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This section contains the discussion on naming realms and principals from the Kerberos specification.
Although realm names are encoded as GeneralStrings and although a realm can technically select any name it chooses, interoperability across realm boundaries requires agreement on how realm names are to be assigned, and what information they imply.
To enforce these conventions, each realm MUST conform to the conventions itself, and it MUST require that any realms with which inter-realm keys are shared also conform to the conventions and require the same from its neighbors.
Kerberos realm names are case sensitive. Realm names that differ only in the case of the characters are not equivalent. There are presently three styles of realm names: domain, X500, and other. Examples of each style follow:
domain: ATHENA.MIT.EDU
X500: C=US/O=OSF
other: NAMETYPE:rest/of.name=without-restrictions
Domain syle realm names MUST look like domain names: they consist of components separated by periods (.) and they contain neither colons (:) nor slashes (/). Though domain names themselves are case insensitive, in order for realms to match, the case must match as well. When establishing a new realm name based on an internet domain name it is recommended by convention that the characters be converted to upper case.
X.500 names contain an equal (=) and cannot contain a colon (:) before the equal. The realm names for X.500 names will be string representations of the names with components separated by slashes. Leading and trailing slashes will not be included. Note that the slash separator is consistent with Kerberos implementations based on RFC1510, but it is different from the separator recommended in RFC2253.
Names that fall into the other category MUST begin with a prefix that contains no equal (=) or period (.) and the prefix MUST be followed by a colon (:) and the rest of the name. All prefixes must be assigned before they may be used. Presently none are assigned.
The reserved category includes strings which do not fall into the first three categories. All names in this category are reserved. It is unlikely that names will be assigned to this category unless there is a very strong argument for not using the ’other’ category.
These rules guarantee that there will be no conflicts between the various name styles. The following additional constraints apply to the assignment of realm names in the domain and X.500 categories: the name of a realm for the domain or X.500 formats must either be used by the organization owning (to whom it was assigned) an Internet domain name or X.500 name, or in the case that no such names are registered, authority to use a realm name MAY be derived from the authority of the parent realm. For example, if there is no domain name for E40.MIT.EDU, then the administrator of the MIT.EDU realm can authorize the creation of a realm with that name.
This is acceptable because the organization to which the parent is assigned is presumably the organization authorized to assign names to its children in the X.500 and domain name systems as well. If the parent assigns a realm name without also registering it in the domain name or X.500 hierarchy, it is the parent’s responsibility to make sure that there will not in the future exist a name identical to the realm name of the child unless it is assigned to the same entity as the realm name.
As was the case for realm names, conventions are needed to ensure that all agree on what information is implied by a principal name. The name-type field that is part of the principal name indicates the kind of information implied by the name. The name-type SHOULD be treated only as a hint to interpreting the meaning of a name. It is not significant when checking for equivalence. Principal names that differ only in the name-type identify the same principal. The name type does not partition the name space. Ignoring the name type, no two names can be the same (i.e. at least one of the components, or the realm, MUST be different). The following name types are defined:
name-type value meaning NT-UNKNOWN 0 Name type not known NT-PRINCIPAL 1 Just the name of the principal as in DCE, or for users NT-SRV-INST 2 Service and other unique instance (krbtgt) NT-SRV-HST 3 Service with host name as instance (telnet, rcommands) NT-SRV-XHST 4 Service with host as remaining components NT-UID 5 Unique ID NT-X500-PRINCIPAL 6 Encoded X.509 Distingished name [RFC 2253] NT-SMTP-NAME 7 Name in form of SMTP email name (e.g. user@foo.com) NT-ENTERPRISE 10 Enterprise name - may be mapped to principal name
When a name implies no information other than its uniqueness at a particular time the name type PRINCIPAL SHOULD be used. The principal name type SHOULD be used for users, and it might also be used for a unique server. If the name is a unique machine generated ID that is guaranteed never to be reassigned then the name type of UID SHOULD be used (note that it is generally a bad idea to reassign names of any type since stale entries might remain in access control lists).
If the first component of a name identifies a service and the remaining components identify an instance of the service in a server specified manner, then the name type of SRV-INST SHOULD be used. An example of this name type is the Kerberos ticket-granting service whose name has a first component of krbtgt and a second component identifying the realm for which the ticket is valid.
If the first component of a name identifies a service and there is a single component following the service name identifying the instance as the host on which the server is running, then the name type SRV- HST SHOULD be used. This type is typically used for Internet services such as telnet and the Berkeley R commands. If the separate components of the host name appear as successive components following the name of the service, then the name type SRV-XHST SHOULD be used. This type might be used to identify servers on hosts with X.500 names where the slash (/) might otherwise be ambiguous.
A name type of NT-X500-PRINCIPAL SHOULD be used when a name from an X.509 certificate is translated into a Kerberos name. The encoding of the X.509 name as a Kerberos principal shall conform to the encoding rules specified in RFC 2253.
A name type of SMTP allows a name to be of a form that resembles a SMTP email name. This name, including an "@" and a domain name, is used as the one component of the principal name.
A name type of UNKNOWN SHOULD be used when the form of the name is not known. When comparing names, a name of type UNKNOWN will match principals authenticated with names of any type. A principal authenticated with a name of type UNKNOWN, however, will only match other names of type UNKNOWN.
Names of any type with an initial component of ’krbtgt’ are reserved for the Kerberos ticket granting service. See Name of the TGS, for the form of such names.
The principal identifier for a server on a host will generally be composed of two parts: (1) the realm of the KDC with which the server is registered, and (2) a two-component name of type NT-SRV-HST if the host name is an Internet domain name or a multi-component name of type NT-SRV-XHST if the name of the host is of a form such as X.500 that allows slash (/) separators. The first component of the two- or multi-component name will identify the service and the latter components will identify the host. Where the name of the host is not case sensitive (for example, with Internet domain names) the name of the host MUST be lower case. If specified by the application protocol for services such as telnet and the Berkeley R commands which run with system privileges, the first component MAY be the string ’host’ instead of a service specific identifier.
The principal identifier of the ticket-granting service shall be composed of three parts: (1) the realm of the KDC issuing the TGS ticket (2) a two-part name of type NT-SRV-INST, with the first part "krbtgt" and the second part the name of the realm which will accept the ticket-granting ticket. For example, a ticket-granting ticket issued by the ATHENA.MIT.EDU realm to be used to get tickets from the ATHENA.MIT.EDU KDC has a principal identifier of "ATHENA.MIT.EDU" (realm), ("krbtgt", "ATHENA.MIT.EDU") (name). A ticket-granting ticket issued by the ATHENA.MIT.EDU realm to be used to get tickets from the MIT.EDU realm has a principal identifier of "ATHENA.MIT.EDU" (realm), ("krbtgt", "MIT.EDU") (name).
The Kerberos protocol provides the means for verifying (subject to the assumptions in Environmental Assumptions) that the entity with which one communicates is the same entity that was registered with the KDC using the claimed identity (principal name). It is still necessary to determine whether that identity corresponds to the entity with which one intends to communicate.
When appropriate data has been exchanged in advance, this determination may be performed syntactically by the application based on the application protocol specification, information provided by the user, and configuration files. For example, the server principal name (including realm) for a telnet server might be derived from the user specified host name (from the telnet command line), the "host/" prefix specified in the application protocol specification, and a mapping to a Kerberos realm derived syntactically from the domain part of the specified hostname and information from the local Kerberos realms database.
One can also rely on trusted third parties to make this determination, but only when the data obtained from the third party is suitably integrity protected while resident on the third party server and when transmitted. Thus, for example, one should not rely on an unprotected domain name system record to map a host alias to the primary name of a server, accepting the primary name as the party one intends to contact, since an attacker can modify the mapping and impersonate the party with which one intended to communicate.
Implementations of Kerberos and protocols based on Kerberos MUST NOT use insecure DNS queries to canonicalize the hostname components of the service principal names. In an environment without secure name service, application authors MAY append a statically configured domain name to unqualified hostnames before passing the name to the security mechanisms, but should do no more than that. Secure name service facilities, if available, might be trusted for hostname canonicalization, but such canonicalization by the client SHOULD NOT be required by KDC implementations.
Implementation note: Many current implementations do some degree of canonicalization of the provided service name, often using DNS even though it creates security problems. However there is no consistency among implementations about whether the service name is case folded to lower case or whether reverse resolution is used. To maximize interoperability and security, applications SHOULD provide security mechanisms with names which result from folding the user-entered name to lower case, without performing any other modifications or canonicalization.
Principal names consist of a sequence of strings, which is often tedious to parse. Therefor, Shishi often uses a “printed” form of principal which embed the entire principal name string sequence, and optionally also the realm, into one string. The format is taken from the Kerberos 5 GSS-API mechanism (RFC 1964).
The elements included within this name representation are as follows, proceeding from the beginning of the string:
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