ietf-nntp HDR replacement text

[Russ Allbery]

9.5.3  The HDR Extension

  This extension provides one new command, HDR.  The label for this
  extension is HDR.

9.5.3.1  HDR

      HDR header
      HDR header range
      HDR header <message-id>

  The HDR command retrieves specific headers from a specified range of
  articles in the currently selected group or an article specified by
  message-id.

  The required header parameter is the name of a header (e.g. "subject")
  in an article and is case-insensitive.  The range parameter of the
  second form of the HDR command may be any of the following:

   * an article number
   * an article number followed by a dash to indicate all following
   * an article number followed by a dash followed by another article
     number.

  The message-id parameter of the third form of the HDR command indicates
  a specific article.  The range and message-id arguments are mutually
  exclusive.  If neither are specified, information from the current
  article is displayed.

  A successful response consists of a 225 response followed by the
  requested list of headers.  This list is returned as a multiline
  response, with each line consisting of the article number, a US-ASCII
  space, and the contents of the header (without the header name or the
  colon and space that follow it).  The contents are modified as follows:
  all US-ASCII CRLF pairs are removed, and then each remaining US-ASCII
  NUL, TAB, CR, or LF character is replaced with a single US-ASCII space.
  (Note that this is the same transformation as is performed by the OVER
  extension.)

  The header content is in all cases taken from the article.  This means
  that, for example, a request for the header "Lines" returns the contents
  of the "Lines" header of the specified articles, not the Lines field
  returned by the OVER extension or any other server-generated value.

  If the header occurs in a given article multiple times, only the value
  of the first occurrence is returned by HDR.

  If the article is specified by message-id rather than by article range,
  the article number is given as "0".  If the requested header is present
  but empty in a given article, a line for that article is included in the
  output but the header content portion of the line is empty.  If the
  requested header is not present in a given article, no line for that
  article is given in the output.  An empty response is therefore valid,
  indicating that the requested header is not present in any of the
  specified articles.

  If the optional argument is a message-id and no such article exists, a
  430 error response shall be returned.  If the optional argument is an
  article number and no article with that number exists in the current
  group, a 423 error response shall be returned.  If HDR is sent without
  any arguments and no article is currently selected, a 420 error response
  shall be returned.

  A server MAY only allow HDR commands for a limited set of headers (such
  as those present in the overview database).  If it does so, it MUST
  respond with a 503 error response to attempts to request other headers
  rather than returning erroneous results such as a successful empty
  response.

9.5.3.1.1  Responses

      225  Headers follow (multiline response)
      412  No newsgroup selected
      420  No article selected
      423  No article with that number
      430  No article with that message ID
      500  Command not recognized
      502  Permission denied
      503  HDR not allowed on that header

9.5.3.1.2  Examples

  Example of a successful retrieval of subject lines from a range of
  articles:

      [S] 200 NNTP Service Ready
      [C] GROUP misc.test
      [S] 211 1234 3000234 3002322 misc.test
      [C] HDR Subject 3000234-300238
      [S] 225 Headers follow
      [S] 3000234 I am just a test article
      [S] 3000237 Re: I am just a test article
      [S] 3000238 Ditto
      [S] .

  Example of a successful retrieval of the subject line from an article by
  message-id:

      [S] 200 NNTP Service Ready
      [C] GROUP misc.test
      [S] 211 1234 3000234 3002322 misc.test
      [C] HDR subject <i.am.a.test.article at example.com>
      [S] 225 Header information follows
      [S] 0 I am just a test article
      [S] .

  Example of a successful retrieval of the subject line from the current
  article:

      [S] 200 NNTP Service Ready
      [C] GROUP misc.test
      [S] 211 1234 3000234 3002322 misc.test
      [C] HDR subject
      [S] 225 Header information follows
      [S] 3000234 I am just a test article
      [S] .

  Example of an unsuccessful retrieval of a header from an article by
  message-id:

      [S] 200 NNTP Service Ready
      [C] HDR subject <i.am.not.there at example.com>
      [S] 430 No Such Article Found

  Example of an unsuccessful retrieval of headers from articles by number
  because no newsgroup was selected first:

      [S] 200 NNTP Service Ready
      [C] HDR subject 300256-
      [S] 412 No newsgroup selected

  Example of retrieving header information when the current group selected
  is empty:

      [S] 200 NNTP Service Ready
      [C] GROUP example.empty.newsgroup
      [S] 211 0 0 0 example.empty.newsgroup
      [C] HDR subject 1-
      [S] 225 Headers follow
      [S] .

  Example of an unsuccessful retrieval of headers because the server does
  not allow HDR commands for that header:

      [S] 200 NNTP Service Ready
      [C] GROUP misc.test
      [S] 211 1234 3000234 3002322 misc.test
      [C] HDR Content-Type 3000234-300238
      [S] 503 HDR not permitted on "Content-Type"

  Example of a failure due to restrictions configured into the server:

      [S] 200 NNTP Service Ready
      [C] GROUP news.group
      [S] 211 1234 3000234 3002322 misc.test
      [C] HDR Subject 3000234-300238
      [S] 502 Service Unavailable

-- 
Russ Allbery (rra at stanford.edu)             <http://www.eyrie.org/~eagle/>