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Re: Multiple Mail domains for reverse ptr records? I'm confused

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  • Greg A. Woods
    At Fri, 27 Nov 2009 13:18:15 -0500 (EST), wietse@porcupine.org (Wietse Venema) wrote: Subject: Re: Multiple Mail domains for reverse ptr records? I m confused
    Message 1 of 16 , Nov 27, 2009
      At Fri, 27 Nov 2009 13:18:15 -0500 (EST), wietse@... (Wietse Venema) wrote:
      Subject: Re: Multiple Mail domains for reverse ptr records? I'm confused
      >
      > Wietse Venema:
      > > Greg A. Woods:
      > > > There _should_ be one PTR for every _valid_ hostname using a given IP
      > > > address.
      > >
      > > Statements such as above remind me of silly knights fighting windmills.
      > >
      > > There is a difference between "right" and "useful", and it even
      > > depends on where they are used - server or client side.

      Indeed -- I do not disagree. However I tried to emphasise the important
      words above in what you quoted.

      Also, don't forget the human use of the DNS either.

      I.e. it is _always_ useful from _some_ perspective to have a valid PTR
      for every valid hostname which may be considered to be legitimately
      pointing to a given address.

      Other hostnames which point to an address but which are not pointed to
      by a corresponding PTR _may_ be useful in some context, however neither
      man nor machine will be able to identify their validity from afar. Only
      the legitimate "owner" of the IP address will be able to claim anything
      about the validity of the hostnames which may be pointing to that
      address, and by definition without using PTRs that cannot be done in the
      context of the DNS.


      > > Multiple server A records are useful. More in the case of HTTP,
      > > less in the case of SMTP which uses indirection via MX records.
      > > (For a discussion about domain-in-a-box applications, see some
      > > actual measurements that I did earlier this year).
      > >
      > > One PTR per A record is not useful. The server end will not know
      > > what client name to use.
      >
      > That is, one PTR per A record, in the case of multiple A records
      > for the same IP address.

      Well it all depends on how the client/server protocol interaction works,
      doesn't it?

      As you suggest, for SMTP the client tells the server which hostname it
      should care about, so the server need not wonder which client name is
      being used.


      > > On the other hand, it is right when every PTR record has a matching
      > > A record that resolves to (among others) the PTR record's address.

      Indeed. Orthogonality goes both ways! :-)

      --
      Greg A. Woods

      +1 416 218-0098 VE3TCP RoboHack <woods@...>
      Planix, Inc. <woods@...> Secrets of the Weird <woods@...>
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