Network Working Group L. Daigle
Internet-Draft A. Newton
Expires: April 25, 2004 VeriSign, Inc.
October 26, 2003
Domain-based Application Service Location Using SRV RRs and the
Dynamic Delegation Discovery Service (DDDS)
draft-daigle-napstr-03.txt
Status of this Memo
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This Internet-Draft will expire on April 25, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo defines a Dynamic Delegation Discovery System (DDDS) [5]
Application for domain name based discovery of application services.
Essentially, this uses DNS NAPTR resource records [6] to provide one
more layer of redirection for service lookup than is feasible with
SRV ([4]) records. It is proposed because real-life use is
demonstrating a need for something slightly more substantial than
SRV, and alternatively SRV usage may become twisted out of its
intended shape.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 What this document means for application protocol developers 4
2. Basic Proposal . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Service Discovery within a Domain . . . . . . . . . . . . . 6
3.2 Multiple Protocols . . . . . . . . . . . . . . . . . . . . . 6
3.3 Remote Hosting . . . . . . . . . . . . . . . . . . . . . . . 7
3.4 Sets of NAPTR RRs . . . . . . . . . . . . . . . . . . . . . 8
4. Motivation and Discussion . . . . . . . . . . . . . . . . . 9
4.1 So, why not just SRV records? . . . . . . . . . . . . . . . 9
4.2 So, why not just NAPTR records? . . . . . . . . . . . . . . 9
4.3 Open Questions . . . . . . . . . . . . . . . . . . . . . . . 10
5. Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1 Guidelines for Application Protocol Developers . . . . . . . 10
5.1.1 Registration of application service and protocol tags . . . 10
5.1.2 Definition of conditions for retry/failure . . . . . . . . . 10
5.1.3 Server identification and handshake . . . . . . . . . . . . 11
5.2 Guidelines for Domain Administrators . . . . . . . . . . . . 11
5.3 Guidelines for Client Software Writers . . . . . . . . . . . 12
5.4 Pseudo pseudocode for S-NAPTR . . . . . . . . . . . . . . . 12
5.4.1 Finding the first (best) target . . . . . . . . . . . . . . 12
5.4.2 Finding subsequent targets . . . . . . . . . . . . . . . . . 13
5.5 Sample unrolling of the S-NAPTR process . . . . . . . . . . 13
5.6 Sample sequence diagram . . . . . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . 16
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
References . . . . . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 18
A. Application Service Location Application of DDDS . . . . . . 18
A.1 Application Unique String . . . . . . . . . . . . . . . . . 18
A.2 First Well Known Rule . . . . . . . . . . . . . . . . . . . 18
A.3 Expected Output . . . . . . . . . . . . . . . . . . . . . . 18
A.4 Flags . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
A.5 Service Parameters . . . . . . . . . . . . . . . . . . . . . 19
A.5.1 Application Services . . . . . . . . . . . . . . . . . . . . 19
A.5.2 Application Protocols . . . . . . . . . . . . . . . . . . . 20
A.6 Valid Rules . . . . . . . . . . . . . . . . . . . . . . . . 20
A.7 Valid Databases . . . . . . . . . . . . . . . . . . . . . . 20
Full Copyright Statement . . . . . . . . . . . . . . . . . . 21
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1. Introduction
Increasingly, application protocol standards are using domain names
to identify server targets, and stipulating that clients should look
up SRV ([4]) 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:
o the server may be operated by a completely different organization
without having to delegate some portion of the zone
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 is quite useful, but Section 4.1 outlines some of the
limitations inherent in the approach.
To address some of the limitations, this document defines a DDDS [5]
Application to map service+protocol+domain to specific server
addresses using both NAPTR [6] and SRV 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.
That is, while 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. And, while
the DDDS specification and use of NAPTR allows multiple levels of
redirection before locating the target server machine with an SRV
record, 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 (prefetchable, cachable) than with some uses of NAPTR
records. This is dubbed "S-NAPTR" -- a "S"traightforward use of
NAPTR records.
For the purposes of this document:
o an "application service" is a generic term for some generic type
of application, independent of the protocol that may be used to
offer it.
o an "application protocol" is an IANA-registered tag associated
with a protocol (and application tranpsort protocol, if several
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are available for the application) used to implement one or
several services
For example, "e-mail" is an application service; "SMTP" is the
protocol that is used to implement it. "Instant Messaging" is an
application service, for which there are several existing and
proposed application protocols ("jabber", "simple", etc). "LDAP" is
an application protocol which can be used to implement several
different application services (e.g., a "whitepages" service,
directory enabled networking service, etc).
1.1 What this document means for application protocol developers
The purpose of this document 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.
Specifically not addressed in this document is how to select the
domain for which the service+protocol is being sought. It is up to
other conventions to define how that might be used (e.g., instant
messaging standards can define what domain to use from IM URIs, how
to step down from foobar.example.com to example.com, and so on, if
that is applicable).
Although this document proposes a DDDS application that does not use
all the features of NAPTR resource records, it does not mean to imply
that DNS resolvers should fail to implement all aspects of the NAPTR
RR standard. A DDDS application is a client use convention.
2. Basic Proposal
The precise details of the specification of this DDDS application are
given in Appendix A. In general, the proposal is to store
application service and protocol descriptions in NAPTR records for
individual domains. This will enable domain administrators to
provide redirection to other domains that provision individual
services, with appropriate weightings and preferences.
2.1 Key Terms
Each "application service" will be associated with an IANA-registered
tag. For example, instant messaging is a type of application, which
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is implemented by many different application-layer protocols, and the
tag "IM" (used as an illustration here) could be registered for it.
An "application protocol" tag represents a protocol used to implement
the application service. In the case where 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., "IM:ProtC") is sufficient for a client to identify a server
with which it can communicate.
2.2 Use Cases
The basic intended use cases for which S-NAPTR has been developed
are:
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 3.1).
o Multiple protocols. This is increasingly common as new
application services are defined. This includes the case of
instant messaging (a service) which can be offered with multiple
protocols (see Section 3.2).
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 3.3 for an example that cannot be served by
SRV records alone.
3. Examples
Section 3.1 illustrates the use of S-NAPTR to express the list of
available application protocols to provide a particular service
within a domain. A chief purpose of S-NAPTR is to allow a layer of
indirection between service naming and service operation that is not
possible with SRV RRs alone. The example in Section 3.2 illustrates
this. Section 3.3 revisits the first example to illustrate the power
of S-NAPTR in separating the choice of available protocols (which the
service owner selects) from the choice and preference of running them
(which the hosting service establishes).
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3.1 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", because 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. Here's a hypothetical case for storing application key or
certificate data for a given domain. The premise is that some
credentials registry (CredReg) service has been defined to be a leaf
node service holding the keys/certs for the servers operated by (or
for) the domain. Furthermore, 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.com might look like
this:
thinkingcat.com.
;; order pref flags service regexp replacement
IN NAPTR 100 10 "" "CREDREG:ldap:iris-beep" "" credreg.thinkingcat.com.
Note that another domain, offering the same application service,
might offer it using a different set of application protocols:
anotherdomain.com.
;; order pref flags service regexp replacement
IN NAPTR 100 10 "" "CREDREG:iris-lw:iris-beep" "" credreg.anotherdomain.com.
3.2 Multiple Protocols
As it stands, there are several different protocols proposed for
offering "instant message" services. Assuming that "IM" was
registered as an application service, this DDDS application could be
used to determine the available services for delivering to a target.
Two particular features of instant messaging should be noted:
1. gatewaying is expected to bridge communications across protocols
2. instant messaging servers are likely to be operated out of a
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different domain than the instant messaging address, and servers
of different protocols may be offered by independent
organizations
For example, "thinkingcat.com" may support its own servers for the
"ProtA" instant messaging protocol, but rely on outsourcing from
"example.com" for "ProtC" and "ProtB" servers.
Using this DDDS-based approach, thinkingcat.com can indicate a
preference ranking for the different types of servers for the instant
messaging service, and 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 IM services for thinkingcat.com, the NAPTR records
for thinkingcat.com are retrieved:
thinkingcat.com.
;; order pref flags service regexp replacement
IN NAPTR 100 10 "s" "IM:ProtA" "" _ProtA._tcp.thinkingcat.com.
IN NAPTR 100 20 "s" "IM:ProtB" "" _ProtB._tcp.example.com.
IN NAPTR 100 30 "s" "IM:ProtC" "" _ProtC._tcp.example.com.
and 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.im.example.com
IN SRV 30 0 10001 nuclearfallout.example.com.au
3.3 Remote Hosting
In the Instant Message hosting example in Section 3.2, the service
owner (thinkingcat.com) had to host pointers to the hosting service's
SRV records in the thinkingcat.com domain.
If the owner domain does not have any preference about which
application protocol is used, a better way to approach this is to
have one NAPTR RR in the thinkingcat.com domain pointing to all the
hosted services, and the hosting domain has NAPTR records for each
service to map them to whatever local hosts it chooses (and may
change from time to time).
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thinkingcat.com.
;; order pref flags service regexp replacement
IN NAPTR 100 10 "s" "IM:ProtA" "" _ProtA._tcp.thinkingcat.c
om.
IN NAPTR 100 20 "" "IM:ProtB:ProtC" "" thinkingcat.example.com.
and 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 service regexp replacement
IN NAPTR 100 10 "s" "IM:ProtC" "" _ProtC._tcp.example.c
om.
IN NAPTR 100 20 "s" "IM:ProtB" "" _ProtB._tcp.example.com.
_ProtB._tcp.example.com.
;; Pref Weight Port Target
IN SRV 10 0 10001 bigiron.example.com
IN SRV 20 0 10001 backup.im.example.com
IN SRV 30 0 10001 nuclearfallout.example.com.au
3.4 Sets of NAPTR RRs
Note that the above sections assumed that there was one service
available (via S-NAPTR) per domain. Often, that will not be the
case. Assuming thinkingcat.com had the CredReg service set up as
described in Section 3.1 and the instant messaging service set up as
described in Section 3.3, then a client querying for the NAPTR RR set
from thinkingcat.com would get the following answer:
thinkingcat.com.
;; order pref flags service regexp replacement
IN NAPTR 100 10 "s" "IM:ProtA" "" _ProtA._tcp.thinkingcat.c
om.
IN NAPTR 100 20 "" "IM:ProtB:ProtC:" "" thinkingcat.example.com.
IN NAPTR 200 10 "" "CREDREG:ldap:iris-beep" "" credreg.thinkingcat.com.
Sorting them by increasing "ORDER", the client would look through the
SERVICE strings to determine if there was a NAPTR RR that matched the
application service it was looking for, with an application protocol
it could use. The first (lowest PREF) record that so matched is the
one the client would use to continue.
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4. Motivation and Discussion
4.1 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:
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 focused on individual server administration,
not application naming: as stated in [4] "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
fact 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 that is more complex than is necessary for
most applications of SRV.
4.2 So, why not just NAPTR records?
That's a trick question. NAPTR records cannot appear in the wild --
see [5]. 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 -- some simple IANA registrations and it's done.
Also, NAPTR has very powerful tools for expressing "rewrite" rules.
That power (==complexity) makes some protocol designers and service
administrators nervous. The concern is that it can translate into
unintelligible, noodle-like rule sets that are difficult to test and
administer.
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This proposed DDDS application specifically uses a subset of NAPTR's
abilities. Only "replacement" expressions are allowed, not "regular
expressions".
4.3 Open Questions
There remain some open questions about the specific approach proposed
for S-NAPTR.
o Is the 32-character limit for the "service" field too limiting?
o Is it better-practice to put each application service/protocol
pair in a separate NAPTR RR?
o Note that the previous 2 points trade off against each other --
separate RRs for each application service/protocol reduces the
likelihood of blowing the 32-character limit, but increases the
size of the RRset.
o Is this going to generate RRsets that are likely to exceed the UDP
packet size for the response?
o As described in Section 5, the client will not try to "negotiate"
protocol preferences with the what is listed in the NAPTR records
-- it will try its favoured protocols in turn. The only issue
with that is that it overrides the preferences expressed by the
server. Is there any way to allow this "negotiation" without
creating an ever-more-complex S-NAPTR resolution process?
5. Guidelines
5.1 Guidelines for Application Protocol Developers
This section outlines the specific elements that protocol developers
must determine and document in order to make use of S-NAPTR.
5.1.1 Registration of application service and protocol tags
Application protocol developers that wish to make use of S-NAPTR must
make provision to register any relevant application service and
application protocol tags, as described in Section 6.
5.1.2 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
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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 (a less preferred target)?
For example, if the client gets a "connection refused" 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.
5.1.3 Server identification and handshake
As noted in Section 7, 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.
5.2 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 that are "reachable" through
the S-NAPTR process for a particular application service can be
thought of as a "tree". Each NAPTR RR 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, etc), but don't look for
reasons to provide more.
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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).
5.3 Guidelines for Client Software Writers
To properly understand DDDS/NAPTR, an implementor must read [5].
However, the most important aspect to keep in mind is that, if one
target fails to work for the application, it is expected that the
application will continue through the S-NAPTR tree to try the (less
preferred) alternatives.
5.4 Pseudo pseudocode for S-NAPTR
5.4.1 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.
(N.B. -- this is brittle! no error checking!).
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
}
if ([FLAG in cur-rr is "U"])
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return target ; the replacement field of the NAPTR record
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)
If a client actually supports multiple protocols for a particular
application service, it could apply the algorithm above for each
protocol in turn, in order of its preference.
5.4.2 Finding subsequent targets
The pseudocode in Section 5.4 is crafted to find the first, most
preferred, host-port pair for a particular application service an
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 should either
o entwine the application-specific protocol with the DNS lookup and
RRset processing described in the pseudocode; or
o use callbacks for the S-NAPTR processing.
5.5 Sample unrolling of the S-NAPTR process
The following is an example of using the S-NAPTR process. It uses
the tag 'WEB' to denote the web browsing application service and the
tag 'SOAP' to denote an XML-based RPC service. It uses the
application protocol tag of 'http' to denote the protocol used for
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the above services.
1. The client requests all the NAPTR records for the target DNS
label.
2. For the S-NAPTR use of NAPTR RRs, the NAPTR regexp field will
always be empty. Therefore, the client ignores all NAPTR RRs
with anything in the regexp field, or flags that are not defined
for use in S-NAPTR.
3. From the remaining NAPTR records, the client selects the lowest-
order and then lowest preference NAPTR record with the desired
application service (e.g., 'WEB' or 'SOAP'). An empty flag
field indicates the next lookup should be for a NAPTR record (as
opposed to SRV, or A, etc...). The replacement field is used as
the label for which the NAPTR records are retrieved.
4. From here, the client follows the NAPTR record for the
application service using the desired application protocol from
the original list of application protocols given for that service
type (e.g. 'WEB:http' or 'SOAP:http').
5. Supposing the matching NAPTR record has "s" in the flag field,
the next lookup is for SRV records using the value of the
replacement field as the DNS label. The client selects the SRV
records with the appropriate priority and weight, and uses that
to determine the target of the next DNS lookup (for an A or AAAA
record).
6. Once the client has determined the IP address of the server using
an A or AAAA record, the client attempts a connection for each
address record for each server name on the specified port with
the selected protocol. If the connection is refused, then the
client should repeat the previous step. If all SRV records from
the previous step are exhausted, then the client should signal an
exception state.
5.6 Sample sequence diagram
Consider the example in Section 3.2. Visually, the sequence of steps
required for the client to reach the final server for a "ProtB"
service for IM for the thinkingcat.com domain is as follows:
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Client NS for NS for
thinkingcat.com example.com backup.im.example.com
| | |
1 -------->| | |
2 <--------| | |
3 ------------------------------>| |
4 <------------------------------| |
5 ------------------------------>| |
6 <------------------------------| |
7 ------------------------------>| |
8 <------------------------------| |
9 ------------------------------------------------->|
10 <-------------------------------------------------|
11 ------------------------------------------------->|
12 <-------------------------------------------------|
(...)
1. the name server (NS) for thinkingcat.com is reached with a
request for all NAPTR records
2. the server responds with the NAPTR records shown in Section 3.2.
3. the second NAPTR record matches the desired criteria; that 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 3.2.
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.im.example.com
8. the client gets the A record with the IP address for
backup.im.example.com from example.com's NS.
9. the client connects to that IP address, on port 10001 (from the
SRV record), using ProtB over tcp.
10. the server responds with an "OK" message.
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11. the client uses ProtB to challenge that this server has
credentials to operate the service for the original domain
(thinkingcat.com)
12. the server responds, and the rest is IM.
6. IANA Considerations
This document calls for 2 IANA registries: one for application
service tags, and one for application protocol tags.
Application service and protocol tags should be defined in an RFC
(unless the "x-" experimental form is used, in which case they are
unregistered). There are no restrictions placed on the tags other
than that they must conform with the syntax defined below (Appendix
A.5). The IANA registries should list the tags and the RFC that
defines their use.
7. Security Considerations
The security of this approach to application service location is only
as good as the security of the DNS servers 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).
To protect against DNS-vectored attacks, 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, IMAP, etc... define the necessary handshake mechansims
to accomplish this task.
The basic mechanism works in the following way:
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 either
be in the subjectDN or subjectAltName fields. For Kerberos, the
name would be a service principle name.
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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, there is reasonable assurance that the
correct end point has been reached.
It is important to note that this document does not define either the
handshake mechanism, the specific credenential naming fields, nor the
name matching semantics. Definitions of S-NAPTR for particular
application protocols MUST define these.
8. Acknowledgements
Many thanks to Dave Blacka, Patrik Faltstrom, Sally Floyd for
discussion and input that has (hopefully!) provoked clarifying
revisions of this document.
References
[1] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
[2] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[3] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[4] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[5] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
One: The Comprehensive DDDS", RFC 3401, October 2002.
[6] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Three: The Domain Name System (DNS) Database", RFC 3403, October
2002.
[7] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Four: The Uniform Resource Identifiers (URI)", RFC 3404, October
2002.
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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
Appendix A. Application Service Location Application of DDDS
This section defines the DDDS application, as described in [5].
A.1 Application Unique String
The Application Unique String is domain label for which an
authoritative server for a particular service is sought.
A.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.
A.3 Expected Output
The expected output of this Application is the information necessary
to connect to authoritative server(s) (host, port, protocol) for an
application service within a given a given domain.
A.4 Flags
This DDDS Application uses only 3 of the Flags defined for the
URI/URN Resolution Application ([7]): "S", "A" and "U". No other
Flags are valid.
All three are for terminal lookups. This means that the Rule is the
last one and that the flag determines what the next stage should be.
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The "S" flag means that the output of this Rule is a domain label for
which one or more SRV [4] 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. "U" means that the output of the Rule is a
URI which should be resolved.
Consistent with the DDDS algorithm, if the Flag string is empty the
next lookup is for another NAPTR record (for the replacement target).
A.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, just 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 ([7]). 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 [1]). Since "+" (the separator used in the
RFC3404 service parameter string) is an allowed character for URI
scheme names, ":" is chosen as the separator here.
A.5.1 Application Services
The "app-service" must be a registered service [this will be an IANA
registry; this is not the IANA port registry, because we want to
define services for which there is no single protocol, and we don't
want to use up port space for nothing].
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A.5.2 Application Protocols
The protocol identifiers that are valid for the "app-protocol"
production are any standard, registered protocols [IANA registry
again -- is this the list of well known/registered ports?].
A.6 Valid Rules
Only substitution Rules are permitted for this application. That is,
no regular expressions are allowed.
A.7 Valid Databases
At present only one DDDS Database is specified for this Application.
[6] specifies a DDDS Database that uses the NAPTR DNS resource record
to 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
that is used for the first lookup -- 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 [6] for more
information on NAPTR records and the Additional Information section
of a DNS response packet.
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