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5.7 The certmap.conf File

Web Server 7 can be configured to allow client authentication through the use of an X.509 digital certificate. This can be performed under the SSL settings for a particular HTTP listener.

When a server receives a request from a client, it can ask for the client’s certificate before proceeding. A client is programmed to respond by sending a client certificate to the server.

After checking that a client certificate chain ends with a trusted Certificate Authority (CA), the Web Server can optionally determine which user is identified by the client certificate and then look up that user’s entry in a directory server. The Web Server authenticates the user by comparing the information in the certificate with the data in the user’s directory entry.

To locate user entries in the directory, a server must know how to interpret certificates from different CAs. You provide the server with interpretation information by means of the certificate mapping configuration file (certmap.conf). This file provides three kinds of information for each listed CA:

  • It maps the distinguished name (DN) in the certificate to a branch point in the LDAP directory.
  • It specifies which DN values from the certificate (username, e-mail address, and so on) the server should use for the purpose of searching the directory.
  • It specifies whether the server should go through an additional verification process. This process involves comparing the certificate presented by the client for authentication with the certificate stored in the user’s directory entry. By comparing the certificate, the server determines whether to allow access or whether to revoke a certificate by removing it from the user’s directory entry.

If more than one directory entry contains the information in the user’s certificate, the server can examine all matching entries to determine which user is trying to authenticate. When examining a directory entry, the server compares the presented certificate with the certificate stored in the entry. If the presented certificate doesn’t match any directory entries or if matching entries don’t contain matching certificates, client authentication fails.

After the server finds a matching entry and certificate in the directory, it can determine the appropriate kind of authorization for the client. For example, some servers use information from a user’s entry to determine group membership, which in turn can be used during evaluation of access control instruction ACIs to determine what resources the user is authorized to access.

5.7.1 File Structure

The certmap.conf file contains information on how a client certificate is mapped to a directory server entry. Example 5.8 demonstrates the certificate mapping configuration file for the default Administration Node.

Example 5.8. Default certmap.conf File

#
# Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
# Use is subject to license terms.
#

#
# This file configures how a certificate is mapped to an LDAP entry.
# See the documentation for more information on this file.
#
# The format of this file is as follows:
#    certmap <name> <issuerDN>
#    <name>:<prop1> [<val1>]
#    <name>:<prop2> [<val2>]
#
# Notes:
#
# 1.  Mapping can be defined per issuer of a certificate.  If mapping
#     doesn't exists for a particular 'issuerDN' then the server uses
#     the default mapping.
#
# 2.  There must be an entry for <name>=default and issuerDN "default".
#     This mapping is the default mapping.
#
# 3.  '#' can be used to comment out a line.
#
# 4.  DNComps & FilterComps are used to form the base DN and filter
#     resp. for performing an LDAP search while mapping the cert to a
#     user entry.
#
# 5.  DNComps can be one of the following:
#    commented out - take the user's DN from the cert as is
#    empty        - search the entire LDAP tree (DN == suffix)
#    attr names    - a comma separated list of attributes to form DN
#
# 6.  FilterComps can be one of the following:
#    commented out - set the filter to "objectclass=*"
#    empty        - set the filter to "objectclass=*"
#    attr names    - a comma separated list of attributes to form the
#                  filter
#

certmap default         default
#default:DNComps
#default:FilterComps    e, uid
#default:verifycert     on
#default:CmapLdapAttr   certSubjectDN
#default:library        <path_to_shared_lib_or_dll>
#default:InitFn       <Init function's name>

5.7.2 Syntax

The file contains one or more named mappings, each applying to a different certificate authority (CA).

Hash symbols (#) at the beginning of a line indicate that the line is a comment. These are ignored by Web Server. Additional lines have the following syntax:

    certmap <name> <issuerDN>
    <name>:<property> [<value>]

The first line uses the name parameter to specify a name for the certificate mapping. It uses the issuerDN parameter to specify the attributes that form the distinguished name found in the CA certificate.

The name attribute is arbitrary; you can call it whatever you want it to be. The issuerDN attribute, however, must exactly match the distinguished name of the CA who issued the client certificate. For example, the following two issuerDN lines differ only in the spaces separating the attributes, but the server treats these two entries as different:

    certmap name ou=Example CA,o=Example,c=US
    certmap name ou=Example CA, o=Example, c=US

The second and subsequent lines of a mapping identify the rules that the server should use when searching the directory for information extracted from a certificate:

    certmap name issuerDN
    name:property1 [value1]
    name:property2 [value2]
    ...

These rules are specified through the use of one or more of the following properties: DNComps, FilterComps, VerifyCert, CmapLdapAttr, Library, and InitFn. Additionally, you could use the certificate API to customize your own properties.

Table 5.4 describes each of the properties contained in the certificate mapping configuration file.

Table 5.4. certmap.conf Properties

Property

Description

DNComps

A list of comma-separated attributes used to determine where in the LDAP directory the server should start searching for entries that match the user’s information (that is, the owner of the client certificate). The server gathers values for these attributes from the client certificate and uses the values to form an LDAP DN, which then determines where the server starts its search in the LDAP directory. For example, if you set DNComps to use the organization (o) and country attributes of the DN, the server starts the search from the o=<organization>, c=<country> entry in the LDAP directory, where <organization> and <country> are replaced with values from the DN in the certificate.

Note the following situations:

  • If there isn’t a DNComps entry in the mapping, the server uses either the CmapLdapAttr setting or the entire subject DN in the client certificate.
  • If the DNComps entry is present but has no value, the server searches the entire LDAP tree for entries matching the filter.

FilterComps

A list of comma-separated attributes used to create a filter by gathering information from the user’s DN in the client certificate. The server uses the values for these attributes to form the search criteria used to match entries in the LDAP directory. If the server finds one or more entries in the LDAP directory that match the user’s information gathered from the certificate, the search is successful and the server optionally performs a verification.

For example, if FilterComps is set to use the e-mail and user ID attributes (FilterComps=e,uid), the server searches the directory for an entry whose values for e-mail and user ID match the end user’s information gathered from the client certificate. E-mail addresses and user IDs are good filters because they are usually unique entries in the directory. The filter needs to be specific enough to match only one entry in the LDAP database.

The following provides a list of valid X.509v3 certificate attributes:

  • c—Country
  • o—Organization
  • cn—Common name
  • l—Location
  • st—State
  • ou—Organizational unit
  • uid—User ID
  • e or mail—E-mail address

Note that the attribute names for the filters must be attribute names from the certificate, not from the LDAP directory. For example, some certificates have an e attribute for the user’s e-mail address; whereas LDAP calls that attribute mail.

verifycert

Tells the server whether it should compare the client’s certificate with the certificate found in the LDAP directory. It takes two values, on and off. You should use this property only if your LDAP directory contains certificates. This feature is useful to ensure your end users have a valid, unrevoked certificate.

CmapLdapAttr

A name for the attribute in the LDAP directory that contains subject DNs from all certificates belonging to the user. The default for this property is certSubjectDN. This attribute isn’t a standard LDAP attribute, so to use this property, you must extend the LDAP schema. Additionally, the search takes place more quickly if the attribute specified by CmapLdapAttr is indexed.

If this property exists in the certmap.conf file, the server searches the entire LDAP directory for an entry whose attribute (named with this property) matches the subject’s full DN (taken from the certificate). If the search doesn’t find any entries, the server retries the search, using the DNComps and FilterComps mappings.

This approach to matching a certificate to an LDAP entry is useful when it’s difficult to match entries using DNComps and FilterComps.

Library

A property whose value is a pathname to a shared library or DLL. You need to use this property only if you create your own properties using the certificate API. For more information, see the Sun Java System Web Server 7.0 Developer’s Guide.

InitFn

A property whose value is the name of an initialization (init) function from a custom library. You need to use this property only if you create your own properties by using the certificate API.

If you are using the Library and InitFn properties, a complete mapping might look like this:

    certmap Example CA       ou=Example CA, o=Example, c=US
    Example CA:CmapLdapAttr  certSubjectDN
    Example CA:DNComps       o, c
    Example CA:FilterComps   e, uid
    Example CA:VerifyCert    on

In this example, the name of the certificate mapping is Example CA, and it is mapped to the issuer’s distinguished name of ou=Example CA, o=Example, c=US.

The CmapLdapAttr property tells the server to search the entire directory for an entry whose certSubjectDN matches the subject’s full DN as listed in the client certificate. If the search doesn’t yield any entries, the server retries the search, using the DNComps and FilterComps mappings.

The DNComps property indicates that the base distinguished name (baseDN) of the search should be created from the organization and country attributes that are extracted from the client certificate.

The FilterComps property indicates that the Web Server should search for entries that match the e-mail and user ID attributes extracted from the client certificate.

The VerifyCert property tells the server whether it should compare the client’s certificate with the certificate found in the user’s directory entry. A value of on ensures that the server does not authenticate the client unless the certificate presented exactly matches the certificate stored in the directory.

5.7.3 Context

Each server instance has its own certificate mapping configuration file; therefore, there is a one-to-one correspondence between the server instance and this file.

5.7.4 Modifications

The Administration Console and command line interface do not provide a method for managing the certificate mapping configuration file. As such, entries must be added to the certmap.conf file directly on a particular Administration Node. After this has been performed, the modifications must be pulled back into the configuration and then pushed out to additional Administration Nodes as appropriate.

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