RFC2384 - POP URL Scheme

Network Working Group R. Gellens

Request for Comments: 2384 QUALCOMM, Incorporated

Category: Standards Track August 1998

POP 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.

1. IntrodUCtion

[POP3] is a widely-deployed mail Access protocol. Many programs

access POP3 message stores, and thus need POP3 configuration

information. Since there are multiple configuration elements which

are required in order to access a mailbox, a single string

representation is convenient.

A POP3 mailbox (like an [IMAP4] mailbox) is a network resource, and

URLs are a widely-supported generalized representation of network

resources.

A means of specifying a POP3 mailbox as a URL will likely be useful

in many programs and protocols. [ACAP] is one case where a string

encapsulation of elements required to access network services is

needed. For example, an [IMAP4] message store is usually specified

in ACAP datasets as an [IMAP-URL].

This memo defines a URL scheme for referencing a POP mailbox.

2. Conventions Used in this Document

The key Words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"

in this document are to be interpreted as defined in "Key words for

use in RFCs to Indicate Requirement Levels" [KEYWORDS].

3. POP Scheme

The POP URL scheme designates a POP server, and optionally a port

number, authentication mechanism, authentication ID, and/or

authorization ID.

The POP URL follows the common Internet scheme syntax as defined in

RFC1738 [BASIC-URL] except that clear text passwords are not

permitted. If :<port> is omitted, the port defaults to 110.

The POP URL is described using [ABNF] in Section 8.

A POP URL is of the general form:

pop://<user>;auth=<auth>@<host>:<port>

Where <user>, <host>, and <port> are as defined in RFC1738, and some

or all of the elements, except "pop://" and <host>, may be omitted.

4. POP User Name and Authentication Mechanism

An authorization (which mailbox to access) and authentication (whose

password to check against) identity (referred to as "user name" for

simplicity) and/or authentication mechanism name may be supplied.

These are used in a "USER", "APOP", "AUTH" [POP-AUTH], or extension

command after making the connection to the POP server. If the URL

doesn't supply an authentication identifier, the program interpreting

the POP URL SHOULD request one from the user.

An authentication mechanism can be eXPressed by adding ";AUTH=<enc-

auth-type>" to the end of the user name. If the authentication

mechanism name is not preceded by a "+", it is a SASL POP [SASL]

mechanism. If it is preceded by a "+", it is either "APOP" or an

extension mechanism.

When an <enc-auth-type> is specified, the client SHOULD request

appropriate credentials from that mechanism and use the "AUTH",

"APOP", or extension command instead of the "USER" command. If no

user name is specified, one SHOULD be oBTained from the mechanism or

requested from the user as appropriate.

The string ";AUTH=*" indicates that the client SHOULD select an

appropriate authentication mechanism. It MAY use any mechanism

supported by the POP server.

If an <enc-auth-type> other than ";AUTH=*" is specified, the client

SHOULD NOT use a different mechanism without explicit user

permission.

If a user name is included with no authentication mechanism, then

";AUTH=*" is assumed.

Since URLs can easily come from untrusted sources, care must be taken

when resolving a URL which requires or requests any sort of

authentication. If authentication credentials are supplied to the

wrong server, it may compromise the security of the user's account.

The program resolving the URL should make sure it meets at least one

of the following criteria in this case:

(1) The URL comes from a trusted source, such as a referral server

which the client has validated and trusts according to site policy.

Note that user entry of the URL may or may not count as a trusted

source, depending on the experience level of the user and site

policy.

(2) Explicit local site policy permits the client to connect to the

server in the URL. For example, if the client knows the site domain

name, site policy may dictate that any hostname ending in that domain

is trusted.

(3) The user confirms that connecting to that domain name with the

specified credentials and/or mechanism is permitted.

(4) A mechanism is used which validates the server before passing

potentially compromising client credentials.

(5) An authentication mechanism is used which will not reveal

information to the server which could be used to compromise future

connections.

A URL containing ";AUTH=*" should be treated with extra care since it

might fall back on a weaker security mechanism. Finally, clients are

discouraged from using a plain text password as a fallback with

";AUTH=*" unless the connection has strong encryption (e.g., a key

length of greater than 56 bits).

Note that if unsafe or reserved characters such as " " or ";" are

present in the user name or authentication mechanism, they MUST be

encoded as described in RFC1738 [BASIC-URL].

5. Relative POP URLs

Relative POP URLs are not permitted.

6. Multinational Considerations

Since 8-bit characters are not permitted in URLs, [UTF8] characters

are encoded as required by the URL specification [BASIC-URL].

7. Examples

The following examples demonstrate how a POP client program might

translate various POP URLs into a series of POP commands. Commands

sent from the client to the server are prefixed with "C:", and

responses sent from the server to the client are prefixed with "S:".

The URL:

<pop://rg@mailsrv.qualcomm.com>

Results in the following client commands:

S: +OK POP3 server ready <1896.697170952@mailsrv.qualcomm.com>

C: USER rg

S: +OK

C: PASS secret

S: +OK rg's mailbox has 2 messages (320 octets)

The URL:

<pop://rg;AUTH=+APOP@mail.eudora.com:8110>

Results in the following client commands:

S: +OK POP3 server ready <1896.697170952@mail.eudora.com>

C: APOP rg c4c9334bac560ecc979e58001b3e22fb

S: +OK mailbox has 1 message (369 octets)

The URL:

<pop://baz;AUTH=SCRAM-MD5@foo.bar>

Results in the following client commands:

S: +OK POP3 server ready <1896.697170952@foo.bar>

C: AUTH SCRAM-MD5 AGNocmlzADx0NG40UGFiOUhCMEFtL1FMWEI3MmVnQGVsZW

Fub3IuaW5ub3NvZnQuY29tPg==

S: + dGVzdHNhbHQBAAAAaW1hcEBlbGVhbm9yLmlubm9zb2Z0LmNvbQBq

aGNOWmxSdVBiemlGcCt2TFYrTkN3

C: AQAAAMg9jU8CeB4KOFk7sUhSQPs=

S: + U0odqYw3B7XIIW0oSz65OQ==

S: +OK mailbox has 1 message (369 octets)

8. ABNF for POP URL scheme

The POP URL scheme is described using [ABNF]:

achar = uchar / "&" / "=" / "~"

; see [BASIC-URL] for "uchar" definition

auth = ";AUTH=" ( "*" / enc-auth-type )

enc-auth-type = enc-sasl / enc-ext

enc-ext = "+" ("APOP" / 1*achar)

;APOP or encoded extension mechanism name

enc-sasl = 1*achar

;encoded version of [SASL] "auth_type"

enc-user = 1*achar

;encoded version of [POP3] mailbox

pop-url = "pop://" server

server = [user-auth "@"] hostport

;See [BASIC-URL] for "hostport" definition

user-auth = enc-user [auth]

9. Security Considerations

Security considerations discussed in the [POP3] specification and the

[BASIC-URL] specification are relevant. Security considerations

related to authenticated URLs are discussed in section 4 of this

document.

Many email clients store the plain text password for later use after

logging into a POP server. Such clients MUST NOT use a stored

password in response to a POP URL without explicit permission from

the user to supply that password to the specified host name.

10. Acknowledgements

This document borrows heavily from Chris Newman's [IMAP-URL]

specification, and has attempted to follow the advice in [URL-

GUIDELINES].

11. References

[ABNF] Crocker, D., and P. Overell, "Augmented BNF for

Syntax Specifications: ABNF", RFC2234, November

1997.

[ACAP] Newman, C., and J. Myers, "ACAP -- Application

Configuration Access Protocol", RFC2244, November

1997.

[BASIC-URL] Berners-Lee, T., Masinter, L., and M. McCahill,

"Uniform Resource Locators (URL)", RFC1738,

December 1994.

[IMAP-URL] Newman, C., "IMAP URL Scheme", RFC2192, September

1997.

[IMAP4] Crispin, M., "Internet Message Access Protocol -

Version 4rev1", RFC2060, December 1996.

[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC2119, March 1997.

[POP-AUTH] Myers, J., "POP3 AUTHentication command", RFC1734,

December 1994.

[POP3] Myers, J., and M. Rose, "Post Office Protocol --

Version 3", STD 53, RFC1939, May 1996.

[SASL] Myers, J., "Simple Authentication and Security Layer

(SASL)", RFC2222, October 1997.

[URL-GUIDELINES] Masinter, Alvestrand, Zigmond, "Guidelines for new

URL Schemes", Work in Progress.

[UTF8] Yergeau, F., "UTF-8, a transformation format of ISO

10646", RFC2279, January 1998.

12. Author's Address

Randall Gellens

QUALCOMM, Incorporated

6455 Lusk Blvd.

San Diego, CA 92121-2779

U.S.A.

Phone: +1 619 651 5115

Fax: +1 619 651 5334

EMail: Randy@Qualcomm.Com

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