There are two phases in granting access to a user. The first
phase is authentication, in which mod_auth_ldap
verifies that the user's credentials are valid. This also called
the search/bind phase. The second phase is
authorization, in which mod_auth_ldap determines
if the authenticated user is allowed access to the resource in
question. This is also known as the compare
During the authentication phase, mod_auth_ldap
searches for an entry in the directory that matches the username
that the HTTP client passes. If a single unique match is found,
then mod_auth_ldap attempts to bind to the
directory server using the DN of the entry plus the password
provided by the HTTP client. Because it does a search, then a
bind, it is often referred to as the search/bind phase. Here are
the steps taken during the search/bind phase.
Generate a search filter by combining the attribute and
filter provided in the AuthLDAPURL directive with
the username passed by the HTTP client.
Search the directory using the generated filter. If the
search does not return exactly one entry, deny or decline
Fetch the distinguished name of the entry retrieved from
the search and attempt to bind to the LDAP server using the
DN and the password passed by the HTTP client. If the bind is
unsuccessful, deny or decline access.
The following directives are used during the search/bind
During the authorization phase, mod_auth_ldap
attempts to determine if the user is authorized to access the
resource. Many of these checks require
mod_auth_ldap to do a compare operation on the
LDAP server. This is why this phase is often referred to as the
compare phase. mod_auth_ldap accepts the
directives to determine if the credentials are acceptable:
The require user directive specifies what
usernames can access the resource. Once
mod_auth_ldap has retrieved a unique DN from the
directory, it does an LDAP compare operation using the username
specified in the require user to see if that username
is part of the just-fetched LDAP entry. Multiple users can be
granted access by putting multiple usernames on the line,
separated with spaces. If a username has a space in it, then it
must be surrounded with double quotes. Multiple users can also be
granted access by using multiple require user
directives, with one user per line. For example, with a AuthLDAPURL of
ldap://ldap/o=Airius?cn (i.e., cn is
used for searches), the following require directives could be used
to restrict access:
require user "Barbara Jenson"
require user "Fred User"
require user "Joe Manager"
Because of the way that mod_auth_ldap handles this
directive, Barbara Jenson could sign on as Barbara
Jenson, Babs Jenson or any other cn that
she has in her LDAP entry. Only the single require
user line is needed to support all values of the attribute
in the user's entry.
If the uid attribute was used instead of the
cn attribute in the URL above, the above three lines
could be condensed to
The require dn directive allows the administrator
to grant access based on distinguished names. It specifies a DN
that must match for access to be granted. If the distinguished
name that was retrieved from the directory server matches the
distinguished name in the require dn, then
authorization is granted.
The following directive would grant access to a specific
The next example is similar to the previous one, but is
uses the common name instead of the UID. Note that this
could be problematical if multiple people in the directory
share the same cn, because a search on cnmust return exactly one entry. That's why
this approach is not recommended: it's a better idea to
choose an attribute that is guaranteed unique in your
directory, such as uid.
Grant access to anybody in the Administrators group. The
users must authenticate using their UID.
require group cn=Administrators, o=Airius
The next example assumes that everyone at Airius who
carries an alphanumeric pager will have an LDAP attribute
of qpagePagerID. The example will grant access
only to people (authenticated via their UID) who have
The next example demonstrates the power of using filters
to accomplish complicated administrative requirements.
Without filters, it would have been necessary to create a
new LDAP group and ensure that the group's members remain
synchronized with the pager users. This becomes trivial
with filters. The goal is to grant access to anyone who has
a filter, plus grant access to Joe Manager, who doesn't
have a pager, but does need to access the same
This last may look confusing at first, so it helps to
evaluate what the search filter will look like based on who
connects, as shown below. The text in blue is the part that
is filled in using the attribute specified in the URL. The
text in red is the part that is filled in using the filter
specified in the URL. The text in green is filled in using
the information that is retrieved from the HTTP client. If
Fred User connects as fuser, the filter would look
The above search will only succeed if fuser has a
pager. When Joe Manager connects as jmanager, the
filter looks like
Normally, FrontPage uses FrontPage-web-specific user/group
files (i.e., the mod_auth module) to handle all
authentication. Unfortunately, it is not possible to just
change to LDAP authentication by adding the proper directives,
because it will break the Permissions forms in
the FrontPage client, which attempt to modify the standard
text-based authorization files.
Once a FrontPage web has been created, adding LDAP
authentication to it is a matter of adding the following
directives to every.htaccess file
that gets created in the web
AuthLDAPURL "the url"
AuthLDAPAuthoritative must be
off to allow mod_auth_ldap to decline group
authentication so that Apache will fall back to file
authentication for checking group membership. This allows the
FrontPage-managed group file to be used.
FrontPage restricts access to a web by adding the require
valid-user directive to the .htaccess
files. If AuthLDAPFrontPageHack is not
on, the require valid-user directive will succeed for
any user who is valid as far as LDAP is
concerned. This means that anybody who has an entry in
the LDAP directory is considered a valid user, whereas FrontPage
considers only those people in the local user file to be
valid. The purpose of the hack is to force Apache to consult the
local user file (which is managed by FrontPage) - instead of LDAP
- when handling the require valid-user directive.
Once directives have been added as specified above,
FrontPage users will be able to perform all management
operations from the FrontPage client.
When choosing the LDAP URL, the attribute to use for
authentication should be something that will also be valid
for putting into a mod_auth user file.
The user ID is ideal for this.
When adding users via FrontPage, FrontPage administrators
should choose usernames that already exist in the LDAP
directory (for obvious reasons). Also, the password that the
administrator enters into the form is ignored, since Apache
will actually be authenticating against the password in the
LDAP database, and not against the password in the local user
file. This could cause confusion for web administrators.
Apache must be compiled with mod_auth in order to
use FrontPage support. This is because Apache will still use
the mod_auth group file for determine the extent of a
user's access to the FrontPage web.
The directives must be put in the .htaccess
files. Attempting to put them inside <Location> or <Directory> directives won't work. This
is because mod_auth_ldap has to be able to grab
directive that is found in FrontPage .htaccess
files so that it knows where to look for the valid user list. If
the mod_auth_ldap directives aren't in the same
.htaccess file as the FrontPage directives, then
the hack won't work, because mod_auth_ldap will
never get a chance to process the .htaccess file,
and won't be able to find the FrontPage-managed user file.
Set to off if this module should let other
authentication modules attempt to authenticate the user, should
authentication with this module fail. Control is only passed on
to lower modules if there is no DN or rule that matches the
supplied user name (as passed by the client).
A bind password to use in conjunction with the bind DN. Note
that the bind password is probably sensitive data, and should be
properly protected. You should only use the AuthLDAPBindDN and AuthLDAPBindPassword if you
absolutely need them to search the directory.
The AuthLDAPCharsetConfig directive sets the location
of the language to charset conversion configuration file. File-path is relative
to the ServerRoot. This file specifies
the list of language extensions to character sets.
Most administrators use the provided charset.conv
file, which associates common language extensions to character sets.
The file contains lines in the following format:
Language-Extensioncharset [Language-String] ...
The case of the extension does not matter. Blank lines, and lines
beginning with a hash character (#) are ignored.
When set, mod_auth_ldap will use the LDAP
server to compare the DNs. This is the only foolproof way to
compare DNs. mod_auth_ldap will search the
directory for the DN specified with the require dn directive, then,
retrieve the DN and compare it with the DN retrieved from the user
entry. If this directive is not set,
mod_auth_ldap simply does a string comparison. It
is possible to get false negatives with this approach, but it is
much faster. Note the mod_ldap cache can speed up
DN comparison in most situations.
This directive specifies which LDAP attributes are used to
check for group membership. Multiple attributes can be used by
specifying this directive multiple times. If not specified,
then mod_auth_ldap uses the member and
When set on, this directive says to use the
distinguished name of the client username when checking for group
membership. Otherwise, the username will be used. For example,
assume that the client sent the username bjenson,
which corresponds to the LDAP DN cn=Babs Jenson,
o=Airius. If this directive is set,
mod_auth_ldap will check if the group has
cn=Babs Jenson, o=Airius as a member. If this
directive is not set, then mod_auth_ldap will
check if the group has bjenson as a member.
If this directive is set to on, the value of the
REMOTE_USER environment variable will be set to the full
distinguished name of the authenticated user, rather than just
the username that was passed by the client. It is turned off by
An RFC 2255 URL which specifies the LDAP search parameters
to use. The syntax of the URL is
For regular ldap, use the
string ldap. For secure LDAP, use ldaps
instead. Secure LDAP is only available if Apache was linked
to an LDAP library with SSL support.
The name/port of the ldap server (defaults to
localhost:389 for ldap, and
localhost:636 for ldaps). To
specify multiple, redundant LDAP servers, just list all
servers, separated by spaces. mod_auth_ldap
will try connecting to each server in turn, until it makes a
Once a connection has been made to a server, that
connection remains active for the life of the
httpd process, or until the LDAP server goes
If the LDAP server goes down and breaks an existing
connection, mod_auth_ldap will attempt to
re-connect, starting with the primary server, and trying
each redundant server in turn. Note that this is different
than a true round-robin search.
The DN of the branch of the
directory where all searches should start from. At the very
least, this must be the top of your directory tree, but
could also specify a subtree in the directory.
The attribute to search for.
Although RFC 2255 allows a comma-separated list of
attributes, only the first attribute will be used, no
matter how many are provided. If no attributes are
provided, the default is to use uid. It's a good
idea to choose an attribute that will be unique across all
entries in the subtree you will be using.
The scope of the search. Can be either one or
sub. Note that a scope of base is
also supported by RFC 2255, but is not supported by this
module. If the scope is not provided, or if base scope
is specified, the default is to use a scope of
A valid LDAP search filter. If
not provided, defaults to (objectClass=*), which
will search for all objects in the tree. Filters are
limited to approximately 8000 characters (the definition of
MAX_STRING_LEN in the Apache source code). This
should be than sufficient for any application.
When doing searches, the attribute, filter and username passed
by the HTTP client are combined to create a search filter that
For example, consider an URL of
a client attempts to connect using a username of Babs
Jenson, the resulting search filter will be