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SMTP Service Extension for Authentication
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.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
1. Introduction
This document defines an SMTP service extension [ESMTP] whereby an SMTP client may indicate an authentication mechanism to the server,perform an authentication protocol exchange, and optionally negotiatea security layer for subsequent protocol interactions. This extension is a profile of the Simple Authentication and Security Layer [SASL].
2. Conventions Used in this Document
In examples, "C:" and "S:" indicate lines sent by the client and server respectively. 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. The Authentication service extension
(1) the name of the SMTP service extension is "Authentication"
(2) the EHLO keyword value associated with this extension is "AUTH"
(3) The AUTH EHLO keyword contains as a parameter a space separated list of the names of supported SASL mechanisms.
(4) a new SMTP verb "AUTH" is defined
(5) an optional parameter using the keyword "AUTH" is added to the MAIL FROM command, and extends the maximum line length of the MAIL FROM command by 500 characters.
(6) this extension is appropriate for the submission protocol [SUBMIT].
4. The AUTH command AUTH mechanism [initial-response]
Arguments:
a string identifying a SASL authentication mechanism. an optional base64-encoded response
Restrictions:
After an AUTH command has successfully completed, no more AUTH commands may be issued in the same session. After a successful AUTH command completes, a server MUST reject any further AUTH commands with a 503 reply. The AUTH command is not permitted during a mail transaction.
Discussion:
The AUTH command indicates an authentication mechanism to the server. If the server supports the requested authentication mechanism, it performs an authentication protocol exchange to authenticate and identify the user. Optionally, it also negotiates a security layer for subsequent protocol interactions. If the requested authentication mechanism is not supported, the server rejects the AUTH command with a 504 reply.
The authentication protocol exchange consists of a series of server challenges and client answers that are specific to the authentication mechanism. A server challenge, otherwise known as a ready response, is a 334 reply with the text part containing a BASE64 encoded string. The client answer consists of a line containing a BASE64 encoded string. If the client wishes to cancel an authentication exchange, it issues a line with a single "*". If the server receives such an answer, it MUST reject the AUTH command by sending a 501 reply.
The optional initial-response argument to the AUTH command is used to save a round trip when using authentication mechanisms that are defined to send no data in the initial challenge.
When the initial-response argument is used with such a mechanism, the initial empty challenge is not sent to the client and the server uses the data in the initial-response argument as if it were sent in response to the empty challenge. Unlike a zero-length client answer to a 334 reply, a zero- length initial response is sent as a single equals sign ("="). If the client uses an initial-response argument to the AUTH command with a mechanism that sends data in the initial challenge, the server rejects the AUTH command with a 535 reply.
If the server cannot BASE64 decode the argument, it rejects the AUTH command with a 501 reply. If the server rejects the authentication data, it SHOULD reject the AUTH command with a 535 reply unless a more specific error code, such as one listed in section 6, is appropriate. Should the client successfully complete the authentication exchange, the SMTP server issues a 235 reply.
The service name specified by this protocol's profile of SASL is "smtp".
If a security layer is negotiated through the SASL authentication exchange, it takes effect immediately following the CRLF that concludes the authentication exchange for the client, and the CRLF of the success reply for the server. Upon a security layer's taking effect, the SMTP protocol is reset to the initial state (the state in SMTP after a server issues a 220 service ready greeting). The server MUST discard any knowledge obtained from the client, such as the argument to the EHLO command, which was not obtained from the SASL negotiation itself. The client MUST discard any knowledge obtained from the server, such as the list of SMTP service extensions, which was not obtained from the SASL negotiation itself (with the exception that a client MAY compare the list of advertised SASL mechanisms before and after authentication in order to detect an active down-negotiation attack). The client SHOULD send an EHLO command as the first command after a successful SASL negotiation which results in the enabling of a security layer.
The server is not required to support any particular authentication mechanism, nor are authentication mechanisms required to support any security layers. If an AUTH command fails, the client may try another authentication mechanism by issuing another AUTH command.
If an AUTH command fails, the server MUST behave the same as if the client had not issued the AUTH command.
The BASE64 string may in general be arbitrarily long. Clients and servers MUST be able to support challenges and responses that are as long as are generated by the authentication mechanisms they support, independent of any line length limitations the client or server may have in other parts of its protocol implementation.
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