Cisco ASA 5505 - Manual (Page 823)

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Cisco ASA 5500 Series Configuration Guide using the CLI
Chapter 41 Configuring Digital Certificates
Information About Digital Certificates
The ASA can retrieve CRLs from CAs using HTTP, SCEP, or LDAP. CRLs retrieved for each trustpoint
are cached for a configurable amount of time for each trustpoint.
When the ASA has cached a CRL for longer than the amount of time it is configured to cache CRLs, the
ASA considers the CRL too old to be reliable, or “stale.” The ASA tries to retrieve a newer version of
the CRL the next time that a certificate authentication requires a check of the stale CRL.
The ASA caches CRLs for an amount of time determined by the following two factors:
• The number of minutes specified with the cache-time command. The default value is 60 minutes.
• The NextUpdate field in the CRLs retrieved, which may be absent from CRLs. You control whether
the ASA requires and uses the NextUpdate field with the enforcenextupdate command.
The ASA uses these two factors in the following ways:
• If the NextUpdate field is not required, the ASA marks CRLs as stale after the length of time defined
by the cache-time command.
• If the NextUpdate field is required, the ASA marks CRLs as stale at the sooner of the two times
specified by the cache-time command and the NextUpdate field. For example, if the cache-time
command is set to 100 minutes and the NextUpdate field specifies that the next update is 70 minutes
away, the ASA marks CRLs as stale in 70 minutes.
If the ASA has insufficient memory to store all CRLs cached for a given trustpoint, it deletes the least
recently used CRL to make room for a newly retrieved CRL.
OCSP
OCSP provides the ASA with a way of determining whether a certificate that is within its valid time
range has been revoked by the issuing CA. OCSP configuration is part of trustpoint configuration.
OCSP localizes certificate status on a validation authority (an OCSP server, also called the responder)
which the ASA queries for the status of a specific certificate. This method provides better scalability and
more up-to-date revocation status than does CRL checking, and helps organizations with large PKI
installations deploy and expand secure networks.
Note The ASA allows a five-second time skew for OCSP responses.
You can configure the ASA to make OCSP checks mandatory when authenticating a certificate by using
the revocation-check ocsp command. You can also make the OCSP check optional by using the
revocation-check ocsp none command, which allows the certificate authentication to succeed when the
validation authority is unavailable to provide updated OCSP data.
OCSP provides three ways to define the OCSP server URL. The ASA uses these servers in the following
order:
1. The OCSP URL defined in a match certificate override rule by using the match certificate
command).
2. The OCSP URL configured by using the ocsp url command.
3. The AIA field of the client certificate.
Note To configure a trustpoint to validate a self-signed OCSP responder certificate, you import the self-signed
responder certificate into its own trustpoint as a trusted CA certificate. Then you configure the match
certificate command in the client certificate validating trustpoint to use the trustpoint that includes the
self-signed OCSP responder certificate to validate the responder certificate. Use the same procedure for