Network Working Group
Request for Comments: 5178
Category: Standards Track
N. Williams
A. Melnikov
Isode Ltd.
May 2008

Generic Security Service Application Program Interface (GSS-API) Internationalization and Domain-Based Service Names and Name Type

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.


This document describes domain-name-based service principal names and the corresponding name type for the Generic Security Service Application Programming Interface (GSS-API). Internationalization of the GSS-API is also covered.

Domain-based service names are similar to host-based service names, but using a domain name (not necessarily an Internet domain name) in addition to a hostname. The primary purpose of domain-based names is to provide a measure of protection to applications that utilize insecure service discovery protocols. This is achieved by providing a way to name clustered services after the "domain" which they service, thereby allowing their clients to authorize the service's servers based on authentication of their service names.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Conventions Used in This Document . . . . . . . . . . . . . . . 4
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
     3.1.  Name Type OID . . . . . . . . . . . . . . . . . . . . . . . 4
     3.2.  Name Type OID and Symbolic Name . . . . . . . . . . . . . . 4
   4.  Query and Display Syntaxes  . . . . . . . . . . . . . . . . . . 4
     4.1.  Examples of Domain-Based Names  . . . . . . . . . . . . . . 5
   5.  Internationalization (I18N) Considerations  . . . . . . . . . . 5
     5.1.  Importing Internationalized Names . . . . . . . . . . . . . 5
     5.2.  Displaying Internationalized Names  . . . . . . . . . . . . 5
   6.  Application Protocol Examples . . . . . . . . . . . . . . . . . 6
     6.1.  NFSv4 Domain-Wide Namespace Root Server Discovery . . . . . 6
     6.2.  LDAP Server Discovery . . . . . . . . . . . . . . . . . . . 6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 8

1. Introduction

Some applications need to discover the names of servers for a specific resource. Some common methods for server discovery are insecure, e.g., queries for DNS [RFC1035] SRV resource records [RFC2782] without using DNSSEC [RFC4033], and are subject to attacks whereby a client can be re-directed to incorrect and possibly malicious servers. A client may even be re-directed to a server that has credentials for itself and thus may authenticate itself to the client, and yet it could be incorrect or malicious (because it has been compromised, say).

Domain-based names allow for GSS-API [RFC2743] initiator applications (clients) to authorize acceptor principals (servers) to serve the resource for which the client used insecure server discovery without either securing the server discovery method or requiring an additional protocol for server authorization. That is, either a discovered server has credentials for authenticating the domain-based service names that it is intended to respond to, or it does not. Availability of valid credentials for authenticating domain-based names embodies the authorization of a given server to a domain-wide service.

A domain-based name consists of three required elements:

  • a service name
  • a domain name
  • a hostname

The domain name and the hostname should be Domain Name System (DNS) names, though domain-based names could be used in non-DNS environments. Because of the use of DNS names we must also provide for internationalization of the GSS-API.

Note that domain-based naming isn't new. According to a report to the KITTEN WG mailing list, there exists at least one implementation of LDAP which uses domain-based service naming, and the DIGEST-MD5 HTTP / Simple Authentication and Security Layer (SASL) mechanism [RFC2831] describes a similar notion. (See section 2.1.2 of [RFC2831] for a description of the "serv-name" field of the digest- response.)

2. Conventions Used in This Document

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 [RFC2119].

3. IANA Considerations

3.1. Name Type OID

The IANA has recorded the following new name-type OID in IANA's "SMI Security for Name System Designators Codes (nametypes)" registry:

   5 gss-domain-based-services [RFC5178]

3.2. Name Type OID and Symbolic Name

This document creates a new GSS-API name-type, with a symbolic name of "GSS_C_NT_DOMAINBASED_SERVICE" and this OID:

   {iso(1) org(3) dod(6) internet(1) security(5) nametypes(6) gss-

4. Query and Display Syntaxes

There is a single name syntax for domain-based names. It is expressed using the ABNF [RFC5234].

The syntax is:

domain-based-name = service "@" domain "@" hostname

         hostname          = domain

domain = sub-domain 1*("." sub-domain)

         sub-domain        = Let-dig [Ldh-str]
         Let-dig           = ALPHA / DIGIT

Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig

Where <service> is defined in Section 4.1 of [RFC2743]. Other rules not defined above are defined in Appendix B.1 of [RFC5234].

4.1. Examples of Domain-Based Names

These examples are not normative:

   o  ldap@somecompany.example@ds1.somecompany.example
   o  nfs@somecompany.example@nfsroot1.somecompany.example

The .example top-level domain is used here in accordance with [RFC2606].

5. Internationalization (I18N) Considerations

We introduce new versions of GSS_Import_name() and GSS_Display_name() to better support Unicode. Additionally, we provide for the use of ASCII Compatible Encoding (ACE)-encoded DNS in the non- internationalized interfaces [RFC3490].

5.1. Importing Internationalized Names

When the input_name_type parameter is the GSS_C_NT_DOMAINBASED_SERVICE OID, then GSS_Import_name() implementations and GSS-API mechanisms MUST accept ACE-encoded internationalized domain names in the hostname and domain name slots of the given domain-based name string.

Support for non-ASCII internationalized domain names SHOULD also be provided through a new function, GSS_Import_name_utf8(), that operates exactly like GSS_Import_name() (with the same input and output parameters and behavior), except that it MUST accept internationalized domain names both as UTF-8 strings and as ACE- encoded strings via its input_name_string argument.

5.2. Displaying Internationalized Names

Implementations of GSS_Display_name() MUST only output US-ASCII or ACE-encoded internationalized domain names in the hostname and domain name slots of domain-based names (or mechanism names (MN) that conform to the mechanism's form for domain-based names).

Support for non-ASCII internationalized domain names SHOULD also be provided through a new function, GSS_Display_name_utf8(), that operates exactly like GSS_Display_name() (with the same input and output parameters and behavior), except that it outputs UTF-8 strings via its name_string output argument. GSS_Display_name_utf8() MUST NOT output ACE-encoded internationalized domain names.

6. Application Protocol Examples

The following examples are not normative. They describe how the authors envision two applications' use of domain-based names.

6.1. NFSv4 Domain-Wide Namespace Root Server Discovery

Work is ongoing to provide a method for constructing domain-wide NFSv4 [RFC3530] filesystem namespaces where there is a single "root" with one or more servers (replicas) and multiple filesystems glued into the namespace through use of "referrals". Clients could then construct a "global" namespace through use of the DNS domain hierarchy.

Here, clients would always know, from context, when they need to find the root servers for a given DNS domain. Root server discovery would be performed using DNS SRV RR lookups, without using DNSSEC where DNSSEC has not been deployed.

When using RPCSEC_GSS [RFC2203] for security, NFSv4 clients would use domain-based names to ensure that the servers named in the SRV RRs are in fact authorized to be the NFSv4 root servers for the target domain.

6.2. LDAP Server Discovery

LDAP clients using the GSS-API through SASL would also benefit from use of domain-based names to protect server discovery through insecure DNS SRV RR lookups, much as described above.

Unlike NFSv4 clients, not all LDAP clients always know from context when they should use domain-based names. That's because existing clients may use host-based naming to authenticate servers discovered through SRV RR lookups. Changing such clients to use domain-based naming when domain-based acceptor credentials have not been deployed to LDAP servers, or when LDAP servers have not been modified to allow use of domain-based naming, would break interoperability. That is, there is a legacy server interoperability issue here. Therefore, LDAP clients may require additional configuration at deployment time to enable (or disable) use of domain-based naming.

Note: whether SASL [RFC4422] or its GSS-API bridges [RFC4752] [GS2] require updates in order allow use of domain-based names is not relevant to the theory of how domain-based naming would protect LDAP clients' server discovery.

7. Security Considerations

Use of GSS-API domain-based names may not be negotiable by some GSS- API mechanisms, and some acceptors may not support GSS-API domain- based names. In such cases, the initiators are left to fall back on the use of host-based names, so the initiators MUST also verify that the acceptor's host-based name is authorized to provide the given service for the domain that the initiator had wanted.

The above security consideration also applies to all GSS-API initiators who lack support for domain-based service names.

Note that, as with all service names, the mere existence of a domain- based service name conveys meaningful information that may be used by initiators for making authorization decisions; therefore, administrators of distributed authentication services should be aware of the significance of the service names for which they create acceptor credentials.

8. References

8.1. Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.
   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.
   [RFC2831]  Leach, P. and C. Newman, "Using Digest Authentication as a
              SASL Mechanism", RFC 2831, May 2000.
   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.
   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

8.2. Informative References

   [GS2]      Josefsson, S., "Using GSS-API Mechanisms in SASL: The GS2
              Mechanism Family", Work in Progress, October 2007.
   [RFC2203]  Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
              Specification", RFC 2203, September 1997.
   [RFC2606]  Eastlake, D. and A. Panitz, "Reserved Top Level DNS
              Names", BCP 32, RFC 2606, June 1999.
   [RFC3530]  Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
              Beame, C., Eisler, M., and D. Noveck, "Network File System
              (NFS) version 4 Protocol", RFC 3530, April 2003.
   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.
   [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.
   [RFC4752]  Melnikov, A., "The Kerberos V5 ("GSSAPI") Simple
              Authentication and Security Layer (SASL) Mechanism",
              RFC 4752, November 2006.

Authors' Addresses

   Nicolas Williams
   Sun Microsystems
   5300 Riata Trace Ct.
   Austin, TX  78727
   Alexey Melnikov
   Isode Ltd.
   5 Castle Business Village,
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   United Kingdom

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