INTERNET-DRAFT                                        Paul Gauthier
   Expires: December 1999                          Inktomi Corporation
   Category: Standards Track                                Josh Cohen
   draft-ietf-wrec-wpad-01.txt                   Microsoft Corporation
                                                       Martin Dunsmuir
                                                    RealNetworks, Inc.
                                                       Charles Perkins
                                                Sun Microsystems, Inc.

                     Web Proxy Auto-Discovery Protocol

Status of This Memo

   This document is a submission by the WREC Working Group of the
   Internet Engineering Task Force (IETF).  Comments should be
   submitted to the mailing list.

   Distribution of this memo is unlimited.

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time.  It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as "work in progress."

     The list of current Internet-Drafts can be accessed at:

     The list of Internet-Draft Shadow Directories can be accessed at:


   A mechanism is needed to permit web clients to locate nearby web
   proxy caches. Current best practice is for end users to hand
   configure their web client (i.e., browser) with the URL of an "auto
   configuration file". In large environments this presents a
   formidable support problem.  It would be much more manageable for
   the web client software to automatically learn the configuration
   information for its web proxy settings. This is typically referred
   to as a resource discovery problem.

   Web client implementers are faced with a dizzying array of resource
   discovery protocols at varying levels of implementation and
   deployment. This complexity is hampering deployment of a "web proxy
   auto-discovery "facility.  This document proposes a pragmatic
   approach to web proxy auto-discovery.  It draws on a number of
   proposed standards in the light of practical deployment concerns. It
   proposes an escalating strategy of resource discovery attempts in
   order to find a nearby web proxy server. It attempts to provide rich

   Gauthier, Cohen, Dunsmuir, Perkins                         [Page 1]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

   mechanisms for supporting a complex environment, which may contain
   multiple web proxy servers.

Table of Contents

Status of This Memo...................................................1
Table of Contents.....................................................2
1.   Conventions used in this document................................2
2.   Introduction.....................................................2
3.   Defining Web Proxy Auto-Discovery................................3
4.   The Discovery Process............................................4
 4.1.  WPAD Overview................................................4
 4.2.  When to Execute WPAD.........................................6
   4.2.1.  Upon Startup of the Web Client............................7
   4.2.2.  Network Stack Events......................................7
   4.2.3.  Expiration of the CFILE...................................7
 4.3.  WPAD Protocol Specification..................................7
 4.4.  Discovery Mechanisms.........................................9
   4.4.1.  DHCP......................................................9
   4.4.2.  SVRLOC/SLP...............................................10
   4.4.3.  DNS A/CNAME  "Well Known Aliases”........................10
   4.4.4.  DNS SRV Records..........................................10
   4.4.5.  DNS TXT service: Entries.................................11
   4.4.6.  Fallback.................................................11
   4.4.7.  Timeouts.................................................11
 4.5.  Composing a Candidate CURL..................................12
 4.6.  Retrieving the CFILE at the CURL............................12
 4.7.  Resuming Discovery..........................................12
5.   Client Implementation Considerations............................12
6.   Proxy Server Considerations.....................................13
7.   Administrator Considerations....................................13
8.   Conditional Compliance..........................................14
 8.1.  Class 0 - Minimally compliant...............................15
 8.2.  Class 1 - Compliant.........................................15
 8.3.  Class 2 - Maximally compliant...............................15
9.   Security Considerations.........................................15
10.  Acknowledgements................................................16
11.  Copyright.......................................................16
12.  References......................................................16
13.  Author Information..............................................17

1.   Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in "Key words for use in
   RFCs to Indicate Requirement Levels" [KEYWORDS].

2.   Introduction

   The problem of locating nearby web proxy cache servers can not wait
   for the implementation and large scale deployment of various

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                         [Page 2]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

   upcoming resource discovery protocols. The widespread success of the
   HTTP protocol and the recent popularity of streaming media has
   placed unanticipated strains on the networks of corporations, ISPs
   and backbone providers. There currently is no effective method for
   these organizations to realize the obvious benefits of web caching
   without tedious and error prone configuration by each and every end

   The de-facto mechanism for specifying a web proxy server
   configuration in web clients is the download of a script or
   configuration file named by a URL. Users are currently expected to
   hand configure this URL into their Browser or other web client.
   This mechanism suffers from a number of drawbacks:

   - Difficulty in supporting a large body of end-users. Many users
   misconfigure their proxy settings and are unable to diagnose the
   cause of their problems.

   - Lack of support for mobile clients who require a different proxy
   as their point of access changes.

   - Lack of support for complex proxy environments where there may
   exist a number of proxy servers with different affinities for
   different clients (based on network proximity, for example).
   Currently, clients would have to "know" which proxy server was
   optimal for their use.

   Currently available methods for resource discovery need to be
   exploited in the context of a well defined framework. Simple,
   functional and efficient mechanisms stand a good chance of solving
   this pressing and basic need. As new resource discovery mechanisms
   mature they can be folded into this framework with little

   This document is a specification for implementers of web client
   software. It defines a protocol for automatically configuring those
   clients to use a local proxy. It also defines how an administrator
   should configure various resource discovery services in their
   network to support WPAD compatible web clients.

   While it does contain suggestions for web proxy server implementers,
   it does not make any specific demands of those parties.

3.   Defining Web Proxy Auto-Discovery

   As mentioned above, currently web client software needs to be
   configured with the URL of a proxy auto-configuration file or
   script. The contents of this script are vendor specific and not
   currently standardized. This document does not attempt to discuss
   the contents of these files (see[8] for an example file format).

   Thus, the Web Proxy Auto-Discovery (WPAD) problem reduces to
   providing the web client a mechanism for discovering the URL of the

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                         [Page 3]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

   Configuration File. Once this Configuration URL (CURL) is known, the
   client software already contains mechanisms for retrieving and
   interpreting the Configuration File (CFILE) to enable access to the
   specified proxy cache servers.

   It is worth carefully noting that the goal of the WPAD process is to
   discover the correct CURL at which to retrieve the CFILE. The client
   is *not* trying to directly discover the name of the proxy server.
   That would circumvent the additional capabilities provided by proxy
   Configuration Files (such as load balancing, request routing to an
   array of servers, automated fail-over to backup proxy server [6,8]).

   It is worth noting that different clients requesting the CURL may
   receive completely different CFILEs in response. The web server may
   send back different CFILES based on a number of criteria such as the
   "User-Agent" header, "Accept" headers, client IP address/subnet,
   etc.  The same client could conceivably receive a different CFILE on
   successive retrievals (as a method of round-robin load balancing,
   for example).

   This document will discuss a range of mechanisms for discovering the
   Configuration URL. The client will attempt them in a predefined
   order, until one succeeds. Existing widely deployed facilities may
   not provide enough expressiveness to specify a complete URL. As
   such, we will define default values for portions of the CURL which
   may not be expressible by some discovery mechanisms:


   <HOST> - There is no default for this potion. Any succeeding
        discovery mechanism will provide a value for the <HOST> portion
        of the CURL. The client MUST NOT provide a default.

   <PORT> - The client MUST assume port 80 if the successful discovery
        mechanism does not provide a port component.

   <PATH> - The client MUST assume a path of "/wpad.dat" if the
        successful discovery mechanism does not provide a path

4.   The Discovery Process

4.1. WPAD Overview

   This sub-section will present a descriptive overview of the WPAD
   protocol. It is intended to introduce the concepts and flow of the
   protocol. The remaining sub-sections (3.2-3.7) will provide the
   rigorous specification of the protocol details. WPAD uses a
   collection of pre-existing Internet resource discovery mechanisms to
   perform web proxy auto-discovery. Readers may wish to refer to [1]
   for a similar approach to resource discovery, since it was a basis
   for this strategy.  The WPAD protocol specifies the following:

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                         [Page 4]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

   - how to use each mechanism for the specific purpose of web proxy
   - the order in which the mechanisms should be performed
   - the minimal set of mechanisms which must be attempted by a WPAD
        compliant web client

   The resource discovery mechanisms utilized by WPAD are as follows.
   - Dynamic Host Configuration Protocol (DHCP, [3,7]).
   - Service Location Protocol (SLP, [4]).
   - "Well Known Aliases” using DNS A records [5,9].
   - DNS SRV records [2,9].
   - "service: URLs" in DNS TXT records [10].

   Of all these mechanisms only the DHCP and “Well Known Aliases” are
   required in WPAD clients. This decision is based on three reasons:
   these facilities are currently widely deployed in existing vendor
   hardware and software; they represent functionality that should
   cover most real world environments; they are relatively simple to

   DNS servers supporting A records are clearly the most widely
   deployed of the services outlined above. It is reasonable to expect
   API support inside most web client development environments (POSIX
   C, Java, etc). The hierarchical nature of DNS makes it possible to
   support hierarchies of proxy servers.

   DNS is not suitable in every environment, unfortunately.
   Administrators often choose a DNS domain name hierarchy that does
   not correlate to network topologies, but rather with some
   organizational model (for example, and DHCP servers, on the other hand, are
   frequently deployed with concern for network topologies. DHCP
   servers provide support for making configuration decisions based on
   subnets, which are directly related to network topology.

   Full client support for DHCP is not as ubiquitous as for DNS. That
   is, not all clients are equipped to take advantage of DHCP for their
   essential network configuration (assignment of IP address, network
   mask, etc). APIs for DHCP are not as widely available. Luckily,
   using DHCP for WPAD does not require either of these facilities. It
   is relatively easy for web client developers to speak just the
   minimal DHCP protocol to perform resource discovery. It entails
   building a simple UDP packet, sending it to the subnet broadcast
   address, and parsing the reply UDP packet(s) which are received to
   extract the WPAD option field. A reference implementation of this
   code in C is available [11].

   The WPAD client attempts a series of resource discovery requests,
   using the discovery mechanisms mentioned above, in a specific order.
   Clients only attempt mechanisms that they support (obviously). Each
   time the discovery attempt succeeds; the client uses the information
   obtained to construct a CURL. If a CFILE is successfully retrieved

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                         [Page 5]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

   at that CURL, the process completes. If not, the client resumes
   where it left of in the predefined series of resource discovery
   requests. If no untried mechanisms remain and a CFILE has not been
   successfully retrieved, the WPAD protocol fails and the client is
   configured to use no proxy server.

   First the client tries DHCP, followed by SLP. If no CFILE has been
   retrieved the client moves on to the DNS based mechanisms. The
   client will cycle through the DNS SRV, “Well Known Aliases” and DNS
   TXT record methods multiple times. Each time through the QNAME being
   used in the DNS query is made less and less specific. In this manner
   the client can locate the most specific configuration information
   possible, but can fall back on less specific information. Every DNS
   lookup has the QNAME prefixed with “wpad” to indicate the resource
   type being requested.

   As an example, consider a client with hostname johns- Assume the web client software supports
   all of the mechanisms listed above. This is the sequence of
   discovery attempts the client would perform until one succeeded in
   locating a valid CFILE:

   - DHCP
   - SLP
   - DNS A lookup on
   - DNS SRV lookup on
   - DNS TXT lookup on
   - DBS A lookup on
   - DNS SRV lookup on
   - DNS TXT lookup on

4.2. When to Execute WPAD

   Web clients need to perform the WPAD protocol periodically to
   maintain correct proxy settings. This should occur on a regular
   basis corresponding to initialization of the client software or the
   networking stack below the client. As well, WPAD will need to occur
   in response to expiration of existing configuration data.  The
   following sections describe the details of these scenarios.  3.2.1.
   Periodic Discovery

   The web proxy auto-discovery process MUST occur at least as
   frequently as one of the following two options. A web client can use
   either option depending on which makes sense in their environment.
   Clients MUST use at least one of the following options. They MAY
   also choose to implement both options.
   - Upon startup of the web client.
   - Whenever there indication from the networking stack that the  IP
   address of the client host either has, or could have, changed.

   In addition, the client MUST attempt a discovery cycle upon
   expiration of a previously downloaded CFILE in accordance with

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                         [Page 6]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

4.2.1.    Upon Startup of the Web Client

   For many types of web client (like web browsers) there can be many
   instances of the client operating for a given user at one time. This
   is often to allow display of multiple web pages in different
   windows, for example. There is no need to re-perform WPAD every time
   a new instance of the web client is opened. WPAD MUST be performed
   when the number of web client instances transitions from 0 to 1. It
   SHOULD NOT be performed as additional instances are created.

4.2.2.    Network Stack Events

   Another option for clients is to tie the execution of WPAD to
   changes in the networking environment. If the client can learn about
   the change of the local host’s IP address, or the possible change of
   the IP address, it MUST re-perform the WPAD protocol.  Many
   operating systems provide indications of “network up” events, for
   example. Those types of events and system-boot events might be the
   triggers for WPAD in many environments.

4.2.3.    Expiration of the CFILE

   The HTTP retrieval of the CURL may return HTTP headers specifying a
   valid lifetime for the CFILE returned. The client MUST obey these
   timeouts and rerun the PAD process when it expires. A client MAY
   rerun the WPAD process if it detects a failure of the currently
   configured proxy (which is not otherwise recoverable via the
   inherent mechanisms provided by the currently active Configuration

   Whenever the client decides to invalidate the current CURL or CFILE,
   it MUST rerun the entire WPAD protocol to ensure it discovers the
   currently correct CURL. Specifically, if the valid lifetime of the
   CFILE ends(as specified by the HTTP headers provided when it was
   retrieved),the complete WPAD protocol MUST be rerun. The client MUST
   NOT simply re-use the existing CURL to obtain a fresh copy of the

   A number of network round trips, broadcast and/or multicast
   communications may be required during the WPAD protocol. The WPAD
   protocol SHOULD NOT be invoked at a more frequent rate than
   specified above (such as per-URL retrieval).

4.3. WPAD Protocol Specification

   The following pseudo-code defines the WPAD protocol.  If a
   particular discovery mechanism is not supported, treat it as a
   failed discovery attempt in the pseudo-code.

   In addition, this logic is expressed below in pseudo-code.
   The following pseudo-code fragment defines WPAD.  Unsupported
   discovery mechanisms are treated as failure in the pseudo-code.

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                         [Page 7]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

   Two subroutines need explanation. The subroutine
   strip_leading_component(dns_string) strips off the leading
   characters, up to and including the first dot (`.') in the string
   which is passed as a parameter, and is expected to contain DNS name.
   The Boolean subroutine is_not_canonical(dns_string) returns FALSE if
   dns_string is one of the canonical domain suffixes defined in RFC
   1591 [13] (for example, "com").

   The slp_list and dns_list elements below are assumed to be linked
   lists containing a data field and a pointer to the next element.
   The data field contains the elements used to override the default
   values in creating a CURL, as detailed in section 3.5.

      load_CFILE() {
        /* MUST use DHCP */
        curl = dhcp_query(/*WPAD option  (section 4.4.1) */);
        if (curl != null) {  /* DHCP succeeded */
           if isvalid (read_CFILE(curl))
              return SUCCESS;  /* valid CFILE */

        /* Should use SLP */
        slp_list = slp_query(/*(WPAD attributes  (Section 4.4.2)*/);
        while (slp_list != null) {  /* test each curl  */
           if isvalid(read_CFILE(slp_list.curl_data))
              return SUCCESS;   /* valid CFILE */
              slp_list =;

        /* all the DNS mechanisms */
        TGTDOM = gethostbyname(me);
        TGTDOM = strip_leading_component(TGTDOM);

        while (is_not_canonical(TGTDOM)) {

           /* SHOULD try DNS SRV records */
           dns_list = dns_query(/*QNAME=wpad.TGTDOM.,
                                        QTYPE=SRV (section 4.4.4)*/);
           while (dns_list != null) { /* each TXT record */
              if isvalid(read_CFILE(dns_list, curl_data))
                 return SUCCESS;   /* valid CFILE */
                 dns_list =;

           /* SHOULD try DNS TXT records */
           dns_list = dns_query(/*QNAME=wpad.TGTDOM.,
                                        QTYPE=TXT (section 4.4.5)*/);
           while (dns_list != null) { /* each TXT record */
              if isvalid(read_CFILE(dns_list, curl_data))
                 return SUCCESS;   /* valid CFILE */

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                         [Page 8]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

                 dns_list =;

           /* MUST try DNS A records */
           dns_list = dns_query(/*QNAME=wpad.TGTDOM.,
                                    QTYPE=A  (Section 4.4.3)*/);

           while (dns_list != null) { /* check each A record */
              if isvalid(read_CFILE(dns_list, curl_data))
                 return SUCCESS;   /* valid CFILE */
                 dns_list =;

           /* Still no match, remove leading component and iterate */
           TGTDOM = strip_leading_component(TGTDOM);

        } /* no A, TXT or SRV records for wpad.* */

        return FAILED;  /* could not locate valid CFILE */

4.4. Discovery Mechanisms

   Each of the resource discovery methods will be marked as to whether
   the client MUST, SHOULD, MAY, or MUST NOT implement them to be
   compliant. Client implementers are encouraged to implement as many
   mechanisms as possible, to promote maximum interoperability.

   | Discovery               |        | Document |
   | Mechanism               | Status | Section  |
   | DHCP                    | MUST   | 4.4.1    |
   | SLP                     | SHOULD | 4.4.2    |
   | "Well Known Alias"      | MUST   | 4.4.3    |
   | DNS SRV Records         | SHOULD | 4.4.4    |
   | DNS TXT "service: URLs" | SHOULD | 4.4.5    |


4.4.1.    DHCP

   Client implementations MUST support DHCP. DHCP has widespread
   support innumerous vendor hardware and software implementations, and
   is widely deployed. It is also perfectly suited to this task, and is
   used to discover other network resources (such a time servers,
   printers, etc). The DHCP protocol is detailed in RFC 2131 [3].
   We propose a new DHCP option with code 252 for use in web proxy
   auto-discovery. See RFC 2132 [7] for a list of existing DHCP

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                         [Page 9]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

   options. See "Conditional Compliance" for more information on DHCP

   The client should obtain the value of the DHCP option code 252 as
   returned by the DHCP server. If the client has already conducted
   DHCP protocol during its initialization, the DHCP server may already
   have supplied that value. If the value is not available through a
   client OS API, the client SHOULD use a DHCPINFORM message to query
   the DHCP server to obtain the value.

   The DHCP option code for WPAD is 252 by agreement of the DHC working
   group chair.  This option is of type STRING.  This string contains a
   URL which points to an appropriate config file.  The STRING is of
   arbitrary size.
   An example STRING value would be:

4.4.2.    Service Location Protocol /SLP

   The Service Location Protocol [RFC2608] is a Proposed Standard for
   discovering services in the Internet.  SLP has several reference
   implementations available; for details, check the following web

   A service type for use with WPAD has been defined and is available
   as an Internet Draft.

   Client implementations SHOULD implement SLP.   SLP Service Replies
   will provide one or more complete CURLs. Each candidate CURL so
   created should be pursued as specified in section 4.5 and beyond.

4.4.3.    DNS A/CNAME  "Well Known Aliases”

   Client implementations MUST support this mechanism. This should be
   straightforward since only basic DNS lookup of A records is
   required. See RFC 2219 [5] for a description of using "well known"
   DNS aliases for resource discovery. We propose the "well known
   alias” of "wpad" for web proxy auto-discovery.

   The client performs the following DNS lookup:

   Each A RR, which is returned, contains an IP address which is used
   to replace the <HOST> default in the CURL.

   Each candidate CURL so created should be pursued as specified in
   section 4.5 and beyond.

4.4.4.    DNS SRV Records

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                        [Page 10]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

   Client implementations SHOULD support the DNS SRV mechanism. Details
   of the protocol can be found in RFC 2052 [2]. If the implementation
   language/environment provides the ability to perform DNS lookups on
   QTYPEs other than A, client implementers are strongly encouraged to
   provide this support. It is acknowledged that not all resolver APIs
   provide this functionality.

   The client issues the following DNS lookup:

   If it receives SRV RRs in response, the client should use each valid
   RR in the order specified in RFC 2052 [2]. Each valid record will
   specify both a <HOST> and a <PORT> to override the CURL defaults.

   Each candidate CURL so created should be pursued as specified in
   section 4.5 and beyond.

4.4.5.    DNS TXT service: Entries

   Client implementation SHOULD support this mechanism.  If the
   implementation language/environment provides the ability to perform
   DNS lookups on QTYPEs other than A, the vendor is strongly
   encouraged to provide this support. It is acknowledged that not all
   resolver APIs provide this functionality.
   The client should attempt to retrieve TXT RRs from the DNS to obtain
   “service: URLs” contained therein. The “service: URL” will be of the
   following format, specifying a complete candidate CURL for each
   record located:

   service: wpad:http://<HOST>:<PORT><PATH>

   The client should first issue the following DNS query:

   It should process each TXT RR it receives (if any) using each
   service:URL found (if any) to generate a candidate CURL. These CURLs
   should be pursued as described in section 3.5 and beyond.
   Readers familiar with [1] should note that WPAD clients MUST NOT
   perform the QNAME=TGTDOM., QCLASS=IN, QTYPE=TXT lookup which would
   be suggested by that document.

4.4.6.    Fallback

   Clients MUST NOT implement the "Fallback" mechanism described in
   [1]. It is unlikely that a client will find a web server prepared to
   handle the CURL request at a random suffix of its FQDN.  This will
   only increase the number of DNS probes and introduce an excess of
   spurious "GET" requests on those hapless web servers.

   Instead, the "Well Known Aliases” method of section 3.4.4 provides
   equivalent functionality.

4.4.7.    Timeouts

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                        [Page 11]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

   Implementers are encouraged to limit the time elapsed in each
   discovery phase.  When possible, limiting each phase to 10 seconds
   is considered reasonable.  Implementers may choose a different value
   which is more appropriate to their network properties.  For example,
   a device implementation, which operated over a wireless network, may
   use a much larger timeout to account for low bandwidth or high

4.5. Composing a Candidate CURL

   Any successful discovery mechanism response will provide a
   <HOST>(perhaps in the form of an IP address). Some mechanisms will
   also provide a <PORT> and/or a <PATH>. The client should override
   the default CURL fields with all of those supplied by the discovery

4.6. Retrieving the CFILE at the CURL

   The client then requests the CURL via HTTP.
   When making the request it MUST transmit HTTP "Accept" headers
   indicating what CFILE formats it is capable of accepting. For
   example, Netscape Navigator browsers with versions 2.0 and beyond
   might include the following line in the HTTP Request:

   Accept: application/x-ns-proxy-autoconfig

   The client MUST follow HTTP redirect directives (response codes 3xx)
   returned by the server. The client SHOULD send a valid "User-Agent"

4.7. Resuming Discovery

   If the HTTP request fails for any reason (fails to connect, server
   error response, etc) the client MUST resume the search for a
   successful CURL where it left off. It should continue attempting
   other sub-steps in a specific discovery mechanism, and then move on
   to the next mechanism or TGTDOM iteration, etc.

5.   Client Implementation Considerations

   The large number of discovery mechanisms specified in this document
   may raise concerns about network traffic and performance. The DHCP
   portion of the process will result in a single broadcast by the
   client, and perhaps a few replies by listening DHCP servers.

   The remaining mechanisms are all DNS based. All DNS queries should
   have the QNAME terminated with a trailing '.' to indicate a FQDN and
   expedite the lookup. As such each TGTDOM iteration will cause 3 DNS
   lookups, each a unicast UDP packet and a reply. Most clients will
   have fewer than 2TGTDOM iterations, limiting the total number of DNS
   request/replies to6.

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                        [Page 12]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

   All total, 7 UDP request/reply packets on client startup is quite a
   low overhead.  The first web page downloaded by the client will
   likely dwarf that packet count. Each of the DNS lookups should stand
   a high chance of hitting the cache in the client's DNS server, since
   other clients will have likely looked them up recently, providing a
   low total elapsed time.

   This is of course the worst case, where no CURLS are obtained, and
   assuming a long client FQDN. Often, a successful CURL will be found
   early in the protocol, reducing the total packet count.
   Client implementations are encouraged to overlap this protocol work
   with other startup activities. Also, client implementers with
   concerns about performance can choose to implement only the
   discovery mechanisms listed as MUST in section 3.4.

   A longer delay could occur if a CURL is obtained, but the hosting
   web server is down. The client could spend considerable time waiting
   for the TCP “connect ()” call to fail. Luckily this is an extremely
   rare case where the web server hosting the CFILE has failed. See
   section 5, where proxy server implementers are encouraged to provide
   support for hosting CURLs on the proxy itself (acting as web
   server).  Since proxy servers are often deployed with considerable
   attention to fault tolerance, this corner case can be further

6.   Proxy Server Considerations

   As mentioned in the previous section, it is suggested that proxy
   servers be capable of acting as a web server, so that they can host
   the CURL directly.

   The implementers of proxy servers are most likely to understand the
   deployment situations of proxy caches, the formats of proxy
   configuration files, etc. They can also build in the ability select
   a CFILE based on all the various inputs at the time of the CURL
   request("User-Agent", "Accept", client IP address/subnet/hostname,
   topological distribution of nearby proxy servers, etc).

7.   Administrator Considerations

   Administrators should configure at least one of the DHCP or DNS A RR
   methods in their environment (since those are the only two all
   compatible clients MUST implement). Beyond that, configuring to
   support mechanisms earlier in the search order will improve client
   startup time.

   One of the major motivations for this protocol structure was to
   support client location of "nearby" proxy servers. In many
   environments there may be a number of proxy servers (workgroup,
   corporate gateway, ISP, backbone). There are a number of possible
   points at which "nearness" decisions can be made in this framework:

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                        [Page 13]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

   - DHCP servers for different subnets can return different answers.
        They can also base decisions on the client cipaddr field or the
        client identifier option.

   - DNS servers can be configured to return different SRV/A/TXT RRs
        for Different domain suffixes (for example, QNAMEs and

   - The web server handling the CURL request can make decisions based
        on the "User-Agent", "Accept", client IP
        address/subnet/hostname, and the topological distribution of
        nearby proxy servers, etc.  This can occur inside a CGI
        executable created to handle the CURL.  As mentioned above it
        could be a proxy server itself handing the CURL request and
        making those decisions.

   - The CFILE may be expressive enough to select from a set of
        alternatives at "runtime" on the client. CARP [6] is based on
        this premise for an array of caches. It is not inconceivable
        that the CFILE could compute some network distance or fitness
        metrics to a set of candidate proxy servers and then select the
        "closest" or "most responsive" server.

   Note that it is valid to configure a DHCP daemon to respond only to
   INFORM option queries in static IP environments

   Not all of the above mechanisms can be supported in all currently
   deployed vendor hardware and software. The hope is that enough
   flexibility is provided in this framework that administrators can
   select which mechanisms will work in their environments.

8.   Conditional Compliance

   In light of the fact that many of the discovery technologies
   described in this document are not well deployed or not available on
   all platforms, this specification permits conditional compliance.
   Conditional compliance is designated by three class identifications.

   Additionally, due to the possible security implications of a DHCP
   broadcast request, it is onerous to REQUIRE an implementer to put
   himself or his implementation at undue risk.  It is quite common to
   have rogue DHCP servers on a network which may fool a DHCP broadcast
   implementation into using a malicious configuration file.  On
   platforms which do not support DHCP natively and cannot get the WPAD
   option along with its IP address, and which cannot support the DHCP
   INFORM unicast request, presumably to a known and trusted DHCP
   server, the likelihood of an undetected spoofing attack is
   increased.  Having an individual program, such as a browser, trying
   to detect a DHCP server on a network is unreasonable, in the
   authors' opinion.  On platforms which use DHCP for their system IP
   address and have previously trusted a DHCP server, a unicast DHCP
   INFORM to that same trusted server does not introduce any additional
   trust to that server.

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                        [Page 14]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

8.1. Class 0 - Minimally compliant

   A WPAD implementation which implements only the following discovery
   mechanisms and interval schemes is considered class 0 compliant:

   DNS A record queries
   Browser or System session refresh intervals

   Class 0 compliance is only applicable to systems or implementations
   which do not natively support DHCP and or cannot securely determine
   a trusted local DHCP server.

8.2. Class 1 - Compliant

   A WPAD implementation which implements only the following discovery
   mechanisms and interval schemes is considered class 1 compliant:

   DNS A record queries
   DHCP INFORM Queries

   Network stack change refresh intervals
   CFILE expiration refresh intervals

8.3. Class 2 - Maximally compliant

   A WPAD implementation which implements only the following discovery
   mechanisms and interval schemes is considered class 1 compliant:

   DNS A record queries
   DHCP INFORM Queries
   DNS TXT service: queries
   DNS SRV RR queries
   SVRLOC Queries
   Network stack change refresh intervals
   CFILE expiration refresh intervals

   To be considered compliant with a given class, an implementation
   MUST support the features listed above corresponding to that class.

9.   Security Considerations

   This document does not address security of the protocols involved.
   The WPAD protocol is vulnerable to existing identified weaknesses in
   DHCP and DNS. The groups driving those standards, as well as the SLP
   protocol standards, are addressing security.

   When using DHCP discovery, clients are encouraged to use unicast
   DHCP INFORM queries instead of broadcast queries which are more
   easily spoofed in insecure networks.

   Minimally, it can be said that the WPAD protocol does not create new
   security weaknesses.

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                        [Page 15]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

10.  Acknowledgements

   The authors' work on this specification would be incomplete without
   the assistance of many people.  Specifically, the authors would like
   the express their gratitude to the following people:

   Chuck Neerdaels, Inktomi, for providing assistance in the design of
   the WPAD protocol as well as for providing reference

   Arthur Bierer, Darren Mitchell, Sean Edmison, Mario Rodriguez, Danpo
   Zhang, and Yaron Goland, Microsoft, for providing implementation
   insights as well as testing and deployment.

   Ari Luotonen, Netscape, for his role in designing the first web

   In addition, the authors are grateful for the feedback provided by
   the following people:

     Jeremy Worley - RealNetworks
     Eric Twitchell - United Parcel Service

11.  Copyright

   Copyright (C) The Internet Society 1998. All Rights Reserved. This
   document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English. The limited permissions granted above are perpetual and
   will not be revoked by the Internet Society or its successors or
   assigns. This document and the information contained herein is
   provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE

12.  References

   [1] Moats, R., Hamilton, M., and P. Leach, "Finding Stuff (How to
        discover services)", Internet Draft, October 1997.

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                        [Page 16]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99

   [2] Gulbrandsen, A., and P. Vixie, "A DNS RR for specifying the
        location    of services (DNS SRV)", RFC 2052, October 1996

   [3] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
        March    1997.

   [4] Veizades, J., Guttman, E., Perkins, C., and M. Day, "Service
        Location Protocol," Internet Draft, October 1997.

   [5] Hamilton, M., and R. Wright, "Use of DNS Aliases for Network
        Services", RFC 2219, October 1997.

   [6] Valloppillil, V., and K. Ross, "Cache Array Routing Protocol",
        Internet Draft, October 1997.

   [7] Alexander, S., and R. Droms, "DHCP Options and BOOTP Vendor
        Extensions", RFC 2132, March 1997.

   [8] Luotonen, A., "Navigator Proxy Auto-Config File Format",
        Netscape    Corporation,
        demo/proxy-live.html, March 1996.

   [9] Mockapetris, P., "Domain Names - Concepts and Facilities",
        RFC 1034, November 1987.

   [10] Perkins, C., Guttman, E., and J. Kempf, "Service Templates and
        service: Schemes", Internet Draft, December 1997.

   [11] “A Sample DHCP Implementation for WPAD”, Inktomi Corporation,, February 1998.

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
        Requirement Levels", BCP 14, RFC 2119, March 1997.

13.  Author Information

   Paul Gauthier
   Inktomi Corporation
   1900 South Norfolk Street Suite 310, San Mateo, CA 94403-1151
   Phone: (650) 653-2800

   Josh Cohen
   Microsoft Corporation
   One Microsoft Way, Redmond, WA 98052
   Phone: (425) 703-5812

   Martin Dunsmuir
   RealNetworks, Inc.
   1111 3rd Ave, Suite 2900, Seattle, WA 98101
   Phone: (206) 674-2237

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                        [Page 17]

   INTERNET-DRAFT Web Proxy Auto-Discovery Protocol           7/28/99


   Charles Perkins
   Sun Microsystems, Inc.
   15 Network Circle, Menlo Park, CA  94025
   Phone: (650) 786-6464
   Email: charles.perkins@Sun.COM

   Category: Standards Track                    Expires: December 1999
   Gauthier, Cohen, Dunsmuir, Perkins                        [Page 18]