Referrals

Referrals point directory clients to another directory container, which can be another directory server running elsewhere, or another container on the same server. The client receiving a referral must use the other container to complete the request.

Some clients follow referrals on your behalf by default. The DS commands do not follow referrals.

Referrals are used, for example, when directory data is temporarily unavailable due to maintenance. Referrals can also be used when a container holds only some of the directory data for a base DN, and points to other containers for branches whose data is not available locally.

Referrals are entries with https://tools.ietf.org/html/rfc4516ref attribute values that point elsewhere. The ref attribute type is required by the referral object class. The referral object class is structural. By default, you cannot add it to an entry that already has a structural object class defined. (When adding a ref attribute to an entry with an existing structural object class, use the extensibleObject auxiliary object class.)

DS servers return referrals to requests that target the affected entry or its child entries. Client applications must be capable of following the referral returned. When the directory responds with a referral, the client can construct new operations and try them again.

The Manage DSAIT control lets you access the referral entry, rather than get a referral. It has OID 2.16.840.1.113730.3.4.2. The control is described in RFC 3296.

Manage Referrals

Suppose the entries below ou=Subscribers,dc=example,dc=com are stored on a different directory server at referral.example.com:1636. You can create a LDAP referral to reference the remote entries.

Before creating the LDAP referral, give directory administrators access to use the Manage DSAIT control, and to manage the ref attribute:

$ dsconfig \
 set-access-control-handler-prop \
 --hostname localhost \
 --port 4444 \
 --bindDN uid=admin \
 --bindPassword password \
 --add global-aci:"(targetcontrol=\"ManageDsaIt\")\
 (version 3.0; acl \"Allow Manage DSAIT control\"; allow(read)\
 groupdn=\"ldap:///cn=Directory Administrators,ou=Groups,dc=example,dc=com\";)" \
 --usePkcs12TrustStore /path/to/opendj/config/keystore \
 --trustStorePassword:file /path/to/opendj/config/keystore.pin \
 --no-prompt

$ ldapmodify \
 --hostname localhost \
 --port 1636 \
 --useSsl \
 --usePkcs12TrustStore /path/to/opendj/config/keystore \
 --trustStorePassword:file /path/to/opendj/config/keystore.pin \
 --bindDN uid=admin \
 --bindPassword password << EOF
dn: dc=example,dc=com
changetype: modify
add: aci
aci: (target="ldap:///dc=example,dc=com")(targetattr="ref")
  (version 3.0; acl "Admins can manage referrals"; allow(all)
  (groupdn = "ldap:///cn=Directory Administrators,ou=Groups,dc=example,dc=com");)
EOF

To create an LDAP referral, either create a referral entry, or add the extensibleObject object class and the ref attribute with an LDAP URL to an existing entry. The example above creates a referral entry at ou=Subscribers,dc=example,dc=com:

$ ldapmodify \
 --hostname localhost \
 --port 1636 \
 --useSsl \
 --usePkcs12TrustStore /path/to/opendj/config/keystore \
 --trustStorePassword:file /path/to/opendj/config/keystore.pin \
 --bindDN uid=kvaughan,ou=people,dc=example,dc=com \
 --bindPassword bribery << EOF
dn: ou=Subscribers,dc=example,dc=com
objectClass: top
objectClass: extensibleObject
objectClass: organizationalUnit
ou: Subscribers
ref: ldaps://referral.example.com:1636/ou=Subscribers,dc=example,dc=com
EOF

DS servers can now return a referral for operations under ou=Subscribers:

$ ldapsearch \
 --hostname localhost \
 --port 1636 \
 --useSsl \
 --usePkcs12TrustStore /path/to/opendj/config/keystore \
 --trustStorePassword:file /path/to/opendj/config/keystore.pin \
 --baseDN ou=subscribers,dc=example,dc=com \
 "(uid=*)"

# The LDAP search request failed: 10 (Referral)
# Additional Information:  A referral entry ou=Subscribers,dc=example,dc=com indicates that the operation must be processed at a different server
# Matched DN:  ou=Subscribers,dc=example,dc=com

To access the entry instead of the referral, use the Manage DSAIT control:

$ ldapsearch \
 --control 2.16.840.1.113730.3.4.2:true \
 --hostname localhost \
 --port 1636 \
 --useSsl \
 --usePkcs12TrustStore /path/to/opendj/config/keystore \
 --trustStorePassword:file /path/to/opendj/config/keystore.pin \
 --bindDN uid=kvaughan,ou=people,dc=example,dc=com \
 --bindPassword bribery \
 --baseDN ou=subscribers,dc=example,dc=com \
 "(&)" \
 ref

dn: ou=Subscribers,dc=example,dc=com
ref: ldaps://referral.example.com:1636/ou=Subscribers,dc=example,dc=com

You can use the Manage DSAIT control to change the referral with the ldapmodify command:

$ ldapmodify \
 --control 2.16.840.1.113730.3.4.2:true \
 --hostname localhost \
 --port 1636 \
 --useSsl \
 --usePkcs12TrustStore /path/to/opendj/config/keystore \
 --trustStorePassword:file /path/to/opendj/config/keystore.pin \
 --bindDN uid=kvaughan,ou=people,dc=example,dc=com \
 --bindPassword bribery << EOF
dn: ou=Subscribers,dc=example,dc=com
changetype: modify
replace: ref
ref: ldap://localhost:1636/ou=People,dc=example,dc=com
EOF