LDAP DN strings: Lightweight Directory Access Protocol (v3): UTF-8 String Representation of Distinguished Names
How a directory name is written as text, and read back again.
Wahl, M., Kille, S., and T. Howes, "Lightweight Directory Access Protocol (v3): UTF-8 String Representation of Distinguished Names", RFC 2253, DOI 10.17487/RFC2253, December 1997, <https://www.rfc-editor.org/info/rfc2253>.
@misc{rfc2253,
series = {Request for Comments},
number = 2253,
howpublished = {RFC 2253},
publisher = {RFC Editor},
doi = {10.17487/RFC2253},
url = {https://www.rfc-editor.org/info/rfc2253},
author = {M. Wahl and S. Kille and T. Howes},
title = {{Lightweight Directory Access Protocol (v3): UTF-8 String Representation of Distinguished Names}},
pagetotal = 10,
year = 1997,
month = dec,
} RFC text
Contents
Network Working Group M. Wahl
Request for Comments: 2253 Critical Angle Inc.
Obsoletes: 1779 S. Kille
Category: Standards Track Isode Ltd.
T. Howes
Netscape Communications Corp.
December 1997
Lightweight Directory Access Protocol (v3):
UTF-8 String Representation of Distinguished Names
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1997). All Rights Reserved.
IESG Note
This document describes a directory access protocol that provides
both read and update access. Update access requires secure
authentication, but this document does not mandate implementation of
any satisfactory authentication mechanisms.
In accordance with RFC 2026, section 4.4.1, this specification is
being approved by IESG as a Proposed Standard despite this
limitation, for the following reasons:
a. to encourage implementation and interoperability testing of
these protocols (with or without update access) before they
are deployed, and
b. to encourage deployment and use of these protocols in read-only
applications. (e.g. applications where LDAPv3 is used as
a query language for directories which are updated by some
secure mechanism other than LDAP), and
c. to avoid delaying the advancement and deployment of other Internet
standards-track protocols which require the ability to query, but
not update, LDAPv3 directory servers.
Readers are hereby warned that until mandatory authentication
mechanisms are standardized, clients and servers written according to
this specification which make use of update functionality are
UNLIKELY TO INTEROPERATE, or MAY INTEROPERATE ONLY IF AUTHENTICATION
IS REDUCED TO AN UNACCEPTABLY WEAK LEVEL.
Implementors are hereby discouraged from deploying LDAPv3 clients or
servers which implement the update functionality, until a Proposed
Standard for mandatory authentication in LDAPv3 has been approved and
published as an RFC.
Abstract
The X.500 Directory uses distinguished names as the primary keys to
entries in the directory. Distinguished Names are encoded in ASN.1
in the X.500 Directory protocols. In the Lightweight Directory
Access Protocol, a string representation of distinguished names is
transferred. This specification defines the string format for
representing names, which is designed to give a clean representation
of commonly used distinguished names, while being able to represent
any distinguished name.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [6].
1. Background
This specification assumes familiarity with X.500 [1], and the
concept of Distinguished Name. It is important to have a common
format to be able to unambiguously represent a distinguished name.
The primary goal of this specification is ease of encoding and
decoding. A secondary goal is to have names that are human readable.
It is not expected that LDAP clients with a human user interface
would display these strings directly to the user, but would most
likely be performing translations (such as expressing attribute type
names in one of the local national languages).
2. Converting DistinguishedName from ASN.1 to a String
In X.501 [2] the ASN.1 structure of distinguished name is defined as:
DistinguishedName ::= RDNSequence
RDNSequence ::= SEQUENCE OF RelativeDistinguishedName
RelativeDistinguishedName ::= SET SIZE (1..MAX) OF
AttributeTypeAndValue
AttributeTypeAndValue ::= SEQUENCE {
type AttributeType,
value AttributeValue }
The following sections define the algorithm for converting from an
ASN.1 structured representation to a UTF-8 string representation.
2.1. Converting the RDNSequence
If the RDNSequence is an empty sequence, the result is the empty or
zero length string.
Otherwise, the output consists of the string encodings of each
RelativeDistinguishedName in the RDNSequence (according to 2.2),
starting with the last element of the sequence and moving backwards
toward the first.
The encodings of adjoining RelativeDistinguishedNames are separated
by a comma character (',' ASCII 44).
2.2. Converting RelativeDistinguishedName
When converting from an ASN.1 RelativeDistinguishedName to a string,
the output consists of the string encodings of each
AttributeTypeAndValue (according to 2.3), in any order.
Where there is a multi-valued RDN, the outputs from adjoining
AttributeTypeAndValues are separated by a plus ('+' ASCII 43)
character.
2.3. Converting AttributeTypeAndValue
The AttributeTypeAndValue is encoded as the string representation of
the AttributeType, followed by an equals character ('=' ASCII 61),
followed by the string representation of the AttributeValue. The
encoding of the AttributeValue is given in section 2.4.
If the AttributeType is in a published table of attribute types
associated with LDAP [4], then the type name string from that table
is used, otherwise it is encoded as the dotted-decimal encoding of
the AttributeType's OBJECT IDENTIFIER. The dotted-decimal notation is
described in [3]. As an example, strings for a few of the attribute
types frequently seen in RDNs include:
String X.500 AttributeType
------------------------------
CN commonName
L localityName
ST stateOrProvinceName
O organizationName
OU organizationalUnitName
C countryName
STREET streetAddress
DC domainComponent
UID userid
2.4. Converting an AttributeValue from ASN.1 to a String
If the AttributeValue is of a type which does not have a string
representation defined for it, then it is simply encoded as an
octothorpe character ('#' ASCII 35) followed by the hexadecimal
representation of each of the bytes of the BER encoding of the X.500
AttributeValue. This form SHOULD be used if the AttributeType is of
the dotted-decimal form.
Otherwise, if the AttributeValue is of a type which has a string
representation, the value is converted first to a UTF-8 string
according to its syntax specification (see for example section 6 of
[4]).
If the UTF-8 string does not have any of the following characters
which need escaping, then that string can be used as the string
representation of the value.
o a space or "#" character occurring at the beginning of the
string
o a space character occurring at the end of the string
o one of the characters ",", "+", """, "\", "<", ">" or ";"
Implementations MAY escape other characters.
If a character to be escaped is one of the list shown above, then it
is prefixed by a backslash ('\' ASCII 92).
Otherwise the character to be escaped is replaced by a backslash and
two hex digits, which form a single byte in the code of the
character.
Examples of the escaping mechanism are shown in section 5.
3. Parsing a String back to a Distinguished Name
The structure of the string is specified in a BNF grammar, based on
the grammar defined in RFC 822 [5]. Server implementations parsing a
DN string generated by an LDAPv2 client MUST also accept (and ignore)
the variants given in section 4 of this document.
distinguishedName = [name] ; may be empty string
name = name-component *("," name-component)
name-component = attributeTypeAndValue *("+" attributeTypeAndValue)
attributeTypeAndValue = attributeType "=" attributeValue
attributeType = (ALPHA 1*keychar) / oid
keychar = ALPHA / DIGIT / "-"
oid = 1*DIGIT *("." 1*DIGIT)
attributeValue = string
string = *( stringchar / pair )
/ "#" hexstring
/ QUOTATION *( quotechar / pair ) QUOTATION ; only from v2
quotechar = <any character except "\" or QUOTATION >
special = "," / "=" / "+" / "<" / ">" / "#" / ";"
pair = "\" ( special / "\" / QUOTATION / hexpair )
stringchar = <any character except one of special, "\" or QUOTATION >
hexstring = 1*hexpair
hexpair = hexchar hexchar
hexchar = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
/ "a" / "b" / "c" / "d" / "e" / "f"
ALPHA = <any ASCII alphabetic character>
; (decimal 65-90 and 97-122)
DIGIT = <any ASCII decimal digit> ; (decimal 48-57)
QUOTATION = <the ASCII double quotation mark character '"' decimal 34>
4. Relationship with RFC 1779 and LDAPv2
The syntax given in this document is more restrictive than the syntax
in RFC 1779. Implementations parsing a string generated by an LDAPv2
client MUST accept the syntax of RFC 1779. Implementations MUST NOT,
however, generate any of the RFC 1779 encodings which are not
described above in section 2.
Implementations MUST allow a semicolon character to be used instead
of a comma to separate RDNs in a distinguished name, and MUST also
allow whitespace characters to be present on either side of the comma
or semicolon. The whitespace characters are ignored, and the
semicolon replaced with a comma.
Implementations MUST allow an oid in the attribute type to be
prefixed by one of the character strings "oid." or "OID.".
Implementations MUST allow for space (' ' ASCII 32) characters to be
present between name-component and ',', between attributeTypeAndValue
and '+', between attributeType and '=', and between '=' and
attributeValue. These space characters are ignored when parsing.
Implementations MUST allow a value to be surrounded by quote ('"'
ASCII 34) characters, which are not part of the value. Inside the
quoted value, the following characters can occur without any
escaping:
",", "=", "+", "<", ">", "#" and ";"
5. Examples
This notation is designed to be convenient for common forms of name.
This section gives a few examples of distinguished names written
using this notation. First is a name containing three relative
distinguished names (RDNs):
CN=Steve Kille,O=Isode Limited,C=GB
Here is an example name containing three RDNs, in which the first RDN
is multi-valued:
OU=Sales+CN=J. Smith,O=Widget Inc.,C=US
This example shows the method of quoting of a comma in an
organization name:
CN=L. Eagle,O=Sue\, Grabbit and Runn,C=GB
An example name in which a value contains a carriage return
character:
CN=Before\0DAfter,O=Test,C=GB
An example name in which an RDN was of an unrecognized type. The
value is the BER encoding of an OCTET STRING containing two bytes
0x48 and 0x69.
1.3.6.1.4.1.1466.0=#04024869,O=Test,C=GB
Finally, an example of an RDN surname value consisting of 5 letters:
Unicode Letter Description 10646 code UTF-8 Quoted
=============================== ========== ====== =======
LATIN CAPITAL LETTER L U0000004C 0x4C L
LATIN SMALL LETTER U U00000075 0x75 u
LATIN SMALL LETTER C WITH CARON U0000010D 0xC48D \C4\8D
LATIN SMALL LETTER I U00000069 0x69 i
LATIN SMALL LETTER C WITH ACUTE U00000107 0xC487 \C4\87
Could be written in printable ASCII (useful for debugging purposes):
SN=Lu\C4\8Di\C4\87
6. References
[1] The Directory -- overview of concepts, models and services.
ITU-T Rec. X.500(1993).
[2] The Directory -- Models. ITU-T Rec. X.501(1993).
[3] Wahl, M., Howes, T., and S. Kille, "Lightweight Directory
Access Protocol (v3)", RFC 2251, December 1997.
[4] Wahl, M., Coulbeck, A., Howes, T. and S. Kille, "Lightweight
Directory Access Protocol (v3): Attribute Syntax Definitions",
RFC 2252, December 1997.
[5] Crocker, D., "Standard of the Format of ARPA-Internet Text
Messages", STD 11, RFC 822, August 1982.
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119.
7. Security Considerations
7.1. Disclosure
Distinguished Names typically consist of descriptive information
about the entries they name, which can be people, organizations,
devices or other real-world objects. This frequently includes some
of the following kinds of information:
- the common name of the object (i.e. a person's full name)
- an email or TCP/IP address
- its physical location (country, locality, city, street address)
- organizational attributes (such as department name or affiliation)
Most countries have privacy laws regarding the publication of
information about people.
7.2. Use of Distinguished Names in Security Applications
The transformations of an AttributeValue value from its X.501 form to
an LDAP string representation are not always reversible back to the
same BER or DER form. An example of a situation which requires the
DER form of a distinguished name is the verification of an X.509
certificate.
For example, a distinguished name consisting of one RDN with one AVA,
in which the type is commonName and the value is of the TeletexString
choice with the letters 'Sam' would be represented in LDAP as the
string CN=Sam. Another distinguished name in which the value is
still 'Sam' but of the PrintableString choice would have the same
representation CN=Sam.
Applications which require the reconstruction of the DER form of the
value SHOULD NOT use the string representation of attribute syntaxes
when converting a distinguished name to the LDAP format. Instead,
they SHOULD use the hexadecimal form prefixed by the octothorpe ('#')
as described in the first paragraph of section 2.4.
8. Authors' Addresses
Mark Wahl
Critical Angle Inc.
4815 W. Braker Lane #502-385
Austin, TX 78759
USA
EMail: M.Wahl@critical-angle.com
Steve Kille
Isode Ltd.
The Dome
The Square
Richmond, Surrey
TW9 1DT
England
Phone: +44-181-332-9091
EMail: S.Kille@ISODE.COM
Tim Howes
Netscape Communications Corp.
501 E. Middlefield Rd, MS MV068
Mountain View, CA 94043
USA
Phone: +1 650 937-3419
EMail: howes@netscape.com
9. Full Copyright Statement
Copyright (C) The Internet Society (1997). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
ISO Plain Language
On this page
§1Background and Goals
The X.500 Directory uses distinguished names as the primary keys to its entries, and encodes them in ASN.1 in the X.500 directory protocols. In the Lightweight Directory Access Protocol, a string representation of distinguished names is transferred instead. This specification defines that string format, and it is designed to give a clean representation of commonly used distinguished names while still being able to represent any distinguished name.
A common format matters because a distinguished name has to be represented unambiguously. The primary goal of the specification is ease of encoding and decoding. The secondary goal is names that are human readable. The RFC does not expect an LDAP client with a user interface to show these strings directly, but rather to translate them, for example by expressing attribute type names in the user's own language.
The document also carries the same warning as the rest of the LDAPv3 series. Until mandatory authentication mechanisms are standardised, clients and servers that make use of update functionality are unlikely to interoperate, or may interoperate only if authentication is reduced to an unacceptably weak level. Implementors are discouraged from deploying such clients and servers in the meantime.
Directory entries are looked up by a name that is a list of facts about the entry, such as the country and the organisation. The list has to be written down in one agreed way, so that every program reading it gets the same answer.
§2Turning a Distinguished Name into Text
In X.501 the ASN.1 structure of a distinguished name is a sequence of relative distinguished names, each of which is a set of attribute type and value pairs.
To convert that structure into a string, an empty sequence becomes the empty string. Otherwise the output is the string encoding of each relative distinguished name, starting with the last element of the sequence and moving backwards toward the first. The encodings of adjoining relative distinguished names are separated by a comma character, ASCII 44.
Each relative distinguished name becomes the string encodings of its attribute type and value pairs, in any order, and where the name has more than one pair the outputs of adjoining pairs are separated by a plus character, ASCII 43.
Each attribute type and value pair is written as the string representation of the type, an equals character, ASCII 61, and then the string representation of the value. If the type appears in the published table of attribute types associated with LDAP, the short name from that table is used. Otherwise the type is written as the dotted-decimal encoding of its object identifier. The table gives CN for commonName, L for localityName, ST for stateOrProvinceName, O for organizationName, OU for organizationalUnitName, C for countryName, STREET for streetAddress, DC for domainComponent and UID for userid.
The value itself is written in one of two ways. If the attribute value has no string representation defined for it, it is written as an octothorpe character, ASCII 35, followed by the hexadecimal representation of each byte of the BER encoding of the value. This form should be used when the attribute type is in the dotted-decimal form. Otherwise the value is converted to a UTF-8 string according to its syntax specification.
That UTF-8 string needs escaping if it has a space or a hash character at the beginning, a space character at the end, or one of the characters comma, plus, quotation mark, backslash, less-than, greater-than or semicolon. Implementations may escape other characters. A character on the list is prefixed with a backslash. Any other character to be escaped is replaced by a backslash and two hexadecimal digits that give the byte for that character.
The name is written from the end backwards, so the most specific piece comes first. Each piece is a short name, an equals sign and a value. A comma separates the pieces and a plus sign separates the parts of a piece that has more than one part.
| String | X.500 AttributeType |
|---|---|
| CN | commonName |
| L | localityName |
| ST | stateOrProvinceName |
| O | organizationName |
| OU | organizationalUnitName |
| C | countryName |
| STREET | streetAddress |
| DC | domainComponent |
| UID | userid |
- Leading space or hashA space or the hash character at the beginning of the value must be escaped.
- Trailing spaceA space at the end of the value must be escaped.
- Special charactersA comma, plus sign, quotation mark, backslash, less-than, greater-than or semicolon must be escaped.
- How to escapePut a backslash before the character, or a backslash and two hexadecimal digits giving the byte for it.
§3Parsing Text back into a Distinguished Name
The structure of the string is given as a BNF grammar based on the grammar in RFC 822. A distinguished name may be the empty string. A name is one or more name components separated by commas. A name component is one or more attribute type and value pairs separated by plus signs, and each pair is an attribute type, an equals sign and a value. The attribute type is either a name made of a letter followed by letters, digits or hyphens, or an object identifier written as numbers separated by dots.
The value is a string, which is either a run of ordinary characters and escape pairs, or a hash followed by a hex string, or, only from version 2, a quotation mark, a run of characters and a closing quotation mark. Inside the quoted value the comma, equals, plus, less-than, greater-than, hash and semicolon may all appear without escaping. A pair is a backslash followed by one of the special characters, a backslash, a quotation mark, or a pair of hexadecimal digits.
Server implementations that parse a DN string generated by an LDAPv2 client must also accept, and ignore, the variants given in the next section.
To read a name back, you split it on the commas, split each piece on the plus signs, and split each of those on the first equals sign. What is left is the type on one side and the value on the other, with any backslash escapes undone.
distinguishedName = [name]\nname = name-component *("," name-component)\nname-component = attributeTypeAndValue *("+" attributeTypeAndValue)\nattributeTypeAndValue = attributeType "=" attributeValue\nattributeType = (ALPHA 1*keychar) / oid§4Relationship with RFC 1779 and LDAPv2
The syntax in this document is more restrictive than the syntax in RFC 1779. Implementations that parse a string generated by an LDAPv2 client must accept the RFC 1779 syntax. Implementations must not, however, generate any of the RFC 1779 encodings that are not described in the conversion section.
Four older forms have to be accepted on input. A semicolon may be used instead of a comma to separate relative distinguished names, and whitespace may appear on either side of the comma or semicolon; the whitespace is ignored and the semicolon is replaced with a comma. An object identifier in the attribute type may be prefixed with the character string "oid." or "OID.". Spaces may appear between a name component and a comma, between an attribute type and value pair and a plus sign, between an attribute type and the equals sign, and between the equals sign and the value. And a value may be surrounded by quotation marks, which are not part of the value, inside which the comma, equals, plus, less-than, greater-than, hash and semicolon may all appear without escaping.
Older programs wrote names in a slightly looser way, using a semicolon instead of a comma, or putting spaces in odd places, or quoting the value. A reader has to understand those, but a writer must use the tidy form.
§5Examples
The notation is designed to be convenient for the common forms of name. A name with three relative distinguished names is written CN=Steve Kille,O=Isode Limited,C=GB.
An example with three relative distinguished names in which the first is multi-valued is written OU=Sales+CN=J. Smith,O=Widget Inc.,C=US.
An example showing how a comma inside an organisation name is quoted is written CN=L. Eagle,O=Sue\, Grabbit and Runn,C=GB. An example with a carriage return character in the value is written CN=Before\0DAfter,O=Test,C=GB.
An example in which a relative distinguished name was of an unrecognised type, so the value is the BER encoding of an OCTET STRING containing the two bytes 0x48 and 0x69, is written 1.3.6.1.4.1.1466.0=#04024869,O=Test,C=GB.
The last example shows a surname value of five letters, including two that are not in plain ASCII. The Latin small letter C with a caron has the 10646 code U0000010D, is 0xC48D in UTF-8, and is written \C4\8D. The Latin small letter C with an acute has the code U00000107, is 0xC487 in UTF-8, and is written \C4\87. Together the five letters can be written in printable ASCII, which is useful for debugging, as SN=Lu\C4\8Di\C4\87.
The examples show a plain name, a name with two parts in one piece, a name with a comma in it, and a name with letters outside plain English. The last of those can also be written using only plain ASCII characters.
§7Security Considerations
Distinguished names typically consist of descriptive information about the entries they name, which can be people, organizations, devices or other real-world objects. That frequently includes the common name of the object, such as a person's full name, an email or TCP/IP address, its physical location such as country, locality, city and street address, and organisational attributes such as a department name or affiliation. Most countries have privacy laws regarding the publication of information about people.
The RFC also warns that turning an attribute value from its X.501 form into an LDAP string is not always reversible back to the same BER or DER form. The example is a distinguished name with one relative distinguished name holding one attribute type and value, where the type is commonName and the value is the text Sam. If that value came from the TeletexString choice the LDAP form is CN=Sam, and if it came from the PrintableString choice the LDAP form is also CN=Sam, even though the two original values were different. An application that needs to reconstruct the DER form of the value, such as one verifying an X.509 certificate, should not use the string representation of attribute syntaxes, but the hexadecimal form prefixed by the octothorpe instead.
Names often contain personal details, and privacy laws apply to publishing them. There is also a trap: the readable form of a name can lose which of several string types a value had, so anything that has to rebuild the original bytes should use the hexadecimal form instead.