Web Proxy Auto Discovery Next Generation
draft-joshco-wpadng-01
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draft-joshco-wpadng-01
Network Working Group J. Cohen
Internet-Draft Self
Intended status: Informational 8 July 2024
Expires: 9 January 2025
Web Proxy Auto Discovery Next Generation
draft-joshco-wpadng-01
Abstract
This specification aims to modernize Web Proxy Automatic Discovery
([WPAD]) which was defined in 1997. At that time, the World Wide Web
was much earlier in its evolution. This specification provides more
modern discovery mechanisms and incorporates [PVD]
Discussion Venues
This note is to be removed before publishing as an RFC.
Source for this draft and an issue tracker can be found at
https://github.com/joshco/wpadng.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 9 January 2025.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. CURLs and URNs . . . . . . . . . . . . . . . . . . . . . . . 3
3. The Discovery Process . . . . . . . . . . . . . . . . . . . . 3
3.1. WPADNG Overview . . . . . . . . . . . . . . . . . . . . . 3
3.2. When to Execute WPADNG . . . . . . . . . . . . . . . . . 5
3.2.1. Periodic Discovery . . . . . . . . . . . . . . . . . 5
3.2.2. Upon Startup of the Web Client . . . . . . . . . . . 5
3.2.3. Network Stack Events . . . . . . . . . . . . . . . . 5
3.2.4. Expiration of the CFILE . . . . . . . . . . . . . . . 6
3.3. Discovery Mechanisms . . . . . . . . . . . . . . . . . . 6
3.3.1. DNS Serice Discovery (DNS-SD) . . . . . . . . . . . . 6
3.3.2. DHCP v4 . . . . . . . . . . . . . . . . . . . . . . . 8
3.3.3. DHCP v6 (TBD) . . . . . . . . . . . . . . . . . . . . 8
3.3.4. Timeouts . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Retrieving the CFILE at the CURL . . . . . . . . . . . . 9
3.4.1. Content Negotiation for PVD . . . . . . . . . . . . . 9
3.5. Resuming Discovery . . . . . . . . . . . . . . . . . . . 9
4. Proxy Server Considerations . . . . . . . . . . . . . . . . . 9
5. Conventions and Definitions . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7.1. DHCPv6 Option Code . . . . . . . . . . . . . . . . . . . 10
7.2. DHCPv4 Option Code . . . . . . . . . . . . . . . . . . . 10
8. Normative References . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 11
A.1. Acknowledgements for current versions . . . . . . . . . . 11
A.2. Acknowledgements from previous versions . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Web Proxy Automatic Discovery Next Generation (WPADNG) defines a
mechanism for Web Browsers, applications, or operating system
services to discover nearby Web Proxy Severs.
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The Web Proxy Auto-Discovery (WPADNG) problem reduces to providing
the web client a mechanism for discovering the URL of the
Configuration File (CFILE). Once this Configuration File URL (CURL)
is known, the client software already contains mechanisms for
retrieving and interpreting the CFILE to enable access to the
specified proxy cache servers.
The goal of the wpadng 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.)
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 set of mechanisms for discovering the
Configuration URL. The client will attempt them in a predefined
order, until one succeeds.
2. CURLs and URNs
When a CURL is returned by discovery, it SHOULD be either a URL which
is retrievable or a URN.
This specification defines the following URN for use with WPADNG.
urn:wpadng:none
When this URN is returned as the CURL, the meaning is that the
current network has no proxy server, and the client SHOULD stop any
further discovery. This can be helpful in avoiding unnecessary
network round trips and preventing discovery of rogue proxy servers.
3. The Discovery Process
3.1. WPADNG Overview
WPADNG uses a collection of pre-existing Internet resource discovery
mechanisms to perform web proxy auto-discovery. The WPADNG protocol
specifies the following:
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* how to use each mechanism for the specific purpose of web proxy
auto-discovery
* the order in which the mechanisms should be performed
* the minimal set of mechanisms which must be attempted by a WPADNG
compliant web client
The resource discovery mechanisms utilized by WPADNG, in order, are
as follows.
* Dynamic Host Configuration Protocol v4 or v6 depending on network.
* DNS-SD
Clients only attempt mechanisms that they support. Each time the
discovery attempt succeeds; the client uses the information obtained
to construct a CURL. If the CURL is the URN urn:wpadng:none or if a
CFILE is successfully retrieved, the process is complete.
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 WPADNG protocol
fails and the client is configured to use no proxy server.
First the client tries DHCP, followed by DNSSD. If no CFILE has been
retrieved the client will retry the DNSSD mechanism multiple times.
Each time through the QNAME being used in the DNSSD 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 DNSSD lookup has the QNAME
prefixed with “_wpadng._tcp” to indicate the resource type being
requested.
As an example, consider a client with hostname johns-
desktop.development.example.com. 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:
* DNSSD PTR lookup on QNAME=_wpadng._tcp.development.example.com.
* DNSSD PTR lookup on QNAME=_wpadng._tcp.example.com.
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3.2. When to Execute WPADNG
Web clients need to perform the WPADNG 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, WPADNG 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
HTTP/1.1.
3.2.2. 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 WPADNG every time a new
instance of the web client is opened. WPADNG 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.
3.2.3. Network Stack Events
Another option for clients is to tie the execution of WPADNG 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 WPADNG 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 WPADNG in many environments.
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3.2.4. 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 WPADNG 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 File).
Whenever the client decides to invalidate the current CURL or CFILE,
it MUST rerun the entire WPADNG 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 WPADNG protocol MUST be rerun. The client
MUST NOT simply re-use the existing CURL to obtain a fresh copy of
the CFILE.
A number of network round trips, broadcast and/or multicast
communications may be required during the WPADNG protocol. The
WPADNG protocol SHOULD NOT be invoked at a more frequent rate than
specified above (such as per-URL retrieval).
3.3. 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 Mechanism | Status |
+=====================+========+
| DHCP | MUST |
+---------------------+--------+
| DNS-SD | MUST |
+---------------------+--------+
Table 1
3.3.1. DNS Serice Discovery (DNS-SD)
Client implementations MUST support [DNSSD] discovery of proxy
configuration files. To suport this, a DNS server advertising WPADNG
will have the following resource records:
SHOULD advertise PTR records which may return multiple advertised
service instances or a single instance.
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MUST advertise TXT records conformant with [DNSSD]
SHOULD advertise SRV records conformant with [DNSSD]
3.3.1.1. PTR Record Definition
For example purposes, we assume a client has attached to the
enterprise network at Example Coporation, which uses the dommain
example.org
WPADNG PTR records should have the following naming scheme:
_wpadng._tcp.example.org
When a client queries for the PTR record, the DNS server replies will
contain zero, one or more responses. These responses contain WPADNG
instance names, and priorities.
+====================================+
| Instance Name |
+====================================+
| primary._wpadng._tcp.example.org |
+------------------------------------+
| secondary._wpadng._tcp.example.org |
+------------------------------------+
| _wpadng._tcp.example.org |
+------------------------------------+
Table 2
In the above example. the enterprise advertises instance names for
proxy servers This allows an enterprise to provide redundancy and
failover.
3.3.1.2. TXT Record Definition
WPADNG DNS TXT records MUST have the following format
txtvers=1
url=CURL
The CURL value is a URN or the URL to retrieve a Proxy Configuration
File.
3.3.1.3. DNSSD Client behavior
When attempting to discover web proxy servers via [DNSSD], the
following sequence should be used:
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1. Query PTR records
2. Query TXT records
3. Compose Candidate URL
4. Retreive configuration file
3.3.2. DHCP v4
Client implementations MUST support DHCP. DHCP has widespread
support in numerous 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 [DHCP]. We propose
a new DHCP option with code 252 for use in web proxy auto-discovery.
See [DHCPOPT] for a list of existing DHCP options.
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.
Example values:
http://server.domain/proxyconfig.pac
urn:wpadng:none
3.3.3. DHCP v6 (TBD)
3.3.4. Timeouts
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
latency.
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3.4. Retrieving the CFILE at the CURL
The client then requests the CURL via HTTP.
3.4.1. Content Negotiation for PVD
When making the request it MUST transmit HTTP "Accept" headers
indicating what CFILE formats it is capable of accepting.
For PVD, the Accept header value is application/pvd_json
For existing WPAD clients, the most commonly used format is
Netscape's Proxy Auto Config (PAC) file, the value is 'application/x-
ns-proxy-autoconfig'
Clients that support PVD can use content negotiation to prefer PVD,
while accepting PAC as a fallback.
:method = GET
:authority = HOST
:path = PATH
accept = application/pvd+json
accept = application/x-ns-proxy-autoconfig
Clients that do not support PVD will continue to use PAC, and not
include PVD in the accept header.
When receiving a GET at the CURL, a proxy server can decide wether to
return PVD or PAC, providing backward compatibility as well as an
upgrade path.
The client MUST follow HTTP redirect directives (response codes 3xx)
returned by the server. The client SHOULD send a valid "User-Agent"
header.
3.5. 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.
4. 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.
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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).
5. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
6. 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.
7. IANA Considerations
7.1. DHCPv6 Option Code
This document requests a new DHCPv6 Option.
7.2. DHCPv4 Option Code
This document seeks to register DHCPv4 option code 252.
8. Normative References
[DHCP] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/rfc/rfc2131>.
[DHCPOPT] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
<https://www.rfc-editor.org/rfc/rfc2132>.
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[DNSSD] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/rfc/rfc6763>.
[PVD] Perkins, C. E., Cohen, J., Dunsmuir, M., Gauthier, P. A.,
Cooper, I., and J. W. C. M.A., "Web Proxy Auto-Discovery
Protocol", Work in Progress, Internet-Draft, draft-ietf-
wrec-wpad-01, 29 July 1999,
<https://datatracker.ietf.org/doc/html/draft-ietf-wrec-
wpad-01>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[WPAD] Perkins, C. E., Cohen, J., Dunsmuir, M., Gauthier, P. A.,
Cooper, I., and J. W. C. M.A., "Web Proxy Auto-Discovery
Protocol", Work in Progress, Internet-Draft, draft-ietf-
wrec-wpad-01, 29 July 1999,
<https://datatracker.ietf.org/doc/html/draft-ietf-wrec-
wpad-01>.
Appendix A. Acknowledgments
A.1. Acknowledgements for current versions
The editor(s) of this specification would like acknowledge the
contributions of the previous authors.
Paul Gauthier Inktomi Corporation
Martin Dunsmuir RealNetworks, Inc.
Charles Perkins Sun Microsystems, Inc.
A.2. Acknowledgements from previous versions
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:
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Chuck Neerdaels, Inktomi, for providing assistance in the design of
the WPADNG protocol as well as for providing reference
implementations.
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
proxy.
In addition, the authors are grateful for the feedback provided by
the following people:
* Jeremy Worley - RealNetworks
* Eric Twitchell - United Parcel Service
Author's Address
Josh Cohen
Self
Email: joshco@gmail.com
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