RFC 3958
Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS)
Network Working Group L. Daigle Request for Comments: 3958 A. Newton Category: Standards Track VeriSign, Inc. January 2005 Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS) 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 (2005).Abstract
This memo defines a generalized mechanism for application service naming that allows service location without relying on rigid domain naming conventions (so-called name hacks). The proposal defines a Dynamic Delegation Discovery System (DDDS) Application to map domain name, application service name, and application protocol dynamically to target server and port.Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Straightforward-NAPTR (S-NAPTR) Specification . . . . . . . . 3 2.1. Key Terms. . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. S-NAPTR DDDS Application Usage . . . . . . . . . . . . . 4 2.2.1. Ordering and Preference. . . . . . . . . . . . . 4 2.2.2. Matching and Non-matching NAPTR Records. . . . . 4 2.2.3. Terminal and Non-terminal NAPTR Records. . . . . 5 2.2.4. S-NAPTR and Successive Resolution. . . . . . . . 5 2.2.5. Clients Supporting Multiple Protocols. . . . . . 6 3. Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Guidelines for Application Protocol Developers . . . . . 6 3.1.1. Registration of Application Service and Protocol Tags. . . . . . . . . . . . . . . . . . 7 3.1.2. Definition of Conditions for Retry/Failure . . . 7 3.1.3. Server Identification and Handshake . . . . . . 8 3.2. Guidelines for Domain Administrators . . . . . . . . . . 8
3.3. Guidelines for Client Software Writers . . . . . . . . . 8
4. Illustrations . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Service Discovery within a Domain . . . . . . . . . . . 9
4.3. Multiple Protocols . . . . . . . . . . . . . . . . . . . 10
4.4. Remote Hosting . . . . . . . . . . . . . . . . . . . . . 11
4.5. Sets of NAPTR RRs . . . . . . . . . . . . . . . . . . . 12
4.6. Sample Sequence Diagram . . . . . . . . . . . . . . . . 13
5. Motivation and Discussion . . . . . . . . . . . . . . . . . . 14
5.1. So Why Not Just SRV Records? . . . . . . . . . . . . . . 15
5.2. So Why Not Just NAPTR Records? . . . . . . . . . . . . . 15
6. Formal Definition of <Application Service Location>
Application of DDDS . . . . . . . . . . . . . . . . . . . . . 16
6.1. Application-Unique String . . . . . . . . . . . . . . . 16
6.2. First Well-Known Rule . . . . . . . . . . . . . . . . . 16
6.3. Expected Output . . . . . . . . . . . . . . . . . . . . 16
6.4. Flags . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.5. Service Parameters . . . . . . . . . . . . . . . . . . . 17
6.5.1. Application Services . . . . . . . . . . . . . . 17
6.5.2. Application Protocols . . . . . . . . . . . . . 17
6.6. Valid Rules . . . . . . . . . . . . . . . . . . . . . . 17
6.7. Valid Databases . . . . . . . . . . . . . . . . . . . . 18
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7.1. Application Service Tag IANA Registry . . . . . . . . . 18
7.2. Application Protocol Tag IANA Registry . . . . . . . . . 18
7.3. Registration Process . . . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Normative References . . . . . . . . . . . . . . . . . . 21
10.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
A. Pseudo-pseudocode for S-NAPTR. . . . . . . . . . . . . . . 22
A.1. Finding the First (Best) Target. . . . . . . . . . . 22
A.2. Finding Subsequent Targets . . . . . . . . . . . . . 23
B. Availability of Sample Code. . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
This memo defines a generalized mechanism for application service
naming that allows service location without relying on rigid domain
naming conventions (so-called name hacks). The proposal defines a
Dynamic Delegation Discovery System (DDDS -- see [4]) Application to
map domain name, application service name, and application protocol
dynamically to target server and port.
As discussed in section 5, existing approaches to using DNS records for dynamically determining the current host for a given application service are limited in terms of the use cases supported. To address some of the limitations, this document defines a DDDS Application to map service+protocol+domain to specific server addresses by using both NAPTR [5] and SRV ([3]) DNS resource records. This can be viewed as a more general version of the use of SRV and/or a very restricted application of the use of NAPTR resource records. 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, RFC 2119 [1].2. Straightforward-NAPTR (S-NAPTR) Specification
The precise details of the specification of this DDDS application are given in Section 6. This section defines the usage of the DDDS application.2.1. Key Terms
"Application service" is a generic term for some type of application, independent of the protocol that may be used to offer it. Each application service will be associated with an IANA-registered tag. For example, retrieving mail is a type of application service that can be implemented by different application-layer protocols (e.g., POP3, IMAP4). A tag, such as "RetMail", could be registered for it. (Note that this has not been done, and there are no plans to do so at the time of this writing.) An "application protocol" is used to implement the application service. These are also associated with IANA-registered tags. Using the mail example above, "POP3" and "IMAP4" could be registered as application protocol tags. If multiple transports are available for the application, separate tags should be defined for each transport. The intention is that the combination of application service and protocol tags should be specific enough that finding a known pair (e.g., "RetMail:POP3" would be sufficient for a client to identify a server with which it can communicate. Some protocols support multiple application services. For example, LDAP is an application protocol and can be found supporting various services (e.g., "whitepages", "directory enabled networking".
2.2. S-NAPTR DDDS Application Usage
As defined in section 6, NAPTR records are used to store application service+protocol information for a given domain. Following the DDDS standard, these records are looked up, and the rewrite rules (contained in the NAPTR records) are used to determine the successive DNS lookups until a desirable target is found. For the rest of this section, refer to the set of NAPTR resource records for example.com, shown in the figure below, where "WP" is the imagined application service tag for "white pages" and "EM" is the application service tag for an imagined "Extensible Messaging" application service. example.com. ;; order pref flags IN NAPTR 100 10 "" "WP:whois++" ( ; service "" ; regexp bunyip.example. ; replacement ) IN NAPTR 100 20 "s" "WP:ldap" ( ; service "" ; regexp _ldap._tcp.myldap.example.com. ; replacement ) IN NAPTR 200 10 "" "EM:protA" ( ; service "" ; regexp someisp.example. ; replacement ) IN NAPTR 200 30 "a" "EM:protB" ; service "" ; regexp myprotB.example.com.; replacement )2.2.1. Ordering and Preference
A client retrieves all the NAPTR records associated with the target domain name (example.com, above). These are to be sorted in terms of increasing ORDER and increasing PREF within each ORDER.2.2.2. Matching and Non-Matching NAPTR Records
Starting with the first sorted NAPTR record, the client examines the SERVICE field to find a match. In the case of the S-NAPTR DDDS application, this means a SERVICE field that includes the tags for the desired application service and a supported application protocol. If more than one NAPTR record matches, they are processed in increasing sort order.
2.2.3. Terminal and Non-terminal NAPTR Records
A NAPTR record with an empty FLAG field is "non-terminal" -- that is, more NAPTR RR lookups are to be performed. Thus, to process a NAPTR record with an empty FLAG field in S-NAPTR, the REPLACEMENT field is used as the target of the next DNS lookup -- for NAPTR RRs. In S-NAPTR, the only terminal flags are "S" and "A". These are called "terminal" NAPTR lookups because they denote the end of the DDDS/NAPTR processing rules. In the case of an "S" flag, the REPLACEMENT field is used as the target of a DNS query for SRV RRs, and normal SRV processing is applied. In the case of an "A" flag, an address record is sought for the REPLACEMENT field target (and the default protocol port is assumed).2.2.4. S-NAPTR and Successive Resolution
As shown in the example set above, it is possible to have multiple possible targets for a single application service+protocol pair. These are to be pursued in order until a server is successfully contacted or all possible matching NAPTR records have been successively pursued through terminal lookup and server contact. That is, a client must backtrack and attempt other resolution paths in the case of failure. "Failure" is declared, and backtracking must be used, when o the designated remote server (host and port) fails to provide appropriate security credentials for the *originating* domain; o connection to the designated remote server otherwise fails -- the specifics terms of which are defined when an application protocol is registered; or o the S-NAPTR-designated DNS lookup fails to yield expected results -- e.g., no A RR for an "A" target, no SRV record for an "S" target, or no NAPTR record with appropriate application service and protocol for a NAPTR lookup. Except in the case of the very first NAPTR lookup, this last is a configuration error: the fact that example.com has a NAPTR record pointing to "bunyip.example" for the "WP:Whois++" service and protocol means the administrator of example.com believes that service exists. If bunyip.example has no "WP:Whois++" NAPTR record, the application client MUST backtrack and try the next available "WP:Whois++" option from example.com. As there is none, the whole resolution fails.
An application client first queries for the NAPTR RRs for the domain of a named application service. The first DNS query is for the NAPTR RRs in the original target domain (example.com, above).2.2.5. Clients Supporting Multiple Protocols
In the case of an application client that supports more than one protocol for a given application service, it MUST pursue S-NAPTR resolution completely for one protocol, exploring all potential terminal lookups in PREF and ORDER ranking, until the application connects successfully or there are no more possibilities for that protocol. That is, the client MUST NOT start looking for one protocol, observe that a successive NAPTR RR set supports another of its preferred protocols, and continue the S-NAPTR resolution based on that protocol. For example, even if someisp.example offers the "EM" service with protocol "ProtB", there is no reason to believe that it does so on behalf of example.com (as there is no such pointer in example.com's NAPTR RR set). It MAY choose which protocol to try first based on its own preference, or on the PREF ranking in the first set of NAPTR records (i.e., those for the target named domain). However, the chosen protocol MUST be listed in that first NAPTR RR set. It MAY choose to run simultaneous DDDS resolutions for more than one protocol, in which case the requirements above apply for each protocol independently. That is, do not switch protocols mid- resolution.3. Guidelines
3.1. Guidelines for Application Protocol Developers
The purpose of S-NAPTR is to provide application standards developers with a more powerful framework (than SRV RRs alone) for naming service targets, without requiring each application protocol (or service) standard to define a separate DDDS application. Note that this approach is intended specifically for use when it makes sense to associate services with particular domain names (e.g., e-mail addresses, SIP addresses, etc). A non-goal is having all manner of label mapped into domain names in order to use this.
This document does not address how to select the domain for which the service+protocol is being sought. Other conventions will have to define how this might be used (e.g., new messaging standards can define what domain to use from their URIs or how to step down from foobar.example.com to example.com, if applicable). Although this document proposes a DDDS application that does not use all the features of NAPTR resource records, it is not intended to imply that DNS resolvers should fail to implement all aspects of the NAPTR RR standard. A DDDS application is a client use convention. The rest of this section outlines the specific elements that protocol developers must determine and document to make use of S-NAPTR.3.1.1. Registration of Application Service and Protocol Tags
Application protocol developers who wish to make use of S-NAPTR must make provisions for registering any relevant application service and application protocol tags, as described in section 7.3.1.2. Definition of Conditions for Retry/Failure
One other important aspect that must be defined is the expected behaviour for interacting with the servers that are reached via S- NAPTR. Specifically, under what circumstances should the client retry a target that was found via S-NAPTR? What should it consider a failure that causes it to return to the S-NAPTR process to determine the next serviceable target, which by definition will have a lower preference ranking. For example, if the client gets a "connection refused" message from a server, should it retry for some (protocol-dependent) period of time? Or should it try the next-preferred target in the S-NAPTR chain of resolution? Should it only try the next-preferred target if it receives a protocol-specific permanent error message? The most important thing is to select one expected behaviour and document it as part of the use of S-NAPTR. As noted earlier, failure to provide appropriate credentials to identify the server as being authoritative for the original target domain is always considered a failure condition.
3.1.3. Server Identification and Handshake
As noted in section 8, use of the DNS for server location increases the importance of using protocol-specific handshakes to determine and confirm the identity of the server that is eventually reached. Therefore, application protocol developers using S-NAPTR should identify the mechanics of the expected identification handshake when the client connects to a server found through S-NAPTR.3.2. Guidelines for Domain Administrators
Although S-NAPTR aims to provide a "straightforward" application of DDDS and use of NAPTR records, it is still possible to create very complex chains and dependencies with the NAPTR and SRV records. Therefore, domain administrators are called upon to use S-NAPTR with as much restraint as possible while still achieving their service design goals. The complete set of NAPTR, SRV, and A RRs "reachable" through the S- NAPTR process for a particular application service can be thought of as a "tree". Each NAPTR RR that is retrieved points to more NAPTR or SRV records; each SRV record points to several A record lookups. Even though a particular client can "prune" the tree to use only those records referring to application protocols supported by the client, the tree could be quite deep, and retracing the tree to retry other targets can become expensive if the tree has many branches. Therefore, o fewer branches is better: For both NAPTR and SRV records, provide different targets with varying preferences where appropriate (e.g., to provide backup services) but don't look for reasons to provide more; and o shallower is better: Avoid using NAPTR records to "rename" services within a zone. Use NAPTR records to identify services hosted elsewhere (i.e., where you cannot reasonably provide the SRV records in your own zone).3.3. Guidelines for Client Software Writers
To understand DDDS/NAPTR properly, an implementor must read [4]. However, the most important aspect to keep in mind is that if the application cannot successfully connect to one target, the application will be expected to continue through the S-NAPTR tree to try the (less preferred) alternatives.
4. Illustrations
4.1. Use Cases
The basic intended use cases for which S-NAPTR has been developed are as follows o Service discovery within a domain. For example, this can be used to find the "authoritative" server for some type of service within a domain (see the specific example in section 4.2). o Multiple protocols. This is already common today as new application services are defined, and is increasingly a problem. It includes the case of extensible messaging (a hypothetical service), which can be offered with multiple protocols (see section 4.3). o Remote hosting. Each of the above use cases applies within the administration of a single domain. However, one domain operator may elect to engage another organization to provide an application service. See section 4.4 for an example that cannot be served by SRV records alone.4.2. Service Discovery within a Domain
There are occasions when it is useful to be able to determine the "authoritative" server for a given application service within a domain. This is "discovery", as there is no a priori knowledge as to whether or where the service is offered; it is therefore important to determine the location and characteristics of the offered service. For example, there is growing discussion of having a generic mechanism for locating the keys or certificates associated with particular application (servers) operated in (or for) a particular domain. The following is a hypothetical case for storing application key or certificate data for a given domain: the premise is that a credentials registry (CredReg) service has been defined as a leaf node service holding the keys/certs for the servers operated by (or for) the domain. It is assumed that more than one protocol is available to provide the service for a particular domain. This DDDS-based approach is used to find the CredReg server that holds the information.
Thus, the set of NAPTR records for thinkingcat.example might look
like this:
thinkingcat.example.
;; order pref flags
IN NAPTR 100 10 "" "CREDREG:ldap:iris.beep" ( ; service
"" ; regexp
theserver.thinkingcat.example. ; replacement
Note that the application service might be offered in another domain
using a different set of application protocols:
anotherdomain.example.
;; order pref flags
IN NAPTR 100 10 "" "CREDREG:iris.lwz:iris.beep" ( ; service
"" ; regexp
foo.anotherdomain.example. ; replacement
)
4.3. Multiple Protocols
Extensible messaging, a hypothetical application service, will be
used for illustrative purposes. (For an example of a real
application service with multiple protocols, see [9] and [10]).
Assuming that "EM" was registered as an application service, this
DDDS application could be used to determine the available services
for delivery to a target.
Two particular features of this hypothetical extensible messaging
should be noted:
1. Gatewaying is expected to bridge communications across protocols.
2. Extensible messaging servers are likely to be operated out of a
different domain than that of the extensible messaging address,
and servers of different protocols may be offered by independent
organizations.
For example, "thinkingcat.example" may support its own servers for
the "ProtA" extensible messaging protocol but rely on outsourcing
from "example.com" for "ProtC" and "ProtB" servers.
Using this DDDS-based approach, thinkingcat.example can indicate a
preference ranking for the different types of servers for the
extensible messaging service, yet the out-sourcer can independently
rank the preference and ordering of servers. This independence is
not achievable through the use of SRV records alone.
Thus, to find the EM services for thinkingcat.example, the NAPTR
records for thinkingcat.example are retrieved:
thinkingcat.example.
;; order pref flags
IN NAPTR 100 10 "s" "EM:ProtA" ( ; service
"" ; regexp
_ProtA._tcp.thinkingcat.example. ; replacement
)
IN NAPTR 100 20 "s" "EM:ProtB" ( ; service
"" ; regexp
_ProtB._tcp.example.com. ; replacement
)
IN NAPTR 100 30 "s" "EM:ProtC" ( ; service
"" ; regexp
_ProtC._tcp.example.com. ; replacement
)
Then the administrators at example.com can manage the preference
rankings of the servers they use to support the ProtB service:
_ProtB._tcp.example.com.
;; Pref Weight Port Target
IN SRV 10 0 10001 bigiron.example.com.
IN SRV 20 0 10001 backup.em.example.com.
IN SRV 30 0 10001 nuclearfallout.australia-isp.example.
4.4. Remote Hosting
In the Instant Message hosting example in Section 4.3, the service
owner (thinkingcat.example) had to host pointers to the hosting
service's SRV records in the thinkingcat.example domain.
A better approach is to have one NAPTR RR in the thinkingcat.example
domain point to all the hosted services. The hosting domain has
NAPTR records for each service to map them to whatever local hosts it
chooses (this may change from time to time).
thinkingcat.example.
;; order pref flags
IN NAPTR 100 10 "s" "EM:ProtA" ( ; service
"" ; regexp
_ProtA._tcp.thinkingcat.example. ; replacement
)
IN NAPTR 100 20 "" "EM:ProtB:ProtC" ( ; service
"" ; regexp
thinkingcat.example.com. ; replacement
)
Then the administrators at example.com can break out the individual
application protocols and manage the preference rankings of the
servers they use to support the ProtB service (as before):
thinkingcat.example.com.
;; order pref flags
IN NAPTR 100 10 "s" "EM:ProtC" ( ; service
"" ; regexp
_ProtC._tcp.example.com. ; replacement
)
IN NAPTR 100 20 "s" "EM:ProtB" ( ; service
"" ; regexp
_ProtB._tcp.example.com. ; replacement
)
_ProtC._tcp.example.com.
;; Pref Weight Port Target
IN SRV 10 0 10001 bigiron.example.com.
IN SRV 20 0 10001 backup.em.example.com.
IN SRV 30 0 10001 nuclearfallout.australia-isp.example.
4.5. Sets of NAPTR RRs
Note that the above sections assume that there was one service
available (via S-NAPTR) per domain. Often, this will not be the
case. Assuming that thinkingcat.example had the CredReg service set
up as described in Section 4.2 and had the extensible messaging
service set up as described in Section 4.4, then a client querying
for the NAPTR RR set from thinkingcat.com would get the following
answer:
thinkingcat.example.
;; order pref flags
IN NAPTR 100 10 "s" "EM:ProtA" ( ; service
"" ; regexp
_ProtA._tcp.thinkingcat.example. ; replacement
)
IN NAPTR 100 20 "" "EM:ProtB:ProtC" ( ; service
"" ; regexp
thinkingcat.example.com. ; replacement
)
IN NAPTR 200 10 "" "CREDREG:ldap:iris-beep" ( ; service
"" ; regexp
bouncer.thinkingcat.example. ; replacement
)
Sorting them by increasing "ORDER", the client would look through the SERVICE strings to determine whether there was a NAPTR RR that matched the application service it was looking for, with an application protocol it could use. The client would use the first (lowest PREF) record that matched to continue.4.6. Sample sequence diagram
Consider the example in section 4.3. Visually, the sequence of steps required for the client to reach the final server for a "ProtB" service for EM for the thinkingcat.example domain is as follows: Client NS for NS for thinkingcat.example example.com backup.em.example.com | | | 1 -------->| | | 2 <--------| | | 3 ------------------------------>| | 4 <------------------------------| | 5 ------------------------------>| | 6 <------------------------------| | 7 ------------------------------>| | 8 <------------------------------| | 9 ------------------------------------------------->| 10 <-------------------------------------------------| 11 ------------------------------------------------->| 12 <-------------------------------------------------| (...) 1. The name server (NS) for thinkingcat.example is reached with a request for all NAPTR records. 2. The server responds with the NAPTR records shown in section 4.3. 3. The second NAPTR record matches the desired criteria; it has an "s" flag and a replacement fields of "_ProtB._tcp.example.com". So the client looks up SRV records for that target, ultimately making the request of the NS for example.com. 4. The response includes the SRV records listed in Section 4.3. 5. The client attempts to reach the server with the lowest PREF in the SRV list -- looking up the A record for the SRV record's target (bigiron.example.com). 6. The example.com NS responds with an error message -- no such machine!
7. The client attempts to reach the second server in the SRV list
and looks up the A record for backup.em.example.com.
8. The client gets the A record with the IP address for
backup.em.example.com from example.com's NS.
9. The client connects to that IP address, on port 10001 (from the
SRV record), by using ProtB over tcp.
10. The server responds with an "OK" message.
11. The client uses ProtB to challenge that this server has
credentials to operate the service for the original domain
(thinkingcat.example)
12. The server responds, and the rest is EM.
5. Motivation and Discussion
Increasingly, application protocol standards use domain names to
identify server targets and stipulate that clients should look up SRV
resource records to determine the host and port providing the server.
This enables a distinction between naming an application service
target and actually hosting the server. It also increases
flexibility in hosting the target service, as follows:
o The server may be operated by a completely different organization
without having to list the details of that organization's DNS
setup (SRVs).
o Multiple instances can be set up (e.g., for load balancing or
secondaries).
o It can be moved from time to time without disrupting clients'
access, etc.
This approach is quite useful, but section 5.1 outlines some of its
inherent limitations.
That is, although SRV records can be used to map from a specific
service name and protocol for a specific domain to a specific server,
SRV records are limited to one layer of indirection and are focused
on server administration rather than on application naming.
Furthermore, although the DDDS specification and use of NAPTR allows
multiple levels of redirection before the target server machine with
an SRV record is located, this proposal requires only a subset of
NAPTR strictly bound to domain names, without making use of the
REGEXP field of NAPTR. These restrictions make the client's
resolution process much more predictable and efficient than it would be with some potential uses of NAPTR records. This is dubbed "S- NAPTR" -- a "S"traightforward use of NAPTR records.5.1. So Why Not Just SRV Records?
An expected question at this point is: this is so similar in structure to SRV records, why are we doing this with DDDS/NAPTR? Limitations of SRV include the following: o SRV provides a single layer of indirection; the outcome of an SRV lookup is a new domain name for which the A RR is to be found. o the purpose of SRV is to address individual server administration issues, not to provide application naming: As stated in [3], "The SRV RR allows administrators to use several servers for a single domain, to move services from host to host with little fuss, and to designate some hosts as primary servers for a service and others as backups". o Target servers by "service" (e.g., "ldap") and "protocol" (e.g., "tcp") in a given domain. The definition of these terms implies specific things (e.g., that protocol should be one of UDP or TCP) without being precise. Restriction to UDP and TCP is insufficient for the uses described here. The basic answer is that SRV records provide mappings from protocol names to host and port. The use cases described herein require an additional layer -- from some service label to servers that may in be hosted within different administrative domains. We could tweak SRV to say that the next lookup could be something other than an address record, but this is more complex than is necessary for most applications of SRV.5.2. So Why Not Just NAPTR Records?
This is a trick question. NAPTR records cannot appear in the wild; see [4]. They must be part of a DDDS application. The purpose here is to define a single, common mechanism (the DDDS application) to use NAPTR when all that is desired is simple DNS- based location of services. This should be easy for applications to use -- a few simple IANA registrations, and it's done.
Also, NAPTR has very powerful tools for expressing "rewrite" rules. This power (==complexity) makes some protocol designers and service administrators nervous. The concern is that these rewrites can translate into unintelligible, noodle-like rule sets that are difficult to test and administer. The proposed DDDS application specifically uses a subset of NAPTR's abilities. Only "replacement" expressions are allowed, not "regular expressions".6. Formal Definition of <Application Service Location> Application of DDDS
This section formally defines the DDDS application, as described in [4].6.1. Application-Unique String
The Application Unique String is domain label for which an authoritative server for a particular service is sought.6.2. First Well-Known Rule
The "First Well-Known Rule" is identity -- that is, the output of the rule is the Application-Unique String, the domain label for which the authoritative server for a particular service is sought.6.3. Expected Output
The expected output of this Application is the information necessary for a client to connect to authoritative server(s) (host, port, protocol) for a particular application service within a given domain.6.4. Flags
This DDDS Application uses only 2 of the Flags defined for the URI/ URN Resolution Application ([6]): "S" and "A". No other Flags are valid. Both are for terminal lookups. This means that the Rule is the last one and that the flag determines what the next stage should be. The "S" flag means that the output of this Rule is a domain label for which one or more SRV [3] records exist. "A" means that the output of the Rule is a domain name and should be used to lookup address records for that domain. Consistent with the DDDS algorithm, if the Flag string is empty the next lookup is for another NAPTR record (for the replacement target).
6.5. Service Parameters
Service Parameters for this Application take the form of a string of characters that follow this ABNF ([2]): service-parms = [ [app-service] *(":" app-protocol)] app-service = experimental-service / iana-registered-service app-protocol = experimental-protocol / iana-registered-protocol experimental-service = "x-" 1*30ALPHANUMSYM experimental-protocol = "x-" 1*30ALPHANUMSYM iana-registered-service = ALPHA *31ALPHANUMSYM iana-registered-protocol = ALPHA *31ALPHANUM ALPHA = %x41-5A / %x61-7A ; A-Z / a-z DIGIT = %x30-39 ; 0-9 SYM = %x2B / %x2D / %x2E ; "+" / "-" / "." ALPHANUMSYM = ALPHA / DIGIT / SYM ; The app-service and app-protocol tags are limited to 32 ; characters and must start with an alphabetic character. ; The service-parms are considered case-insensitive. Thus, the Service Parameters may consist of an empty string, an app- service, or an app-service with one or more app-protocol specifications separated by the ":" symbol. Note that this is similar to, but not the same as the syntax used in the URI DDDS application ([6]). The DDDS DNS database requires each DDDS application to define the syntax of allowable service strings. The syntax here is expanded to allow the characters that are valid in any URI scheme name (see [8]). As "+" (the separator used in the RFC3404 service parameter string) is an allowed character for URI scheme names, ":" is chosen as the separator here.6.5.1. Application Services
The "app-service" must be an IANA-registered service; see Section 7 for instructions on registering new application service tags.6.5.2. Application Protocols
The protocol identifiers valid for the "app-protocol" production are standard, registered protocols; see section 7 for instructions on registering new application protocol tags.6.6. Valid Rules
Only substitution Rules are permitted for this application. That is, no regular expressions are allowed.
6.7. Valid Databases
At present only one DDDS Database is specified for this Application. [5] specifies that a DDDS Database using the NAPTR DNS resource record contain the rewrite rules. The Keys for this database are encoded as domain-names. The First Well-Known Rule produces a domain name, and this is the Key used for the first look up. The NAPTR records for that domain are requested. DNS servers MAY interpret Flag values and use that information to include appropriate NAPTR, SRV, or A records in the Additional Information portion of the DNS packet. Clients are encouraged to check for additional information but are not required to do so. See the Additional Information Processing section of [5] for more information on NAPTR records and the Additional Information section of a DNS response packet.7. IANA Considerations
This document calls for two IANA registries: one for application service tags, and one for application protocol tags.7.1. Application Service Tag IANA Registry
IANA has established and will maintain a registry for S-NAPTR Application Service Tags, listing at least the following information for each such tag: o Application Service Tag: A string conforming with the IANA- registered-service defined in section 6.5. o Defining publication: The RFC used to define the Application Service Tag, as defined in the registration process, below. An initial Application Service Tag registration is contained in [9].7.2. Application Protocol Tag IANA Registry
IANA has established and will maintain a registry for S-NAPTR Application Protocol Tags, listing at least the following information for each such tag: o Application Protocol Tag: A string conforming with the iana- registered-protocol defined in section 6.5.
o Defining publication: The RFC used to define the Application
Protocol Tag, as defined in the registration process, below.
An initial Application Protocol Tag registration is defined in [10].
7.3. Registration Process
All application service and protocol tags that start with "x-" are
considered experimental, and no provision is made to prevent
duplicate use of the same string. Implementors use them at their own
risk.
All other application service and protocol tags are registered based
on the "specification required" option defined in [7], with the
further stipulation that the "specification" is an RFC (of any
category).
No further restrictions are placed on the tags except that they must
conform with the syntax defined below (Section 6.5).
The defining RFC must clearly identify and describe, for each tag
being registered,
o application protocol or service tag,
o intended usage,
o interoperability considerations,
o security considerations (see section 8 of this document for
further discussion of the types of considerations that are
applicable), and
o any relevant related publications.
8. Security Considerations
The security of this approach to application service location is only
as good as the security of the DNS queries along the way. If any of
them is compromised, bogus NAPTR and SRV records could be inserted to
redirect clients to unintended destinations. This problem is hardly
unique to S-NAPTR (or NAPTR in general). A full discussion of the
security threats pertaining to DNS can be found in [11].
To protect against DNS-vectored attacks, secured DNS (DNSSEC) [12]
can be used to ensure the validity of the DNS records received.
Whether or not DNSSEC is used, applications should define some form
of end-to-end authentication to ensure that the correct destination
has been reached. Many application protocols such as HTTPS, BEEP,
and IMAP define the necessary handshake mechanisms to accomplish this
task. Newly defined application protocols should take this into
consideration and incorporate appropriate mechanisms.
The basic mechanism works as follows:
1. During some portion of the protocol handshake, the client sends to
the server the original name of the desired destination (i.e., no
transformations that may have resulted from NAPTR replacements,
SRV targets, or CNAME changes). In certain cases where the
application protocol does not have such a feature but TLS may be
used, it is possible to use the "server_name" TLS extension.
2. The server sends back to the client a credential with the
appropriate name. For X.509 certificates, the name would be in
either the subjectDN or the subjectAltName field. For Kerberos,
the name would be a service principle name.
3. Using the matching semantics defined by the application protocol,
the client compares the name in the credential with the name sent
to the server.
4. If the names match and the credentials have integrity, there is
reasonable assurance that the correct end point has been reached.
5. The client and server establish an integrity-protected channel.
Note that this document does not define either the handshake
mechanism, the specific credential naming fields, nor the name-
matching semantics. Definitions of S-NAPTR for particular
application protocols MUST define these.
9. Acknowledgements
Many thanks to Dave Blacka, Patrik Faltstrom, Sally Floyd, and Ted
Hardie for discussion and input that have (hopefully!) provoked
clarifying revisions to this document.
10. References
10.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. [3] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000. [4] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part One: The Comprehensive DDDS", RFC 3401, October 2002. [5] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part Three: The Domain Name System (DNS) Database", RFC 3403, October 2002. [6] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part Four: The Uniform Resource Identifiers (URI)", RFC 3404, October 2002. [7] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.10.2. Informative References
[8] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998. [9] Newton, A. and M. Sanz, "IRIS: A Domain Registry (dreg) Type for the Internet Registry Information Service (IRIS)", RFC 3982, January 2005. [10] Newton, A. and M. Sanz, "Using the Internet Registry Information Service (IRIS) over the Blocks Extensible Exchange Protocol (BEEP)", RFC 3983, January 2005. [11] Atkins, D. and R. Austein, "Threat Analysis Of The Domain Name System", Work in Progress, April 2004. [12] Arends, R., Larson, M., Austein, R., and D. Massey, "Protocol Modifications for the DNS Security Extensions", Work in Progress, May 2004.
Appendix A. Pseudo-Pseudocode for S-NAPTR
A.1. Finding the First (Best) Target
Assuming the client supports 1 protocol for a particular application service, the following pseudocode outlines the expected process to find the first (best) target for the client, using S-NAPTR. target = [initial domain] naptr-done = false while (not naptr-done) { NAPTR-RRset = [DNSlookup of NAPTR RRs for target] [sort NAPTR-RRset by ORDER, and PREF within each ORDER] rr-done = false cur-rr = [first NAPTR RR] while (not rr-done) if ([SERVICE field of cur-rr contains desired application service and application protocol]) rr-done = true target= [REPLACEMENT target of NAPTR RR] else cur-rr = [next rr in list] if (not empty [FLAG in cur-rr]) naptr-done = true } port = -1 if ([FLAG in cur-rr is "S"]) { SRV-RRset = [DNSlookup of SRV RRs for target] [sort SRV-RRset based on PREF] target = [target of first RR of SRV-RRset] port = [port in first RR of SRV-RRset] } ; now, whether it was an "S" or an "A" in the NAPTR, we ; have the target for an A record lookup
host = [DNSlookup of target] return (host, port)A.2. Finding Subsequent Targets
The pseudocode in Appendix A is crafted to find the first, most preferred host-port pair for a particular application service and protocol. If, for any reason, that host-port pair did not work (connection refused, application-level error), the client is expected to try the next host-port in the S-NAPTR tree. The pseudocode above does not permit retries -- once complete, it sheds all context of where in the S-NAPTR tree it finished. Therefore, client software writers could o entwine the application-specific protocol with the DNS lookup and RRset processing described in the pseudocode and continue the S- NAPTR processing if the application code fails to connect to a located host-port pair; o use callbacks for the S-NAPTR processing; or o use an S-NAPTR resolution routine that finds *all* valid servers for the required application service and protocol from the originating domain and that provides them in a sorted order for the application to try.Appendix B. Availability of Sample Code
Sample Python code for S-NAPTR resolution is available from http://www.verisignlabs.com/pysnaptr-0.1.tgz
Authors' Addresses
Leslie Daigle VeriSign, Inc. 21355 Ridgetop Circle Dulles, VA 20166 US EMail: leslie@verisignlabs.com; leslie@thinkingcat.com Andrew Newton VeriSign, Inc. 21355 Ridgetop Circle Dulles, VA 20166 US EMail: anewton@verisignlabs.com
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