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Versions: 00 01                                                         
Network Working Group                                      M. Nottingham
Intended status: Informational                             July 04, 2014
Expires: January 05, 2015

                     Problems with Proxies in HTTP


   This document discusses the use and configuration of proxies in HTTP,
   pointing out problems in the currently deployed Web infrastructure
   along the way.  It then offers a few principles to base further
   discussion upon, and lists some potential avenues for further

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on January 05, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Why Proxy?  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Application Layer Gatewaying  . . . . . . . . . . . . . .   4
     2.2.  Caching . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Network Policy Enforcement  . . . . . . . . . . . . . . .   4
     2.4.  Content Filtering (a.k.a. Content Policy Enforcement) . .   4
     2.5.  Content Modification  . . . . . . . . . . . . . . . . . .   5
   3.  How Proxies are Interposed  . . . . . . . . . . . . . . . . .   6
     3.1.  Manual Configuration  . . . . . . . . . . . . . . . . . .   6
     3.2.  proxy.pac and WPAD  . . . . . . . . . . . . . . . . . . .   6
     3.3.  Interception  . . . . . . . . . . . . . . . . . . . . . .   7
     3.4.  Configuration As Side Effect  . . . . . . . . . . . . . .   8
   4.  Second-Order Effects of Proxy Deployment  . . . . . . . . . .   8
     4.1.  Proxies and HTTP  . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Proxies and TLS . . . . . . . . . . . . . . . . . . . . .   9
   5.  Principles for Consideration  . . . . . . . . . . . . . . . .   9
     5.1.  Proxies Have a Legitimate Place . . . . . . . . . . . . .  10
     5.2.  Security Should be Encouraged . . . . . . . . . . . . . .  10
     5.3.  Users Need to be Informed of Proxies  . . . . . . . . . .  10
     5.4.  Users Need to be able to Tunnel through Proxies . . . . .  11
     5.5.  Proxies Can say "No"  . . . . . . . . . . . . . . . . . .  11
     5.6.  Changes Need to be Detectable . . . . . . . . . . . . . .  11
     5.7.  Proxies Need to be Easy . . . . . . . . . . . . . . . . .  11
     5.8.  Proxies Need to Communicate to Users  . . . . . . . . . .  11
     5.9.  Users Require Simple Interfaces . . . . . . . . . . . . .  12
     5.10. User Agents Are Diverse . . . . . . . . . . . . . . . . .  12
     5.11. RFC2119 Doesn't Define Reality  . . . . . . . . . . . . .  12
     5.12. It Needs to be Deployable . . . . . . . . . . . . . . . .  13
   6.  Potential Areas to Investigate  . . . . . . . . . . . . . . .  13
     6.1.  Improving Proxy.Pac . . . . . . . . . . . . . . . . . . .  13
     6.2.  TLS Errors for Proxies  . . . . . . . . . . . . . . . . .  13
     6.3.  HTTP Errors for Proxies . . . . . . . . . . . . . . . . .  13
     6.4.  TLS for Proxy Connections . . . . . . . . . . . . . . . .  14
     6.5.  Improved Network Information  . . . . . . . . . . . . . .  14
     6.6.  Improving Trust . . . . . . . . . . . . . . . . . . . . .  14
     6.7.  HTTP Signatures . . . . . . . . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

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   HTTP/1.1 [RFC7230] was designed to accommodate proxies.  It allows
   them (and other components) to cache content expansively, and allows
   for proxies to break "semantic transparency" by changing message
   content, within broad constraints.

   As the Web has matured, more networks have taken advantage of this by
   deploying proxies for a variety of reasons, in a number of different
   ways.  Section 2 is a survey of the different ways that proxies are
   used, and Section 3 shows how they are interposed into communication.

   Some uses of proxies cause problems (or the perception of them) for
   origin servers and end users.  While some uses are obviously
   undesirable from the perspective of an end users and/or origin
   server, other effects of their deployment are more subtle; these are
   examined in Section 4.

   These tensions between the interests of the stakeholders in every
   HTTP connection - the end users, the origin servers and the networks
   they use - has led to decreased trust for proxies, then increasing
   deployment of encryption, then workarounds for encryption, and so

   Left unchecked, this escalation can erode the value of the Web
   itself.  Therefore, Section 5 proposes straw-man principals to base
   further discussion upon.

   Finally, Section 6 proposes some areas of technical investigation
   that might yield solutions (or at least mitigations) for some of
   these problems.

   Note that this document is explicitly about "proxies" in the sense
   that HTTP defines them.  Intermediaries that are interposed by the
   server (e.g., gateways and so-called "Reverse Proxies", as used in
   Content Delivery Networks) are out of scope.

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Why Proxy?

   HTTP proxies are interposed between user agents and origin servers
   for a variety of purposes; some of them are with the full knowledge
   and consent of end users, to their benefit, and some are solely for
   the purposes of the network operator - sometimes even against the
   interests of the end users.

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   This section attempts to identify the different motivations networks
   have for deploying proxies.

2.1.  Application Layer Gatewaying

   Some networks do not have direct Internet connectivity for Web
   browsing.  These networks can deploy proxies that do have Internet
   connectivity and then configure clients to use them.

   Such gatewaying between networks were some of the first uses for

2.2.  Caching

   An extremely common use of proxies is to interpose a HTTP cache, in
   order to save bandwidth, improve end-user perceived latency, increase
   reliability, or some combination of these purposes.

   HTTP defines a detailed model for caching (see [RFC7234]); however,
   some lesser-known aspects of the caching model can cause operational
   issues.  For example, it allows caches to go into an "offline" mode
   where most content can be served stale.

   Also, proxy caches sometimes fail to honor the HTTP caching model,
   reusing content when it should not have been.  This can cause
   interoperability issues, with the end user seeing overly "stale"
   content, or applications not operating correctly.

2.3.  Network Policy Enforcement

   Some proxies are deployed to aid in network policy enforcement; for
   example, to control access to the network, requiring a login (as
   allowed explicitly by HTTP's proxy authentication mechanism),
   bandwidth shaping of HTTP access, quotas, etc.  This includes so-
   called "Captive Portals" used for network login.

   Some uses of proxies for policy enforcement cause problems; e.g.,
   when a proxy uses URL rewriting to send a user a message (e.g., a
   "blocked" page), they can make it appear as if the origin server is
   sending that message - especially when the user agent isn't a browser
   (e.g., a software update process).

2.4.  Content Filtering (a.k.a. Content Policy Enforcement)

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   Some networks attempt to filter HTTP messages (both request and
   response) based upon network-specific criteria.  For example, they
   might wish to stop users from downloading content that contains
   malware, or that violates site policies on appropriate content, or
   that violates local law.

   Intermediary proxies as a mechanism for enforcing content
   restrictions are often easy to circumvent.  For example, a device
   might become infected by using a different network, or a VPN.
   Nevertheless, they are commonly used for this purpose.

   Some content policy enforcement is also done locally to the user
   agent; for example, several Operating Systems have machine-local
   proxies built in that scan content.

   Content filtering is often seen as controversial, often depending on
   the context it is used within and how it is performed.

2.5.  Content Modification

   Some networks modify HTTP messages (both request and response) as
   they pass through proxies.  This might include the message body,
   headers, request-target, method or status code.

   Motivation for content modification varies.  For example, some mobile
   networks interpose proxies that modify content in an attempt to save
   bandwidth, improve perceived performance, or transcode content to
   formats that limited-resource devices can more easily consume.

   Modifications also include adding metadata in headers for accounting
   purposes, or removing metadata such as Accept-Encoding to make virus
   scanning easier.

   In other cases, content modification is performed to make more
   substantial modifications.  This could include inserting
   advertisements, or changing the layout of content in an attempt to
   make it easier to use.

   Content modification is very controversial, often depending on the
   context it is used within and how it is performed.  Many feel that,
   without the explicit consent of either the end user or the origin
   server, a proxy that modifies content violates their relationship,
   thereby degrading trust in the Web overall.

   However, it should be noted that Section 5.7.2 of [RFC7230]
   explicitly allows "non-transparent" proxies that modify content in
   certain ways.  Such proxies are required to honor the "no-transform"
   directive, giving both user agents and origin servers a mechanism to

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   "opt out" of modifications ([RFC7234], Section; however, it
   is not technically enforced.

   [W3C.NOTE-ct-guidelines-20101026] is a product of the W3C Mobile Web
   Best Practices Working Group that attempts to set guidelines for
   content modification proxies.  Again, it is a policy document,
   without technical enforcement measures.

3.  How Proxies are Interposed

   How a proxy is interposed into a network flow often has great affect
   on perceptions of its operation by end users and origin servers.
   This section catalogues the ways that this happens, and potential
   problems with each.

3.1.  Manual Configuration

   The original way to interpose a proxy was to manually configure it
   into the user agent.  For example, most browsers still have the
   ability to have a proxy hostname and port configured for HTTP; many
   Operating Systems have system-wide proxy settings.

   Unfortunately, manual configuration suffers from several problems:

   o  Users often lack the expertise to manually configure proxies.

   o  When the user changes networks, they must manually change proxy
      settings, a laborious task.  This makes manual configuration
      impractical in a modern, mobile-driven world.

   o  Not all HTTP stacks support manual proxy configuration.
      Therefore, a proxy administrator cannot rely upon this method.

3.2.  proxy.pac and WPAD

   The limitations of manual configuration were recognized long ago.
   The solution that evolved was a format called "proxy.pac" [proxypac]
   that allowed the proxy configuration to be automated, once the user
   agent had loaded it.

   Proxy.pac is a JavaScript format; before each request is made, it is
   dispatched to a function in the file that returns a string that
   denotes whether a proxy is to be used, and if so, which one to use.

   Discovery of the appropriate proxy.pac file for a given network can
   be made using a DHCP extension, [wpad].  WPAD started as a simple
   protocol; it conveys a URL that locates the proxy.pac file for the

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   Unfortunately, the proxy.pac/WPAD combination has several operational
   issues that limit its deployment:

   o  The proxy.pac format does not define timeouts or failover behavior
      precisely, leading to wide divergence between implementations.
      This makes supporting multiple user agents reliably difficult for
      the network.

   o  WPAD is not widely implemented by user agents; some only implement

   o  In those user agents where it is implemented, WPAD is often not
      the default, meaning that users need to configure its use.

   o  Neither proxy.pac nor WPAD have been standardized, leading to
      implementation divergence and resulting interoperability problems.

   o  There are DNS-based variants of WPAD, adding to to confusion.

   o  DHCP options generally require tight integration with the
      operating system to pass the results to HTTP-based applications.
      While this level of integration is found between O/Ses and their
      provided applications, the interface may or may not be available
      to third parties.

   o  WPAD can be spoofed, allowing attackers to interpose a proxy and
      intercept traffic.  This is a blocking issue for implementation.

3.3.  Interception

   The problems with manual configuration and proxy.pac/WPAD have led to
   the wide deployment of a third style of interposition; interception

   Interception occurs when lower-layer protocols are configured to
   route HTTP traffic to a host other than the origin server for the URI
   in question.  It requires no client configuration (hence its
   popularity over other methods).  See [RFC3040] for an example of an
   interception-related protocol.

   Interception is also strongly motivated when it is necessary to
   assure that the proxy is always used, e.g., to enforce policy.

   Interception is problematic, however, because it is often done
   without the consent of either the end user or the origin server.
   This means that a response that appears to be coming from the origin
   server is actually coming from the intercepting proxy.  This makes it
   difficult to support features like proxy authentication, as the

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   unexpected status code breaks many clients (e.g., non-interactive
   applications like software installers).

   Furthermore, interception is a "layer violation"; i.e., misusing
   lower-layer protocols to enforce a higher-layer (often expressed as
   "layer 8") requirement.

   In addition, as adoption of multi-path TCP (MPTCP) [RFC6824]
   increases, the ability of intercepting proxies to offer a consistent
   service degrades.

3.4.  Configuration As Side Effect

   More recently, it's become more common for a proxy to be interposed
   as a side effect of another choice by the user.

   For example, the user might decide to add virus scanning - either as
   installed software, or a service that they configure from their
   provider - that is interposed as a proxy.  Indeed, almost all desktop
   virus scanners and content filters operate in this fashion.

   This approach has the merits of both being easy and obtaining
   explicit user consent.  However, in some cases, the end user might
   not understand the consequences of use of the proxy, especially upon
   security and interoperability.

4.  Second-Order Effects of Proxy Deployment

4.1.  Proxies and HTTP

   Deployment of proxies has an effect on the HTTP protocol itself.
   Because a proxy implements both a server and a client, any
   limitations or bugs in their implementation impact the protocol's

   For example, HTTP has a defined mechanism for upgrading the protocol
   of a connection, to aid in the deployment of new versions of HTTP
   (such as HTTP/2) or completely different protocol (e.g., [RFC6455]).

   However, operational experience has shown that a significant number
   of proxy implementations do not correctly implement it, leading to
   dangerous situations where two ends of a HTTP connection think
   different protocols are being spoken.

   Another example is the Expect/100-continue mechanism in HTTP/1.1,
   which is often incorrectly implemented.  Likewise, differences in
   support for trailers limits protocol extensions.

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4.2.  Proxies and TLS

   It has become more common for Web sites to use TLS [RFC5246] in an
   attempt to avoid many of the problems above.  Many have advocated use
   of TLS more broadly; for example, see the EFF's HTTPS Everywhere
   [https-everywhere] program, and SPDY's default use of TLS

   However, doing so engenders a few problems.

   Firstly, TLS as used on the Web is not a perfectly secure protocol,
   and using it to protect all traffic gives proxies a strong incentive
   to work around it, e.g., by deploying a certificate authority
   directly into browsers, or buying a sub-root certificate.

   Secondly, it removes the opportunity for the proxy to inform the user
   agent of relevant information; for example, conditions of access,
   access denials, login interfaces, and so on.  User Agents currently
   do not display any feedback from proxy, even in the CONNECT response
   (e.g., a 4xx or 5xx error), limiting their ability to have informed
   users of what's going on.

   Finally, it removes the opportunity for services provided by a proxy
   that the end user might wish to opt into.  For example, consider when
   a remote village shares a proxy server to cache content, thereby
   helping to overcome the limitations of their Internet connection.
   TLS-protected HTTP traffic cannot be cached by intermediaries,
   removing much of the benefit of the Web to what is arguably one of
   its most important target audiences.

   It is now becoming more common for a proxy to man-in-the-middle TLS
   connections (see [tls-mitm] for an overview), to gain access to the
   application message flows.  This represents a serious degradation in
   the trust infrastructure of the Web.

   Worse is the situation where proxies provide a certificate where they
   inure the user to a certificate warning that they then need to ignore
   in order to receive service.

5.  Principles for Consideration

   Every HTTP connection has at least three major stakeholders; the user
   (through their agent), the origin server (possibly using gateways
   such as a CDN) and the networks between them.

   Currently, the capabilities of these stakeholders are defined by how
   the Web is deployed.  Most notably, networks sometimes change
   content.  If they change it too much, origin servers will start using

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   encryption.  Changing the way that HTTP operates therefore has the
   potential to re-balance the capabilities of the various stakeholders.

   This section proposes several straw-man principles for consideration
   as the basis of those changes.  Their sole purpose here is to provoke

5.1.  Proxies Have a Legitimate Place

   As illustrated above, there are many legitimate uses for proxies, and
   they are a necessary part of the architecture of the Web. While all
   uses of proxies are not legitimate - especially when they're
   interposed without the knowledge or consent of the end user and the
   origin - undesirable intermediaries (i.e., those that break the
   reasonable expectations of other stakeholders) are a small portion of
   those deployed used.

   Note that while proxies have a legitimate place, it does not imply
   that they are an equal stakeholder to other parties in all ways;
   e.g., they do not have a natural right to access encrypted content,
   for example.

5.2.  Security Should be Encouraged

   Any solution needs to give all stakeholders - end users, networks and
   origin servers - a strong incentive towards security.

   This has subtle implications.  If networks are disempowered
   disproportionately, they might react by blocking secure connections,
   discouraging origin servers (who often have even stronger profit
   incentives) from deploying encryption, which would result in a net
   loss of security.

   On the other hand, if networks are given carte blanche, it can
   destroy trust in the Web altogether.  In particular, making it too
   easy to interpose a proxy (even if the user is "informed" by clicking
   through a dialogue) degrades the infrastructure in an unacceptable

5.3.  Users Need to be Informed of Proxies

   When a proxy is interposed, the user needs to be informed about it,
   so they have the opportunity to change their configuration (e.g.,
   attempt to introduce encryption), or not use the network at all.

   Proxies also need to be strongly authenticated; i.e., users need to
   be able to verify who the proxy is.

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5.4.  Users Need to be able to Tunnel through Proxies

   When a proxy is interposed, the user needs to be able to tunnel any
   request through it without its content (or that of the response)
   being exposed to the proxy.

   This includes both "https://" and "http://" URIs.

5.5.  Proxies Can say "No"

   A proxy can refuse to forward any request.  Currently, the
   granularity of that "no" is per-URI for unencrypted requests, and
   per-IP (perhaps per-SNI) for encrypted requests.

5.6.  Changes Need to be Detectable

   Any changes to the message body, request URI, method, status code, or
   representation header fields of an HTTP message need to be detectable
   by the origin server or user agent, as appropriate, if they desire

   This allows a proxy to be trusted, but its integrity to be verified.

5.7.  Proxies Need to be Easy

   It must be possible to configure a proxy extremely easily; the
   adoption of interception over proxy.pac/WPAD illustrates this very

5.8.  Proxies Need to Communicate to Users

   There are many situations where a proxy needs to communicate with the
   end user; for example, to gather network authentication credentials,
   communicate network policy, report that access to content has been
   denied, and so on.

   Currently, HTTP has poor facilities for doing so.  The proxy
   authentication mechanism is extremely limited, and while there are a
   few status codes that are defined as being from a proxy rather than
   the origin, they do not cover all necessary situations.

   The Warning header field ([RFC7234], Section 5.5) was designed as a
   very limited form of communication between proxies and end users, but
   it has not been widely adopted, nor exposed by User Agents.

   Importantly, proxies also need a limited communication channel when
   TLS is in use, for similar purposes.

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   Equally as important, the communication needs to clearly come from
   the proxy, rather than the origin, and be strongly authenticated.

5.9.  Users Require Simple Interfaces

   While some users are sophisticated in their understanding of Web
   security, they are in a vanishingly small minority.  The concepts and
   implications of many decisions regarding security are subtle, and
   require an understanding of how the Web works; describing these
   trade-offs in a modal dialogue box that gets in the way of the
   content the user wants has been proven not to work.

   Similarly, while some Web publishers are sophisticated regarding
   security, the vast majority are not (as can be proven by the
   prevalence of cross-site scripting attacks).

   Therefore, any changes cannot rely upon perfect understanding by
   these parties, or even any great effort upon their part.  This
   implies that user interface will be one of the biggest challenges
   faced, both in the browser and for any changes server-side.

   Notably, the most widely understood indicator of security today is
   the "lock icon" that shows when a connection is protected by TLS.
   Any erosion of the commonly-understood semantics of that indicator,
   as well as "https://" URIs, is likely to be extremely controversial,
   because it changes the already-understood security properties of the

   Another useful emerging convention is that of "Incognito" or
   "private" mode, where the end user has requested enhanced privacy and
   security.  This might be used to introduce higher requirements for
   the interposition of intermediaries, or even to prohibit their use
   without full encryption.

5.10.  User Agents Are Diverse

   HTTP is used in a wide variety of environments.  As such there can be
   no assumption that a user is sitting on the other end to interpret
   information or answer questions from proxies.

5.11.  RFC2119 Doesn't Define Reality

   It's very tempting for a committee to proclaim that proxies "MUST" do
   this and "SHOULD NOT" do that, but the reality is that the proxies,
   like any other actor in a networked system, will do what they can,
   not what they're told to do, if they have an incentive to do it.

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   Therefore, it's not enough to say that (for example), "proxies have
   to honor no-transform" as HTTP/1.1 does.  Instead, the protocol needs
   to be designed in a way so that either transformations aren't
   possible, or if they are, they can be detected (with appropriate
   handling by User Agents defined).

5.12.  It Needs to be Deployable

   Any improvements to the proxy ecosystem MUST be incrementally
   deployable, so that existing clients can continue to function.

6.  Potential Areas to Investigate

   Finally, this section lists some areas of potential future
   investigation, bearing the principles suggested above in mind.

6.1.  Improving Proxy.Pac

   Many of the flaws in proxy.pac can be fixed by careful specification
   and standardization, with active participation by both implementers
   and those that deploy it.

6.2.  TLS Errors for Proxies

   HTTP's use of TLS [RFC2818] currently offers no way for an
   interception proxy to communicate with the user agent on its own
   behalf.  This might be necessary for network authentication,
   notification of filtering by hostname, etc.

   The challenge in defining such a mechanism is avoiding the opening of
   new attack vectors; if unauthenticated content can be served as if it
   were from the origin server, or the user can be encouraged to "click
   through" a dialog, it has severe security implications.  As such, the
   user experience would need to be carefully considered.

6.3.  HTTP Errors for Proxies

   HTTP currently defines two status codes that are explicitly generated
   by a proxy:

   o  504 Gateway Timeout ([RFC7231], Section 6.6.5) - when a proxy (or
      gateway) times out going forward

   o  511 Network Authentication Required ([RFC6585], Section 6) - when
      authentication information is necessary to access the network

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   It might be interesting to discuss whether a separate user experience
   can be formed around proxy-specific status codes, along with the
   definition of new ones as necessary.

6.4.  TLS for Proxy Connections

   While TLS can be used end-to-end for "https://" URIs, support for
   connecting to a proxy itself using TLS (e.g., for "http://" URIs) is
   spotty.  Using a proxy without strong proof of its identity
   introduces security issues, and if a proxy can legitimately insert
   itself into communication, its identity needs to be verifiable.

6.5.  Improved Network Information

   Many of the use cases for proxies that modify content is transcoding
   or otherwise adapting that which is too "heavy" for the network it is
   transiting through.

   If network operators made better, more fine-grained and timely
   information about their operational characteristics freely available,
   endpoints (server and client) could adapt requests and responses to
   reflect it, thereby removing the need for intermediation.

6.6.  Improving Trust

   Currently, it is possible to exploit the mismatched incentives and
   other flaws in the CA system to cause a browser to trust a proxy as
   authoritative for a "https://" URI without full user knowledge.  This
   needs to be remedied; otherwise, proxies will continue to man-in-the-
   middle TLS.

6.7.  HTTP Signatures

   Signatures for HTTP content - both requests and responses - have been
   discussed on and off for some time.

   Of particular interest here, signed responses would allow a user-
   agent to verify that the origin's content has not been modified in
   transit, whilst still allowing it to be cached by intermediaries.

   Likewise, if header values can be signed, the caching policy (as
   expressed by Cache-Control, Date, Last-Modified, Age, etc.) can be
   signed, meaning it can be verified as being adhered to.

   Note that properly designed, a signature mechanism could work over
   TLS, separating the trust relationship between the UA and the origin
   server and that of the UA and its proxy (with appropriate consent).

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   There are significant challenges in designing a robust, widely-
   deployable HTTP signature mechanism.  One of the largest is an issue
   of user interface - what ought the UA do when encountering a bad

7.  Security Considerations

   Plenty of them, I suspect.

8.  Acknowledgements

   This document benefits from conversations and feedback from many
   people, including Amos Jeffries, Willy Tarreau, Patrick McManus,
   Roberto Peon, Guy Podjarny, Eliot Lear, Brad Hill, Martin Nilsson and
   Julian Reschke.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

9.2.  Informative References

              Belshe, M. and R. Peon, "SPDY Protocol", draft-mbelshe-
              httpbis-spdy-00 (work in progress), February 2012.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3040]  Cooper, I., Melve, I., and G. Tomlinson, "Internet Web
              Replication and Caching Taxonomy", RFC 3040, January 2001.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC6455]  Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC
              6455, December 2011.

   [RFC6585]  Nottingham, M. and R. Fielding, "Additional HTTP Status
              Codes", RFC 6585, April 2012.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, January 2013.

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   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230, June

   [RFC7231]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content", RFC 7231, June 2014.

   [RFC7234]  Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
              Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June

              Rabin, J., "Guidelines for Web Content Transformation
              Proxies 1.0", World Wide Web Consortium NOTE NOTE-ct-
              guidelines-20101026, October 2010,

              EFF, ., "HTTPS Everywhere", 2013, <https://www.eff.org/

              various, ., "Proxy Auto-Config", 2013,

              Jarmoc, J., "SSL/TLS Interception Proxies and Transitive
              Trust", 2012, <https://www.grc.com/miscfiles/

   [wpad]     Cohen, J., "Web Proxy Auto-Discovery Protocol", 1999,

Author's Address

   Mark Nottingham

   Email: mnot@mnot.net
   URI:   http://www.mnot.net/

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