination IP address, or click the ellipsis (...) to choose an IP address already defined in ASDM.

Step 7 In the Service field, enter an IP service name or number for the destination service, or click ellipsis (...) button to choose a service.

Step 8 (Optional) In the Description field, add a description.

Step 9 (Optional) Click More Options to do any of the following:

To specify a source service for TCP or UDP, enter a TCP or UDP service in the Source Service field.

The destination service and source service must be the same. Copy and paste the destination Service field to the Source Service field.

To make the rule inactive, uncheck Enable Rule.

You may not want to remove a rule, but instead turn it off.

To set a time range for the rule, from the Time Range drop-down list, choose an existing time range. To add a new time range, click the ellipsis (...). For more information, see Configuring Time Ranges, page 19-15.

Step 10 Click OK.

The dialog box closes and the rule appears in the AAA Rules table.

Step 11 Click Apply.

The changes are saved to the running configuration.


Enabling the Redirection Method of Authentication for HTTP and HTTPS

This method of authentication enables HTTP(S) listening ports to authenticate network users. When you enable a listening port, the security appliance serves an authentication page for direct connections and, by enabling redirection, for through traffic. This method also prevents the authentication credentials from continuing to the destination server. See the "Security Appliance Authentication Prompts" section for more information about the redirection method versus the basic method.

To enable a AAA listener, perform the following steps:


Step 1 From the Configuration > Firewall > AAA Rules pane, click Advanced.

The AAA Rules Advanced Options dialog box appears.

Step 2 Under Interactive Authentication, click Add.

The Add Interactive Authentication Entry dialog box appears.

Step 3 For the Protocol, choose either HTTP or HTTPS. You can enable both by repeating this procedure and creating two separate rules.

Step 4 From the Interface drop-down list, choose the interface on which you want to enable the listener.

Step 5 From the Port drop-down list, choose the port or enter a number.

This is the port that the security appliance listens on for direct or redirected traffic; the defaults are 80 (HTTP) and 443 (HTTPS). You can use any port number and retain the same functionality, but be sure your direct authentication users know the port number; redirected traffic is sent to the correct port number automatically, but direct authenticators must specify the port number manually.

Step 6 (Optional) Check Redirect network users for authentication request.

This option redirects through traffic to an authentication web page served by the security appliance. Without this option, only traffic directed to the security appliance interface can access the authentication web pages.


Note If you enable the redirect option, you cannot also configure static PAT for the same interface where you translate the interface IP address and the same port that is used for the listener; NAT succeeds, but authentication fails.


Step 7 Click OK, and then click OK to exit the AAA Rules Advanced Options dialog box.

Step 8 Click Apply.


Enabling Secure Authentication of Web Clients

If you use HTTP authentication, by default the username and password are sent from the client to the security appliance in clear text; in addition, the username and password are sent on to the destination web server as well. The security appliance provides several methods of securing HTTP authentication, including the following methods:

Enable the redirection method of authentication for HTTP—See the "Enabling the Redirection Method of Authentication for HTTP and HTTPS" section. This method prevents the authentication credentials from continuing to the destination server.

Enabling Virtual HTTP—Virtual HTTP lets you authenticate separately with the security appliance and with the HTTP server. Even if the HTTP server does not need a second authentication, this feature achieves the effect of stripping the basic authentication credentials from the HTTP GET request. See the "Authenticating HTTP(S) Connections with a Virtual Server" section for more information.

Enabling the Exchange of Usernames and Passwords Using HTTPS—To enable the exchange of usernames and passwords between a web client and the security appliance with HTTPS, perform the following steps:

a. From the Configuration > Firewall > AAA Rules pane, click Advanced. The AAA Rules Advanced Options dialog box appears.

b. Under Secure HTTP, click Enable Secure HTTP.

c. Click OK, and then click OK to exit the AAA Rules Advanced Options dialog box. Click Apply.

This is the only method that protects credentials between the client and the security appliance, as well as between the security appliance and the destination server. You can use this method alone, or in conjunction with either of the other methods so you can maximize your security.

After enabling this feature, when a user requires authentication when using HTTP, the security appliance redirects the HTTP user to an HTTPS prompt. After you authenticate correctly, the security appliance redirects you to the original HTTP URL.

Secured web-client authentication has the following limitations:

A maximum of 16 concurrent HTTPS authentication sessions are allowed. If all 16 HTTPS authentication processes are running, a new connection requiring authentication will not succeed.

When the uauth timeout is set to unlimited, HTTPS authentication might not work. If a browser initiates multiple TCP connections to load a web page after HTTPS authentication, the first connection is let through, but the subsequent connections trigger authentication. As a result, users are continuously presented with an authentication page, even if the correct username and password are entered each time. To work around this, set the uauth timeout to 1 second (see the Configuration > Firewall > Advanced > Global Timeouts pane). However, this workaround opens a 1-second window of opportunity that might allow non-authenticated users to go through the firewall if they are coming from the same source IP address.

Because HTTPS authentication occurs on the SSL port 443, users must not configure an Access Rule to block traffic from the HTTP client to HTTP server on port 443. Furthermore, if static PAT is configured for web traffic on port 80, it must also be configured for the SSL port.

Authenticating Directly with the Security Appliance

If you do not want to allow HTTP, HTTPS, Telnet, or FTP through the security appliance but want to authenticate other types of traffic, you can authenticate with the security appliance directly using HTTP, HTTPS, or Telnet.

Authenticating Telnet Connections with a Virtual Server

Authenticating HTTP(S) Connections with a Virtual Server

Authenticating Telnet Connections with a Virtual Server

Although you can configure network access authentication for any protocol or service (see the "Configuring Authentication for Network Access" section), you can authenticate directly with HTTP, Telnet, or FTP only. A user must first authenticate with one of these services before other traffic that requires authentication is allowed through. If you do not want to allow HTTP, Telnet, or FTP through the security appliance, but want to authenticate other types of traffic, you can configure virtual Telnet; the user Telnets to a given IP address configured on the security appliance, and the security appliance provides a Telnet prompt.

You must configure authentication for Telnet access to the virtual Telnet address as well as the other services you want to authenticate according to the "Configuring Authentication for Network Access" section.

When an unauthenticated user connects to the virtual Telnet IP address, the user is challenged for a username and password, and then authenticated by the AAA server. Once authenticated, the user sees the message "Authentication Successful." Then, the user can successfully access other services that require authentication.

For inbound users (from lower security to higher security), you must also include the virtual Telnet address as a destination interface in the Access Rule applied to the source interface. Moreover, you must add a static NAT rule for the virtual Telnet IP address, even if NAT is not required. An identity NAT rule is typically used (where you translate the address to itself).

For outbound users, there is an explicit permit for traffic, but if you apply an Access Rule to an inside interface, be sure to allow access to the virtual Telnet address. A static NAT rule is not required.

To logout from the security appliance, reconnect to the virtual Telnet IP address; you are prompted to log out.

To enable direct authentication using Telnet, perform the following steps:


Step 1 From the Configuration > Firewall > Advanced > Virtual Access > Virtual Telnet Server area, check the Enable check box.

Step 2 In the Virtual Telnet Server field, add the IP address of the virtual Telnet server.

Make sure this address is an unused address that is routed to the security appliance. For example, if you perform NAT for inside addresses accessing an outside server, and you want to provide outside access to the virtual HTTP server, you can use one of the global NAT addresses for the virtual HTTP server address.

Step 3 Click Apply.

The virtual server is added and the changes are saved to the running configuration.


Authenticating HTTP(S) Connections with a Virtual Server

When you use HTTP authentication on the security appliance (see the"Configuring Authentication for Network Access" section), the security appliance uses basic HTTP authentication by default. You can change the authentication method so that the security appliance redirects HTTP connections to web pages generated by the security appliance itself using the "Configuring HTTP Redirect" section on page 6-4.

However, if you continue to use basic HTTP authentication, then you might need the virtual HTTP server when you have cascading HTTP authentications.

If the destination HTTP server requires authentication in addition to the security appliance, then virtual HTTP lets you authenticate separately with the security appliance (via a AAA server) and with the HTTP server. Without virtual HTTP, the same username and password you used to authenticate with the security appliance is sent to the HTTP server; you are not prompted separately for the HTTP server username and password. Assuming the username and password is not the same for the AAA and HTTP servers, then the HTTP authentication fails.

This feature redirects all HTTP connections that require AAA authentication to the virtual HTTP server on the security appliance. The security appliance prompts for the AAA server username and password. After the AAA server authenticates the user, the security appliance redirects the HTTP connection back to the original server, but it does not include the AAA server username and password. Because the username and password are not included in the HTTP packet, the HTTP server prompts the user separately for the HTTP server username and password.

For inbound users (from lower security to higher security), you must also include the virtual HTTP address as a destination interface in the Access Rule applied to the source interface. Moreover, you must add a static NAT rule for the virtual HTTP IP address, even if NAT is not required. An identity NAT rule is typically used (where you translate the address to itself).

For outbound users, there is an explicit permit for traffic, but if you apply an Access Rule to an inside interface, be sure to allow access to the virtual HTTP address. A static NAT rule is not required.


Note Do not set the uauth timeout duration to 0 seconds when using virtual HTTP, because this setting prevents HTTP connections to the real web server. See the "Configuring Global Timeouts" section on page 27-23.


You can authenticate directly with the security appliance at the following URLs when you enable AAA for the interface:

http://interface_ip[:port]/netaccess/connstatus.html
https://interface_ip[:port]/netaccess/connstatus.html

To allow users to authenticate with the security appliance virtual server separately from the HTTP server, perform the following steps:


Step 1 From the Configuration > Firewall > Advanced > Virtual Access > Virtual HTTP Server area, check the Enable check box.

Step 2 In the Virtual HTTP Server field, add the IP address of the virtual HTTP server.

Make sure this address is an unused address that is routed to the security appliance. For example, if you perform NAT for inside addresses accessing an outside server, and you want to provide outside access to the virtual HTTP server, you can use one of the global NAT addresses for the virtual HTTP server address.

Step 3 (Optional) If you are using text-based browsers, where redirection does not happen automatically, check the Display redirection warning check box. This enables an alert to notify users when the HTTP connection is being redirected.

Step 4 Click Apply.

The virtual server is added and the changes are saved to the running configuration.


Configuring the Authentication Proxy Limit

You can manually configure the uauth session limit by setting the maximum number of concurrent proxy connections allowed per user.

To set the proxy limit, perform the following steps:


Step 1 From the Configuration > Firewall > AAA Rules pane, click Advanced.

The AAA Rules Advanced Options dialog box appears.

Step 2 In the Proxy Limit area, check Enable Proxy Limit.

Step 3 In the Proxy Limit field, enter the number of concurrent proxy connections allowed per user, from 1 to 128.

Step 4 Click OK, and then click Apply.


Configuring Authorization for Network Access

After a user authenticates for a given connection, the security appliance can use authorization to further control traffic from the user.

This section includes the following topics:

Configuring TACACS+ Authorization

Configuring RADIUS Authorization

Configuring TACACS+ Authorization

You can configure the security appliance to perform network access authorization with TACACS+.

Authentication and authorization rules are independent; however, any unauthenticated traffic matched by an authorization rule will be denied. For authorization to succeed:

1. A user must first authenticate with the security appliance.

Because a user at a given IP address only needs to authenticate one time for all rules and types, if the authentication session hasn't expired, authorization can occur even if the traffic is not matched by an authentication rule.

2. After a user authenticates, the security appliance checks the authorization rules for matching traffic.

3. If the traffic matches the authorization rule, the security appliance sends the username to the TACACS+ server.

4. The TACACS+ server responds to the security appliance with a permit or a deny for that traffic, based on the user profile.

5. The security appliance enforces the authorization rule in the response.

See the documentation for your TACACS+ server for information about configuring network access authorizations for a user.

To configure TACACS+ authorization, perform the following steps:


Step 1 Enable authentication. For more information, see the "Configuring Network Access Authentication" section. If you have already enabled authentication, continue to the next step.

Step 2 From the Configuration > Firewall > AAA Rules pane, choose Add > Add Authorization Rule.

The Add Authorization Rule dialog box appears.

Step 3 From the Interface drop-down list, choose the interface for applying the rule.

Step 4 In the Action field, click one of the following, depending on the implementation:

Authorize

Do not Authorize.

Step 5 From the AAA Server Group drop-down list, choose a server group. To add a AAA server to the server group, click Add Server. See the "Configuring AAA Server Groups" section on page 14-9 for more information.

Only TACACS+ servers are supported.

Step 6 In the Source field, add the source IP address, or click the ellipsis (...) to choose an IP address already defined in ASDM.

Step 7 In the Destination field, enter the destination IP address, or click the ellipsis (...) to choose an IP address already defined in ASDM.

Step 8 In the Service field, enter an IP service name or number for the destination service, or click ellipsis (...) button to choose a service.

Step 9 (Optional) In the Description field, add a description.

Step 10 (Optional) Click More Options to do any of the following:

To specify a source service for TCP or UDP, enter a TCP or UDP service in the Source Service field.

The destination service and source service must be the same. Copy and paste the destination Service field to the Source Service field.

To make the rule inactive, uncheck Enable Rule.

You may not want to remove a rule, but instead turn it off.

To set a time range for the rule, from the Time Range drop-down list, choose an existing time range. To add a new time range, click the ellipsis (...). For more information, see Configuring Time Ranges, page 19-15.

Step 11 Click OK.

The dialog box closes and the rule appears in the AAA Rules table.

Step 12 Click Apply.

The changes are saved to the running configuration.


Configuring RADIUS Authorization

When authentication succeeds, the RADIUS protocol returns user authorizations in the access-accept message sent by a RADIUS server. For more information about configuring authentication, see the "Configuring Authentication for Network Access" section.

When you configure the security appliance to authenticate users for network access, you are also implicitly enabling RADIUS authorizations; therefore, this section contains no information about configuring RADIUS authorization on the security appliance. It does provide information about how the security appliance handles access list information received from RADIUS servers.

You can configure a RADIUS server to download an access list to the security appliance or an access list name at the time of authentication. The user is authorized to do only what is permitted in the user-specific access list.


Note If you have enabled the Per User Override Setting (see the Configuration > Firewall > Access Rules > Advanced > Access Rules Advanced Options dialog box), be aware of the following effects of this feature on authorization by user-specific access lists:

Without the per-user-override feature, traffic for a user session must be permitted by both the interface access list and the user-specific access list.

With the per-user-override feature, the user-specific access list determines what is permitted.


This section includes the following topics:

Configuring a RADIUS Server to Send Downloadable Access Control Lists

Configuring a RADIUS Server to Download Per-User Access Control List Names

Configuring a RADIUS Server to Send Downloadable Access Control Lists

This section describes how to configure Cisco Secure ACS or a third-party RADIUS server, and includes the following topics:

About the Downloadable Access List Feature and Cisco Secure ACS

Configuring Cisco Secure ACS for Downloadable Access Lists

Configuring Any RADIUS Server for Downloadable Access Lists

Converting Wildcard Netmask Expressions in Downloadable Access Lists

About the Downloadable Access List Feature and Cisco Secure ACS

Downloadable access lists is the most scalable means of using Cisco Secure ACS to provide the appropriate access lists for each user. It provides the following capabilities:

Unlimited access list size—Downloadable access lists are sent using as many RADIUS packets as required to transport the full access list from Cisco Secure ACS to the security appliance.

Simplified and centralized management of access lists—Downloadable access lists enable you to write a set of access lists once and apply it to many user or group profiles and distribute it to many security appliances.

This approach is most useful when you have very large access list sets that you want to apply to more than one Cisco Secure ACS user or group; however, its ability to simplify Cisco Secure ACS user and group management makes it useful for access lists of any size.

The security appliance receives downloadable access lists from Cisco Secure ACS using the following process:

1. The security appliance sends a RADIUS authentication request packet for the user session.

2. If Cisco Secure ACS successfully authenticates the user, Cisco Secure ACS returns a RADIUS access-accept message that contains the internal name of the applicable downloadable access list. The Cisco IOS cisco-av-pair RADIUS VSA (vendor 9, attribute 1) contains the following attribute-value pair to identify the downloadable access list set:

ACS:CiscoSecure-Defined-ACL=acl-set-name

where acl-set-name is the internal name of the downloadable access list, which is a combination of the name assigned to the access list by the Cisco Secure ACS administrator and the date and time that the access list was last modified.

3. The security appliance examines the name of the downloadable access list and determines if it has previously received the named downloadable access list.

If the security appliance has previously received the named downloadable access list, communication with Cisco Secure ACS is complete and the security appliance applies the access list to the user session. Because the name of the downloadable access list includes the date and time it was last modified, matching the name sent by Cisco Secure ACS to the name of an access list previous downloaded means that the security appliance has the most recent version of the downloadable access list.

If the security appliance has not previously received the named downloadable access list, it may have an out-of-date version of the access list or it may not have downloaded any version of the access list. In either case, the security appliance issues a RADIUS authentication request using the downloadable access list name as the username in the RADIUS request and a null password attribute. In a cisco-av-pair RADIUS VSA, the request also includes the following attribute-value pairs:

AAA:service=ip-admission
AAA:event=acl-download

In addition, the security appliance signs the request with the Message-Authenticator attribute (IETF RADIUS attribute 80).

4. Upon receipt of a RADIUS authentication request that has a username attribute containing the name of a downloadable access list, Cisco Secure ACS authenticates the request by checking the Message-Authenticator attribute. If the Message-Authenticator attribute is missing or incorrect, Cisco Secure ACS ignores the request. The presence of the Message-Authenticator attribute prevents malicious use of a downloadable access list name to gain unauthorized network access. The Message-Authenticator attribute and its use are defined in RFC 2869, RADIUS Extensions, available at http://www.ietf.org.

5. If the access list required is less than approximately 4 KB in length, Cisco Secure ACS responds with an access-accept message containing the access list. The largest access list that can fit in a single access-accept message is slightly less than 4 KB because some of the message must be other required attributes.

Cisco Secure ACS sends the downloadable access list in a cisco-av-pair RADIUS VSA. The access list is formatted as a series of attribute-value pairs that each contain an ACE and are numbered serially:

ip:inacl#1=ACE-1
ip:inacl#2=ACE-2
.
.
.
ip:inacl#n=ACE-n

An example of an attribute-value pair follows:

ip:inacl#1=permit tcp 10.1.0.0 255.0.0.0 10.0.0.0 255.0.0.0

6. If the access list required is more than approximately 4 KB in length, Cisco Secure ACS responds with an access-challenge message that contains a portion of the access list, formatted as described above, and an State attribute (IETF RADIUS attribute 24), which contains control data used by Cisco Secure ACS to track the progress of the download. Cisco Secure ACS fits as many complete attribute-value pairs into the cisco-av-pair RADIUS VSA as it can without exceeding the maximum RADIUS message size.

The security appliance stores the portion of the access list received and responds with another access-request message containing the same attributes as the first request for the downloadable access list plus a copy of the State attribute received in the access-challenge message.

This repeats until Cisco Secure ACS sends the last of the access list in an access-accept message.

Configuring Cisco Secure ACS for Downloadable Access Lists

You can configure downloadable access lists on Cisco Secure ACS as a shared profile component and then assign the access list to a group or to an individual user.

The access list definition consists of one or more security appliance commands that are similar to the extended access-list command, except without the following prefix:

access-list acl_name extended

The following example is a downloadable access list definition on Cisco Secure ACS version 3.3:

+--------------------------------------------+
| Shared profile Components                  |
|                                            |
|      Downloadable IP ACLs Content          |
|                                            |
| Name:    acs_ten_acl                       |
|                                            |
|      ACL Definitions                       |
|                                            |
| permit tcp any host 10.0.0.254             |
| permit udp any host 10.0.0.254             |
| permit icmp any host 10.0.0.254            |
| permit tcp any host 10.0.0.253             |
| permit udp any host 10.0.0.253             |
| permit icmp any host 10.0.0.253            |
| permit tcp any host 10.0.0.252             |
| permit udp any host 10.0.0.252             |
| permit icmp any host 10.0.0.252            |
| permit ip any any                          |
+--------------------------------------------+

For more information about creating downloadable access lists and associating them with users, see the user guide for your version of Cisco Secure ACS.

On the security appliance, the downloaded access list has the following name:

#ACSACL#-ip-acl_name-number

The acl_name argument is the name that is defined on Cisco Secure ACS (acs_ten_acl in the preceding example), and number is a unique version ID generated by Cisco Secure ACS.

The downloaded access list on the security appliance consists of the following lines:

access-list #ACSACL#-ip-asa-acs_ten_acl-3b5385f7 permit tcp any host 10.0.0.254
access-list #ACSACL#-ip-asa-acs_ten_acl-3b5385f7 permit udp any host 10.0.0.254
access-list #ACSACL#-ip-asa-acs_ten_acl-3b5385f7 permit icmp any host 10.0.0.254
access-list #ACSACL#-ip-asa-acs_ten_acl-3b5385f7 permit tcp any host 10.0.0.253
access-list #ACSACL#-ip-asa-acs_ten_acl-3b5385f7 permit udp any host 10.0.0.253
access-list #ACSACL#-ip-asa-acs_ten_acl-3b5385f7 permit icmp any host 10.0.0.253
access-list #ACSACL#-ip-asa-acs_ten_acl-3b5385f7 permit tcp any host 10.0.0.252
access-list #ACSACL#-ip-asa-acs_ten_acl-3b5385f7 permit udp any host 10.0.0.252
access-list #ACSACL#-ip-asa-acs_ten_acl-3b5385f7 permit icmp any host 10.0.0.252
access-list #ACSACL#-ip-asa-acs_ten_acl-3b5385f7 permit ip any any

Configuring Any RADIUS Server for Downloadable Access Lists

You can configure any RADIUS server that supports Cisco IOS RADIUS VSAs to send user-specific access lists to the security appliance in a Cisco IOS RADIUS cisco-av-pair VSA (vendor 9, attribute 1).

In the cisco-av-pair VSA, configure one or more ACEs that are similar to the access-list extended command, except that you replace the following command prefix:

access-list acl_name extended

with the following text:

ip:inacl#nnn=

The nnn argument is a number in the range from 0 to 999999999 that identifies the order of the command statement to be configured on the security appliance. If this parameter is omitted, the sequence value is 0, and the order of the ACEs inside the cisco-av-pair RADIUS VSA is used.

The following example is an access list definition as it should be configured for a cisco-av-pair VSA on a RADIUS server:

ip:inacl#1=permit tcp 10.1.0.0 255.0.0.0 10.0.0.0 255.0.0.0
ip:inacl#99=deny tcp any any
ip:inacl#2=permit udp 10.1.0.0 255.0.0.0 10.0.0.0 255.0.0.0
ip:inacl#100=deny udp any any
ip:inacl#3=permit icmp 10.1.0.0 255.0.0.0 10.0.0.0 255.0.0.0

For information about making unique per user the access lists that are sent in the cisco-av-pair attribute, see the documentation for your RADIUS server.

On the security appliance, the downloaded access list name has the following format:

AAA-user-username

The username argument is the name of the user that is being authenticated.

The downloaded access list on the security appliance consists of the following lines. Notice the order based on the numbers identified on the RADIUS server.

access-list  AAA-user-bcham34-79AD4A08 permit tcp 10.1.0.0 255.0.0.0 10.0.0.0 255.0.0.0
access-list  AAA-user-bcham34-79AD4A08 permit udp 10.1.0.0 255.0.0.0 10.0.0.0 255.0.0.0
access-list  AAA-user-bcham34-79AD4A08 permit icmp 10.1.0.0 255.0.0.0 10.0.0.0 255.0.0.0
access-list  AAA-user-bcham34-79AD4A08 deny tcp any any
access-list  AAA-user-bcham34-79AD4A08 deny udp any any

Downloaded access lists have two spaces between the word "access-list" and the name. These spaces serve to differentiate a downloaded access list from a local access list. In this example, "79AD4A08" is a hash value generated by the security appliance to help determine when access list definitions have changed on the RADIUS server.

Converting Wildcard Netmask Expressions in Downloadable Access Lists

If a RADIUS server provides downloadable access lists to Cisco VPN 3000 series concentrators as well as to the security appliance, you may need the security appliance to convert wildcard netmask expressions to standard netmask expressions. This is because Cisco VPN 3000 series concentrators support wildcard netmask expressions but the security appliance only supports standard netmask expressions. Configuring the security appliance to convert wildcard netmask expressions helps minimize the effects of these differences upon how you configure downloadable access lists on your RADIUS servers. Translation of wildcard netmask expressions means that downloadable access lists written for Cisco VPN 3000 series concentrators can be used by the security appliance without altering the configuration of the downloadable access lists on the RADIUS server.

You configure access list netmask conversion on a per-server basis when you add a server to a server group, on the Configuration > Device Management > Users/AAA > AAA Server Groups > AAA Server Groups area. See the "Adding a Server to a Group" section on page 14-10.

Configuring a RADIUS Server to Download Per-User Access Control List Names

To download a name for an access list that you already created on the security appliance (at the CLI) from the RADIUS server when a user authenticates, configure the IETF RADIUS filter-id attribute (attribute number 11) as follows:

filter-id=acl_name


Note In Cisco Secure ACS, the value for filter-id attributes are specified in boxes in the HTML interface, omitting filter-id= and entering only acl_name.


For information about making unique per user the filter-id attribute value, see the documentation for your RADIUS server.

See the Cisco Security Appliance Command Line Configuration Guide to create an access list on the security appliance.

Configuring Accounting for Network Access

The security appliance can send accounting information to a RADIUS or TACACS+ server about any TCP or UDP traffic that passes through the security appliance. If that traffic is also authenticated, then the AAA server can maintain accounting information by username. If the traffic is not authenticated, the AAA server can maintain accounting information by IP address. Accounting information includes when sessions start and stop, username, the number of bytes that pass through the security appliance for the session, the service used, and the duration of each session.

To configure accounting, perform the following steps:


Step 1 If you want the security appliance to provide accounting data per user, you must enable authentication. For more information, see the "Configuring Network Access Authentication" section. If you want the security appliance to provide accounting data per IP address, enabling authentication is not necessary and you can continue to the next step.

Step 2 From the Configuration > Firewall > AAA Rules pane, choose Add > Add Accounting Rule.

The Add Accounting Rule dialog box appears.

Step 3 From the Interface drop-down list, choose the interface for applying the rule.

Step 4 In the Action field, click one of the following, depending on the implementation:

Account

Do not Account.

Step 5 From the AAA Server Group drop-down list, choose a server group. To add a AAA server to the server group, click Add Server. See the "Configuring AAA Server Groups" section on page 14-9 for more information.

Step 6 In the Source field, add the source IP address, or click the ellipsis (...) to choose an IP address already defined in ASDM.

Step 7 In the Destination field, enter the destination IP address, or click the ellipsis (...) to choose an IP address already defined in ASDM.

Step 8 In the Service field, enter an IP service name or number for the destination service, or click ellipsis (...) button to choose a service.

Step 9 (Optional) In the Description field, add a description.

Step 10 (Optional) Click More Options to do any of the following:

To specify a source service for TCP or UDP, enter a TCP or UDP service in the Source Service field.

The destination service and source service must be the same. Copy and paste the destination Service field to the Source Service field.

To make the rule inactive, uncheck Enable Rule.

You may not want to remove a rule, but instead turn it off.

To set a time range for the rule, from the Time Range drop-down list, choose an existing time range. To add a new time range, click the ellipsis (...). For more information, see Configuring Time Ranges, page 19-15.

Step 11 Click OK.

The dialog box closes and the rule appears in the AAA Rules table.

Step 12 Click Apply.

The changes are saved to the running configuration.


Using MAC Addresses to Exempt Traffic from Authentication and Authorization

The security appliance can exempt from authentication and authorization any traffic from specific MAC addresses.

For example, if the security appliance authenticates TCP traffic originating on a particular network but you want to allow unauthenticated TCP connections from a specific server, you would use a MAC exempt rule to exempt from authentication and authorization any traffic from the server specified by the rule. This feature is particularly useful to exempt devices such as IP phones that cannot respond to authentication prompts.

The order of entries matters, because the packet uses the first entry it matches, as opposed to a best match scenario. If you have a permit entry, and you want to deny an address that is allowed by the permit entry, be sure to enter the deny entry before the permit entry.

To use MAC addresses to exempt traffic from authentication and authorization, perform the following steps:

Step 13 From the Configuration > Firewall > AAA Rules pane, choose Add > Add MAC Exempt Rule.

The Add MAC Exempt Rule dialog box appears.

Step 14 From the Action drop-down list, click one of the following, depending on the implementation:

MAC Exempt

No MAC Exempt

The MAC Exempt option allows traffic from the MAC address without having to authenticate or authorize. The No MAC Exempt option specifies a MAC address that is not exempt from authentication or authorization. You might need to add a deny entry if you permit a range of MAC addresses using a MAC address mask such as ffff.ffff.0000, and you want to force a MAC address in that range to be authenticated and authorized.

Step 15 In the MAC Address field, specify the source MAC address in 12-digit hexadecimal form; that is, nnnn.nnnn.nnnn.

Step 16 In the MAC Mask field, specify the portion of the MAC address that should be used for matching. For example, ffff.ffff.ffff matches the MAC address exactly. ffff.ffff.0000 matches only the first 8 digits.

Step 17 Click OK.

The dialog box closes and the rule appears in the AAA Rules table.

Step 18 Click Apply.

The changes are saved to the running configuration.