Network Working Group A. Petersson
Internet-Draft M. Nilsson
Intended status: Standards Track Opera Software
Expires: October 7, 2011 April 5, 2011
Forwarded-For HTTP Extension
draft-petersson-forwarded-for-00
Abstract
This document standardizes an HTTP extension header field that allows
proxy components to disclose the originating IP number of a request.
Given a trusted path of proxying components, each subsequent
component will have access to all IP numbers used in the chain of
proxied HTTP requests.
This document also standardizes ways for a proxy possessor 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.
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This Internet-Draft will expire on October 7, 2011.
Copyright Notice
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document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. Syntax Notations . . . . . . . . . . . . . . . . . . . . . . . 3
4. Forwarded-For . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. IPv4 and IPv6 identifiers . . . . . . . . . . . . . . . . . 5
5.2. The "unknown" identifier . . . . . . . . . . . . . . . . . 6
5.3. The "hidden" identifier . . . . . . . . . . . . . . . . . . 6
5.4. Obfuscated identifier . . . . . . . . . . . . . . . . . . . 6
6. Implentations considerations . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7.1. Header validity . . . . . . . . . . . . . . . . . . . . . . 7
7.2. Information Leak . . . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
9. Normative references . . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Forwarded-For BNF definition . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
In today's HTTP landscape, there are a multitude of different
applications such as caching, content filtering, and content
compression, acting as proxy for the user agent and effectively
anonymizing the requests to look as if they originated from the proxy
IP number. As most of the time anonymizing is not the primary
purpose, or even a desired effect, a way of disclosing the original
IP number on HTTP level instead of depending on the TCP/IP connection
remote IP number is needed.
One common way of disclosing the originator of the HTTP requests and
the proxy path is with the X-Forwarded-For header field. This
document intends to standardize the way of disclosing this
information by formalizing syntax and semantics for a new Forwarded-
For header field. This new header field also extends the de facto
standard of X-Forwarded-For with features for which real life
deployments have shown a need.
This document intentionally does not cover headers specifically
useful in a reverse proxy environment as that is considered as out of
scope for this document.
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 BNF notation defined in Section
2.1 of [RFC2616], including its rules for implied linear whitespace
(LWS).
The syntax for IPv4address and IPv6address is defined in [RFC2373].
4. Forwarded-For
The Forwarded-For HTTP header field is an optional header field that,
when used, contains the identification of the user agent that sent
the requests being proxied. The identification would typically be
the IP number of the host, though in special cases other types of
tokens may be preferred. If the request is passing through several
proxies, the identification of the previous proxy is added to the
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list of identifiers in the header field. Thus, when utilized fully,
the Forwarded-For header field combined with the remote IP of the TCP
connection carrying the HTTP request will form a complete history of
the non-transparent network entities in the requests network path.
The list is represented as a list of HTTP header field-values
[RFC2616], with the client identifier first (leftmost), followed by
any subsequent proxy identifiers. The last proxy in the chain is not
part of the list in the Forwarded-For header field. The last proxy's
IP number is, however, readily available as the remote IP of the
TCP/IP connection. The header field can be formally defined in
augmented BNF syntax as
Forwarded-For = "Forwarded-For" ":" 1#node
Example:
Forwarded-For: 192.0.2.43,[2001:db8:cafe::17],unknown
Given that a proxy wishes to add a Forwarded-For header field to the
outgoing request, if the incoming request has no such header field,
the proxy simply adds the header with the remote IP number of the
TCP/IP connection. If, on the other hand, the incoming request has
such a header field, the proxy simply adds a comma and the remote IP
to the list. A proxy MAY remove all Forwarded-For header fields from
a request. It MUST, however, ensure that the correct header field is
updated in case of multiple Forwarded-For header fields.
Example: A request from a client with IP number 192.0.2.43 passes
through a proxy with IP number 198.51.100.17, then through another
proxy with IP number 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 cache proxy.
o The HTTP request between the client and the first proxy has no
Forwarded-For 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: 192.0.2.43,198.51.100.17" header field.
Note that, at some points in a connection chain, the IP number might
not be correctly updated in the Forwarded-For 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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5. Identifiers
The identifiers are the IP address of the network node, a predefined
token hiding the real identity, but signalling 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" | "hidden" | obfnode
All of the identifiers may optionally have the port identifier, for
example, allowing the identification of the end point in a NAT:ed
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. If a node-port is
appended to an IPv6address the IPv6address MUST be enclosed by square
brackets.
node-port = port | obfport
port = 1*5DIGIT
obfport = 1*(ALPHA | DIGIT)
Note that this also allows portnumbers to be appended to the "hidden"
and the "unknown" identifiers. 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 obfort always should contain at least one ALPHA.
5.1. IPv4 and IPv6 identifiers
The IPv4address and IPv6address BNF tokens is defined as:
IPv6address = addr6 | "[" addr6 "]"
addr6 = hexpart [ ":" IPv4address ]
IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT
hexpart = hexseq | hexseq "::" [ hexseq ] | "::" [ hexseq ]
hexseq = hex4 *( ":" hex4)
hex4 = 1*4HEXDIG
Note that the IP number may be one from the internal nets, as defined
in [RFC1918] and [RFC4193].
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5.2. 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.
5.3. The "hidden" identifier
The "hidden" identifier is used when the administrator of a proxy
server entity would like to keep the identity of that entity secret,
but still disclose that it exists.
5.4. Obfuscated identifier
A generated identifier may be used where there is a wish to keep the
internal IP numbers secret, while still allowing the Forwarded-For
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 )
6. Implentations 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,[2001:db8:cafe::17],unknown
is equivalent to the header field
Forwarded-For: 192.0.2.43, [2001:db8:cafe::17], unknown
which is equivalent to the header fields
Forwarded-For: 192.0.2.43
Forwarded-For: [2001:db8:cafe::17], unknown
Also note that the draft [I-D.ietf-httpbis-p1-messaging] renders the
use of folding within a list obsolete. The use of CRLF within the
field-value list is therefore NOT RECOMMENDED.
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7. Security Considerations
7.1. Header validity
The Forwarded-For 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 white list
them as trusted. This approach has at least two weaknesses. First
the chain of IP numbers listed before the request came to the proxy
cannot be trusted. Secondly, unless the communication between
proxies and end point is secured, the data can be modified by an
attacker with access to the network.
7.2. Information Leak
The Forwarded-For 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.
8. IANA Considerations
There are no IANA Considerations.
9. Normative references
[I-D.ietf-httpbis-p1-messaging]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., Berners-Lee, T., and J. Reschke,
"HTTP/1.1, part 1: URIs, Connections, and Message
Parsing", draft-ietf-httpbis-p1-messaging-13 (work in
progress), March 2011.
[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.
[RFC2373] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
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[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.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
Appendix A. Forwarded-For BNF definition
This appendix defines the Forwarded-For header field.
Forwarded-For = "Forwarded-For" ":" LWS Forwarded-For-v
Forwarded-For-v = 1#node
node = nodename [":" node-port]
nodename = IPv4address | IPv6address |
"unknown" | "hidden" | obfnode
obfnode = "_" 1*( ALPHA | DIGIT )
node-port = port | obfport
port = 1*5DIGIT
obfport = 1*( ALPHA | DIGIT )
IPv6address = addr6 | "[" addr6 "]"
addr6 = hexpart [ ":" IPv4address ]
IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT
hexpart = hexseq | hexseq "::" [ hexseq ] | "::" [ hexseq ]
hexseq = hex4 *( ":" hex4)
hex4 = 1*4HEXDIG
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
Email: nilsson@opera.com
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