SMTP Authentication
SMTP Authentication, often abbreviated SMTP AUTH, is an extension of the
History
SMTP as specified by
John Gardiner Myers published the first draft of SMTP AUTH in 1995,
The Internet Mail Consortium (IMC) reported that 55% of mail servers were open relays in 1998,[5] but less than 1% in 2002.[6]
Role in the mail transport system
Using a
Return-Path
) used for SPF and the From address agree with the authenticated user-id is particularly important for domains that sign messages using DKIMKeywords ending in "A" such as ESMTPA
and ESMTPSA
, are provided for the with
clause of Received
header fields, when messages are received with SMTP AUTH.
Details
As with all SMTP extensions, SMTP AUTH is advertised in the EHLO response, along with a list of supported authentication methods. These methods may change after issuing STARTTLS, typically allowing plain text passwords in the latter case only. RFC 4954 provides the following example ("C:" and "S:" are not part of the protocol, they indicate lines sent by the client and server, respectively):
S: 220 smtp.example.com ESMTP Server C: EHLO client.example.com S: 250-smtp.example.com Hello client.example.com S: 250-AUTH GSSAPI DIGEST-MD5 S: 250-ENHANCEDSTATUSCODES S: 250 STARTTLS C: STARTTLS S: 220 Ready to start TLS ... TLS negotiation proceeds. Further commands protected by TLS layer ... C: EHLO client.example.com S: 250-smtp.example.com Hello client.example.com S: 250 AUTH GSSAPI DIGEST-MD5 PLAIN C: AUTH PLAIN dGVzdAB0ZXN0ADEyMzQ= S: 235 2.7.0 Authentication successful
SMTP AUTH can be used also on port 25. Usually, servers reject RCPT TO commands that imply relaying unless authentication credentials have been accepted. The specification recommends that servers issue 530 5.7.0 Authentication required in response to most commands in case the server is configured to require authentication and the client hasn't done it yet. Only servers listening on port 587, or private servers, should be configured that way, not a Message eXchange (MX). However, the historical trait that SMTP is not authenticated by default results in a different behavior with regard to access protocols, in some cases; for example, when using AUTH EXTERNAL after STARTTLS.[9]
Besides the AUTH command, the extension also provides for an AUTH parameter to the MAIL FROM command, so as to allow to distinguish authentication from authorization. That way, a sender can identify itself and transmit several messages during the same session. While the authentication doesn't need to vary, once established, different messages may be sent according to different agreements and hence require different authorization. For example, messages may be relayed on behalf of different users. Use of this parameter is much less popular than using the command to grant relay privileges.
SMTP Authentication is an "extension" in SMTP terms, so it requires server and client to use EHLO verb for greeting to indicate support for extensions, as opposed to the obsolete HELO greeting.[10] For backward compatibility, HELO greeting may be accepted when no extension is used.
The capitalized text after the AUTH command is a list of the types of authorization that the SMTP server will accept.
Some examples of authorization protocols include:
- PLAIN (Uses Base64 encoding)
- LOGIN (Uses Base64 encoding)[11] (obsoleted in favor of PLAIN)
- GSSAPI (Generic Security Services Application Program Interface)
- DIGEST-MD5 (Digest access authentication)
- MD5
- CRAM-MD5
- OAUTH10A (OAuth 1.0a HMAC-SHA1 tokens as defined in RFC 5849)
- OAUTHBEARER (OAuth 2.0 bearer tokens as defined in RFC 6750)
- XOAUTH2 [12]
Standards
- RFC 3207, SMTP Service Extension for Secure SMTP over Transport Layer Security, Paul Hoffman, February 2002.
- RFC 3848, ESMTP and LMTP Transmission Types Registration, Chris Newman, July 2004.
- RFC 6409, Message Submission for Mail, Randall Gellens and John C. Klensin, November 2011 (obsoletes RFC 4409, from 2006, which in turn replaced RFC 2476, from December 1998).
- RFC 4422, Simple Authentication and Security Layer (SASL), Alexey Melnikov and Kurt D. Zeilenga, June 2006.
- RFC 4616, The PLAIN SASL Mechanism, K. Zeilenga, Ed., August 2006.
- RFC 4954, SMTP Service Extension for Authentication, Robert Siemborski and Alexey Melnikov, July 2007.
- RFC 7628, A Set of Simple Authentication and Security Layer (SASL) Mechanisms for OAuth, W. Mills, T. Showalter and H. Tschofenig, August 2015.
Other
- Erwin Hoffmann, SMTP Authentication [Tutorial], last edit 2017-01-10.
See also
- E-mail authentication
- Simple Mail Transfer Protocol
- Mail submission agent
- Email client port numbers
- Simple Authentication and Security Layer
- Open mail relay
- POP before SMTP
References
- ^ The relevant RFCs for reference are specified in the #Standards section
- ^ The Trustees of Indiana University (2008-04-01). "In Unix, what is an open mail relay?". University Information Technology Services. Indiana University. Archived from the original on 2007-06-17. Retrieved 2008-04-07.
- IETF. Retrieved 2010-05-30.
- .
- ^ Paul Hoffman (February 1, 1998). "Allowing Relaying in SMTP: A Survey". Internet Mail Consortium. Archived from the original on 2016-03-05. Retrieved 2010-05-30.
- ^ Paul Hoffman (August 2002). "Allowing Relaying in SMTP: A Series of Surveys". Internet Mail Consortium. Archived from the original on 2007-01-18. Retrieved 2010-05-30.
- IETF. Retrieved 2019-07-05.
- ^ "Mail parameters". IANA registry. Retrieved 2011-07-23.
- IETF. Retrieved 2010-05-30.
- .
However, for compatibility with older conforming implementations, SMTP clients and servers MUST support the original HELO mechanisms as a fallback.
- ^ K. Murchison and M. Crispin, The LOGIN SASL Mechanism, 28 August 2003, expired draft. LOGIN is described as obsolete in the SASL Mechanisms document but the mechanism is still in use.
- ^ Gmail's XOAuth2 SASL protocol