Network Working Group A. Petersson
Internet-Draft M. Nilsson
Intended status: Standards Track Opera Software
Expires: October 22, 2012 April 20, 2012
Forwarded HTTP Extension
draft-ietf-appsawg-http-forwarded-02
Abstract
This document standardizes an HTTP extension header field that allows
proxy components to disclose information lost in the proxying
process, e.g., the originating IP address of a request or IP number
of the proxy on the user-agent facing interface. Given a trusted
path of proxying components, this makes it possible to arrange so
that each subsequent component will have access to e.g., all IP
addresses used in the chain of proxied HTTP requests.
This document also specifies guidelines for a proxy administrator to
anonymize the origin of a request.
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 October 22, 2012.
Copyright Notice
Copyright (c) 2012 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
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. Syntax Notations . . . . . . . . . . . . . . . . . . . . . . . 3
4. Forwarded . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Forwarded by . . . . . . . . . . . . . . . . . . . . . . . 5
5.2. Forwarded for . . . . . . . . . . . . . . . . . . . . . . 5
5.3. Forwarded host . . . . . . . . . . . . . . . . . . . . . . 5
5.4. Forwarded proto . . . . . . . . . . . . . . . . . . . . . 6
5.5. Extensions . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Node identifiers . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. IPv4 and IPv6 identifiers . . . . . . . . . . . . . . . . 7
6.2. The "unknown" identifier . . . . . . . . . . . . . . . . . 7
6.3. Obfuscated identifier . . . . . . . . . . . . . . . . . . 7
7. Implementation considerations . . . . . . . . . . . . . . . . 8
7.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Security considerations . . . . . . . . . . . . . . . . . . . 9
8.1. Header validity and integrity . . . . . . . . . . . . . . 9
8.2. Information leak . . . . . . . . . . . . . . . . . . . . . 9
9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative references . . . . . . . . . . . . . . . . . . . 10
10.2. Informative references . . . . . . . . . . . . . . . . . . 10
Appendix A. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 10
A.1. Since draft-petersson-forwarded-for-00 . . . . . . . . . . 11
A.2. Since draft-petersson-forwarded-for-01 . . . . . . . . . . 11
A.3. Since draft-petersson-forwarded-for-02 . . . . . . . . . . 11
A.4. Since draft-ietf-appsawg-http-forwarded-00 . . . . . . . . 11
A.5. Since draft-ietf-appsawg-http-forwarded-01 . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
In today's HTTP landscape, there are a multitude of different
applications acting as a proxy for the user agent and effectively
anonymizing the requests to look as if they originated from the proxy
IP address or in other ways changing the information in the original
request. Examples of such applications include caching, content
filtering, content compression, crypto offload, and load balancing.
As most of the time this destructive behavior is not the primary
purpose, or even a desired effect, a way of disclosing the original
information at HTTP level instead of depending on the TCP/IP
connection remote IP address and transport port number is needed.
In addition to the above mentioned problems, there may also be issues
due to the use of NAT. This is further discussed in [RFC6269].
A common way to disclose this information is by using the de facto
standard header fields such as X-Forwarded-For, X-Forwarded-By, and
X-Forwarded-Proto. This document intends to standardize syntax and
semantics for disclosing such information. The header field also
combines all information within one single header field, making it
possible to correlate the header fields to each other. With the
header field format described in this document, it is possible to
know what information belongs together, given that the proxies are
trusted. Such conclusions are not possible to make with the
X-Forwarded class of header fields. This new header field also
extends the de facto standard of, e.g., X-Forwarded-For with features
for which real life deployments have shown a need.
2. 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].
3. Syntax Notations
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with the list rule extension defined in Section
3.2.5 of [I-D.ietf-httpbis-p1-messaging].
4. Forwarded
The Forwarded HTTP header field is an OPTIONAL header field that,
when used, contains a list of parameter-identifier pairs that
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disclose information that is altered or lost when a proxy is involved
in the path of the request. This applies to forwarding proxies, as
well as reverse proxies. Information passed in this header can be,
e.g., the source IP address of the request, the IP address of the
incoming interface on the proxy, or whether HTTP or HTTPS is used.
If the request is passing through several proxies, each proxy MAY add
a set of parameters; it MAY also remove earlier added Forwarded-
header fields.
The top-level list is represented as a list of HTTP header field-
value as defined in Section 3.2 of [I-D.ietf-httpbis-p1-messaging].
The first element in this list holds information added by the first
proxy, followed by information added by any subsequent proxy. Each
field-value is a semicolon-separated list, this sub-list consists of
parameter-identifier pairs. Parameter-identifier pairs are grouped
together by an equals sign. The parameter names are case-
insensitive. The header field can be defined in augmented BNF syntax
as:
Forwarded = "Forwarded" ":" LWS Forwarded-v
Forwarded-v = 1#forwarded-element
forwarded-element =
[ forwarded-pair ] *( ";" [ forwarded-pair ] )
forwarded-pair = token "=" value
value = token / quoted-string
Example:
Forwarded: For=192.0.2.43,"for=[2001:db8:cafe::17]:47011"
Forwarded: proto=https;by=198.51.100.60
Given that a proxy wishes to add a Forwarded header field to the
outgoing request, if the incoming request has no such header field,
the proxy simply adds the header with the list of parameters desired.
If, on the other hand, the incoming request has such a header field,
the proxy adds a comma and the list of parameters. A proxy MAY
remove all Forwarded header fields from a request. It MUST, however,
ensure that the correct header field is updated in case of multiple
Forwarded header fields.
5. Parameters
This document specifies a number of parameters and valid values for
each of them.
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o "by" identifies the user-agent facing interface of the proxy.
o "for" identifies the node making the request to the proxy.
o "host" is the host request header-field as received by the proxy.
o "proto" indicates what protocol was used to make the request.
5.1. Forwarded by
The "by" parameter is used to disclose the interface where the
request came in to the proxy server. Typically, the value of this
parameter is an IP address and optionally a port number, but it can,
however, be some other kind of identifier.
The syntax of a "by" value, after potential quoted-string unescaping,
MUST conform to the "node" ABNF described in Section 6.
This is primarily added by reverse proxies that wish to forward this
information to the backend server.
5.2. Forwarded for
The "for" parameter is used to disclose information about the user
agent that initiated the request. Typically the value of this
parameter is an IP address, but it MAY also be some other kind of
identifier.
The syntax of a "for" value, after potential quoted-string
unescaping, MUST conform to the "node" ABNF described in Section 6.
In a chain of proxy servers where this is fully utilized, the first
for-parameter will disclose the user agent where the request first
was made, followed by any subsequent proxy identifiers. The last
proxy in the chain is not part of the list of for-parameters. The
last proxy's IP address, and optionally a port number, are, however,
readily available as the remote IP address of the TCP/IP connection.
5.3. Forwarded host
The "host" parameter is used to forward the original value of the
"Host" header field. This MAY be used for example by the origin
server if a reverse proxy is rewriting the "Host" header field to
some internal host name.
The syntax for a "host" value, after potential quoted-string
unescaping, MUST conform to the Host ABNF described in Section 5.4 of
[I-D.ietf-httpbis-p1-messaging].
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5.4. Forwarded proto
The "proto" parameter has the value of the used protocol type. The
syntax of a "proto" value, after potential quoted-string unescaping,
MUST conform to the URI scheme name as defined in Section 3.1 in
[RFC3986] and registered to IANA according to [RFC4395]. Typical
values are "http" or "https".
For example, in an environment where a reverse proxy is also used as
a crypto offloader, this allows the origin server to rewrite URLs in
a document to match the type of connection as the user agent
requested, even though all connections to the origin server are
unencrypted HTTP.
5.5. Extensions
Private extensions allow for adding own parameters and values. This
may be particularly useful in a reverse proxy environment. If these
extensions are to be widely spread it is RECOMENDED that they are
standardized. IANA MUST in such cases be notified, and parameters
should be registered according to [RFC3864]. Parameters MUST be
registered with the "Forwarded." prefix.
New parameters and their values MUST conform the forwarded-pair as
defined in ABNF in Section 4.
6. Node identifiers
The node identifiers are the IP address, and optionally port number,
of the network node, a predefined token hiding the real identity, but
signaling that such a component exists in the network path, or a
generated token allowing for tracing and debugging without revealing
network internals.
nodename = IPv4address / IPv6address /
"unknown" / obfnode
All of the identifiers may optionally have the port identifier, for
example, allowing the identification of the end point in a NATted
environment.
node = nodename [ ":" node-port ]
The node-port can be identified either by its TCP port number or by a
generated token obfuscating the real port number.
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node-port = port / obfport
port = 1*5DIGIT
obfport = 1*(ALPHA / DIGIT)
Note that this also allows port numbers to be appended to the the
"unknown" identifier. Interpretation of such notation is, however,
left to the possessor of a proxy adding such a value to the header
field. To distinguish an obfport from a port, we RECOMMEND that an
obfport always should contain at least one ALPHA.
Example:
192.0.2.43:47011
[2001:db8:cafe::17]:47011
6.1. IPv4 and IPv6 identifiers
The ABNF rules for "IPv6address" and "IPv4address" are defined in
[RFC3986] The IPv6address SHOULD comply with textual representation
recommendations [RFC5952] (e.g., lowercase, zero compression).
Note that the IP address may be one from the internal nets, as
defined in [RFC1918] and [RFC4193]. Also, note that an IPv6 adress
always must be enclosed by square brackets.
6.2. The "unknown" identifier
The "unknown" identifier is used when the identity of the preceding
entity is not known. One example would be a proxy server process
generating an outgoing request without direct access to the incoming
request TCP socket.
6.3. Obfuscated identifier
A generated identifier may be used where there is a wish to keep the
internal IP addresses secret, while still allowing the Forwarded
header field to be used for tracing and debugging. The identifiers
can be randomly generated for each request and do not need to be
statically assigned to resources. To distinguish the obfuscated
identifier from other identifiers, it MUST have a leading underscore
"_". Further, it MUST also consist of only US-ASCII letters and US-
ASCII digits.
obfnode = "_" 1*( ALPHA / DIGIT )
Example:
Forwarded: for=_hidden, for=_SEVKISEK
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7. Implementation considerations
Note that an HTTP list allows white spaces to occur between the
identifiers, and the list may be split over multiple header fields.
As an example, the header field
Forwarded: for=192.0.2.43,for=[2001:db8:cafe::17],for=unknown
is equivalent to the header field
Forwarded: for=192.0.2.43, for=[2001:db8:cafe::17], for=unknown
which is equivalent to the header fields
Forwarded: for=192.0.2.43
Forwarded: for=[2001:db8:cafe::17], for=unknown
Note that this header field is relevant on a per request basis and
MUST NOT be cached. Also note that this header field MUST NOT be
preserved across redirects.
7.1. Example
A request from a client with IP address 192.0.2.43 passes through a
proxy with IP address 198.51.100.17, then through another proxy with
IP address 203.0.113.60 before reaching a origin server. This could,
for example, be an office client behind a corporate malware filter
talking to a origin server through a reverse proxy.
o The HTTP request between the client and the first proxy has no
Forwarded header field.
o The HTTP request between the first and second proxy has a
"Forwarded: for=192.0.2.43" header field.
o The HTTP request between the second proxy and the origin server
has a "Forwarded: for=192.0.2.43,
for=198.51.100.17;by=203.0.113.60;proto=http;host=example.com"
header field.
Note that, at some points in a connection chain, the information
might not be correctly updated in the Forwarded header field, either
because of lack of support of this HTTP extension or because of a
policy decision not to disclose information about this network
component.
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8. Security considerations
8.1. Header validity and integrity
The Forwarded HTTP header field cannot be relied upon to be correct,
as it may be modified, whether mistakenly or for malicious reasons,
by every node on the way to the server, including the client making
the request.
One approach is to verify the correctness of proxies and whitelist
them as trusted. This approach has at least two weaknesses. First,
the chain of IP addresses listed before the request came to the proxy
cannot be trusted. Second, unless the communication between proxies
and the end point is secured, the data can be modified by an attacker
with access to the network.
8.2. Information leak
The Forwarded HTTP header field can reveal internal structures of the
network setup behind the NAT or proxy setup, which may be undesired.
This can be addressed either by preventing the internal nodes from
updating the HTTP header field or by having an egress proxy removing
entries that reveals internal network information.
9. IANA considerations
This document specifies the HTTP header listed below, which should be
added to the permanent HTTP header registry defined in [RFC3864]. We
also suggest that substructres are allowed for individual standards
of extension parameters Section 5.5.
Header field: Forwarded
Header field: Forwarded.for
Header field: Forwarded.by
Header field: Forwarded.host
Header field: Forwarded.proto
Applicable protocol: http/https
Status: standard
Author/Change controller:
IETF (iesg@ietf.org)
Internet Engineering Task Force
Specification document(s): this specification (Section 4)
Related information: None
10. References
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10.1. Normative references
[I-D.ietf-httpbis-p1-messaging]
Fielding, R., Lafon, Y., and J. Reschke, "HTTP/1.1, part
1: URIs, Connections, and Message Parsing",
draft-ietf-httpbis-p1-messaging-19 (work in progress),
March 2012.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[RFC4395] Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
Registration Procedures for New URI Schemes", BCP 35,
RFC 4395, February 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, August 2010.
10.2. Informative references
[RFC6269] Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
Roberts, "Issues with IP Address Sharing", RFC 6269,
June 2011.
Appendix A. Change Log (to be removed by RFC Editor before
publication)
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A.1. Since draft-petersson-forwarded-for-00
Added IANA considerations.
Expanded scope and add parameterized list.
A.2. Since draft-petersson-forwarded-for-01
Removed "x-" from private extensions.
Allow for any protocol name.
Rename kv-v to forwarded-element and kv to forwarded-value.
Add informative reference RFC6269.
A.3. Since draft-petersson-forwarded-for-02
Name change to draft-ietf-appsawg-http-forwarded-00.
Updated proto in list under section 5 Parameters.
Remove "hidden" but mention _hidden as an example in 6.3 Obfuscated
identifier.
Clarify that IPv6-addresses must be enclosed by square brackets.
Restrict ext-value: do not allow "," or ";".
A.4. Since draft-ietf-appsawg-http-forwarded-00
Write IP address instead of IP number.
Remove BNF for IP addresses.
A.5. Since draft-ietf-appsawg-http-forwarded-01
Refer to httpbis instead of RFC2616. Thereby also change to RFC5234
ABNF.
Split up ABNF to be more general on top level.
Add some comments on draft-ietf-httpbis-p2-semantics-19#section-3.1
to "Implementation Considerations"
Removal of ABNF appendix.
Merging of the sections "Private extensions" and "Future extensions".
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Authors' Addresses
Andreas Petersson
Opera Software
S:t Larsgatan 12
Linkoping SE-582 24
Email: pettson@opera.com
Martin Nilsson
Opera Software
S:t Larsgatan 12
Linkoping SE-582 24
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