RFC 4468
Message Submission BURL Extension
Message Submission BURL Extension ✨
okay bestie, the title block first 📜 this is RFC 4468, published through the Network Working Group and written by C. Newman of Sun Microsystems in May 2006. It's on the Standards Track, so it's an actual official protocol spec, not just a vibe. It updates RFC 3463 🦋
Status of This Memo 🎀
This document specifies an Internet standards track protocol for the Internet community, and it asks for discussion and suggestions for improvements. For the standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited 💌
Copyright Notice 🌸
Copyright (C) The Internet Society (2006).
Abstract 🦋
Okay so the submission profile of Simple Mail Transfer Protocol (SMTP) already gives email clients a standard way to submit a complete message for delivery 📬 This spec levels that up by adding a new BURL command, which can fetch submission data from an Internet Message Access Protocol (IMAP) server. The result is that a mail client can push content from an IMAP server into the SMTP infrastructure without downloading it to the client and uploading it back to the server. It's giving efficiency 💅
Table of Contents 📖
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................2
3. BURL Submission Extension .......................................3
3.1. SMTP Submission Extension Registration .....................3
3.2. BURL Transaction ...........................................3
3.3. The BURL IMAP Options ......................................4
3.4. Examples ...................................................5
3.5. Formal Syntax ..............................................6
4. 8-Bit and Binary ................................................7
5. Updates to RFC 3463 .............................................7
6. Response Codes ..................................................7
7. IANA Considerations .............................................9
8. Security Considerations .........................................9
9. References .....................................................11
9.1. Normative References ......................................11
9.2. Informative References ....................................12
Appendix A. Acknowledgements .....................................131. Introduction 🧚
This spec extends the standard Message Submission [RFC4409] protocol so data can be fetched from an IMAP server at message submission time 💖 This MAY be used together with the CHUNKING [RFC3030] mechanism, so chunks of the message can come from an external IMAP server. The payoff is that you can forward an email message without downloading it to the client first. Main character energy 👑
2. Conventions Used in This Document 📚
Whenever the key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" show up in this document, they're meant to be interpreted as defined in "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119]. Read them carefully, bestie 🫶
The formal syntax uses the Augmented Backus-Naur Form (ABNF) [RFC4234] notation, including the core rules defined in Appendix B of RFC 4234 ✨
3. BURL Submission Extension 💌
This section defines the BURL submission extension 🎀
3.1. SMTP Submission Extension Registration 🌈
- The name of this submission extension is "BURL". This extends the Message Submission protocol on port 587, and it MUST NOT be advertised by a regular SMTP [RFC2821] server on port 25 that acts as a relay for incoming mail from other SMTP relays. Not on the relay, no cap 🙅♀️
- The EHLO keyword value associated with the extension is
BURL. - The
BURLEHLO keyword will have zero or more arguments. The only argument defined at this time is the"imap"argument, which MUST be present in order to use IMAP URLs with BURL. Clients MUST ignore other arguments after theBURLEHLO keyword unless they are defined by a subsequent IETF standards track specification. The arguments that appear after theBURLEHLO keyword may change after SMTP AUTH [RFC2554] is used, so a server that advertisesBURLwith no arguments prior to authentication is saying that BURL is supported but authentication is required to use it 🔐 - This extension adds the
BURLSMTP verb. The verb is used as a replacement for the DATA command, and it's only permitted during a mail transaction after at least one successful RCPT TO 💅
3.2. BURL Transaction 🦋
A simple BURL transaction consists of MAIL FROM, one or more RCPT TO headers, and a BURL command with the "LAST" tag. The BURL command includes an IMAP URL pointing to a fully formed message that's ready for injection into the SMTP infrastructure. If PIPELINING [RFC2920] is advertised, the client MAY send the entire transaction in one round trip. If no valid RCPT TO address is supplied, the BURL command simply fails, and no resolution of the BURL URL argument is performed. If at least one valid RCPT TO address is supplied, the BURL URL argument gets resolved before the server responds to the command 🍓
A more sophisticated BURL transaction MAY occur when the server also advertises CHUNKING [RFC3030]. In this case, the BURL and BDAT commands may be interleaved until one of them terminates the transaction with the "LAST" argument. If PIPELINING [RFC2920] is also advertised, the client may pipeline the entire transaction in one round trip. However, it MUST wait for the results of the "LAST" BDAT or BURL command before starting a new transaction. Patience is a virtue 🧚
The BURL command directs the server to fetch the data object that the URL refers to and include it in the message. If the URL fetch fails, the server fails the entire transaction 💀
3.3. The BURL IMAP Options 🌸
When "imap" is present in the space-separated list of arguments following the BURL EHLO keyword, it means the BURL command supports the URLAUTH [RFC4467] extended form of IMAP URLs [RFC2192], and the submit server is configured with the necessary credentials to resolve "urlauth=submit+" IMAP URLs for the submit server's domain 🔑
Subsequent to a successful SMTP AUTH command, the submission server MAY indicate a prearranged trust relationship with a specific IMAP server by including a BURL EHLO keyword argument of the form "imap://imap.example.com". In this case, the submission server permits a regular IMAP URL that refers to messages or parts of messages on imap.example.com, as long as the user who authenticated to the submit server can access them. Note that this form does not imply that the submit server supports URLAUTH URLs. The submit server must advertise both "imap" and "imap://imap.example.com" to indicate support for both extended and non-extended URL forms ⭐
When the submit server connects to the IMAP server, it acts as an IMAP client, so it's subject to both the mandatory-to-implement IMAP capabilities in Section 6.1.1 of RFC 3501 and the security considerations in Section 11 of RFC 3501. Specifically, this requires that the submit server implement a configuration that uses STARTTLS followed by SASL PLAIN [SASL-PLAIN] to authenticate to the IMAP server. Security is not a vibe you skip 🔥
When the submit server resolves a URLAUTH IMAP URL, it uses submit server credentials when authenticating to the IMAP server. The authentication identity and password used for submit credentials MUST be configurable. The string "submit" is suggested as a default value for the authentication identity, with no default for the password. Typically the authorization identity is empty in this case, so the IMAP server derives the authorization identity from the authentication identity. If the IMAP URL uses the "submit+" access identifier prefix, the submit server MUST refuse the BURL command unless the userid in the URL's <access> token matches the submit client's authorization identity 🙈
When the submit server resolves a regular IMAP URL, it uses the submit client's authorization identity when authenticating to the IMAP server. If both the submit client and the submit server's embedded IMAP client use SASL PLAIN (or the equivalent), the submit server SHOULD forward the client's credentials if and only if the submit server knows that the IMAP server is in the same administrative domain. If the submit server supports SASL mechanisms other than PLAIN, it MUST implement a configuration in which the submit server's embedded IMAP client uses STARTTLS and SASL PLAIN with the submit server's authentication identity and password (for the respective IMAP server) and the submit client's authorization identity. Okay that's a lot, but it's all iconic 👑
3.5. Formal Syntax 🧩
Okay so here's the grammar, bestie 🎀 It builds on ABNF [RFC4234] and Uniform Resource Identifiers [RFC3986], and it's giving precision ✨
burl-param = "imap" / ("imap://" authority)
; parameter to BURL EHLO keyword
burl-cmd = "BURL" SP absolute-URI [SP end-marker] CRLF
end-marker = "LAST"4. 8-Bit and Binary ✨
Okay, the big rule here: if your submit server is flexing BURL, it MUST also advertise 8BITMIME [RFC1652] 💅 It also has to do the down conversion from that spec on the complete message when 8-bit data shows up with the BURL command and gets passed to a 7-bit server. If the URL argument to BURL points at binary data, the submit server MAY refuse the command or down convert it the way Binary SMTP [RFC3030] describes 🧠
The Submit server MAY refuse to accept a BURL command, or a combo of BURL and BDAT commands, that ends up with un-encoded 8-bit data in mail or MIME [RFC2045] headers. Or, if it's feeling generous, the server MAY accept that data and down convert it to MIME header encoding [RFC2047] 🫶
5. Updates to RFC 3463 💌
SMTP or Submit servers that advertise ENHANCEDSTATUSCODES [RFC2034] use the enhanced status codes from RFC 3463 [RFC3463] 📮 BURL introduces new error cases that RFC didn't think about, so this spec adds a few more enhanced status codes:
X.6.6 Message content not available
The message content couldn't be fetched from a remote system. This might be useful as a permanent or persistent temporary notification 🌸
X.7.8 Trust relationship required
The submission server needs a configured trust relationship with a third-party server before it can access the message content 🔑
6. Response Codes 🚦
This section has example response codes for the BURL command. You can use other text with the same codes. This list isn't exhaustive, and BURL clients MUST tolerate any valid SMTP response code 🫠 Most of these examples come with the right enhanced status code [RFC3463].
554 5.5.0 No recipients have been specified
Shows up when BURL is used (like with PIPELINING) and every RCPT TO failed 💀
503 5.5.0 Valid RCPT TO required before BURL
This one's the alternative to the one above. It shows up when BURL is used (like with PIPELINING) and every RCPT TO failed 🙈
554 5.6.3 Conversion required but not supported
This happens when the URL points to binary data and the implementation can't down convert to base64. It can also happen if the URL points to message data with 8-bit content in headers and the server doesn't down convert that content ✨
554 5.3.4 Message too big for system
The message (after URL resolution) is bigger than this server's per-message size limit 📦
554 5.7.8 URL resolution requires trust relationship
The submit server doesn't have a trust relationship with the IMAP server named in the URL argument to BURL 🔐
552 5.2.2 Mailbox full
The recipient is local, the submit server supports direct delivery, and the recipient has gone over their quota, grace period for delivery attempts included 📬
554 5.6.6 IMAP URL resolution failed
The IMAP URLFETCH command returned an error or no data 😭
250 2.5.0 Waiting for additional BURL or BDAT commands
A BURL command without the "LAST" modifier got sent. The URL for this BURL command was resolved successfully, but the content won't necessarily be committed to persistent storage until the rest of the message content comes in. For example, a Unix server might have written the content into a queue file buffer without having run fsync() yet.
If the server loses power in that window, the content can still be lost ⚡
451 4.4.1 IMAP server unavailable
The connection to the IMAP server for resolving the URL failed 🥀
250 2.5.0 Ok.
The URL was resolved successfully, and the complete message data has been committed to persistent storage 🎉
250 2.6.4 MIME header conversion with loss performed
The URL pointed to message data with 8-bit data in mail or MIME headers. That data got converted to MIME header encoding [RFC2047], but the submit server might not have guessed the unlabeled character set correctly 🤭
7. IANA Considerations 🗂️
The "BURL" SMTP extension, as described in Section 3, has been registered. That registration is marked for use by message submission [RFC4409] only in the registry 💖
8. Security Considerations 🚨
Okay this section is SERIOUS bestie 😭 Modern SMTP submission servers often have content-based security and denial-of-service defenses like virus filtering, size limits, server-generated signatures, spam filtering, etc. Implementations of BURL should fetch the URL content before those content-based mechanisms get applied, so they can keep doing their job 🛡️
Clients that send unsolicited bulk email or email with viruses could use this mechanism to make up for a slow link between the client and submit server. Like, this would make it totally doable for a programmable cell phone or other device on a slow link to become a major source of unsolicited bulk email and/or viruses 📱💀 So it matters even more that submit server vendors implementing BURL have auditing and/or defenses against these denial-of-service attacks. That includes mandatory authentication, logging that ties unique client identifiers to mail transactions, limits on reuse of the same IMAP URL, rate limits, recipient count limits, and content filters 🔥
Sending the URLAUTH [RFC4467] form of IMAP URLs in the clear can expose the authorization token to network eavesdroppers 👀 Implementations that support these URLs can fix this by using a strong confidentiality protection mechanism. For example, the SMTP STARTTLS [RFC3207] and IMAP STARTTLS [RFC3501] extensions, combined with a configuration setting that requires their use with these IMAP URLs, would address the concern 🔒
Using a prearranged trust relationship between a submit server and a specific IMAP server brings its own security considerations. A compromise of the submit server should not automatically compromise every account on the IMAP server, so trust relationships involving super-user proxy credentials are strongly discouraged 🙅♀️ A system that makes the submit server authenticate to the IMAP server with submit credentials, and then requires a URLAUTH URL to fetch any content, addresses this concern. A trusted third party model for proxy credentials, like the one Kerberos 5 [RFC4120] provides, would also do the job 🧚
When a client uses SMTP STARTTLS to send a BURL command that references non-public information, users expect the entire message content to be treated confidentially 🫣 To meet that expectation, the message submission server SHOULD use STARTTLS or a mechanism providing equivalent data confidentiality when it fetches the content referenced by that URL 💜
A legit user of a submit server might try to compromise other accounts on the server by giving an IMAP URLAUTH URL that points to a server under their control, one designed to undermine the security of the submit server 😈 So the IMAP client code the submit server uses must be robust with respect to arbitrary input sizes (including large IMAP literals) and arbitrary delays from the IMAP server. Requiring a prearranged trust relationship between a submit server and the IMAP server also handles this concern 🏰
An authorized user of the submit server could set up a fraudulent IMAP server and pass its URL to the submit server. The submit server might then contact that fraudulent IMAP server to authenticate with submit credentials and fetch content. There are several ways to reduce this risk. A submit server that only uses submit credentials with a fixed set of trusted IMAP servers won't expose those credentials. A submit server can also treat the IMAP server as untrusted and include defenses for buffer overflows, denial-of-service slowdowns, and other potential attacks. Finally, because authentication is required to use BURL, it's possible to keep a secure audit trail and use it to detect and punish the offending party 👑
9. References 📚
9.1. Normative References
[RFC1652] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
Crocker, "SMTP Service Extension for
8bit-MIMEtransport", RFC 1652, July 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2192] Newman, C., "IMAP URL Scheme", RFC 2192,
September 1997.
[RFC2554] Myers, J., "SMTP Service Extension for Authentication",
RFC 2554, March 1999.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP
over Transport Layer Security", RFC 3207,
February 2002.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
VERSION 4rev1", RFC 3501, March 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter,
"Uniform Resource Identifier (URI): Generic Syntax",
STD 66, RFC 3986, January 2005.
[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[RFC4409] Gellens, R. and J. Klensin, "Message Submission for
Mail", RFC 4409, April 2006.
[RFC4467] Crispin, M., "Internet Message Access Protocol (IMAP) -
URLAUTH Extension", RFC 4467, May 2006.9.2. Informative References
[RFC2034] Freed, N., "SMTP Service Extension for Returning
Enhanced Error Codes", RFC 2034, October 1996.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail
Extensions) Part Three: Message Header Extensions for
Non-ASCII Text", RFC 2047, November 1996.
[RFC2920] Freed, N., "SMTP Service Extension for Command
Pipelining", STD 60, RFC 2920, September 2000.
[RFC3030] Vaudreuil, G., "SMTP Service Extensions for
Transmission of Large and Binary MIME Messages",
RFC 3030, December 2000.
[RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC 3463, January 2003.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC
4120, July 2005.
[SASL-PLAIN] Zeilenga, K., "The Plain SASL Mechanism", Work in
Progress, March 2005.Appendix A. Acknowledgements ✨
okay bestie, big thank-you train 🚂💖 this whole doc came out of the lemonade WG, and every single person in that group chat gets love for their input 🫶. Mark Crispin was the one who basically dreamed up this mechanism in the first place, so he's a whole legend 👑. Huge shoutout to Randall Gellens, Alexey Melnikov, Sam Hartman, Ned Freed, Dave Cridland, Peter Coates, and Mark Crispin again for reviewing the document 🔍. The RFC Editor also gets a thank-you for fixing the author's grammar mistakes 💅😭. Then there's Ted Hardie, Randall Gellens, Mark Crispin, Pete Resnick, and Greg Vaudreuil, who had extremely intense debates comparing this proposal to the alternatives 🔥🧠. And finally the lemonade WG chairs, Eric Burger and Glenn Parsons, who knew when to end the debate and kept the doc moving until it was finished 🍋🎀
Author's Address
This is where to find the author:
Chris Newman
Sun Microsystems
3401 Centrelake Dr., Suite 410
Ontario, CA 91761
US
EMail: chris.newman@sun.comFull Copyright Statement
Copyright (C) The Internet Society (2006). 🌸
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📦 where this page comes from
- ietf: rfc4468 2006-05
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms