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[solved] Re: postmulti behind NAT

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  • /dev/rob0
    ... You re right, probably none. People who do hairpin NAT wrong (not restricting by interface) can create open relays when the router IP address is in
    Message 1 of 8 , Jul 23, 2013
      On Tue, Jul 23, 2013 at 07:54:38AM +0200, Ulrich Zehl wrote:
      > On Sat, Jul 20, 2013 at 03:45:35PM -0500, /dev/rob0 wrote:
      > > They don't have "hairpin NAT" set up, whereby if I try to
      > > connect to this NATed IP address it would go to the router
      > > and come back to me. I'm fine with that, actually; while
      > > that would solve the instant problem, it could be bad in
      > > other ways.
      >
      > May I ask what badness you expect from that?

      You're right, probably none. People who do "hairpin NAT" wrong (not
      restricting by interface) can create open relays when the router IP
      address is in $mynetworks. Even if not an open relay, it destroys
      your strongest antispam tools by obscuring the client IP address.

      In this case the NAT should only be done for packets from the MSA
      instance to the MX instance, so the usual problem of the wrong IP
      address being logged would not matter: the router IP *is* the MSA.

      > My first thought was to suggest setting up the NAT rules yourself
      > (using iptables or whatever your OS offers). If they're restricted
      > to match only TCP to <externalIP>:25, I don't see much potential
      > for problems (but of course, I may be missing something).

      My ultimate solution was as per the discussion with Wietse: an
      alternate DNS view of the MX instance hostname. I accomplished it
      using dnsmasq and an entry in /etc/hosts, quick and easy. Any
      additional names which might be used for this address must be added
      to hosts as well.

      I should close up this thread by saying that in this case all my
      puzzling was pointless. :) This site uses an external filtering
      service as relayhost. MSA sends everything to relayhost, the
      relayhost delivers.

      Still, it's not entirely useless, because I learned something, and
      these folks might not always want to be at the mercy of their
      filtering service provider. If/when they decide to switch to send
      direct-to-MX, it already works.
      --
      http://rob0.nodns4.us/ -- system administration and consulting
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