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Cisco ASA 5500 Series Configuration Guide using the CLI
Chapter38 Configuring AAA Rules for Network Access
Configuring Authorization for Network Access
•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
ASAs.
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 ASA receives downloadable access lists from Cisco Secure ACS using the following process:
1. The ASA 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 includes the internal name of the applicable downloadable access list.
The Cisco IOS cisco-av-pair RADIUS VSA (vendor 9, attribute 1) includes 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 ASA examines the name of the downloadable access list and determines if it has previously
received the named downloadable access list.
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If the ASA has previously received the named downloadable access list, communication with
Cisco Secure ACS is complete and the ASA applies the access list to the user session. Because
the name of the downloadable access list includes the date and time that it was last modified,
matching the name sent by Cisco Secure ACS to the name of an access list previously
downloaded means that the ASA has the most recent version of the downloadable access list.
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If the ASA 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out-of-date version of the access list or it may not have downloaded any version of the access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 ASA 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 ASA signs the request with the Message-Authenticator attribute (IETF RADIUS
attribute 80).
4. After receipt of a RADIUS authentication request that has a username attribute that includes 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 that includes the access list. The largest access list that can fit in a
single access-accept message is slightly less than 4 KB, because part 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 include an ACE and are numbered
serially:
ip:inacl#1=ACE-1