Network Working Group K. Zeilenga, Ed.

Network Working Group K. Zeilenga, Ed.
Request for Comments: 4514 OpenLDAP Foundation
Obsoletes: 2253 June 2006
Category: Standards Track

Lightweight Directory Access Protocol (LDAP):
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 (2006).

Abstract

The X.500 Directory uses distinguished names (DNs) as primary keys to
entries in the directory. This document defines the string
representation used in the Lightweight Directory Access Protocol
(LDAP) to transfer distinguished names. The string representation is
designed to give a clean representation of commonly used
distinguished names, while being able to represent any distinguished
name.

1. Background and Intended Usage

In X.500-based directory systems [X.500], including those accessed
using the Lightweight Directory Access Protocol (LDAP) [RFC4510],
distinguished names (DNs) are used to unambiguously refer to
directory entries [X.501][RFC4512].

The structure of a DN [X.501] is described in terms of ASN.1 [X.680].
In the X.500 Directory Access Protocol [X.511] (and other ITU-defined
directory protocols), DNs are encoded using the Basic Encoding Rules
(BER) [X.690]. In LDAP, DNs are represented in the string form
described in this document.

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

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implementations with a human user interface would display these
strings directly to the user, but that they would most likely be
performing translations (such as expressing attribute type names in
the local national language).

This document defines the string representation of Distinguished
Names used in LDAP [RFC4511][RFC4517]. Section 2 details the
RECOMMENDED algorithm for converting a DN from its ASN.1 structured
representation to a string. Section 3 details how to convert a DN
from a string to an ASN.1 structured representation.

While other documents may define other algorithms for converting a DN
from its ASN.1 structured representation to a string, all algorithms
MUST produce strings that adhere to the requirements of Section 3.

This document does not define a canonical string representation for
DNs. Comparison of DNs for equality is to be performed in accordance
with the distinguishedNameMatch matching rule [RFC4517].

This document is a integral part of the LDAP technical specification
[RFC4510], which obsoletes the previously defined LDAP technical
specification, RFC 3377, in its entirety. This document obsoletes
RFC 2253. Changes since RFC 2253 are summarized in Appendix B.

This specification assumes familiarity with X.500 [X.500] and the
concept of Distinguished Name [X.501][RFC4512].

1.1. Conventions

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 BCP 14 [RFC2119].

Character names in this document use the notation for code points and
names from the Unicode Standard [Unicode]. For example, the letter
"a" may be represented as either <U+0061> or <LATIN SMALL LETTER A>.

Note: a glossary of terms used in Unicode can be found in [Glossary].
Information on the Unicode character encoding model can be found in
[CharModel].

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2. Converting DistinguishedName from ASN.1 to a String

X.501 [X.501] defines the ASN.1 [X.680] structure of distinguished
name. The following is a variant provided for discussion purposes.

DistinguishedName ::= RDNSequence

RDNSequence ::= SEQUENCE OF RelativeDistinguishedName

RelativeDistinguishedName ::= SET SIZE (1..MAX) OF
AttributeTypeAndValue

AttributeTypeAndValue ::= SEQUENCE {
type AttributeType,
value AttributeValue }

This section defines the RECOMMENDED algorithm for converting a
distinguished name from an ASN.1-structured representation to a UTF-8
[RFC3629] encoded Unicode [Unicode] character string representation.
Other documents may describe other algorithms for converting a
distinguished name to a string, but only strings that conform to the
grammar defined in Section 3 SHALL be produced by LDAP
implementations.

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 Section
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 (',' U+002C) character.

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 Section 2.3), in any order.

Where there is a multi-valued RDN, the outputs from adjoining
AttributeTypeAndValues are separated by a plus sign ('+' U+002B)
character.

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2.3. Converting AttributeTypeAndValue

The AttributeTypeAndValue is encoded as the string representation of
the AttributeType, followed by an equals sign ('=' U+003D) character,
followed by the string representation of the AttributeValue. The
encoding of the AttributeValue is given in Section 2.4.

If the AttributeType is defined to have a short name (descriptor)
[RFC4512] and that short name is known to be registered [REGISTRY]
[RFC4520] as identifying the AttributeType, that short name, a
<descr>, is used. Otherwise the AttributeType is encoded as the
dotted-decimal encoding, a <numericoid>, of its OBJECT IDENTIFIER.
The <descr> and <numericoid> are defined in [RFC4512].

Implementations are not expected to dynamically update their
knowledge of registered short names. However, implementations SHOULD
provide a mechanism to allow their knowledge of registered short
names to be updated.

2.4. Converting an AttributeValue from ASN.1 to a String

If the AttributeType is of the dotted-decimal form, the
AttributeValue is represented by an number sign ('#' U+0023)
character followed by the hexadecimal encoding of each of the octets
of the BER encoding of the X.500 AttributeValue. This form is also
used when the syntax of the AttributeValue does not have an LDAP-
specific ([RFC4517], Section 3.1) string encoding defined for it, or
the LDAP-specific string encoding is not restricted to UTF-8-encoded
Unicode characters. This form may also be used in other cases, such
as when a reversible string representation is desired (see Section
5.2).

Otherwise, if the AttributeValue is of a syntax that has a LDAP-
specific string encoding, the value is converted first to a UTF-8-
encoded Unicode string according to its syntax specification (see
[RFC4517], Section 3.3, for examples). If that UTF-8-encoded Unicode
string does not have any of the following characters that need
escaping, then that string can be used as the string representation
of the value.

- a space (' ' U+0020) or number sign ('#' U+0023) occurring at
the beginning of the string;

- a space (' ' U+0020) character occurring at the end of the
string;

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RFC 4514 LDAP: Distinguished Names June 2006

- one of the characters '"', '+', ',', ';', '<', '>', or '\'
(U+0022, U+002B, U+002C, U+003B, U+003C, U+003E, or U+005C,
respectively);

- the null (U+0000) character.

Other characters may be escaped.

Each octet of the character to be escaped is replaced by a backslash
and two hex digits, which form a single octet in the code of the
character. Alternatively, if and only if the character to be escaped
is one of

' ', '"', '#', '+', ',', ';', '<', '=', '>', or '\'
(U+0020, U+0022, U+0023, U+002B, U+002C, U+003B,
U+003C, U+003D, U+003E, U+005C, respectively)

it can be prefixed by a backslash ('\' U+005C).

Examples of the escaping mechanism are shown in Section 4.

3. Parsing a String Back to a Distinguished Name

The string representation of Distinguished Names is restricted to
UTF-8 [RFC3629] encoded Unicode [Unicode] characters. The structure
of this string representation is specified using the following
Augmented BNF [RFC4234] grammar:

distinguishedName = [ relativeDistinguishedName
*( COMMA relativeDistinguishedName ) ]
relativeDistinguishedName = attributeTypeAndValue
*( PLUS attributeTypeAndValue )
attributeTypeAndValue = attributeType EQUALS attributeValue
attributeType = descr / numericoid
attributeValue = string / hexstring

; The following characters are to be escaped when they appear
; in the value to be encoded: ESC, one of <escaped>, leading
; SHARP or SPACE, trailing SPACE, and NULL.
string = [ ( leadchar / pair ) [ *( stringchar / pair )
( trailchar / pair ) ] ]

leadchar = LUTF1 / UTFMB
LUTF1 = %x01-1F / %x21 / %x24-2A / %x2D-3A /
%x3D / %x3F-5B / %x5D-7F

trailchar = TUTF1 / UTFMB
TUTF1 = %x01-1F / %x21 / %x23-2A / %x2D-3A /

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%x3D / %x3F-5B / %x5D-7F

stringchar = SUTF1 / UTFMB
SUTF1 = %x01-21 / %x23-2A / %x2D-3A /
%x3D / %x3F-5B / %x5D-7F

pair = ESC ( ESC / special / hexpair )
special = escaped / SPACE / SHARP / EQUALS
escaped = DQUOTE / PLUS / COMMA / SEMI / LANGLE / RANGLE
hexstring = SHARP 1*hexpair
hexpair = HEX HEX

where the productions <descr>, <numericoid>, <COMMA>, <DQUOTE>,
<EQUALS>, <ESC>, <HEX>, <LANGLE>, <NULL>, <PLUS>, <RANGLE>, <SEMI>,
<SPACE>, <SHARP>, and <UTFMB> are defined in [RFC4512].

Each <attributeType>, either a <descr> or a <numericoid>, refers to
an attribute type of an attribute value assertion (AVA). The
<attributeType> is followed by an <EQUALS> and an <attributeValue>.
The <attributeValue> is either in <string> or <hexstring> form.

If in <string> form, a LDAP string representation asserted value can
be obtained by replacing (left to right, non-recursively) each <pair>
appearing in the <string> as follows:

replace <ESC><ESC> with <ESC>;
replace <ESC><special> with <special>;
replace <ESC><hexpair> with the octet indicated by the <hexpair>.

If in <hexstring> form, a BER representation can be obtained from
converting each <hexpair> of the <hexstring> to the octet indicated
by the <hexpair>.

There is one or more attribute value assertions, separated by <PLUS>,
for a relative distinguished name.

There is zero or more relative distinguished names, separated by
<COMMA>, for a distinguished name.

Implementations MUST recognize AttributeType name strings
(descriptors) listed in the following table, but MAY recognize other
name strings.

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String X.500 AttributeType
------ --------------------------------------------
CN commonName (2.5.4.3)
L localityName (2.5.4.7)
ST stateOrProvinceName (2.5.4.8)
O organizationName (2.5.4.10)
OU organizationalUnitName (2.5.4.11)
C countryName (2.5.4.6)
STREET streetAddress (2.5.4.9)
DC domainComponent (0.9.2342.19200300.100.1.25)
UID userId (0.9.2342.19200300.100.1.1)

These attribute types are described in [RFC4519].

Implementations MAY recognize other DN string representations.
However, as there is no requirement that alternative DN string
representations be recognized (and, if so, how), implementations
SHOULD only generate DN strings in accordance with Section 2 of this
document.

4. 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):

UID=jsmith,DC=example,DC=net

Here is an example of a name containing three RDNs, in which the
first RDN is multi-valued:

OU=Sales+CN=J. Smith,DC=example,DC=net

This example shows the method of escaping of a special characters
appearing in a common name:

CN=James \"Jim\" Smith\, III,DC=example,DC=net

The following shows the method for encoding a value that contains a
carriage return character:

CN=Before\0dAfter,DC=example,DC=net

In this RDN example, the type in the RDN is unrecognized, and the
value is the BER encoding of an OCTET STRING containing two octets,
0x48 and 0x69.

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RFC 4514 LDAP: Distinguished Names June 2006

1.3.6.1.4.1.1466.0=#04024869

Finally, this example shows an RDN whose commonName value consists of
5 letters:

Unicode Character Code UTF-8 Escaped
------------------------------- ------ ------ --------
LATIN CAPITAL LETTER L U+004C 0x4C L
LATIN SMALL LETTER U U+0075 0x75 u
LATIN SMALL LETTER C WITH CARON U+010D 0xC48D \C4\8D
LATIN SMALL LETTER I U+0069 0x69 i
LATIN SMALL LETTER C WITH ACUTE U+0107 0xC487 \C4\87

This could be encoded in printable ASCII [ASCII] (useful for
debugging purposes) as:

CN=Lu\C4\8Di\C4\87

5. Security Considerations

The following security considerations are specific to the handling of
distinguished names. LDAP security considerations are discussed in
[RFC4511] and other documents comprising the LDAP Technical
Specification [RFC4510].

5.1. 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)

In some cases, such information can be considered sensitive. In many
countries, privacy laws exist that prohibit disclosure of certain
kinds of descriptive information (e.g., email addresses). Hence,
server implementers are encouraged to support Directory Information
Tree (DIT) structural rules and name forms [RFC4512], as these
provide a mechanism for administrators to select appropriate naming
attributes for entries. Administrators are encouraged to use
mechanisms, access controls, and other administrative controls that
may be available to restrict use of attributes containing sensitive
information in naming of entries. Additionally, use of

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authentication and data security services in LDAP [RFC4513][RFC4511]
should be considered.

5.2. 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 (Basic Encoding Rules) or DER (Distinguished Encoding Rules)
form. An example of a situation that 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 is of the PrintableString choice, would have the
same representation <CN=Sam>.

Applications that 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 number sign ('#'
U+0023) as described in the first paragraph of Section 2.4.

6. Acknowledgements

This document is an update to RFC 2253, by Mark Wahl, Tim Howes, and
Steve Kille. RFC 2253 was a product of the IETF ASID Working Group.

This document is a product of the IETF LDAPBIS Working Group.

7. References

7.1. Normative References

[REGISTRY] IANA, Object Identifier Descriptors Registry,
<http://www.iana.org/assignments/ldap-parameters>.

[Unicode] The Unicode Consortium, "The Unicode Standard, Version
3.2.0" is defined by "The Unicode Standard, Version
3.0" (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-
61633-5), as amended by the "Unicode Standard Annex
#27: Unicode 3.1"
(http://www.unicode.org/reports/tr27/) and by the
"Unicode Standard Annex #28: Unicode 3.2"
(http://www.unicode.org/reports/tr28/).

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RFC 4514 LDAP: Distinguished Names June 2006

[X.501] International Telecommunication Union -
Telecommunication Standardization Sector, "The
Directory -- Models," X.501(1993) (also ISO/IEC 9594-
2:1994).

[X.680] International Telecommunication Union -
Telecommunication Standardization Sector, "Abstract
Syntax Notation One (ASN.1) - Specification of Basic
Notation", X.680(1997) (also ISO/IEC 8824-1:1998).

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.

[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.

[RFC4510] Zeilenga, K., Ed., "Lightweight Directory Access
Protocol (LDAP): Technical Specification Road Map", RFC
4510, June 2006.

[RFC4511] Sermersheim, J., Ed., "Lightweight Directory Access
Protocol (LDAP): The Protocol", RFC 4511, June 2006.

[RFC4512] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Directory Information Models", RFC 4512, June
2006.

[RFC4513] Harrison, R., Ed., "Lightweight Directory Access
Protocol (LDAP): Authentication Methods and Security
Mechanisms", RFC 4513, June 2006.

[RFC4517] Legg, S., Ed., "Lightweight Directory Access Protocol
(LDAP): Syntaxes and Matching Rules", RFC 4517, June
2006.

[RFC4519] Sciberras, A., Ed., "Lightweight Directory Access
Protocol (LDAP): Schema for User Applications", RFC
4519, June 2006.

[RFC4520] Zeilenga, K., "Internet Assigned Numbers Authority
(IANA) Considerations for the Lightweight Directory
Access Protocol (LDAP)", BCP 64, RFC 4520, June 2006.

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RFC 4514 LDAP: Distinguished Names June 2006

7.2. Informative References

[ASCII] Coded Character Set--7-bit American Standard Code for
Information Interchange, ANSI X3.4-1986.

[CharModel] Whistler, K. and M. Davis, "Unicode Technical Report
#17, Character Encoding Model", UTR17,
<http://www.unicode.org/unicode/reports/tr17/>, August
2000.

[Glossary] The Unicode Consortium, "Unicode Glossary",
<http://www.unicode.org/glossary/>.

[X.500] International Telecommunication Union -
Telecommunication Standardization Sector, "The
Directory -- Overview of concepts, models and
services," X.500(1993) (also ISO/IEC 9594-1:1994).

[X.511] International Telecommunication Union -
Telecommunication Standardization Sector, "The
Directory: Abstract Service Definition", X.511(1993)
(also ISO/IEC 9594-3:1993).

[X.690] International Telecommunication Union -
Telecommunication Standardization Sector,
"Specification of ASN.1 encoding rules: Basic Encoding
Rules (BER), Canonical Encoding Rules (CER), and
Distinguished Encoding Rules (DER)", X.690(1997) (also
ISO/IEC 8825-1:1998).

[RFC2849] Good, G., "The LDAP Data Interchange Format (LDIF) -
Technical Specification", RFC 2849, June 2000.

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Appendix A. Presentation Issues

This appendix is provided for informational purposes only; it is not
a normative part of this specification.

The string representation described in this document is not intended
to be presented to humans without translation. However, at times it
may be desirable to present non-translated DN strings to users. This
section discusses presentation issues associated with non-translated
DN strings. Issues with presentation of translated DN strings are
not discussed in this appendix. Transcoding issues are also not
discussed in this appendix.

This appendix provides guidance for applications presenting DN
strings to users. This section is not comprehensive; it does not
discuss all presentation issues that implementers may face.

Not all user interfaces are capable of displaying the full set of
Unicode characters. Some Unicode characters are not displayable.

It is recommended that human interfaces use the optional hex pair
escaping mechanism (Section 2.3) to produce a string representation
suitable for display to the user. For example, an application can
generate a DN string for display that escapes all non-printable
characters appearing in the AttributeValue's string representation
(as demonstrated in the final example of Section 4).

When a DN string is displayed in free-form text, it is often
necessary to distinguish the DN string from surrounding text. While
this is often done with whitespace (as demonstrated in Section 4), it
is noted that DN strings may end with whitespace. Careful readers of
Section 3 will note that the characters '<' (U+003C) and '>' (U+003E)
may only appear in the DN string if escaped. These characters are
intended to be used in free-form text to distinguish a DN string from
surrounding text. For example, <CN=Sam\ > distinguishes the string
representation of the DN composed of one RDN consisting of the AVA
(the commonName (CN) value 'Sam ') from the surrounding text. It
should be noted to the user that the wrapping '<' and '>' characters
are not part of the DN string.

DN strings can be quite long. It is often desirable to line-wrap
overly long DN strings in presentations. Line wrapping should be
done by inserting whitespace after the RDN separator character or, if
necessary, after the AVA separator character. It should be noted to
the user that the inserted whitespace is not part of the DN string
and is to be removed before use in LDAP. For example, the following
DN string is long:

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CN=Kurt D. Zeilenga,OU=Engineering,L=Redwood Shores,
O=OpenLDAP Foundation,ST=California,C=US

So it has been line-wrapped for readability. The extra whitespace is
to be removed before the DN string is used in LDAP.

Inserting whitespace is not advised because it may not be obvious to
the user which whitespace is part of the DN string and which
whitespace was added for readability.

Another alternative is to use the LDAP Data Interchange Format (LDIF)
[RFC2849]. For example:

# This entry has a long DN...
dn: CN=Kurt D. Zeilenga,OU=Engineering,L=Redwood Shores,
O=OpenLDAP Foundation,ST=California,C=US
CN: Kurt D. Zeilenga
SN: Zeilenga
objectClass: person

Appendix B. Changes Made since RFC 2253

This appendix is provided for informational purposes only, it is not
a normative part of this specification.

The following substantive changes were made to RFC 2253:

- Removed IESG Note. The IESG Note has been addressed.
- Replaced all references to ISO 10646-1 with [Unicode].
- Clarified (in Section 1) that this document does not define a
canonical string representation.
- Clarified that Section 2 describes the RECOMMENDED encoding
algorithm and that alternative algorithms are allowed. Some
encoding options described in RFC 2253 are now treated as
alternative algorithms in this specification.
- Revised specification (in Section 2) to allow short names of any
registered attribute type to appear in string representations of
DNs instead of being restricted to a "published table". Removed
"as an example" language. Added statement (in Section 3)
allowing recognition of additional names but require recognition
of those names in the published table. The table now appears in
Section 3.
- Removed specification of additional requirements for LDAPv2
implementations which also support LDAPv3 (RFC 2253, Section 4)
as LDAPv2 is now Historic.
- Allowed recognition of alternative string representations.
- Updated Section 2.4 to allow hex pair escaping of all characters
and clarified escaping for when multiple octet UTF-8 encodings

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are present. Indicated that null (U+0000) character is to be
escaped. Indicated that equals sign ('=' U+003D) character may
be escaped as '\='.
- Rewrote Section 3 to use ABNF as defined in RFC 4234.
- Updated the Section 3 ABNF. Changes include:
+ allowed AttributeType short names of length 1 (e.g., 'L'),
+ used more restrictive <oid> production in AttributeTypes,
+ did not require escaping of equals sign ('=' U+003D)
characters,
+ did not require escaping of non-leading number sign ('#'
U+0023) characters,
+ allowed space (' ' U+0020) to be escaped as '\ ',
+ required hex escaping of null (U+0000) characters, and
+ removed LDAPv2-only constructs.
- Updated Section 3 to describe how to parse elements of the
grammar.
- Rewrote examples.
- Added reference to documentations containing general LDAP
security considerations.
- Added discussion of presentation issues (Appendix A).
- Added this appendix.

In addition, numerous editorial changes were made.

Editor's Address

Kurt D. Zeilenga
OpenLDAP Foundation

EMail: [email protected]

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RFC 4514 LDAP: Distinguished Names June 2006

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