usefor-article-03 February 2000

5.6.2.  Adding a path-identity to the Path header
   When an injecting, relaying or serving agent receives an article, it
   MUST prepend its own path-identity followed by a delimiter to the
   beginning of the Path-content. In addition, it SHOULD then add CRLF
   and WSP if it would otherwise result in a line longer than 79
   characters.

   The path-identity added MUST be unique to that agent. To this end it
   SHOULD be one of:

   1. A fully qualified domain name (FQDN) associated (by the Internet
      DNS service [RFC 1034]) with an A record, which SHOULD identify
      the actual machine prepending this path-identity. Ideally, this
      FQDN should also be "mailable" in the sense that it enables the
      construction of a valid E-mail address of the form "usenet@<FQDN>"
      or "news@<FQDN>" [RFC 2142] whereby the administrators of that
      agent may be reached.

   2. A fully qualified domain name (FQDN) associated (by the Internet
      DNS service) with an MX record which MUST then enable the
      construction of a valid E-mail address of the form "usenet@<FQDN>"
      or "news@<FQDN>" whereby the administrators of that agent may be
      reached.

   3. A name registered previously in the UUCP maps database (found in
      the newsgroup comp.mail.maps), containing no '.' character.

   4. An encoding of an IP address - <dotted-quad> [RFC 820] or <ipv6-
      numeric> [RFC 2373] (the requirement to be able to use an <ipv6-
      numeric> is the reason for including ':' as an allowed character
      within a path-identity).

   5. A '.' followed by an arbitrary name not in the UUCP maps database,
      but believed to be unique and registered at least with all sites
      immediately downstream from the given site.

   Of the above options, nos. 1 to 3 are much to be preferred, unless
   there are strong technical reasons dictating otherwise. In
   particular, the injecting agent's path-identity MUST, as a special
   case, be an FQDN mailable address in the sense defined under option
   1, or with an associated MX record as in option 2.

   The injecting agent's path-identity MUST be followed by the special
   delimiter '%' which serves to separate the pre-injection and post-
   injection regions of the Path-content (see 5.6.3).

   In the case of a relaying or serving agent, the delimiter is chosen
   as follows.  When such an agent receives an article, it MUST
   establish the identity of the source and compare it with the leftmost
   path-identity of the Path-content. If it matches, a '/' should be
   used as the delimiter when prepending the agent's own path-identity.
   If it does not match then the agent should prepend two entries to the
   Path-content; firstly the true established path-identity of the
   source followed by a '?'  delimiter, and then, to the left of that,
   the agent's own path-identity followed by a '/' delimiter as usual.
   This prepending of two entries SHOULD NOT be done if the provided and
   established identities match.

   Any method of establishing the identity of the source may be used
   (but see 5.6.5 below), with the consideration that, in the event of
   problems, the agent concerned may be called upon to justify it.

        NOTE: The use of the '%' delimiter marks the position of the
        injecting agent in the chain. In normal circumstances there
        should therefore be only one `%` delimiter present, and
        injecting agents MAY choose to reject proto-articles with a '%'
        already in them. If, for whatever reason, more than one '%' is
        found, then the path-identity in front of the leftmost '%' is to
        be regarded as the true injecting agent.