LDAP (Lightweight Directory Access Protocol) is a protocol that lets clients access directory services. LDAP is not the directory’s data: it is the way a client communicates with a directory. Inside that directory, information is arranged in a tree of entries; each entry has attributes, and its place in the tree is described by a distinguished name (DN).
What is LDAP?
LDAP stands for Lightweight Directory Access Protocol. It defines how clients communicate with directory services and the meaning and encoding of protocol elements. The protocol is specified in RFC 4511.
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The key distinction is between the access protocol and the information service: LDAP is the protocol; the directory stores and organizes information. A client uses LDAP to access that service.
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A directory organizes its information in a Directory Information Tree (DIT). The tree is made up of entries connected by parent-child relationships. The directory model is described in RFC 4512.
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Entries contain attributes
An entry is a named collection of information and the basic unit held in a directory. RFC 4512 puts it this way: “A directory entry, a named collection of information, is the basic unit of information held in the Directory.” An entry contains attributes, and each attribute has an attribute description and one or more values.
Schema defines what entries can contain
A directory’s schema constrains which object classes and attribute types an entry may use, as well as the values those attributes may hold. In other words, an entry is not simply an unrestricted bundle of text: its permitted structure and content depend on the directory’s schema.
What is the difference between an RDN and a DN?
An RDN (Relative Distinguished Name) names an entry relative to its immediate parent. A DN (Distinguished Name) combines that entry’s RDN with the DNs of its parent entries, giving a name that identifies the entry’s place in the tree.
RDN: a name among siblings
An RDN consists of one or more attribute-value assertions (AVAs). It must be unique among the children of the same parent, but it need not be unique across the entire directory. An RDN can also be multi-valued, with more than one AVA contributing to the name.
DN: the entry and its path
Consider CN=John Smith,OU=Sales,O=ACME Limited,L=Moab,ST=Utah,C=US. Its leftmost component, CN=John Smith, is the RDN for the entry. The following components identify successive parent entries in the tree. This is a structural illustration, not a required directory layout; directories do not all use these containers.
The relationship can be read from left to right as entry first, then parent, then the next parent up the tree. That is why an RDN is relative while a DN identifies an entry in its directory context.
How is a DN written?
In LDAP’s DN string form, commas separate RDNs and an equals sign separates an attribute type from its value. In a multi-valued RDN, plus signs join the AVAs. These rules are defined in RFC 4514.
DN values cannot always be copied as plain, unescaped text. RFC 4514 specifies escaping for certain characters and positions, including a space or # at the beginning of a value, a space at its end, and punctuation such as commas, plus signs, quotation marks, backslashes, angle brackets, semicolons, and equals signs. A comma inside an escaped value, for example, is not a separator between RDNs. Treat a DN as structured syntax rather than an arbitrary comma-separated label.
Does the DN string determine whether two names are equal?
No. RFC 4514 does not define a canonical string representation for DNs. Two strings that look different are not necessarily different names, and byte-for-byte comparison alone is not the rule for deciding DN equality. The applicable matching rule is distinguishedNameMatch. Applications that need to compare DNs should use the directory’s defined matching behavior rather than rely only on the printed spelling.
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Can a DN reveal personal or organizational information?
It can. DNs may contain descriptive details such as people’s names, email addresses, locations, or organizational information. RFC 4514 warns that this information can be sensitive. Treat DNs shown in logs, screenshots, or examples as potentially identifying data, and avoid sharing them casually.
LDAP’s directory model and DN syntax do not, by themselves, prescribe a deployment’s authentication or transport-security configuration. Those topics are addressed separately in RFC 4513 and RFC 4511.
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