IDS Working Group Al Grimstad
INTERNET-DRAFT Rick Huber
Sri Sataluri
AT&T
Steve Kille
Isode Ltd.
Mark Wahl
Critical Angle Inc.
March 12, 1997
Naming Plan for an Internet Directory Service
Filename: draft-ietf-ids-dirnaming-01.txt
Status of this Memo
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Abstract
Application of the conventional X.500 approach to naming has, in the
experience of the authors, proven to be an obstacle to the creation of
directory services. We propose a new directory naming plan that
leverages the strengths of the most popular and successful Internet
naming schemes for naming objects in a hierarchical directory. This
plan can, we believe, facilitate the creation of an Internet White
Pages Service (IWPS) and other directory-enabled applications by
overcoming the problems encountered by those using the conventional
recommended X.500 approach to naming.
1.0 EXECUTIVE SUMMARY
The conventional approach to naming taken by the X.500 community has,
in the experience of the authors, shown itself to be an obstacle to
the creation of directory services. The required registration
infrastructure is either non-existent or largely ignored. The
infrastructure that does exist is cumbersome to use and tends to
produce counterproductive results. The attributes used for naming
have been confusing for users and inflexible to managers and operators
of directory services.
This paper describes an alternative directory naming plan for the
construction of the Internet White Pages Service (IWPS) and other
directory-enabled applications. It has three main features. First, it
bases directory name construction on the existing infrastructure of
names in the Internet, that is, names from the Domain Name System
(DNS) and mailbox identifiers structured according to RFC822. Second,
names constructed according to this plan use existing standardized
directory attributes and can co-exist with names constructed according
to traditional X.500 naming practices. And third, the hierarchical
pattern of directory names need not mirror exactly the domain part of
RFC822 mailbox identifiers, but can be structured to support various
requirements such as data partitioning and access control lists (ACLs)
based on the naming hierarchy.
Here, in summary, is our proposal.
For naming entries of leaf directory objects such as users, groups,
server applications and certification authorities in a hierarchical
directory (e.g., one accessed via LDAP, the Lightweight Directory
Access Protocol), we propose the use of the user identifier attribute
"uid" for the relative distinguished name. The value of this
attribute should be an RFC822 address, e.g.,
uid=John.Smith@acme.com
To address situations where it is inconvenient or inappropriate to use
an RFC822 mailbox identifier for a leaf directory object, we propose
the use of the conventional common name attribute, "cn".
The upper portions of the hierarchical directory tree should be
constructed using the components of registered DNS names using the
domain component attribute "dc". The directory name for the
organization having the domain name acme.com will then be, e.g.,
dc=acme, dc=com
Organizations can add additional directory structure, for example to
support implementation of access control lists or partitioning of their
directory information, by using registered subdomains of DNS names,
e.g.,
dc=corporate, dc=acme, dc=com
Directory distinguished names will thus have the following structure,
e.g.,
uid=John.Smith@acme.com, dc=acme, dc=com
uid=Mary.Jones@acme.com, dc=corporate, dc=acme, dc=com
uid=J.Smith@worldnet.att.net, dc=legal, dc=acme, dc=com
cn=Reading Room, dc=physics, dc=national-lab, dc=edu
Searching the directory (for persons, or other similar objects) based
on exact matching of the uid attribute should be optimized in the
directory service such that it is essentially equivalent to searching
using a directory distinguished name.
External mechanisms can be used to locate the proper directory server
to query to obtain directory information.
2.0 THE PROBLEM
The X.500 Directory model [1] can be used to create a world-wide
distributed directory. The Internet X.500 Directory Pilot has been
operational for several years and has grown to a size of about 1.5
million entries of varying quality. The rate of growth of the pilot is
far lower than the rate of growth of the Internet during the pilot
period.
There are a substantial number of contributing factors that have
inhibited the growth of this pilot. The common X.500 approach to
naming, while not the preponderant problem, has contributed in several
ways to limit the growth of an Internet White Pages Service based on
X.500.
2.1 Naming Problems
The conventional way to construct names in the X.500 community is
documented as an informative (i.e., not officially standardized) Annex
B to X.521. The relative distinguished name (RDN) of a user consists of
a common name (cn) attribute. This is meant to be what -- in the user's
particular society -- is customarily understood to be the name of that
user. The distinguished name of a user is the combination of the name
of some general object, such as an organization or a geographical unit,
with the common name. There are two main problems with this style of
name construction.
First, the common name attribute, while seeming to be user-friendly,
cannot be used generally as an RDN in practice. In any significant
set of users to be named under the same Directory Information Tree
(DIT) node there will be collisions on common name. There is no way
to overcome this other than either by forcing uniqueness on common
names, something they do not possess, or by using an additional
attribute to prevent collisions. This additional attribute normally
needs to be unique in a much larger context to have any practical
value. The end result is a RDN that is very long and unpopular with
users.
Second, and more serious, X.500 has not been able to use any
significant number of pre-existing names. Since X.500 naming models
typically use organization names as part of the hierarchy [2, 3],
organization names must be registered. As organization names are
frequently tied to trademarks and are used in sales and promotions,
registration can be a difficult and acrimonious process.
The North American Directory Forum (NADF, now the North Atlantic
Directory Forum but still the NADF) proposed to avoid the problem of
registration by using names that were already registered in the "civil
naming infrastructure" [3]. Directory distinguished names would be
based on an organization's legal name as recognized by some
governmental agency (county clerk, state secretary of state, etc.) or
other registering entity such as ANSI.
This scheme has the significant advantage of keeping directory service
providers out of disputes about the right to use a particular name,
but it leads to rather obscure names. Among these obscurities, the
legal name almost invariably takes a form that is less familiar and
longer than what users typically associate with the organization. For
example, in the US a large proportion of legal organization names end
with the text ", Inc." as in "Acme, Inc." Moreover, in the case of
the US, the civil naming infrastructure does not operate nationally,
so the organization names it provides must be located under state and
regional DIT nodes, making them difficult to find while browsing the
directory. NADF proposes a way to algorithmically derive
multi-attribute RDNs which would allow placement of entries or aliases
in more convenient places in the DIT, but these derived names are
cumbersome and unpopular. For example, suppose Nadir is an
organization that is registered in New Jersey civil naming
infrastructure under the name "Nadir Networks, Inc." Its civil
distinguished name (DN) would then be
o="Nadir Networks, Inc.", st=New Jersey, c=US
while its derived name which is unambiguous under c=US directly is
o="Nadir Networks, Inc." + st=New Jersey, c=US
More generally, the requirement for registration of organizations in
X.500 naming has led to the establishment of national registration
authorities whose function is mainly limited to assignment of X.500
organization names. Because of the very limited attraction of X.500,
interest in registering an organization with one of these national
authorities has been minimal. Finally, multi-national organizations are
frustrated by a lack of an international registration authority.
2.2 Directory Models
The Internet community proposed the Light-weight Directory Access
Protocol (LDAP) [4], initially, as a simplified access method for
X.500 directories. However, more recently, a number of different
directory server implementations have begun to appear that use LDAP as
an access protocol to backend retrieval systems not based on X.500.
The X.500 Directory, via its knowledge model, attempts to build a
world-wide directory in a top-down fashion. This approach has only met
with partial success.
The appearance on the Internet of directory systems supporting LDAP
but not tied into the world of X.500 will, in our opinion, stimulate
the creation of directories in a bottom-up fashion [5]. These
directory servers or directory islands will be loosely tied together
via LDAP referrals and other external mechanisms.
For such loosely coordinated mechanisms to work effectively, a few of
the general properties of X.500 need to be retained. Among these are
(1) an approach to naming that makes the linkage of the directory
islands as simple as possible, and (2) an extendible schema of
directory information with a widely accepted common base. The naming
plan described here supports these two characteristics.
2.3 Addressing the Problems
This paper describes a directory naming plan that is a radical
departure from the traditional X.500 naming scheme -- the X.500 scheme
has not been generally accepted by the Internet community and has been
found to be very clumsy in implementing and administering directory
services by the authors.
This naming plan is straightforward to implement by both classic X.500
systems and islands of stand-alone LDAP servers. Its unique strength
lies in its use of the existing Internet naming schemes -- RFC822
addresses [6] and the Domain Name System [7]. Further, by use of an
attribute, the user ID attribute, and a tree structure based on the
DNS, the plan, if broadly implemented, may well simplify the
construction of external mechanisms to locate appropriate LDAP
servers.
The focus of this naming plan is on naming Internet users,
certification authorities and server applications. It is not
applicable to resolving problems with naming objects such as X.500
residential persons which typically lack e-mail addresses. Because it
is a hierarchical plan using typed attribute values (tags), the naming
plan can co-exist with other hierarchical plans using other typed
attributes such as a plan for residential persons and the conventional
X.521 Annex B plan.
3.0 TECHNOLOGICAL ASSUMPTIONS
We assume that the fundamental protocol interface to systems offering
general directory services will be the TCP/IP-based Lightweight
Directory Access Protocol. Objects -- users, certification
authorities and server applications -- are named in this protocol in a
hierarchical fashion and represented as described in RFC 1779 [8]. We
also assume that user authentication and other security services will
increasingly depend on public key cryptographic techniques such as
those used in the Secure Sockets Layer (SSL) [9]. These techniques use
information such as certificates (certified public cryptographic keys)
in which objects are identified with directory names.
4.0 IDENTIFICATION
The one universally accepted scheme of user identification (actually
user maildrop or mailbox identification) in the Internet today is the
RFC822 syntax for e-mail addresses. The RFC822 syntax contains a
domain name and a user identifier that is local to that domain.
4.1 Domain Identification
The syntax of a domain identifier is defined by the Domain Name System
(DNS) in RFC 1034 [6]. Examples of such identifiers are acme.com and
foo.nj.us.
4.2 RFC822 Identification
The syntax of an RFC822 identifier is:
local@domain
where
"domain" is a hierarchically structured identifier as defined
by the DNS, and
"local" is an identifier for a user (literally a maildrop)
that is valid within the domain "domain".
Examples of such identifiers are J.Smith@acme.com and
S.Johnson@corporate.acme.com.
Each user for whom information is maintained in a directory service
will typically have at least one e-mail address structured according
to RFC822. While the user may well have many such addresses, one will
be selected for the purpose of user identification. We call this the
"distinguished" e-mail address or the "distinguished" RFC822
identifier for the user.
5.0 DIRECTORY DISTINGUISHED NAMES
5.1 Overview of Distinguished Names
The general structure of a name in LDAP, termed a distinguished name
(DN), is:
attribute, attribute, ... attribute
An "attribute" is a pairing of a type identifier and a value separated
by "=". For example, cn=Samuel Johnson is an attribute with type
identifier "cn", short for common name, and value "Samuel Johnson".
The order of the attributes in a DN is significant. Like DNS
identifiers, DNs are hierarchical and "little endian", that is, the
most global piece comes last. An example of a DN is:
cn=Samuel Johnson, o=Famous Lexicographers, c=GB
where:
cn=Samuel Johnson means the attribute type "cn" has the value
"Samuel Johnson",
o=Famous Lexicographers means that the attribute type "o", short
for organization name, has the value "Famous Lexicographers", and
c=GB means that the attribute type "c", short for country name, has
the value "GB", the international abbreviation for the country
Great Britain.
The set of all DNs forms a tree structure that is called the Directory
Information Tree (DIT).
5.2 Directory Distinguished Names
Suppose an organization, e.g., Acme, wants to bring up a directory
service. It either has a registered DNS name or it should get one.
Given a DNS name, we will use a form of DN construction largely based
on RFC1279 [10]. This RFC shows how to map an RFC822 e-mail address to
a DN. We differ from it in two respects: (1) we use a mapping that
starts at the root of the DIT; and (2) we use the attribute type user
ID (userid or uid) for the first attribute of the DN. The value of the
uid attribute is a complete RFC822 address that may but need not be
related to the DN of the parent entry.
The pattern for the organization's DN will be:
dc=A, ..., dc=Z
and the pattern for the DN of a user in that organization will be:
uid=RFC822 identifier, dc=A, ..., dc=Z
We consider each of the two attribute types, dc and uid, used to form
the DN in turn.
5.2.1 Domain Component (dc)
The domain component attribute is defined and registered in RFC1274
[2].
The domain component attributes of a organization's DN will normally be
constructed from the domain name of that organization. That is, for the
organization "Acme, Inc." with domain name "acme.com", the DN will be
dc=acme, dc=com
The domain component attributes of a user's DN will normally be
constructed from the domain name of his/her distinguished e-mail
address. That is, for the user uid=J.Smith@acme.com the domain
component attributes would typically be:
dc=acme, dc=com
The full LDAP DN for this user would then be:
uid=J.Smith@acme.com, dc=acme, dc=com
The algorithm for constructing the uid part of the DN is given in section
5.2.2.
It is important to emphasize that the elements of the domain name of
an RFC822 identifier may, BUT NEED NOT, be the same as the domain
components of the DN. This means that the domain components provide a
degree of freedom to support access control or other directory
structuring requirements that need not be mechanically reflected in
the user's e-mail address. We do not want under any condition to
force the user's e-mail address to change just to facilitate a new
system requirement such as a modification in an access control
structure. It should also be noted that while we do not require that
the domain components match the RFC822 identifier, we DO require that
the concatenated domain components form a registered domain name, that
is, one that is represented in the DNS. This automatically avoids name
conflicts in the directory hierarchy.
To provide an example of a DN which deviates from what might be
considered the default structure, consider the following scenario.
Suppose that J.Smith needs to be granted special permissions to
information in the dc=acme, dc=com part of the LDAP DIT. Since it will
be, in general, easier to organize special users by their name
structure than via groups (an arbitrary collection of DNs), we use
subdomains for this purpose. Suppose the special permissions were
required by users in the MIS organizational unit. A subdomain
"mis.acme.com" is established, if it does not already exist, according
to normal DNS procedures. The special permissions will be granted to
users with the name structure:
uid=*, dc=mis, dc=acme, dc=com
The DN of J.Smith in this case will be:
uid=J.Smith@acme.com, dc=mis, dc=acme, dc=com
In principal, there is nothing to prevent the domain name elements of
the RFC822 identifier from being completely different from the domain
components of the DN. For instance, the DN for a J.Smith could be:
uid=J.Smith@worldnet.att.net, dc=mis, dc=acme, dc=com
While we do not REQUIRE that the domain name part of the uid match the
dc components of the directory distinguished name, we suggest that this
be done where possible. At a minimum, if the most significant pieces of
the DN and the uid are the same (i.e., "dc=acme, dc=com" and
"acme.com") the likelihood, based on a knowledge of a user's e-mail
address, of discovering an appropriate directory system to contact to
find information about the user is greatly enhanced.
The example above represents a situation where this suggestion isn't
possible because some of the users in a population have mailbox
identifiers that differ from the pattern of the rest of the users,
e.g., most mailboxes are of the form local@acme.com, but a
subpopulation have mailboxes from an ISP and therefore mailboxes of
the form local@worldnet.att.net.
5.2.2 User ID (uid)
The userid (uid) attribute is defined and registered in RFC1274 [2].
The value of the user ID attribute for the user's name will be the
user's distinguished e-mail address in RFC822 syntax. For example, if
there is a user affiliated with the organization Acme having
distinguished e-mail address J.Smith@acme.com, the uid attribute will
be:
uid=J.Smith@acme.com
We strongly suggest that uid=J.Smith@acme.com to be a working e-mail
address. The whole idea here is that users will remember a working
e-mail address; we shouldn't plague them with broken RFC822 addresses
constructed for the convenience of the directory service only. The A
or MX record for the domain name can point to either a customer or
network service provider host.
Since an RFC822 identifier unambiguously identifies a user, it can be
used (by system processes) to search a particular directory system
(e.g. an LDAP server or a related set of LDAP servers) to find a user's
entry. The user need not -- and we assume typically will not -- even
know his/her DN. See Section 8.1.
Directory administrators having several RFC822 identifiers to choose
from when constructing a DN for a user should consider the following
factors:
o Machine-independent addresses are likely to be more stable,
resulting in directory names that change less. Thus an identifier
such as:
js@acme.com
may well be preferable to one such as:
js@blaster.third-floor.acme.com.
o Use of some form of "handle" for the "local" part that is distinct
from a user's real name may result in fewer collisions and
thereby lessen user pain and suffering. Thus the identifier:
js@acme.com
may well be preferable to one such as:
J.Smith@acme.com
Practical experience with use of the RFC822 mailbox identifier scheme
described here has shown that there are situations where it is
convenient to use such identifies for all users in a particular
population, although a few users do not, in fact, possess working
mailboxes. For example, an organization may have a existing unique
identification scheme for all employees that is used as a alias to the
employees' real mailboxes -- which may be quite heterogeneous in
structure. The identification scheme works for all employees to
identify unambiguously each employee; it only works as an e-mail alias
for those employees having real mailboxes. For this reason it would
be a bad assumption for directory-enabled applications to use the uid
as a mailbox; the value(s) of the mail attribute should always be
checked.
5.2.3 Common Name
To address situations where it is inconvenient or inappropriate to use
an RFC822 mailbox identifier for the RDN of a leaf directory object,
we propose the use of the conventional common name attribute, "cn".
As an example of the assignment of a DN to a directory object for
which the creation of a uid would be cumbersome, consider naming an
object of class room, specified in 8.3.5 of the COSINE and Internet
X.500 Schema [2]. For example, the following name might be more
natural for the directory administrator to assign and for applications
using this sort of object to use:
cn=Reading Room, dc=physics, dc=national-lab, dc=edu
In certain environments, especially where there are a relatively small
number of users requiring DNs, that is, where name collisions of the
common name will not happen, it may be convenient to use the
traditional X.500 common name attribute as in:
cn=John Smith, dc=mis, dc=acme, dc=com
6.0 NAMING FOR CERTIFICATES
The certified public keys, certificates, used in SSL and other forms of
strong (i.e. cryptographically based) authentication are structured
according to the ITU X.509 standard. A certificate contains two names
of importance, the name of the "subject" and the "issuer". The subject
will be either a user or a server; the issuer will be a certification
authority. The encoding of these names differs significantly from the
encoding of LDAP names which are simply character strings. In
particular, the attribute types used in names are encoded as globally
unique "object identifiers." The object identifiers relevant to the
names considered in this naming plan are assigned in either the X.500
standards (ITU X.520) or RFC1274, the COSINE and Internet X.500 Schema
[2].
6.1 User Certificates
For user certificates issued by a certification authority, the subject
name will be the proper X.509 representation of the user hierarchical
DNs described in section 5 of this paper.
6.2 Server/Site Certificates
As certificates are used more and more for authentication, applications
that run as processes on systems need to be named so that they can also
be located in the directory. Since the name of the subject is encoded
in a certificate, for application instances to be portable, we need a
naming scheme that is independent of the underlying hardware platform
and its IP address or DNS host name.
In existing Internet products that use certificates, the terms "Server
Certificate" and "Site Certificate" have been (mis-)used to identify
application instances.
We propose that application instances be named similar to individuals
as entities under a specific branch of the DIT and be given appropriate
unique RFC822 identifiers. The RFC822 identifier so chosen is not
constrained by the naming plan. As illustrated in the three examples
below, it can be either host-independent (1,2) or host dependent (3).
If host-independent, it can be based on a subdomain (2) of the
organization's domain name or, if such registration is convenient,
based on the domain name (1) itself.
A few examples of application instance names are:
uid=dirsrv0@acme.com, dc=sales, dc=acme, dc=com (1)
uid=certsrv0@sales.acme.com, dc=sales, dc=acme, dc=com (2)
uid=gnarlysrv0@surf.sales.acme.com, dc=sales, dc=acme, dc=com (3)
This name is used in the construction of the server's certificate.
A consequence of this method of constructing names is that application
instances have mailboxes. This can be thought of as an opportunity to
provide users with a predictable contact for resolution of operational
problems with the application much like postmaster@DOMAIN is the
predictable contact for e-mail problems at DOMAIN.
To address situations where it is inconvenient or inappropriate to use
an RFC822 mailbox identifier for the RDN of an application instance,
we propose the use of the conventional common name attribute, "cn".
6.3 Certification Authority Certificates
A certification authority (CA) is the trusted guarantor for a user or
server certificate. The CA's name appears in the "issuer" field of the
certificate it issues. The names of several CAs are already built into
popular Internet Web browsers such as the Netscape Navigator and the
Microsoft Internet Explorer. Some examples of such CA names are:
ou=Directory Services, o=AT&T, c=US
ou=Secure Server Certification Authority, o="RSA Data Security, Inc.",
c=US
These names use the traditional X.500 attributes for naming. These
attributes are organizational unit name (ou), organization name (o)
and country name (c). To maintain compatibility, the appropriate CAs
can be incorporated in any LDAP-based directory service with these
existing names.
In future, new certification authorities may be assigned names
following the structure described in section 5 of this
document. Examples of such new CA names are:
uid=certification-authority@CAs-R-us.com, dc=CAs-R-us, dc=com
cn=certification authority, dc=CAs-R-us, dc=com
dc=certification-authority, dc=CAs-R-us, dc=com
7.0 OTHER DIRECTORY OBJECTS
This subsection of the naming plan is concerned with other
miscellaneous directory objects for which a regular naming structure
is required. This collection of objects consists, at this point, only
of groups. Other documents may define the naming plans for representing
other kinds of objects in the directory.
7.1 Groups
A group is a directory object, and therefore has a directory name. It
expresses membership of other directory objects in a set. Examples of
groups are: a set of users permitted to modify the entries in a
subtree of the DIT; a set of users permitted to view a related set of
web pages; and a set of e-mail recipients interested in a particular
subject. Note that the latter example is, from a UNIX-centric
perspective, at this point a hypothetical directory oriented
example. In practice, groups in the UNIX-centric part of e-mail world,
more commonly termed mailing lists, are not represented by directory
group entries and members of the group are identified by an e-mail
address rather than a directory name.
Group objects in the directory are named following the same structure
as users and servers: the RDN of the object uses a uid attribute value
and the group object is placed under an appropriate domain object. As
in the case of users and servers, the uid attribute value must be a
valid RFC822 mailbox identifier. There is no particular requirement
and, in fact, it might even be undesirable, that e-mail sent to this
mailbox should be exploded to the mailboxes of the members of the group.
On the other hand, it might well be convenient that the uid used as
the RDN of a group object be the mailbox of a list rather than an
individual.
For example, suppose the directory administrators for the dc=acme,
dc=com subtree of the DIT have set up an e-mail list
ds-admin@acme.com. It might be convenient to create a directory group
with the name
uid=ds-admin@acme.com, dc=acme, dc=com
for the construction of access control lists in the acme subtree,
granting members of this list modification permissions on all the
entries. In this case e-mail sent to ds-admin@acme.com would typically
go, in a UNIX-centric environment, not to the members of the directory
group as defined by the attribute of the group's entry that identifies
the members, but to the list of mailboxes exploded at the host
acme.com.
To address situations where it is inconvenient or inappropriate to use
an RFC822 mailbox identifier for the RDN of a group object, we propose
the use of the conventional common name attribute, "cn".
8.0 IMPLEMENTATION ISSUES
8.1 Directory Services Considerations
This naming plan has been designed so that a user's entry can be found
unambiguously using nothing but the user's distinguished e-mail address --
assuming that the query is sent to the right LDAP directory server.
Systems having the user's DN in hand can, of course, directly access
the user's entry via LDAP.
An LDAP search request with the following components will either (1)
find uniquely the entry of a user with the provided distinguished
e-mail address, or (2) indicate that no user has the provided e-mail
address as a distinguished e-mail address:
baseObject: root
scope: wholeSubtree
filter: equalityMatch (uid == distinguished e-mail address)
A search such as this is possible in LDAP servers being planned
because, unlike X.500 servers, the LDAP servers we envision are not
universally interconnected in one global system according to the X.500
knowledge model. In X.500 such a search would propagate to all the
servers in the system; in the envisioned LDAP system it would be
limited to one server (or potentially a small set of servers).
In a world of LDAP islands, the issue of finding the right island to
which to direct a directory query arises. The new version of LDAP
under design in the IETF [11] has a referral mechanism. Given a query,
a server can return a referral to an other LDAP server that might hold
the data. This approach can be used to tie together the servers holding
the distributed data of an LDAP island. By generalizing this concept
one can imagine building a simple referral server that knows about the
LDAP islands of the Internet. It would compare the naming information
in a query it receives with its knowledge of LDAP islands and return a
referral to the appropriate island.
It has been traditional in X.500 and LDAP directory services to employ
the common name (cn) attribute in naming. While this naming plan
doesn't require use of the cn attribute in naming, it should be
stressed that it is still quite important. It will play a significant
role in enabling searches to find user entries of interest. To this
end, what is typically done is to have multiple values of the common
name attribute in the user's entry. Thus the entry for
J.Smith@acme.com might well have the common name attribute values
"John Smith", "John Q. Smith" and "John Quincy Smith" to optimize
matching of search requests.
The attribute rfc822mailbox (or mail) is normally used to hold a user's
e-mail address in X.500 and LDAP directory services. A user with
multiple e-mail addresses will not be assigned multiple DNs simply
because of the multiple addresses. As described above, one of these
e-mail addresses -- a machine-independent and fairly static one -- will
be elected the distinguished e-mail address and used to construct the
DN. If it is desirable to make available the other e-mail addresses,
they can be stored in the user's rfc822mailbox attribute. It is
technically possible, although undesirable because potentially
confusing, that the rfc822mailbox attribute does NOT contain the
distinguished e-mail address as one of its values.
An important matter for directory administrators to consider with
respect to the use of RFC822 mailbox identifiers is that it is
possible to construct more than one DN using the same mailbox
identifier as the RDN. This may or may not have undesirable
consequences, depending on the expectations of specific applications
accessing the directory. The safest practice is probably to avoid such
multiple use of the same mailbox identifier.
Another less safe alternative might be to only permit such multiple
use in cases where the directory objects with the same uid as the RDN
have different base object classes. This would permit applications --
which would need to be aware of this nuance -- to find objects based
on searches for specific uid values to distinguish between meaningful
and non-meaningful directory objects.
Consider the following example. Suppose two directory objects were to
be created by Acme,
uid=ds-dsadmin@acme.com, dc=engr, dc=acme, dc=com (1)
and
uid=ds-admin@acme.com, dc=acme, dc=com (2).
The first object represents an administrative user and is of object
class organizational role. The second represents a group and
represents a list of objects (expressed by the values of the member
attribute) who have been granted some form of administrative
permission. An application wishing to determine the group members and
knowing only the uid value "ds-admin@acme.com" might experience
problems if it naively expected expected the first match found under
dc=acme, dc=com to contain a member attribute.
8.2 Alternative DN Structures
Some organizations may wish to be represented by a scheme based upon a
more classic X.500/NADF naming system [12]. These organizations can be
accommodated in the scheme without significant problems or naming
conflicts.
The approach we take should not preclude participation in a larger
directory service, so names held in the stand-alone LDAP directory
shouldn't collide unnecessarily with names held by other directory
service operators. (If two directory service providers hold
information with respect to the same DN, these are two different views
of the same real world object, not two unrelated objects that
accidentally share a "name.") Naming based on DNS meets this
criterion. Disciplined use of X.500 naming can also meet this
criterion.
To enable a consistent searching strategy in a directory where objects
named according to the plan described here as well as the classic
X.500/NADF naming system, we strongly recommend the user ID (uid)
attribute, although not part of the DN, have a valid RFC822 identifier
for the user. The LDAP DIT structure will follow the typical
X.500/NADF approach as follows.
Organizations wishing to pursue this approach will need to register
their organization's name with the appropriate authority as generally
accepted in the X.500/NADF community. This means that US organizations
will register with ANSI. The resort to the civil naming
infrastructure of states and counties should be actively
discouraged. There may be cases where this sort of name construction
can make sense, so no absolute prohibition is made here. The names
constructed from this civil naming infrastructure (a la NADF), while
meeting the technical criteria for non-ambiguity and right to use, are
typically long and unwieldy.
Suppose the US company "XYZ Widgets" expressed a wish to follow this
approach. A typical user name might be
uid=J.User@xyzwidgets.com, o="XYZ Widgets, Inc.", c=US
where o="XYZ Widgets, Inc.", c=US is an ANSI registered name. (The
"alphanumeric string" registered with ANSI is 'XYZ Widgets, Inc.',
where the single quotes mark off what is registered but are not part of
the registered information.)
To support structured access control within the company, the analogous
form to the registration of a subdomain in the mainline approach is the
creation of an organizational unit node.
8.3 Directory Schema Implications of the Naming Plan
The traditional directory schema(s) developed for the X.500 standard
and its application to the Internet [3] require extension to be used
with the naming plan developed here. The extensions described below
attempt to reuse existing schema elements as much as possible. The
directory objects for which extensions are required are: organization,
organizational unit, ca, server (application) and group.
So as to continue to use existing structural object classes to the
extent possible, we propose supplementing entries based on these
classes with additional information from two new auxiliary object
classes, dcObject and uidObject.
The auxiliary object class dcObject is defined as:
name: dcObject
requires: dc
The auxiliary object class uidObject is defined as:
name: uidObject
requires: uid
In a pure X.500 context, our schema would also require the definition
of new name forms and structure rules. These concepts are not
required, however, for the specification of LDAP schemas.
8.3.1 Organization Schema
The dc attribute is employed to construct the RDN of an organization
object. This is enabled by adding the auxiliary class dcObject to the
organization's objectClass attribute.
8.3.2 Organizational Unit Schema
The dc attribute is employed to construct the RDN of an
organizationalUnit object (which is subordinate in the DIT to either
an organization or an organizationalUnit object). This is enabled by
adding the auxiliary class dcObject to the organizational unit's
objectClass attribute.
8.3.3 Person Schema
No schema extensions are required for person objects if either the
inetOrgPerson (preferred) or the newPilotPerson object classes are
used. The attribute uid is permissible in each class. For consistency,
the uidObject could be added to person entry objectClass attributes
to assist applications filtering on this attribute.
8.3.4 Certification Authority Schema
The certification authority (CA) object class is an auxiliary class,
meaning it is essentially a set of additional attributes for a base
class such as organizationalRole, organization, organizationalUnit or
person. Except in the case where the base structural class is
inetOrgPerson, use of the uid attribute to construct the RDN of a CA
will require the auxiliary class uidObject to permit the uid attribute
to be used. In the cases where organizationalUnit or organization is
the base class for a CA, use of the auxiliary class dcObject will
permit the RDN of the CA to be a domain component.
8.3.5 Server Application Schema
Server applications are typically represented by entries of the object
class applicationProcess (or a class derived from it). Sometimes the
class applicationEntity is used. In either case, the uid attribute
may be employed to construct the RDN of a server application object.
This is enabled by adding the auxiliary class uidObject to the server
application's objectClass attribute.
8.3.6 Group of Names Schema
The uid attribute may be employed to construct the RDN of a groupOfNames
object. This is enabled by adding the auxiliary class uidObject to the
groupOfNames' objectClass attribute.
9.0 SECURITY CONSIDERATIONS
Security considerations are not part of this paper.
10.0 ACKNOWLEDGMENTS
This plan has emerged in the course of a number of fruitful
discussions, especially with David Chadwick, John Dale, Joe Gajewski,
Mark Jackson, Ryan Moats, Tom Spencer and Chris Tzu.
11.0 REFERENCES
[1] X.500: The Directory -- Overview of Concepts, Models, and
Service, CCITT Recommendation X.500, December, 1988.
[2] P.Barker, and S. Kille, "The COSINE and Internet X.500 Schema",
RFC1274, 11/27/1991.
[3] The North American Directory Forum, "A Naming Scheme for c=US",
RFC1255, September 1991.
[4] W. Yeong, T. Howes, and S. Kille, "Lightweight Directory Access
Protocol", RFC1777, 03/28/1995. (Work is also underway in the
IETF to produce an extended version of LDAP.)
[5] J. Postel, and C. Anderson, "White Pages Meeting Report",
RFC1588, February 1994.
[6] P. Mockapetris, "Domain names - concepts and facilities",
RFC1034.
[7] D. Crocker, "Standard for the format of ARPA Internet text
messages", RFC822.
[8] S. Kille, "A String Representation of Distinguished Names",
RFC1779, 03/28/1995.
[9] A. Freier, P. Karlton, and P. Kocher, "The SSL Protocol Version
3.0", Work in Progress, Internet Draft
<draft-freier-ssl-version3-01.txt>, March 1996.
[10] S. Kille, "X.500 and Domains", RFC1279, 11/27/1991.
[11] M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access
Protocol (v3)", Work in Progress, Internet Draft
<draft-ietf-asid-ldapv3-protocol-04.txt>, February 17, 1997.
[12] The North American Directory Forum, "NADF Standing Documents: A
Brief Overview", RFC 1417, The North American Directory Forum",
NADF, February 1993.
12. Authors' Addresses
Al Grimstad
AT&T
Room 1C-429, 101 Crawfords Corner Road
Holmdel, NJ 07733-3030
USA
EMail: alg@att.com
Rick Huber
AT&T
Room 1B-433, 101 Crawfords Corner Road
Holmdel, NJ 07733-3030
USA
EMail: rvh@att.com
Sri Sataluri
AT&T
Room 4G-202, 101 Crawfords Corner Road
Holmdel, NJ 07733-3030
USA
EMail: sri@att.com
Steve Kille
Isode Limited
The Dome, The Square
Richmond
TW9 1DT
UK
Phone: +44-181-332-9091
EMail: S.Kille@isode.com
Mark Wahl
Critical Angle Inc.
4815 W Braker Lane #502-385
Austin, TX 78759
USA
EMail: M.Wahl@critical-angle.com
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