LDAP access control

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The management of what type of access (read, write, etc) users should be granted for resources is known as access control. The configuration directives involved are called access control lists or ACLs.

When we installed the slapd package, various ACLs were set up automatically. We will look at a few important consequences of those defaults and, in so doing, we’ll get an idea of how ACLs work and how they’re configured.

To get the effective ACL for an LDAP query we need to look at the ACL entries of both the database being queried, and those of the special frontend database instance. Note that the ACLs belonging to the frontend database are always appended to the database-specific ACLs, and the first match ‘wins’.

Getting the ACLs

The following commands will give, respectively, the ACLs of the mdb database (dc=example,dc=com) and those of the frontend database:

$ sudo ldapsearch -Q -LLL -Y EXTERNAL -H ldapi:/// -b \
cn=config '(olcDatabase={1}mdb)' olcAccess
    
dn: olcDatabase={1}mdb,cn=config
olcAccess: {0}to attrs=userPassword by self write by anonymous auth by * none
olcAccess: {1}to attrs=shadowLastChange by self write by * read
olcAccess: {2}to * by * read

$ sudo ldapsearch -Q -LLL -Y EXTERNAL -H ldapi:/// -b \
cn=config '(olcDatabase={-1}frontend)' olcAccess
    
dn: olcDatabase={-1}frontend,cn=config
olcAccess: {0}to * by dn.exact=gidNumber=0+uidNumber=0,cn=peercred,cn=external
 ,cn=auth manage by * break
olcAccess: {1}to dn.exact="" by * read
olcAccess: {2}to dn.base="cn=Subschema" by * read

Note:
The Root DN always has full rights to its database and does not need to be included in any ACL.

Interpreting the results

The first two ACLs are crucial:

olcAccess: {0}to attrs=userPassword by self write by anonymous auth by * none
olcAccess: {1}to attrs=shadowLastChange by self write by * read

This can be represented differently for easier reading:

to attrs=userPassword
    by self write
    by anonymous auth
    by * none
    
to attrs=shadowLastChange
    by self write
    by * read

These ACLs enforce the following:

Anonymous ‘auth’ access is provided to the userPassword attribute so that users can authenticate, or bind. Perhaps counter-intuitively, ‘by anonymous auth’ is needed even when anonymous access to the DIT is unwanted, otherwise this would be a chicken-and-egg problem: before authentication, all users are anonymous.
The ‘by self write’ ACL grants write access to the userPassword attribute to users who authenticated as the DN where the attribute lives. In other words, users can update the userPassword attribute of their own entries.
The userPassword attribute is otherwise inaccessible by all other users, with the exception of the Root DN, who always has access and doesn’t need to be mentioned explicitly.
In order for users to change their own password, using passwd or other utilities, the user’s own shadowLastChange attribute needs to be writable. All other directory users get to read this attribute’s contents.

This DIT can be searched anonymously because of to * by * read in this ACL, which grants read access to everything else, by anyone (including anonymous):

to *
    by * read

If this is unwanted then you need to change the ACL. To force authentication during a bind request you can alternatively (or in combination with the modified ACL) use the olcRequire: authc directive.

SASL identity

There is no administrative account (“Root DN”) created for the slapd-config database. There is, however, a SASL identity that is granted full access to it. It represents the localhost’s superuser (root/sudo). Here it is:

dn.exact=gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth

The following command will display the ACLs of the slapd-config database:

$ sudo ldapsearch -Q -LLL -Y EXTERNAL -H ldapi:/// -b \
cn=config '(olcDatabase={0}config)' olcAccess
    
dn: olcDatabase={0}config,cn=config
olcAccess: {0}to * by dn.exact=gidNumber=0+uidNumber=0,cn=peercred,
              cn=external,cn=auth manage by * break

Since this is a SASL identity we need to use a SASL mechanism when invoking the LDAP utility in question – the EXTERNAL mechanism (see the previous command for an example). Note that:

You must use sudo to become the root identity in order for the ACL to match.
The EXTERNAL mechanism works via Interprocess Communication (IPC, UNIX domain sockets). This means you must use the ldapi URI format.

A succinct way to get all the ACLs is like this:

$ sudo ldapsearch -Q -LLL -Y EXTERNAL -H ldapi:/// -b \
cn=config '(olcAccess=*)' olcAccess olcSuffix

Next steps

See how to set up LDAP users and groups.