Re: [ietf-smtp] Stray <LF> in the middle of messages
John C Klensin <john-ietf@jck.com> Sat, 06 June 2020 18:16 UTC
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Date: Sat, 06 Jun 2020 14:15:49 -0400
From: John C Klensin <john-ietf@jck.com>
To: dcrocker@bbiw.net, Leo Gaspard <ietf=40leo.gaspard.io@dmarc.ietf.org>
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Subject: Re: [ietf-smtp] Stray <LF> in the middle of messages
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Actually, the comment in his last paragraph notwithstanding and other histories of disagreement, I completely agree with Dave. I would add one additional cautionary note: we now have several security-related tools, in difference degrees of active use, that digitally sign message bodies, headers, or both. If something sees a bare LF and converts it after those signatures are computed. testing them will typically fail. That is just one more reason for observing Dave's (and Postel's) advice: be really careful that what you send (or allow to be sent) out conforms to a narrow reading of the standard. How much deviation you want to accept in something you receive depends on your circumstances. john --On Saturday, June 6, 2020 10:22 -0700 Dave Crocker <dhc@dcrocker.net> wrote: > On 6/6/2020 10:06 AM, Leo Gaspard wrote: >>> In addition, the appearance of "bare" "CR" or "LF" >>> characters in text (i.e., either without the other) has a >>> long history of causing problems in mail implementations and >>> applications that use the mail system as a tool. SMTP >>> client implementations MUST NOT transmit these characters >>> except when they are intended as line terminators and then >>> MUST, as indicated above, transmit them only as a <CRLF> >>> sequence. >> Should I understand this paragraph as meaning that if I ever >> receive such an ill-formed message, I… can? should? must? >> accept it and… can? should? must? convert the <LF> into >> proper <CRLF>? > > > > Encountering practice that differs from theory -- or, in this > case, specification -- can be an excellent teachable moment. > > I think your opportunity, here, is to appreciate the > pragmatics of Postel's Law. It is often taken to mean that > one can send whatever one wants, but Jon was never that > sloppy. You should try to generate only according to the > strictest interpretation of the spec. > > But there are realities of different interpretations and some > tolerable sloppiness that apply to this case, given a long > history of systems that do use LF as line terminator and > network software that fails to map it to the network standard > CRLF. > > I believe systems still vary on how they respond to an > incoming LF. I even suspect there are arguments for treating > it only as data and arguments for treating it as newline. I > suspect, therefore, the answer to your question will be how > you assess the benefits and detriments of each choice. > > This being an IETF list, we should expect strong, differing > comments from others. > > d/
- [ietf-smtp] Stray <LF> in the middle of messages Leo Gaspard
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Dave Crocker
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Claus Assmann
- Re: [ietf-smtp] Stray <LF> in the middle of messa… John C Klensin
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Valdis Kl ē tnieks
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Paul Smith
- Re: [ietf-smtp] Stray <LF> in the middle of messa… John C Klensin
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Valdis Kl ē tnieks
- Re: [ietf-smtp] Stray <LF> in the middle of messa… John C Klensin
- Re: [ietf-smtp] Stray <LF> in the middle of messa… John Levine
- Re: [ietf-smtp] Stray <LF> in the middle of messa… John C Klensin
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Hector Santos
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Alessandro Vesely
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Keith Moore
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Valdis Kl ē tnieks
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Paul Smith
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Dave Crocker
- Re: [ietf-smtp] Stray <LF> in the middle of messa… John C Klensin
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Dave Crocker
- Re: [ietf-smtp] Stray <LF> in the middle of messa… John Levine
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Ned Freed
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Hector Santos
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Hector Santos
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Valdis Kl ē tnieks
- Re: [ietf-smtp] Stray <LF> in the middle of messa… John C Klensin
- Re: [ietf-smtp] Stray <LF> in the middle of messa… Viktor Dukhovni