Internet DRAFT - draft-urn-dns-rds
draft-urn-dns-rds
Network Working Group M. Mealling
Internet Draft Network Solutions, Inc.
draft-ietf-urn-dns-rds-00.txt R. Daniel
Category: Standards Track DATAFUSION, Inc.
Expires: May, 1999
Resolution of Uniform Resource Identifiers
using the Domain Name System
Status of this Memo
===================
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Abstract:
=========
The architectural principles laid out in RFC2276 [15] defines the
concept of a 'resolver discovery service'. This document describes
an immediately-deployable RDS. It is implemented by a new DNS Resource
Record, NAPTR (Naming Authority PoinTeR) [16], that provides a method
for encoding incrementally discovered rules within DNS. By using
these incrementally discovered rules to re-map parts of a URI, we
can change the host that is contacted to resolve a URI. This will
allow a more graceful handling of URLs over long time periods, and
forms the foundation for a new proposal for Uniform Resource Names.
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In addition to locating resolvers, the NAPTR provides for other
naming systems to be grandfathered into the URN world, provides
independence between the name assignment system and the resolution
protocol system, and allows multiple services (Identifier to
Location, Identifier to Description, Identifier to Resource, ...)
to be offered. In conjunction with the SRV RR, the NAPTR record
allows those services to be replicated for the purposes of fault
tolerance and load balancing.
Introduction:
=============
Uniform Resource Locators have been a significant advance in
retrieving Internet-accessible resources. However, their brittle
nature over time has been recognized for several years. The Uniform
Resource Identifier working group proposed the development of Uniform
Resource Names to serve as persistent, location-independent
identifiers for Internet resources in order to overcome most of the
problems with URLs. RFC-1737 [1] sets forth requirements on URNs.
During the lifetime of the URI-WG, a number of URN proposals were
generated. The developers of several of those proposals met in a
series of meetings, resulting in a compromise known as the Knoxville
framework. The major principle behind the Knoxville framework is
that the resolution system must be separate from the way names are
assigned. This is in marked contrast to most URLs, which identify the
host to contact and the protocol to use. Readers are referred to [2]
for background on the Knoxville framework and for additional
information on the context and purpose of this proposal.
Separating the way names are resolved from the way they are
constructed provides several benefits. It allows multiple naming
approaches and resolution approaches to compete, as it allows
different protocols and resolvers to be used. There is just one
problem with such a separation - how do we resolve a name when it
can't give us directions to its resolver?
For the short term, DNS is the obvious candidate for the resolution
framework, since it is widely deployed and understood. However, it is
not appropriate to use DNS to maintain information on a per-resource
basis. First of all, DNS was never intended to handle that many
records. Second, the limited record size is inappropriate for catalog
information. Third, domain names are not appropriate as URNs.
Therefore our approach is to use DNS to locate "resolvers" that can
provide information on individual resources, potentially including
the resource itself. To accomplish this, we "rewrite" the URI into a
domain name following the rules found in NAPTR records. Rewrite
rules provide considerable power, which is important when trying to
meet the goals listed above. However, collections of rules can become
difficult to understand. To lessen this problem, the NAPTR rules are
*always* applied to the original URI, *never* to the output of
previous rules.
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Terminology
===========
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
RFC 2119.
Overview of the NAPTR Record
============================
The NAPTR record is defined in RFCXXXX [16]. To summarize, the key
fields in the NAPTR RR are Order, Preference, Service, Flags, Regexp,
and Replacement:
* The order field specifies the order in which records MUST be
processed when multiple NAPTR records are returned in response to a
single query. A naming authority may have delegated a portion of
its namespace to another agency. Evaluating the NAPTR records in
the correct order is necessary for delegation to work properly.
* The preference field specifies the order in which records SHOULD be
processed when multiple NAPTR records have the same value of
"order". This field lets a service provider specify the order in
which resolvers are contacted, so that more capable machines are
contacted in preference to less capable ones.
* The service field specifies the resolution protocol and resolution
service(s) that will be available if the rewrite specified by the
regexp or replacement fields is applied. Resolution protocols are
the protocols used to talk with a resolver. They will be specified
in other documents, such as [5]. Resolution services are operations
such as I2R (URI to Resource), I2L (URI to URL), I2C (URI to URC),
etc. These are specified in the URI Resolution Services
document[6], and their behavior in a particular resolution protocol
will be given in the specification for that protocol (see [5] for a
concrete example).
* The flags field contains modifiers that affect what happens in the
next DNS lookup, typically for optimizing the process. Flags may
also affect the interpretation of the other fields in the record,
therefore, clients MUST skip NAPTR records which contain an unknown
flag value.
* The regexp field is one of two fields used for the rewrite rules,
and is the core concept of the NAPTR record. The regexp field is a
String containing a sed-like substitution expression. (The actual
grammar for the substitution expressions is given later in this
draft). The substitution expression is applied to the original URN
to determine the next domain name to be queried. The regexp field
should be used when the domain name to be generated is conditional
on information in the URI. If the next domain name is always known,
which is anticipated to be a common occurrence, the replacement
field should be used instead.
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* The replacement field is the other field that may be used for the
rewrite rule. It is an optimization of the rewrite process for the
case where the next domain name is fixed instead of being
conditional on the content of the URI. The replacement field is a
domain name (subject to compression if a DNS sender knows that a
given recipient is able to decompress names in this RR type's RDATA
field). If the rewrite is more complex than a simple substitution
of a domain name, the replacement field should be set to . and the
regexp field used.
Note that the client applies all the substitutions and performs all
lookups, they are not performed in the DNS servers. Note also that it
is the belief of the developers of this document that regexps should
rarely be used. The replacement field seems adequate for the vast
majority of situations. Regexps are only necessary when portions of a
namespace are to be delegated to different resolvers. Finally, note
that the regexp and replacement fields are, at present, mutually
exclusive. However, developers of client software should be aware
that a new flag might be defined which requires values in both
fields.
The Distinction between URNs and URLs
=====================================
From the point of view of this system, there is no theoretical
difference between resolving URIs in the general case and URNs
in the specific case. Operationally however, there is a difference
that stems from the unknown case of URI resolution not becoming
widespread. If URN resolution is collapsed into generic URI
resolution, URNs may suffer by the lack of adoption of URI resolution.
The technically correct solution however should discourage
such a case.
The solution is to allow for shortcutting for URN resolution. In
the following specification generic URI resolution starts by
inserting rules for known URI shemes into the 'uri.net' registry.
For URN resolution one of the rules would be for the 'urn' URI
scheme. This rule would simply delegate to the 'urn.net' zone
for additional NAPTRS based on the URN namespace.
Since this rule is the basis for the entire URN RDS, it can be
shortcutted by simply starting URN resolution at the 'urn.net'
registry. This the distinction between the 'uri.net' and 'urn.net'
well known keys seen below.
The DNS RDS Algorithm
=====================
Since the general RDS framework was the basis for the original NAPTR
algorithm, the two match very well. The only pieces missing from the
general NAPTR specification are the original key, protocols and
services.
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The First Known Key
-------------------
In the generic URI case, the first known key is created by taking
the URI scheme and appending 'uri.net' to the end. In the specific,
shortcutted URN case, the first known key is created by taking
the Namespace Identifier and appending 'urn.net' to the end.
URI Example
-----------
http://www.foo.com/ would have a first known key of 'http.uri.net'.
URN Example
-----------
urn:foo:12345 would have a first known key of 'foo.urn.net'.
Services
--------
The services that make sense for URI resolution are generic for
both URI and URN resolution since the input value types itself
based on the URI scheme. Some valid services are defined in
RFCXXXX (draft-ietf-urn-resolution-services-07.txt).
Examples of some of these services are:
I2L: given a URI return one URL that identifies a location where the
original URI can be found
I2Ls: given a URI return one or more URLs that identify multiple
locations where the original URI can be found
I2R: given a URI return one instance of the resource identified by
that URI.
I2Rs: given a URI return one or more instances of the resources
identified by that URI.
I2C: given a URI return one instance of a description of that
resource.
I2N: given a URI return one URN that names the resource
(Caution: equality with respect to URNs is non-trivial. See
[15] for examples of why.)
Protocols
---------
The protocols used in the Services field are currently limited to
THTTP [5]. Simply specifying any protocol in the services field
is insufficient since there are additional semantics surrounding URI
resolution that are not specified within the protocols.
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For example, if Z39.50 were to be specified as a valid protocol it
would have to define how it would encode requests for specific
services, how the URI is encoded, and what information is returned.
Thus, for this document the only valid value used in the examples is
'thttp'.
Examples
========
Example 1
---------
Consider a URN that uses the hypothetical DUNS namespace. DUNS
numbers are identifiers for approximately 30 million registered
businesses around the world, assigned and maintained by Dunn and
Bradstreet. The URN might look like:
urn:duns:002372413:annual-report-1997
The first step in the resolution process is to find out about the
DUNS namespace. The namespace identifier [3], "duns", is extracted
from the URN, prepended to urn.net, and the NAPTRs for duns.urn.net
looked up. It might return records of the form:
duns.urn.net
;; order pref flags service regexp replacement
IN NAPTR 100 10 "s" "dunslink+I2L+I2C" "" dunslink.udp.isi.dandb.com
IN NAPTR 100 20 "s" "rcds+I2C" "" rcds.udp.isi.dandb.com
IN NAPTR 100 30 "s" "thttp+I2L+I2C+I2R" "" thttp.tcp.isi.dandb.com
The order field contains equal values, indicating that no name
delegation order has to be followed. The preference field indicates
that the provider would like clients to use the special dunslink
protocol, followed by the RCDS protocol, and that HTTP is offered as
a last resort. All the records specify the "s" flag, which will be
explained momentarily. The service fields say that if we speak
dunslink, we will be able to issue either the I2L or I2C requests to
obtain a URL or a URC (description) of the resource. The Resource
Cataloging and Distribution Service (RCDS)[7] could be used to get a
URC for the resource, while HTTP could be used to get a URL, URC, or
the resource itself. All the records supply the next domain name to
query, none of them need to be rewritten with the aid of regular
expressions.
The general case might require multiple NAPTR rewrites to locate a
resolver, but eventually we will come to the "terminal NAPTR". Once
we have the terminal NAPTR, our next probe into the DNS will be for a
SRV or A record instead of another NAPTR. Rather than probing for a
non-existent NAPTR record to terminate the loop, the flags field is
used to indicate a terminal lookup. If it has a value of "s", the
next lookup should be for SRV RRs, "a" denotes that A records should
sought. A "p" flag is also provided to indicate that the next action
is Protocol-specific, but that looking up another NAPTR will not be
part of it.
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Since our example RR specified the "s" flag, it was terminal.
Assuming our client does not know the dunslink protocol, our next
action is to lookup SRV RRs for rcds.udp.isi.dandb.com, which will
tell us hosts that can provide the necessary resolution service. That
lookup might return:
;; Pref Weight Port Target
rcds.udp.isi.dandb.com IN SRV 0 0 1000 defduns.isi.dandb.com
IN SRV 0 0 1000 dbmirror.com.au
IN SRV 0 0 1000 ukmirror.com.uk
telling us three hosts that could actually do the resolution, and
giving us the port we should use to talk to their RCDS server. (The
reader is referred to the SRV proposal [4] for the interpretation of
the fields above).
There is opportunity for significant optimization here. We can return
the SRV records as additional information for terminal NAPTRs (and
the A records as additional information for those SRVs). While this
recursive provision of additional information is not explicitly
blessed in the DNS specifications, it is not forbidden, and BIND does
take advantage of it [8]. This is a significant optimization. In
conjunction with a long TTL for *.urn.net records, the average number
of probes to DNS for resolving DUNS URNs would approach one.
Therefore, DNS server implementors SHOULD provide additional
information with NAPTR responses. The additional information will be
either SRV or A records. If SRV records are available, their A
records should be provided as recursive additional information.
Note that the example NAPTR records above are intended to represent
the reply the client will see. They are not quite identical to what
the domain administrator would put into the zone files. For one
thing, the administrator should supply the trailing '.' character on
any FQDNs.
Also note that there could have been an additional first step where
the URN was resolved as a generic URI by looking up urn.uri.net.
The resulting rule would have specified that the NID be extracted
from the URN and 'urn.net' appended to it resulting in the
new key 'duns.urn.net' which is the first step from above.
Example 2
---------
Consider a URN namespace based on MIME Content-Ids. The URN might
look like this:
urn:cid:199606121851.1@mordred.gatech.edu
(Note that this example is chosen for pedagogical purposes, and does
not conform to the CID URL scheme.)
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The first step in the resolution process is to find out about the CID
namespace. The namespace identifier, cid, is extracted from the URN,
prepended to urn.net, and the NAPTR for cid.urn.net looked up. It
might return records of the form:
cid.urn.net
;; order pref flags service regexp replacement
IN NAPTR 100 10 "" "" "/urn:cid:.+@([^\.]+\.)(.*)$/\2/i" .
We have only one NAPTR response, so ordering the responses is not a
problem. The replacement field is empty, so we check the regexp
field and use the pattern provided there. We apply that regexp to the
entire URN to see if it matches, which it does. The \2 part of the
substitution expression returns the string "gatech.edu". Since the
flags field does not contain "s" or "a", the lookup is not terminal
and our next probe to DNS is for more NAPTR records:
lookup(query=NAPTR, "gatech.edu").
Note that the rule does not extract the full domain name from the
CID, instead it assumes the CID comes from a host and extracts its
domain. While all hosts, such as mordred, could have their very own
NAPTR, maintaining those records for all the machines at a site as
large as Georgia Tech would be an intolerable burden. Wildcards are
not appropriate here since they only return results when there is no
exactly matching names already in the system.
The record returned from the query on "gatech.edu" might look like:
gatech.edu IN NAPTR
;; order pref flags service regexp replacement
IN NAPTR 100 50 "s" "z3950+I2L+I2C" "" z3950.tcp.gatech.edu
IN NAPTR 100 50 "s" "rcds+I2C" "" rcds.udp.gatech.edu
IN NAPTR 100 50 "s" "thttp+I2L+I2C+I2R" "" thttp.tcp.gatech.edu
Continuing with our example, we note that the values of the order and
preference fields are equal in all records, so the client is free to
pick any record. The flags field tells us that these are the last
NAPTR patterns we should see, and after the rewrite (a simple
replacement in this case) we should look up SRV records to get
information on the hosts that can provide the necessary service.
Assuming we prefer the Z39.50 protocol, our lookup might return:
;; Pref Weight Port Target
z3950.tcp.gatech.edu IN SRV 0 0 1000 z3950.gatech.edu
IN SRV 0 0 1000 z3950.cc.gatech.edu
IN SRV 0 0 1000 z3950.uga.edu
telling us three hosts that could actually do the resolution, and
giving us the port we should use to talk to their Z39.50 server.
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Recall that the regular expression used \2 to extract a domain name
from the CID, and \. for matching the literal '.' characters
seperating the domain name components. Since '\' is the escape
character, literal occurances of a backslash must be escaped by
another backslash. For the case of the cid.urn.net record above, the
regular expression entered into the zone file should be
"/urn:cid:.+@([^\\.]+\\.)(.*)$/\\2/i". When the client code actually
receives the record, the pattern will have been converted to
"/urn:cid:.+@([^.]+\.)(.*)$/\2/i".
Example 3
---------
Even if URN systems were in place now, there would still be a
tremendous number of URLs. It should be possible to develop a URN
resolution system that can also provide location independence for
those URLs. This is related to the requirement in [1] to be able to
grandfather in names from other naming systems, such as ISO Formal
Public Identifiers, Library of Congress Call Numbers, ISBNs, ISSNs,
etc.
The NAPTR RR could also be used for URLs that have already been
assigned. Assume we have the URL for a very popular piece of
software that the publisher wishes to mirror at multiple sites around
the world:
http://www.foo.com/software/latest-beta.exe
We extract the prefix, "http", and lookup NAPTR records for
http.uri.net. This might return a record of the form
http.uri.net IN NAPTR
;; order pref flags service regexp replacement
100 90 "" "" "!http://([^/:]+)!\1!i" .
This expression returns everything after the first double slash and
before the next slash or colon. (We use the '!' character to delimit
the parts of the substitution expression. Otherwise we would have to
use backslashes to escape the forward slashes, and would have a
regexp in the zone file that looked like
"/http:\\/\\/([^\\/:]+)/\\1/i".).
Applying this pattern to the URL extracts "www.foo.com". Looking up
NAPTR records for that might return:
www.foo.com
;; order pref flags service regexp replacement
IN NAPTR 100 100 "s" "thttp+L2R" "" thttp.tcp.foo.com
IN NAPTR 100 100 "s" "ftp+L2R" "" ftp.tcp.foo.com
Looking up SRV records for thttp.tcp.foo.com would return information
on the hosts that foo.com has designated to be its mirror sites. The
client can then pick one for the user.
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Notes:
======
- Registration procedures for the urn.net and uri.net DNS zones
is specified in "Assignment Procedures for the URI Resolution
using DNS (RFC2168)" [17]
- A client MUST process multiple NAPTR records in the order
specified by the "order" field, it MUST NOT simply use the first
record that provides a known protocol and service combination.
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- If a record at a particular order matches the URI, but the
client doesn't know the specified protocol and service, the
client SHOULD continue to examine records that have the same
order. The client MUST NOT consider records with a higher value
of order. This is necessary to make delegation of portions of
the namespace work. The order field is what lets site
administrators say "all requests for URIs matching pattern x go
to server 1, all others go to server 2".
(A match is defined as:
1) The NAPTR provides a replacement domain name
or
2) The regular expression matches the URN
)
- When multiple RRs have the same "order", the client should use
the value of the preference field to select the next NAPTR to
consider. However, because of preferred protocols or services,
estimates of network distance and bandwidth, etc. clients may
use different criteria to sort the records.
- If the lookup after a rewrite fails, clients are strongly
encouraged to report a failure, rather than backing up to pursue
other rewrite paths.
- When a namespace is to be delegated among a set of resolvers,
regexps must be used. Each regexp appears in a separate NAPTR
RR. Administrators should do as little delegation as possible,
because of limitations on the size of DNS responses.
- Note that SRV RRs impose additional requirements on clients.
Acknowledgments:
=================
The editors would like to thank Keith Moore for all his consultations
during the development of this draft. We would also like to thank
Paul Vixie for his assistance in debugging our implementation, and
his answers on our questions. Finally, we would like to acknowledge
our enormous intellectual debt to the participants in the Knoxville
series of meetings, as well as to the participants in the URI and URN
working groups.
References:
===========
[1] Sollins, Karen and Larry Masinter, "Functional Requirements
for Uniform Resource Names", RFC-1737, Dec. 1994.
[2] The URN Implementors, Uniform Resource Names: A Progress Report,
http://www.dlib.org/dlib/february96/02arms.html, D-Lib Magazine,
February 1996.
[3] Moats, Ryan, "URN Syntax", RFC-2141, May 1997.
[4] Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying
the location of services (DNS SRV)", RFC-2052, October 1996.
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RFC nnnn Resolution of URIs Using the DNS November 1998
[5] Daniel, Jr., Ron, "A Trivial Convention for using HTTP in URN
Resolution", RFC-2169, June 1997.
[6] URN-WG, "URN Resolution Services", Work in Progress. RFCXXXX
[7] Moore, Keith, Shirley Browne, Jason Cox, and Jonathan Gettler,
Resource Cataloging and Distribution System, Technical Report
CS-97-346, University of Tennessee, Knoxville, December 1996
[8] Paul Vixie, personal communication.
[9] Orth, Charles and Bill Arms; Handle Resolution Protocol
Specification, http://www.handle.net/docs/client_spec.html
[10] Williamson, S., M. Kosters, D. Blacka, J. Singh, K. Zeilstra,
"Referral Whois Protocol (RWhois)", RFC-2167, June 1997.
[11] Information Retrieval (Z39.50): Application Service Definition
and Protocol Specification, ANSI/NISO Z39.50-1995, July 1995.
[12] IEEE Standard for Information Technology - Portable Operating
System Interface (POSIX) - Part 2: Shell and Utilities (Vol. 1);
IEEE Std 1003.2-1992; The Institute of Electrical and
Electronics Engineers; New York; 1993. ISBN:1-55937-255-9
[13] Braden, R., "Requirements for Internet Hosts - Application and
and Support", RFC-1123, Oct. 1989.
[14] Sollins, Karen, "Architectural Principles of Uniform Resource
Name Resolution", RFC2276. January 1998.
[15] Mealling, Michael, Daniel, Jr., Ron. "The Naming Authority
Pointer (NAPTR) DNS Resource Record",
(draft-urn-naptr-rr-00.txt), November 1998.
[16] Mealling, Michael. "Assignment Procedures for the URI Resolution
using DNS (RFC2168)", (draft-ietf-urn-urn.net-procedures-01.txt),
November 1998.
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IANA Considerations
===================
The use of the "urn.net" and "uri.net" zones requires registration
policies and procedures to be followed and for the operation of
those DNS zones to be maintained. These policies and procedures
are spelled out in a "Assignment Procedures for the URI Resolution
using DNS (RFC2168)" [17]. The operation of those zones imposes
operational and adminstrative responsibilities on the IANA.
The registration methods used for specifying values for the Services
(both protocols and services) and Flags fields that are specific to
URI resolution is for a specification to be published as an RFC
and approved by the IESG.
The registration policies for URLs and URNs are also specified
elsewhere and thus those impacts on the IANA are spelled out there.
Security Considerations
=======================
The use of "urn.net" and "uri.net" as the registry for namespaces
is subject to denial of service attacks, as well as other DNS
spoofing attacks. The interactions with DNSSEC are currently being
studied. It is expected that NAPTR records will be signed with
SIG records once the DNSSEC work is deployed.
The rewrite rules make identifiers from other namespaces subject to
the same attacks as normal domain names. Since they have not been
easily resolvable before, this may or may not be considered a
problem.
Regular expressions should be checked for sanity, not blindly passed
to something like PERL.
This document has discussed a way of locating a resolver, but has not
discussed any detail of how the communication with the resolver takes
place. There are significant security considerations attached to the
communication with a resolver. Those considerations are outside the
scope of this document, and must be addressed by the specifications
for particular resolver communication protocols.
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Author Contact Information:
===========================
Michael Mealling
Network Solutions
505 Huntmar Park Drive
Herndon, VA 22070
voice: (703) 742-0400
fax: (703) 742-9552
email: michaelm@internic.net
URL: http://www.netsol.com/
Ron Daniel Jr.
DATAFUSION, Inc.
139 Townsend Street, Ste. 100
San Francisco, CA 94107
415.222.0100 fax 415.222.0150
rdaniel@datafusion.net
http://www.datafusion.net
Appendix A -- Psuedo Code
=====
For the edification of implementers, pseudocode for a client routine
using NAPTRs is given below. This code is provided merely as a
convience, it does not have any weight as a standard way to process
NAPTR records. Also, as is the case with pseudocode, it has never
been executed and may contain logical errors. You have been warned.
//
// findResolver(URN)
// Given a URN, find a host that can resolve it.
//
findResolver(string URN) {
// prepend prefix to urn.net
sprintf(key, "%s.urn.net", extractNS(URN));
do {
rewrite_flag = false;
terminal = false;
if (key has been seen) {
quit with a loop detected error
}
add key to list of "seens"
records = lookup(type=NAPTR, key); // get all NAPTR RRs for 'key'
discard any records with an unknown value in the "flags" field.
sort NAPTR records by "order" field and "preference" field
(with "order" being more significant than "preference").
n_naptrs = number of NAPTR records in response.
curr_order = records[0].order;
max_order = records[n_naptrs-1].order;
// Process current batch of NAPTRs according to "order" field.
for (j=0; j < n_naptrs && records[j].order <= max_order; j++) {
if (unknown_flag) // skip this record and go to next one
continue;
newkey = rewrite(URN, naptr[j].replacement, naptr[j].regexp);
if (!newkey) // Skip to next record if the rewrite didn't
match continue;
// We did do a rewrite, shrink max_order to current value
// so that delegation works properly
max_order = naptr[j].order;
// Will we know what to do with the protocol and services
// specified in the NAPTR? If not, try next record.
if(!isKnownProto(naptr[j].services)) {
continue;
}
if(!isKnownService(naptr[j].services)) {
continue;
}
// At this point we have a successful rewrite and we will
// know how to speak the protocol and request a known
// resolution service. Before we do the next lookup, check
// some optimization possibilities.
if (strcasecmp(flags, "S")
|| strcasecmp(flags, "P"))
|| strcasecmp(flags, "A")) {
terminal = true;
services = naptr[j].services;
addnl = any SRV and/or A records returned as additional
info for naptr[j].
}
key = newkey;
rewriteflag = true;
break;
}
} while (rewriteflag && !terminal);
// Did we not find our way to a resolver?
if (!rewrite_flag) {
report an error
return NULL;
}
// Leave rest to another protocol?
if (strcasecmp(flags, "P")) {
return key as host to talk to;
}
// If not, keep plugging
if (!addnl) { // No SRVs came in as additional info, look them up
srvs = lookup(type=SRV, key);
}
sort SRV records by preference, weight, ...
foreach (SRV record) { // in order of preference
try contacting srv[j].target using the protocol and one of the
resolution service requests from the "services" field of the
last NAPTR record.
if (successful)
return (target, protocol, service);
// Actually we would probably return a result, but this
// code was supposed to just tell us a good host to talk to.
}
die with an "unable to find a host" error;
}
Mealling & Daniel [Page 15]
