RFC2368 - The mailto URL scheme

Network Working Group P. Hoffman

Request for Comments: 2368 Internet Mail Consortium

Updates: 1738, 1808 L. Masinter

Category: Standards Track Xerox Corporation

J. Zawinski

Netscape Communications

July 1998

The mailto URL scheme

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

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 RFC1738, 'Uniform Resource Locators', and

RFC1808, 'Relative Uniform Resource Locators'. The syntax of

'mailto' URLs from RFC1738 is extended to allow creation of more RFC

822 messages by allowing the URL to eXPress additional header and

body fields.

1. 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.

2. Syntax of a mailto URL

Following the syntax conventions of RFC1738 [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 RFC822 [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 RFC822 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. The "body" hname should contain the content for

the first text/plain body part of the message. The mailto URL is

primarily intended for generation of short text messages that are

actually the content of automatic processing (such as "subscribe"

messages for mailing lists), not general MIME bodies.

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

Because the "&" (ampersand) character is reserved in Html, any mailto

URL which contains an ampersand must be spelled differently in HTML

than in other contexts. A mailto URL which appears in an HTML

document must use "&" instead of "&".

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

8-bit characters in mailto URLs are forbidden. MIME encoded Words (as

defined in [RFC2047]) are permitted in header values, but not for any

part of a "body" hname.

3. 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.

4. 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 the 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 RFC822-compliant mail messages using the "subject" and "body"

headers.

5. Encoding

RFC1738 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 MUST 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.

6. Examples

URLs for an ordinary individual mailing address:

<mailto:chris@example.com>

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>

An interesting use of your mailto URL is when browsing archives of

messages. Each browsed message might contain a mailto URL like:

<mailto:Foobar@example.com?In-Reply-

To=%3c3469A91.D10AF4C@example.com>

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 RFC822, 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>

As described above, the "&" (ampersand) character is reserved in HTML

and must be replacded with "&". Thus, a complex URL that has

internal ampersands might look like:

Click

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

send a greeting message to <i>Joe and Bob</i>.

7. Security Considerations

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.

8. IANA Considerations

This document changes the definition of the mailto: URI scheme; any

registry of URI schemes should refer to this document rather than its

predecessor, RFC1738.

9. References

[RFC822] Crocker, D., "Standard for the Format of ARPA Internet Text

Messages", STD 11, RFC822, August 1982.

[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, Editors,

"Uniform Resource Locators (URL)", RFC1738, December 1994.

[RFC1808] Fielding, R., "Relative Uniform Resource Locators", RFC

1808, June 1995.

[RFC2047] Moore, K., "MIME Part Three: Message Header Extensions for

Non-ASCII Text", RFC2047, November 1996.

A. Change from RFC1738

RFC1738 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. Acknowledgments

This document was derived from RFC1738 and RFC1808 [RFC1808]; the

acknowledgments from those specifications still applies.

The following people contributed to this memo or had and discussed

similar ideas for mailto.

Harald Alvestrand

Bryan Costales

Steve Dorner

Al Gilman

Mark Joseph

Laurence Lundblade

Keith Moore

Jacob Palme

Michael Patton

C. Author Contact Information

Paul E. Hoffman

Internet Mail Consortium

127 Segre Place

Santa Cruz, CA 95060 USA

EMail: phoffman@imc.org

Larry Masinter

Xerox Corporation

3333 Coyote Hill Road

Palo Alto, CA 94304 USA

EMail: masinter@parc.xerox.com

Jamie Zawinski

Netscape Communications Corp.

501 East Middlefield Road

Mountain View, CA 94043 USA

EMail: jwz@netscape.com

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