Network Working Group M. Nottingham
Internet-Draft October 15, 2013
Intended status: Informational
Expires: April 18, 2014
Problems with Proxies in HTTP
draft-nottingham-http-proxy-problem-00
Abstract
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
exploration.
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 April 18, 2014.
Copyright Notice
Copyright (c) 2013 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
Provisions Relating to IETF Documents
(http://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 and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Nottingham Expires April 18, 2014 [Page 1]
Internet-Draft HTTP Proxy Problems October 2013
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
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) . . 5
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 . . . . . . . . . . . . . . . . . . . . . 8
5. Principles for Consideration . . . . . . . . . . . . . . . . . 9
5.1. Proxies Have a Legitimate Place . . . . . . . . . . . . . 10
5.2. Security Should be Encouraged . . . . . . . . . . . . . . 10
5.3. Interoperability is Important Too . . . . . . . . . . . . 10
5.4. Users Need to be Informed of Proxies . . . . . . . . . . . 10
5.5. Users Need to be able to Tunnel through Proxies . . . . . 10
5.6. Proxies Can say "No" . . . . . . . . . . . . . . . . . . . 11
5.7. Changes Need to be Detectable . . . . . . . . . . . . . . 11
5.8. Proxies Need to be Easy . . . . . . . . . . . . . . . . . 11
5.9. Proxies Need to Communicate to Users . . . . . . . . . . . 11
5.10. Users Require Simple Interfaces . . . . . . . . . . . . . 12
5.11. User Agents Are Diverse . . . . . . . . . . . . . . . . . 12
5.12. Choices are Context-Specific . . . . . . . . . . . . . . . 12
5.13. RFC2119 Doesn't Define Reality . . . . . . . . . . . . . . 13
5.14. It Needs to be Deployable . . . . . . . . . . . . . . . . 13
6. Areas to Investigate . . . . . . . . . . . . . . . . . . . . . 13
6.1. Living with Interception . . . . . . . . . . . . . . . . . 13
6.2. Improving Proxy.Pac and WPAD . . . . . . . . . . . . . . . 13
6.3. TLS Errors for Proxies . . . . . . . . . . . . . . . . . . 13
6.4. HTTP Errors for Proxies . . . . . . . . . . . . . . . . . 14
6.5. TLS for Proxy Connections . . . . . . . . . . . . . . . . 14
6.6. TLS for HTTP URIs . . . . . . . . . . . . . . . . . . . . 14
6.7. Improving Trust . . . . . . . . . . . . . . . . . . . . . 14
6.8. HTTP Signatures . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
Nottingham Expires April 18, 2014 [Page 2]
Internet-Draft HTTP Proxy Problems October 2013
1. Introduction
HTTP/1.1 [RFC2616] 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
forth.
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 may 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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
Nottingham Expires April 18, 2014 [Page 3]
Internet-Draft HTTP Proxy Problems October 2013
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.
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
proxies.
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
[I-D.ietf-httpbis-p6-cache]); 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).
Nottingham Expires April 18, 2014 [Page 4]
Internet-Draft HTTP Proxy Problems October 2013
2.4. Content Filtering (a.k.a. Content Policy Enforcement)
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 [RFC2616] explicitly allows "non-
transparent" proxies that modify content in certain ways. Such
Nottingham Expires April 18, 2014 [Page 5]
Internet-Draft HTTP Proxy Problems October 2013
proxies are required to honor the "no-transform" directive, giving
both user agents and origin servers a mechanism to "opt out" of
modifications; however, it is not technically enforced.
[W3C.CR-ct-guidelines-20100617] 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
network.
Nottingham Expires April 18, 2014 [Page 6]
Internet-Draft HTTP Proxy Problems October 2013
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
proxy.pac.
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.
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
proxies.
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
unexpected status code breaks many clients (e.g., non-interactive
applications like software installers).
In addition, as adoption of multi-path TCP (MPTCP)[RFC6824]
increases, the ability of intercepting proxies to offer a consistent
service degrades.
Nottingham Expires April 18, 2014 [Page 7]
Internet-Draft HTTP Proxy Problems October 2013
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
use.
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/2.0) 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.
Anothr 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.
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
[I-D.mbelshe-httpbis-spdy].
However, doing so engenders a few problems.
Nottingham Expires April 18, 2014 [Page 8]
Internet-Draft HTTP Proxy Problems October 2013
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 inform
users of what's going on.
Finally, it removes the opportunity for services provided by a proxy
that the end user may 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 must then 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
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
discussion.
Nottingham Expires April 18, 2014 [Page 9]
Internet-Draft HTTP Proxy Problems October 2013
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.
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.
5.3. Interoperability is Important Too
Security at the expense of long-term interoperability is not a good
trade.
For example, if networks decide to only allow secure connections to
well-known, large origin servers, it creates a "walled garden" that
favours big sites at the expense of less well-known ones.
Likewise, if a jurisdiction cannot use standard-conformant browsers
to impose their legal requirements upon network users, they might
decide to create a separate Web based upon competing technology.
5.4. 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.
5.5. 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.
Nottingham Expires April 18, 2014 [Page 10]
Internet-Draft HTTP Proxy Problems October 2013
This includes both "https://" and "http://" URIs.
5.6. Proxies Can say "No"
A proxy can refuse to forward any request. This includes a request
to a specific URI, or from a specific user, and includes refusing to
allow tunnels as described above.
The "no", however, needs to be explicit, and explicitly from the
proxy.
5.7. Changes Need to be Detectable
Any changes to the message body, request URI, method, status code, or
representation header fields of an HTTP message needs to be
detectable by the origin server or user agent, as appropriate, if
they desire it.
This allows a proxy to be trusted, but its integrity to be verified.
5.8. 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
clearly.
5.9. 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 define as being from a proxy rather than
the origin, they do not cover all necessary situations.
The Warning header field 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.
Equally as important, the communication needs to clearly come from
the proxy, rather than the origin, and be strongly authenticated.
Nottingham Expires April 18, 2014 [Page 11]
Internet-Draft HTTP Proxy Problems October 2013
5.10. 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
Web.
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.
5.11. 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.
5.12. Choices are Context-Specific
Getting consent from users, as well as informing them, can take a
variety of forms. For example, if we require that users consent to
using a proxy, that consent could be obtained through a modal dialog
in the browser, or through a written agreement between an employer
and their employee.
Likewise, a browser vendor may choose not to implement some optional
portions of the specification, based upon how they want to position
their product with their audience.
Nottingham Expires April 18, 2014 [Page 12]
Internet-Draft HTTP Proxy Problems October 2013
5.13. 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.
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).
5.14. It Needs to be Deployable
Any improvements to the proxy ecosystem MUST be incrementally
deployable, so that existing clients can continue to function.
6. Areas to Investigate
Finally, this section lists some areas of potential future
investigation, bearing the principles suggested above in mind.
6.1. Living with Interception
The IETF has long fought against interception proxies, as they are
indistinguishable from Man-In-The-Middle attacks. Nevertheless, they
persist as the preferred method for interposing proxies in many
networks.
Unless another mechanism can be found or defined that offers equally
attractive properties to network operators, we ought to consider that
they'll continue to be deployed, and work to find ways to make their
operation both more verifiable and unnecessary (or at least
legitimate).
6.2. Improving Proxy.Pac and WPAD
Many of the flaws in proxy.pac and WPAD can be fixed by careful
specification and standardization, with active participation by both
implementers and those that deploy them.
6.3. 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,
Nottingham Expires April 18, 2014 [Page 13]
Internet-Draft HTTP Proxy Problems October 2013
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.
6.4. HTTP Errors for Proxies
HTTP currently defines two status codes that are explicitly generated
by a proxy:
o 504 Gateway Timeout [RFC2616] - when a proxy (or gateway) times
out going forward
o 511 Network Authentication Required [RFC6585] - when
authentication information is necessary to access the network
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.
6.5. 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.
6.6. TLS for HTTP URIs
To allow users to tunnel any request through proxies without
revealing its contents, it must be possible to use TLS for HTTP URIs.
Proxies can then choose whether to allow such tunneled traffic, and
if not, the user can choose whether to trust the proxy.
6.7. 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.
Nottingham Expires April 18, 2014 [Page 14]
Internet-Draft HTTP Proxy Problems October 2013
6.8. HTTP Signatures
Signatures for HTTP content - both requests and responses - has 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).
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
signature?
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 and Martin Nilsson.
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
[I-D.ietf-httpbis-p6-cache]
Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
Transfer Protocol (HTTP/1.1): Caching",
draft-ietf-httpbis-p6-cache-24 (work in progress),
Nottingham Expires April 18, 2014 [Page 15]
Internet-Draft HTTP Proxy Problems October 2013
September 2013.
[I-D.mbelshe-httpbis-spdy]
Belshe, M. and R. Peon, "SPDY Protocol",
draft-mbelshe-httpbis-spdy-00 (work in progress),
February 2012.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[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.
[W3C.CR-ct-guidelines-20100617]
Rabin, J., "Guidelines for Web Content Transformation
Proxies 1.0", World Wide Web Consortium CR CR-ct-
guidelines-20100617, June 2010,
<http://www.w3.org/TR/2010/CR-ct-guidelines-20100617>.
[https-everywhere]
EFF, ., "HTTPS Everywhere", 2013,
<https://www.eff.org/https-everywhere>.
[proxypac]
various, ., "Proxy Auto-Config", 2013,
<http://en.wikipedia.org/wiki/Proxy_auto-config>.
[tls-mitm]
Jarmoc, J., "SSL/TLS Interception Proxies and Transitive
Trust", 2012, <https://www.grc.com/miscfiles/
HTTPS_Interception_Proxies.pdf>.
Nottingham Expires April 18, 2014 [Page 16]
Internet-Draft HTTP Proxy Problems October 2013
[wpad] Cohen, J., "Web Proxy Auto-Discovery Protocol", 1999,
<http://tools.ietf.org/html/draft-ietf-wrec-wpad-01>.
Author's Address
Mark Nottingham
Email: mnot@mnot.net
URI: http://www.mnot.net/
Nottingham Expires April 18, 2014 [Page 17]