| Rfc | 7239 |
|---|
| Title | Forwarded HTTP Extension |
|---|
| Author | A. Petersson, M. Nilsson |
|---|
| Date | June 2014 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Internet Engineering Task Force (IETF) A. Petersson
Request for Comments: 7239 M. Nilsson
Category: Standards Track Opera Software
ISSN: 2070-1721 June 2014
Forwarded HTTP Extension
Abstract
This document defines an HTTP extension header field that allows
proxy components to disclose information lost in the proxying
process, for example, the originating IP address of a request or IP
address of the proxy on the user-agent-facing interface. In a path
of proxying components, this makes it possible to arrange it so that
each subsequent component will have access to, for example, 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 is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7239.
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
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.
Table of Contents
1. Introduction ....................................................3
2. Notational Conventions ..........................................4
3. Syntax Notations ................................................4
4. Forwarded HTTP Header Field .....................................4
5. Parameters ......................................................6
5.1. Forwarded By ...............................................6
5.2. Forwarded For ..............................................6
5.3. Forwarded Host .............................................7
5.4. Forwarded Proto ............................................7
5.5. Extensions .................................................7
6. Node Identifiers ................................................8
6.1. IPv4 and IPv6 Identifiers ..................................9
6.2. The "unknown" Identifier ...................................9
6.3. Obfuscated Identifier ......................................9
7. Implementation Considerations ..................................10
7.1. HTTP Lists ................................................10
7.2. Header Field Preservation .................................10
7.3. Relation to Via ...........................................10
7.4. Transition ................................................11
7.5. Example Usage .............................................11
8. Security Considerations ........................................12
8.1. Header Validity and Integrity .............................12
8.2. Information Leak ..........................................12
8.3. Privacy Considerations ....................................12
9. IANA Considerations ............................................14
10. References ....................................................14
10.1. Normative References .....................................14
10.2. Informative References ...................................15
Appendix A. Acknowledgments .......................................16
1. Introduction
In today's HTTP landscape, there are a multitude of different
applications that act as proxies for the user agents. In many cases,
these proxies exists without the action or knowledge of the end-user.
These cases occur, for example, when the proxy exists as a part of
the infrastructure within the organization running the web server.
Such proxies may be used for features such as load balancing or
crypto offload. Another example is when the proxy is used within the
same organization as the user, and the proxy is used to cache
resources. However, these proxies make the requests appear as if
they originated from the proxy's IP address, and they may change
other information in the original request. This represents a loss of
information from the original request.
This loss of information can cause problems for a web server that has
a specific use for the clients' IP addresses that will not be met by
using the address of the proxy or other information changed by the
proxy. The main uses of this information are for diagnostics, access
control, and abuse management. Diagnostic functions can include
event logging, troubleshooting, and statistics gathering, and the
information collected is usually only stored for short periods of
time and only gathered in response to a particular problem or a
complaint from the client. Access control can be operated by
configuring a list of client IP addresses from which access is
permitted, but this approach will not work if a proxy is used, unless
the proxy is trusted and is, itself, configured with a list of
allowed client addresses for the server. Cases of abuse require
identification of the abuser and this uses many of the same features
identified for diagnostics.
Most of the time that a proxy is used, this loss of information is
not the primary purpose, or even a desired effect, of using the
proxy. Thus, to restore the desired functionality when a proxy is in
use, a way of disclosing the original information at the HTTP level
is needed. Clearly, however, when the purpose of using a proxy is to
provide client anonymity, the proxy will not use the feature defined
in this document.
It should be noted that the use of a reverse proxy also hides
information. Again, where the loss of information is not a
deliberate function of the use of the reverse proxy, it can be
desirable to find a way to encode the information within the HTTP
messages so that the consumer can see it.
A common way to disclose this information is by using the non-
standard header fields such as X-Forwarded-For, X-Forwarded-By, and
X-Forwarded-Proto. There are many benefits to using a standardized
approach to commonly desired protocol function: not least is
interoperability between implementations. This document standardizes
a header field called "Forwarded" and provides the syntax and
semantics for disclosing such information. "Forwarded" also combines
all the information within one single header field, making it
possible to correlate that information. With the header field format
described in this document, it is possible to know what information
belongs together, as long as the proxies are trusted. Such
conclusions are not possible to make with the X-Forwarded class of
header fields. The header field defined in this document is optional
such that implementations of proxies that are intended to provide
privacy are not required to operate or implement the header field.
Note that similar issues to those described for proxies also arise
with use of NATs. This is discussed further in [RFC6269].
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
7 of [RFC7230].
4. Forwarded HTTP Header Field
The "Forwarded" HTTP header field is an OPTIONAL header field that,
when used, contains a list of parameter-identifier pairs that
disclose information that is altered or lost when a proxy is involved
in the path of the request. Due to the sensitive nature of the data
passed in this header field (see Sections 8.2 and 8.3), this header
field should be turned off by default. Further, each parameter
should be configured individually. "Forwarded" is only for use in
HTTP requests and is not to be used in HTTP responses. This applies
to forwarding proxies, as well as reverse proxies. Information
passed in this header field can be, for example, the source IP
address of the request, the IP address of the incoming interface on
the proxy, or whether HTTP or HTTPS was used. If the request is
passing through several proxies, each proxy can add a set of
parameters; it can also remove previously added "Forwarded" header
fields.
The top-level list is represented as a list of HTTP header
field-values as defined in Section 3.2 of [RFC7230]. The first
element in this list holds information added by the first proxy that
implements and uses this header field, and each subsequent element
holds information added by each subsequent proxy. Because this
header field is optional, any proxy in the chain may choose not to
update this header field. Each field-value is a semicolon-separated
list; this sublist consists of parameter-identifier pairs.
Parameter-identifier pairs are grouped together by an equals sign.
Each parameter MUST NOT occur more than once per field-value. The
parameter names are case-insensitive. The header field value can be
defined in ABNF syntax as:
Forwarded = 1#forwarded-element
forwarded-element =
[ forwarded-pair ] *( ";" [ forwarded-pair ] )
forwarded-pair = token "=" value
value = token / quoted-string
token = <Defined in [RFC7230], Section 3.2.6>
quoted-string = <Defined in [RFC7230], Section 3.2.6>
Examples:
Forwarded: for="_gazonk"
Forwarded: For="[2001:db8:cafe::17]:4711"
Forwarded: for=192.0.2.60;proto=http;by=203.0.113.43
Forwarded: for=192.0.2.43, for=198.51.100.17
Note that as ":" and "[]" are not valid characters in "token", IPv6
addresses are written as "quoted-string".
A proxy server that wants to add a new "Forwarded" header field value
can either append it to the last existing "Forwarded" header field
after a comma separator or add a new field at the end of the header
block. 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:
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. When proxies choose to use the
"by" parameter, its default configuration SHOULD contain an
obfuscated identifier as described in Section 6.3. If the server
receiving proxied requests requires some address-based functionality,
this parameter MAY instead contain an IP address (and, potentially, a
port number). A third option is the "unknown" identifier described
in Section 6.2.
The syntax of a "by" value, after potential quoted-string unescaping,
conforms 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. It can also be interesting in a
multihomed environment to signal to backend servers from which the
request came.
5.2. Forwarded For
The "for" parameter is used to disclose information about the client
that initiated the request and subsequent proxies in a chain of
proxies. When proxies choose to use the "for" parameter, its default
configuration SHOULD contain an obfuscated identifier as described in
Section 6.3. If the server receiving proxied requests requires some
address-based functionality, this parameter MAY instead contain an IP
address (and, potentially, a port number). A third option is the
"unknown" identifier described in Section 6.2.
The syntax of a "for" value, after potential quoted-string
unescaping, conforms 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 client where the request was first
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 at the transport layer.
It can, however, be more relevant to read information about the last
proxy from preceding "Forwarded" header field's "by" parameter, if
present.
5.3. Forwarded Host
The "host" parameter is used to forward the original value of the
"Host" header field. This can 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
[RFC7230].
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 with 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
Extensions allow for additional parameters and values. Extensions
can be particularly useful in reverse proxy environments. All
extension parameters SHOULD be registered in the "HTTP Forwarded
Parameter" registry. If certain extensions are expected to have
widespread deployment, they SHOULD also be standardized. This is
further discussed in Section 9.
6. Node Identifiers
The node identifier is one of the following:
o The client's IP address, with an optional port number
o A token indicating that the IP address of the client is not known
to the proxy server
o A generated token, allowing for tracing and debugging, while
allowing the internal structure or sensitive information to be
hidden
The node identifier is defined by the ABNF syntax as:
node = nodename [ ":" node-port ]
nodename = IPv4address / "[" IPv6address "]" /
"unknown" / obfnode
IPv4address = <Defined in [RFC3986], Section 3.2.2>
IPv6address = <Defined in [RFC3986], Section 3.2.2>
obfnode = "_" 1*( ALPHA / DIGIT / "." / "_" / "-")
node-port = port / obfport
port = 1*5DIGIT
obfport = "_" 1*(ALPHA / DIGIT / "." / "_" / "-")
DIGIT = <Defined in [RFC5234], Section 3.4>
ALPHA = <Defined in [RFC5234], Section B.1>
Each of the identifiers may optionally have the port identifier, for
example, allowing the identification of the endpoint in a NATed
environment. The "node-port" can be identified either by its port
number or by a generated token obfuscating the real port number. An
obfuscated port may be used in situations where the possessor of the
proxy wants the ability to trace requests -- for example, in debug
purposes -- but does not want to reveal internal information.
Note that the ABNF above also allows port numbers to be appended to
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, the "obfport"
MUST have a leading underscore. Further, it MUST also consist of
only "ALPHA", "DIGIT", and the characters ".", "_", and "-".
It is important to note that an IPv6 address and any nodename with
node-port specified MUST be quoted, since ":" is not an allowed
character in "token".
Examples:
"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] (for example, lowercase,
compression of zeros).
Note that the IP address may be one from the internal nets, as
defined in [RFC1918] and [RFC4193]. Also, note that an IPv6 address
is always enclosed in square brackets.
6.2. The "unknown" Identifier
The "unknown" identifier is used when the identity of the preceding
entity is not known, but the proxy server still wants to signal that
a forwarding of the request was made. 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. This can also be
useful if the proxy uses some sort of interface labels and there is a
desire to pass them rather than an IP address. Unless static
assignment of identifiers is necessary for the server's use of the
identifiers, obfuscated identifiers SHOULD be randomly generated for
each request. If the server requires that identifiers persist across
requests, they SHOULD NOT persist longer than client IP addresses.
To distinguish the obfuscated identifier from other identifiers, it
MUST have a leading underscore "_". Furthermore, it MUST also
consist of only "ALPHA", "DIGIT", and the characters ".", "_", and
"-".
Example:
Forwarded: for=_hidden, for=_SEVKISEK
7. Implementation Considerations
7.1. HTTP Lists
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
7.2. Header Field Preservation
There are some cases when this header field should be kept and some
cases where it should not be kept. A directly forwarded request
should preserve and possibly extend it. If a single incoming request
causes the proxy to make multiple outbound requests, special care
must be taken to decide whether or not the header field should be
preserved. In many cases, the header field should be preserved, but
if the outbound request is not a direct consequence of the incoming
request, the header field should not be preserved. Consider also the
case when a proxy has detected a content mismatch in a 304 response
and is following the instructions in [RFC7232], Section 4.1 to repeat
the request unconditionally, in which case the new request is still
basically a direct consequence of the origin request, and the header
field should probably be kept.
7.3. Relation to Via
The "Via" header field (see [RFC7230], Section 5.7.1) is a header
field with a similar use case as this header field. The "Via" header
field, however, only provides information about the proxy itself, and
thereby leaves out the information about the client connecting to the
proxy server. The "Forwarded" header field, on the other hand, has
relaying information from the client-facing side of the proxy server
as its main purpose. As "Via" is already widely deployed, its format
cannot be changed to address the problems that "Forwarded" addresses.
Note that it is not possible to combine information from this header
field with the information from the Via header field. Some proxies
will not update the "Forwarded" header field, some proxies will not
update the Via header field, and some proxies will update both.
7.4. Transition
If a proxy gets incoming requests with X-Forwarded-* header fields
present, it is encouraged to convert these into the header field
described in this document, if it can be done in a sensible way. If
the request only contains one type -- for example, X-Forwarded-For --
this can be translated to "Forwarded", by prepending each element
with "for=". Note that IPv6 addresses may not be quoted in
X-Forwarded-For and may not be enclosed by square brackets, but they
are quoted and enclosed in square brackets in "Forwarded".
X-Forwarded-For: 192.0.2.43, 2001:db8:cafe::17
becomes:
Forwarded: for=192.0.2.43, for="[2001:db8:cafe::17]"
However, special care must be taken if, for example, both
X-Forwarded-For and X-Forwarded-By exist. In such cases, it may not
be possible to do a conversion, since it is not possible to know in
which order the already existing fields were added. Also, note that
removing the X-Forwarded-For header field may cause issues for
parties that have not yet implemented support for this new header
field.
7.5. Example Usage
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 an 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 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.
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 to ensure that the "Forwarded" HTTP header field is
correct is to verify the correctness of proxies and to 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 endpoint 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 using obfuscated
elements, by preventing the internal nodes from updating the HTTP
header field, or by having an egress proxy remove entries that reveal
internal network information.
This header field should never be copied into response messages by
origin servers or intermediaries, as it can reveal the whole proxy
chain to the client. As a side effect, special care must be taken in
hosting environments not to allow the TRACE request where the
"Forwarded" field is used, as it would appear in the body of the
response message.
8.3. Privacy Considerations
In recent years, there have been growing concerns about privacy.
There is a trade-off between ensuring privacy for users versus
disclosing information that is useful, for example, for debugging,
statistics, and generating location-dependent content. The
"Forwarded" HTTP header field, by design, exposes information that
some users consider privacy sensitive, in order to allow for such
uses. For any proxy, if the HTTP request contains header fields that
specifically request privacy semantics, the proxy SHOULD NOT use the
"Forwarded" header field, nor in any other manner pass private
information, such as IP addresses, on to the next hop.
The client's IP address, that may be forwarded in the "for" parameter
of this header field, is considered to be privacy sensitive by many
people, as the IP address may be able to uniquely identify a client,
what operator the user is using, and possibly a rough estimation of
where the user is geographically located.
Proxies using this extension will preserve the information of a
direct connection. This has an end-user privacy impact regardless of
whether the end-user or deployer knows or expects that this is the
case.
Implementers and deployers of such proxies need to consider whether,
and how, deploying this extension affects user privacy.
The default configuration for both the "by" and "for" parameters
SHOULD contain obfuscated identifiers. These identifiers SHOULD be
randomly generated per request. If identifiers that persist across
requests are required, their lifetimes SHOULD be limited and they
SHOULD NOT persist longer than client IP addresses. When generating
obfuscated identifiers, care must be taken not to include potentially
sensitive information in them.
Note that users' IP addresses may already be forwarded by proxies
using the header field X-Forwarded-For, which is widely used. It
should also be noted that if the user were doing the connection
directly without passing the proxy, the client's IP address would be
sent to the web server. Users that do not actively choose an
anonymizing proxy cannot rely on having their IP address shielded.
These users who want to minimize the risk of being tracked must also
note that there are other ways information may leak, for example, by
browser header field fingerprinting. The Forwarded header field
itself, even when used without a uniquely identifying client
identifier, may make fingerprinting more feasible by revealing the
chain of proxies traversed by the client's request.
9. IANA Considerations
This document specifies the HTTP header field listed below, which has
been added to the "Permanent Message Header Field Names" registry
defined in [RFC3864].
Header field: Forwarded
Applicable protocol: http
Status: standard
Author/Change controller:
IETF (iesg@ietf.org)
Internet Engineering Task Force
Specification document(s): this specification (Section 4)
Related information: None
The "Forwarded" header field contains parameters for which IANA has
created and now maintains a new registry entitled "HTTP Forwarded
Parameters". Initial registrations are given below. For future
assignments, the registration procedure is IETF Review [RFC5226].
The security and privacy implications of all new parameters should be
thoroughly documented. New parameters and their values MUST conform
with the forwarded-pair as defined in ABNF in Section 4. Further, a
short description should be provided in the registration.
+-------------+---------------------------------------+-------------+
| Parameter | Description | Reference |
| name | | |
+-------------+---------------------------------------+-------------+
| by | IP address of incoming interface of a | Section 5.1 |
| | proxy | |
| for | IP address of client making a request | Section 5.2 |
| | through a proxy | |
| host | Host header field of the incoming | Section 5.3 |
| | request | |
| proto | Application protocol used for | Section 5.4 |
| | incoming request | |
+-------------+---------------------------------------+-------------+
Table 1: Initial Assignments
10. References
10.1. Normative References
[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.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[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.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, June 2014.
[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
June 2014.
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. Acknowledgments
Thanks to Per Cederqvist, Alissa Cooper, Adrian Farrel, Stephen
Farrell, Ned Freed, Per Hedbor, Amos Jeffries, Poul-Henning Kamp,
Murray S. Kucherawy, Barry Leiba, Salvatore Loreto, Alexey Melnikov,
S. Moonesamy, Susan Nichols, Mark Nottingham, Julian Reschke, John
Sullivan, Willy Tarreau, and Dan Wing for their feedback.
Authors' Addresses
Andreas Petersson
Opera Software
EMail: andreas@sbin.se
Martin Nilsson
Opera Software
S:t Larsgatan 12
Linkoping SE-582 24
EMail: nilsson@opera.com