Network Working Group K. Zeilenga
Request for Comments: 3088 OpenLDAP Foundation
Category: EXPerimental April 2001
OpenLDAP Root Service
An experimental LDAP referral service
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
The OpenLDAP Project is operating an experimental LDAP (Lightweight
Directory Access Protocol) referral service known as the "OpenLDAP
Root Service". The automated system generates referrals based upon
service location information published in DNS SRV RRs (Domain Name
System location of services resource records). This document
describes this service.
1. Background
LDAP [RFC2251] directories use a hierarchical naming scheme inherited
from X.500 [X500]. Traditionally, X.500 deployments have used a
geo-political naming scheme (e.g., CN=Jane
Doe,OU=Engineering,O=Example,ST=CA,C=US). However, registration
infrastrUCture and location services in many portions of the naming
hierarchical are inadequate or nonexistent.
The construction of a global directory requires a robust registration
infrastructure and location service. Use of Internet domain-based
naming [RFC2247] (e.g., UID=jdoe,DC=eng,DC=example,DC=net) allows
LDAP directory services to leverage the existing DNS [RFC1034]
registration infrastructure and DNS SRV [RFC2782] resource records
can be used to locate services [LOCATE].
1.1. The Glue
Most existing LDAP implementations do not support location of
directory services using DNS SRV resource records. However, most
servers support generation of referrals to "superior" server(s).
This service provides a "root" LDAP service which servers may use as
their superior referral service.
Client may also use the service directly to locate services
associated with an arbitrary Distinguished Name [RFC2253] within the
domain based hierarchy.
Notice:
The mechanisms used by service are experimental. The descriptions
provided by this document are not definitive. Definitive
mechanisms shall be published in a Standard Track document(s).
2. Generating Referrals based upon DNS SRV RRs
This service returns referrals generated from DNS SRV resource
records [RFC2782].
2.1. DN to Domain Name Mapping
The service maps a DN [RFC2253] to a fully qualified domain name
using the following algorithm:
domain = null;
foreach RDN left-to-right // [1]
{
if not multi-valued RDN and
RDN.type == domainComponent
{
if ( domain == null domain == "." )
{ // start
domain = "";
}
else
{ // append separator
domain .= ".";
}
if ( RDN.value == "." )
{ // root
domain = ".";
}
else
{ // append domainComponent
domain .= RDN.value;
}
continue;
}
domain = null;
}
Examples:
Distinguished Name Domain
----------------------------- ------------
DC=example,DC=net example.net
UID=jdoe,DC=example,DC=net example.net
DC=. . [2]
DC=example,DC=net,DC=. . [3]
DC=example,DC=.,DC=net net [4]
DC=example.net example.net [5]
CN=Jane Doe,O=example,C=US null
UID=jdoe,DC=example,C=US null
DC=example,O=example,DC=net net
DC=example+O=example,DC=net net
DC=example,C=US+DC=net null
Notes:
0) A later incarnation will use a Standard Track mechanism.
1) A later incarnation of this service may use a right-to-left
algorithm.
2) RFC2247 does not state how one can map the domain representing
the root of the domain tree to a DN. We suggest the root of the
domain tree be mapped to "DC=." and that this be reversable.
3) RFC2247 states that domain "example.net" should be mapped to the
DN "DC=example,DC=net", not to "DC=example,DC=net,DC=.". As it is
not our intent to introduce or support an alternative domain to DN
mapping, the algorithm ignores domainComponents to the left of
"DC=.".
4) RFC2247 states that domain "example.net" should be mapped to the
DN "DC=example,DC=net", not to "DC=example,DC=.,DC=net". As it is
not our intent to introduce or support an alternative domain to DN
mapping, the algorithm ignores domainComponents to the left of
"DC=." and "DC=." itself if further domainComponents are found to
the right.
5) RFC2247 states that value of an DC attribute type is a domain
component. It should not contain multiple domain components. A
later incarnation of this service may map this domain to null or
be coded to return invalid DN error.
If the domain is null or ".", the service aborts further processing
and returns noSuchObject. Later incarnation of this service may
abort processing if the resulting domain is a top-level domain.
2.2. Locating LDAP services
The root service locates services associated with a given fully
qualified domain name by querying the Domain Name System for LDAP SRV
resource records. For the domain example.net, the service would do a
issue a SRV query for the domain "_ldap._tcp.example.net". A
successful query will return one or more resource records of the
form:
_ldap._tcp.example.net. IN SRV 0 0 389 ldap.example.net.
If no LDAP SRV resource records are returned or any DNS error occurs,
the service aborts further processing and returns noSuchObject.
Later incarnations of this service will better handle transient
errors.
2.3. Constructing an LDAP Referrals
For each DNS SRV resource record returned for the domain, a LDAP URL
[RFC2255] is constructed. For the above resource record, the URL
would be:
ldap://ldap.example.net:389/
These URLs are then returned in the referral. The URLs are currently
returned in resolver order. That is, the server itself does not make
use of priority or weight information in the SRV resource records. A
later incarnation of this service may.
3. Protocol Operations
This section describes how the service performs basic LDAP
operations. The service supports operations extended through certain
controls as described in a later section.
3.1. Basic Operations
Basic (add, compare, delete, modify, rename, search) operations
return a referral result if the target (or base) DN can be mapped to
a set of LDAP URLs as described above. Otherwise a noSuchObject
response or other appropriate response is returned.
3.2. Bind Operation
The service accepts "anonymous" bind specifying version 2 or version
3 of the protocol. All other bind requests will return a non-
successful resultCode. In particular, clients which submit clear
text credentials will be sent an unwillingToPerform resultCode with a
cautionary text regarding providing passWords to strangers.
As this service is read-only, LDAPv3 authentication [RFC2829] is not
supported.
3.3. Unbind Operations
Upon receipt of an unbind request, the server abandons all
outstanding requests made by client and disconnects.
3.4. Extended Operations
The service currently does recognize any extended operation. Later
incarnations of the service may support Start TLS [RFC2830] and other
operations.
3.5. Update Operations
A later incarnation of this service may return unwillingToPerform for
all update operations as this is an unauthenticated service.
4. Controls
The service supports the ManageDSAit control. Unsupported controls
are serviced per RFC2251.
4.1. ManageDSAit Control
The server recognizes and honors the ManageDSAit control [NAMEDREF]
provided with operations.
If DNS location information is available for the base DN itself, the
service will return unwillingToPerform for non-search operations.
For search operations, an entry will be returned if within scope and
matches the provided filter. For example:
c: searchRequest {
base="DC=example,DC=net"
scope=base
filter=(objectClass=*)
ManageDSAit
}
s: searchEntry {
dn: DC=example,DC=net
objectClass: referral
objectClass: extensibleObject
dc: example
ref: ldap://ldap.example.net:389/
associatedDomain: example.net
}
s: searchResult {
success
}
If DNS location information is available for the DC portion of a
subordinate entry, the service will return noSuchObject with the
matchedDN set to the DC portion of the base for search and update
operations.
c: searchRequest {
base="CN=subordinate,DC=example,DC=net"
scope=base
filter=(objectClass=*)
ManageDSAit
}
s: searchResult {
noSuchObject
matchedDN="DC=example,DC=net"
}
5. Using the Service
Servers may be configured to refer superior requests to
<ldap://root.openldap.org:389>.
Though clients may use the service directly, this is not encouraged.
Clients should use a local service and only use this service when
referred to it.
The service supports LDAPv3 and LDAPv2+ [LDAPv2+] clients over
TCP/IPv4. Future incarnations of this service may support TCP/IPv6
or other transport/internet protocols.
6. Lessons Learned
6.1. Scaling / Reliability
This service currently runs on a single host. This host and
associated network resources are not yet exhausted. If they do
become exhausted, we believe we can easily scale to meet the demand
through common distributed load balancing technics. The service can
also easily be duplicated locally.
6.2. Protocol interoperability
This service has able avoided known interoperability issues in
supporting variants of LDAP.
6.2.1. LDAPv3
The server implements all features of LDAPv3 [RFC2251] necessary to
provide the service.
6.2.2. LDAPv2
LDAPv2 [RFC1777] does not support the return of referrals and hence
may not be referred to this service. Though a LDAPv2 client could
connect and issue requests to this service, the client would treat
any referral returned to it as an unknown error.
6.2.3. LDAPv2+
LDAPv2+ [LDAPv2+] provides a number of extensions to LDAPv2,
including referrals. LDAPv2+, like LDAPv3, does not require a bind
operation before issuing of other operations. As the referral
representation differ between LDAPv2+ and LDAPv3, the service returns
LDAPv3 referrals in this case. However, as commonly deployed LDAPv2+
clients issue bind requests (for compatibility with LDAPv2 servers),
this has not generated any interoperability issues (yet).
A future incarnation of this service may drop support for LDAPv2+
(and LDAPv2).
6.2.4. CLDAP
CLDAP [RFC1798] does not support the return of referrals and hence is
not supported.
7. Security Considerations
This service provides information to "anonymous" clients. This
information is derived from the public directories, namely the Domain
Name System.
The use of authentication would require clients to disclose
information to the service. This would be an unnecessary invasion of
privacy.
The lack of encryption allows eavesdropping upon client requests and
responses. A later incarnation of this service may support
encryption (such as via Start TLS [RFC2830]).
Information integrity protection is not provided by the service. The
service is subject to varies forms of DNS spoofing and attacks. LDAP
session or operation integrity would provide false sense of security
concerning the integrity of DNS information. A later incarnation of
this service may support DNSSEC and provide integrity protection (via
SASL, TLS, or IPSEC).
The service is subject to a variety of denial of service attacks.
The service is capable of blocking access by a number of factors.
This capability have yet to be used and likely would be ineffective
in preventing sophisticated attacks. Later incarnations of this
service will likely need better protection from such attacks.
8. Conclusions
DNS is good glue. By leveraging of the Domain Name System, global
LDAP directories may be built without requiring a protocol specific
registration infrastructures.
In addition, use of DNS service location allows global directories to
be built "ad hoc". That is, anyone with a domain name can
participate. There is no requirement that the superior domain
participate.
9. Additional Information
Additional information about the OpenLDAP Project and the OpenLDAP
Root Service can be found at <http://www.openldap.org/>.
10. Author"s Address
Kurt Zeilenga
OpenLDAP Foundation
EMail: kurt@openldap.org
11. Acknowledgments
Internet hosting for this experiment is provided by the Internet
Software Consortium <http://www.isc.org/>. Computing resources were
provided by Net Boolean Incorporated <http://www.boolean.net/>. This
experiment would not have been possible without the contributions of
numerous volunteers of the open source community. Mechanisms
described in this document are based upon those introduced in
[RFC2247] and [LOCATE].
References
[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
STD 13, RFC1034, November 1987.
[RFC1777] Yeong, W., Howes, T. and S. Kille, "Lightweight Directory
Access Protocol", RFC1777, March 1995.
[RFC1798] Young, A., "Connection-less Lightweight Directory Access
Protocol", RFC1798, June 1995.
[RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.
[RFC2247] Kille, S., Wahl, M., Grimstad, A., Huber, R. and S.
Sataluri, "Using Domains in LDAP/X.500 Distinguished
Names", RFC2247, January 1998.
[RFC2251] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory
Access Protocol (v3)", RFC2251, December 1997.
[RFC2253] Wahl, M., Kille, S. and T. Howes, "Lightweight Directory
Access Protocol (v3): UTF-8 String Representation of
Distinguished Names", RFC2253, December 1997.
[RFC2255] Howes, T. and M. Smith, "The LDAP URL Format", RFC2255,
December 1997.
[RFC2782] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC2782,
February 2000.
[RFC2829] Wahl, M., Alvestrand, H., Hodges, J. and R. Morgan,
"Authentication Methods for LDAP", RFC2829, May 2000.
[RFC2830] Hodges, J., Morgan, R. and M. Wahl, "Lightweight Directory
Access Protocol (v3): Extension for Transport Layer
Security", RFC2830, May 2000.
[LOCATE] IETF LDAPext WG, "Discovering LDAP Services with DNS",
Work in Progress.
[LDAPv2+] University of Michigan LDAP Team, "Referrals within the
LDAPv2 Protocol", August 1996.
[NAMEDREF] Zeilenga, K. (editor), "Named Subordinate References in
LDAP Directories", Work in Progress.
[X500] ITU-T Rec. X.500, "The Directory: Overview of Concepts,
Models and Service", 1993.
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