X-FOOBAR: style headers (What are they?)

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2 total messages Started by Paul Lutt Tue, 21 Jul 1992 16:06

Author: Paul Lutt
Date: Tue, 21 Jul 1992 16:06

17 lines
898 bytes

While not a newcomer to email or netnews, I'm a bit confused about the
origin and use of email headers of the form:

    X-FOOBAR:

Where FOOBAR may be some commonly used header field, such as
FROM.

I've looked through the RFCs that we have locally and can find no
mention of this practice.  Could some kind soul please enlighten me?
Thanks.
--
	Paul Lutt
Domain:	pwl@tc.fluke.COM
 Voice:	+1 206 356 5059
  UUCP:	uunet!fluke!pwl
 Snail:	John Fluke Mfg. Co. / P.O. Box 9090 / Everett, WA 98206-9090

Re: X-FOOBAR: style headers (What are they?)

Author: Christopher Davis
Date: Tue, 21 Jul 1992 21:09

46 lines
2604 bytes

Paul> == Paul Lutt <pwl@tc.fluke.COM> 

 Paul> While not a newcomer to email or netnews, I'm a bit confused about the
 Paul> origin and use of email headers of the form:

 Paul>     X-FOOBAR:

 Paul> Where FOOBAR may be some commonly used header field, such as
 Paul> FROM.

 Paul> I've looked through the RFCs that we have locally and can find no
 Paul> mention of this practice.  Could some kind soul please enlighten me?
 Paul> Thanks.

RFC822:
     4.7.4.  EXTENSION-FIELD

             A limited number of common fields have  been  defined  in
        this  document.   As  network mail requirements dictate, addi-
        tional fields may be standardized.   To  provide  user-defined
        fields  with  a  measure  of  safety,  in name selection, such
        extension-fields will never have names  that  begin  with  the
        string "X-".

     4.7.5.  USER-DEFINED-FIELD

             Individual users of network mail are free to  define  and
        use  additional  header  fields.   Such fields must have names
        which are not already used in the current specification or  in
        any definitions of extension-fields, and the overall syntax of
        these user-defined-fields must conform to this specification's
        rules   for   delimiting  and  folding  fields.   Due  to  the
        extension-field  publishing  process,  the  name  of  a  user-
        defined-field may be pre-empted

        Note:  The prefatory string "X-" will never  be  used  in  the
               names  of Extension-fields.  This provides user-defined
               fields with a protected set of names.

Basically, that means a simple and guaranteed method of "commenting out"
a header is to add "X-" to the beginning.
--
Christopher Davis * ckd@eff.org * System Administrator, EFF * +1 617 864 0665
   ``The First Amendment is often inconvenient.  But that is besides the
  point.  Inconvenience does not absolve the government of its obligation
         to tolerate speech.'' --Justice Anthony Kennedy, in 91-155

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