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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.
Further reading
- See the man page for slapd.access.
- The access control topic in the OpenLDAP administrator’s guide.