Internet-Draft                              Paul Hoffman
draft-hoffman-mailto-url-02                 Internet Mail Consortium
October 19, 1997                            Larry Masinter
Expires in six months                       Xerox Corporation
                                            Jamie Zawinski
                                            Netscape Communications

                       The mailto URL scheme

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Abstract

This document defines the format of Uniform Resource Locators (URL)
for designating electronic mail addresses. It is one of a suite of
documents which replace RFC 1738, 'Uniform Resource Locators', and RFC
1808, 'Relative Uniform Resource Locators'. The syntax of 'mailto'
URLs from RFC 1738 is extended to allow creation of more RFC 822
messages by allowing the URL to express additional header and body
fields.

. Introduction

The mailto URL scheme is used to designate the Internet mailing
address of an individual or service. In its simplest form, a mailto
URL contains an Internet mail address.

For greater functionality, because interaction with some resources may
require message headers or message bodies to be specified as well as
the mail address, the mailto URL scheme is extended to allow setting
mail header fields and the message body.

. Syntax of a mailto URL

Following the syntax conventions of RFC 1738 [RFC1738], a "mailto"
URL has the form:

   mailtoURL  =  "mailto:" [ to ] [ headers ]
   to         =  #mailbox
   headers    =  "?" header *( "&" header )
   header     =  hname "=" hvalue
   hname      =  *urlc
   hvalue     =  *urlc

"#mailbox" is as specified in RFC 822 [RFC822]. This means that it consists
of zero or more comma-separated mail addresses, possibly including
"phrase" and "comment" components. Note that all URL reserved
characters in "to" must be encoded: in particular, parentheses, commas,
and the percent sign ("%"), which commonly occur in the "mailbox"
syntax.

"hname" and "hvalue" are encodings of an RFC 822 header name and value,
respectively. As with "to", all URL reserved characters must be encoded.

The special hname "body" indicates that the associated hvalue is the
body of the message.

Within mailto URLs, the characters "?", "=", "&" are reserved.

Also note that it is legal to specify both "to" and an "hname" whose
value is "to". That is,

     mailto:addr1%2C%20addr2

is equivalent to

     mailto:?to=addr1%2C%20addr2

is equivalent to

     mailto:addr1?to=addr2


. Semantics and operations

A mailto URL designates an "internet resource", which is the mailbox
specified in the address. When additional headers are supplied, the
resource designated is the same address, but with an additional
profile for accessing the resource. While there are Internet resources
that can only be accessed via electronic mail, the mailto URL is not
intended as a way of retrieving such objects automatically.

In current practice, resolving URLs such as those in the "http" scheme
causes an immediate interaction between client software and a host
running an interactive server. The "mailto" URL has unusual semantics
because resolving such a URL does not cause an immediate interaction.
Instead, the client creates a message to the designated address with
the various header fields set as default. The user can edit the
message, send this message unedited, or choose not to send the
message. The operation of how any URL scheme is resolved is not
mandated by the URL specifications.

. Unsafe headers

The user agent interpreting a mailto URL SHOULD choose not to create a
message if any of the headers are considered dangerous; it may also
choose to create a message with only a subset of the headers given in
the URL. Only the Subject, Keywords, and Body headers are believed to
be both safe and useful.

The creator of a mailto URL cannot expect that a resolver of a URL to
understand more than the "subject" and "body" headers. Clients that
resolve mailto URLs into mail messages should be able to correctly
create RFC 822-compliant mail messages using the "subject" and "body"
headers.

. Encoding

RFC 1738 requires that many characters in URLs be encoded. This
affects the mailto scheme for some common characters that might appear
in addresses, headers or message contents. One such character is space
(" ", ASCII hex 20). Note the examples above that use "%20" for space
in the message body. Also note that line breaks in the body of a
message should be encoded with "%0D%0A".

People creating mailto URLs must be careful to encode any reserved
characters that are used in the URLs so that properly-written URL
interpreters can read them. Also, client software that reads URLs must
be careful to decode strings before creating the mail message so that
the mail messages appear in a form that the recipient will understand.
These strings should be decoded before showing the user the message.

The mailto URL scheme is limited in that it does not provide for
substitution of variables. Thus, a message body that must include a
user's email address can not be encoded using the mailto URL. This
limitation also prevents mailto URLs that are signed with public keys
and other such variable information.

. Examples

URLs for an ordinary individual mailing address:

   <mailto:masinter@parc.xerox.com>
   <mailto:ietf-url@imc.org>

A URL for a mail response system that requires the name of the file
in the subject:

   <mailto:infobot@example.com?subject=current-issue>

A mail response system that requires a "send" request in the body:

   <mailto:infobot@example.com?body=send%20current-issue>

A similar URL could have two lines with different "send" requests (in
this case, "send current-issue" and, on the next line, "send index".)

   <mailto:infobot@example.com?body=send%20current-issue%0D%0Asend%20index>

A request to subscribe to a mailing list:

   <mailto:majordomo@example.com?body=subscribe%20bamboo-l>

A URL for a single user which includes a CC of another user:

   <mailto:joe@example.com?cc=bob@example.com&body=hello>

Another way of expressing the same thing:

   <mailto:?to=joe@example.com&cc=bob@example.com&body=hello>

Note the use of the "&" reserved character, above. The following
example, by using "?" twice, is incorrect:

   <mailto:joe@example.com?cc=bob@example.com?body=hello>   ; WRONG!

According to RFC 822, the characters "?", "&", and even "%" may occur
in addr-specs. The fact that they are reserved characters in this URL
scheme is not a problem: those characters may appear in mailto URLs,
they just may not appear in unencoded form. The standard URL encoding
mechanisms ("%" followed by a two-digit hex number) must be used in
certain cases.

To indicate the address "gorby%kremvax@example.com" one would do:

   <mailto:gorby%25kremvax@example.com>

To indicate the address "unlikely?address@example.com", and include
another header, one would do:

   <mailto:unlikely%3Faddress@example.com?blat=foop>

. Security

The mailto scheme can be used to send a message from one user to
another, and thus can introduce many security concerns. Mail messages
can be logged at the originating site, the recipient site, and
intermediary sites along the delivery path. If the messages are not
encoded, they can also be read at any of those sites.

A mailto URL gives a template for a message that can be sent by mail
client software. The contents of that template may be opaque or
difficult to read by the user at the time of specifying the URL. Thus,
a mail client should never send a message based on a mailto URL
without first showing the user the full message that will be sent
(including all headers that were specified by the mailto URL), fully
decoded, and asking the user for approval to send the message as
electronic mail. The mail client should also make it clear that the
user is about to send an electronic mail message, since the user may
not be aware that this is the result of a mailto URL.

A mail client should never send anything without complete disclosure
to the user of what is will be sent; it should disclose not only the
message destination, but also any headers. Unrecognized headers, or
headers with values inconsistent with those the mail client would
normally send should be especially suspect. MIME headers
(MIME-Version, Content-*) are most likely inappropriate, as are those
relating to routing (From, Bcc, Apparently-To, etc.)

Note that some headers are inherently unsafe to include in a message
generated from a URL. For example, headers such as "From:", "Bcc:",
and so on, should never be interpreted from a URL. In general, the
fewer headers interpreted from the URL, the less likely it is that a
sending agent will create an unsafe message.

Examples of problems with sending unapproved mail include:

  * mail that breaks laws upon delivery, such as making illegal
    threats;

  * mail that identifies the sender as someone interested in
    breaking laws;

  * mail that identifies the sender to an unwanted third party;

  * mail that causes a financial charge to be incurred on the sender;

  * mail that causes an action on the recipient machine that causes
    damage that might be attributed to the sender.

Programs that interpret mailto URLs should ensure that the SMTP "From"
address is set and correct.

. Acknowledgments

This document was derived from RFC 1738 and RFC 1808 [RFC1808]; the
acknowledgments from those specifications still applies.

The following people contributed to this draft or had and discussed
similar ideas for mailto.

Bryan Costales
Steve Dorner
Al Gilman
Mark Joseph
Michael Patton

. References

[RFC822] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", RFC 822, University of Delaware, August 1982.

[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, Editors, "Uniform
Resource Locators (URL)", RFC 1738, CERN, Xerox Corporation,
University of Minnesota, December 1994.

[RFC1808] Fielding, R., "Relative Uniform Resource Locators", RFC 1808, UC
Irvine, June 1995.


Appendixes

A. Change from RFC 1738

RFC 1738 defined only a simple 'mailto' with no headers, just an
addr-spec (not a full mailbox.)  However, required usage and
implementation has led to the development of an extended syntax that
included more header fields.

B. Author contact information:

Paul E. Hoffman
Internet Mail Consortium
 Segre Place
Santa Cruz, CA  95060 USA
Tel: 408-426-9827
phoffman@imc.org

Larry Masinter
Xerox Corporation
 Coyote Hill Road
Palo Alto, CA 94304 USA
masinter@parc.xerox.com

Jamie Zawinski
Netscape Communications Corp.
 East Middlefield Road
Mountain View, CA 94043 USA
jwz@netscape.com