[26617] in North American Network Operators' Group
Re: Selection of Appropriate Local SMTP Relay
daemon@ATHENA.MIT.EDU (Daniel Senie)
Mon Jan 10 12:19:00 2000
Message-ID: <387A13E9.643834AF@senie.com>
Date: Mon, 10 Jan 2000 12:16:25 -0500
From: Daniel Senie <dts@senie.com>
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To: Joe Abley <jabley@patho.gen.nz>
Cc: nanog@merit.edu
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There exists a DHCP option for specifying the SMTP server for a client
to use (option 69). This could be used in the case of DSL and cable
modems, but I'm unaware of anyone actually using it. I also haven't seen
anything to lead me to believe mail client applications have any way to
get this from the local system should the value be negotiated.
PPP IPCP presently does not have such a value defined. Adding one, while
possible, would result in the same issue with getting the address to the
application as for DHCP. The right solution for both cases is to get
some work going in this area, but it won't help anything short term.
Using DNS, as you propose, has some problems. In your example, you rely
on the topology of the network, and make assumptions about classful
structure. This isn't workable in the present network environment. One
idea that comes to mind which may or may not be workable is for clients
to do an MX lookup on the DNS servers specified using PPP IPCP or DHCP
and assuming those MX records point to a client-usable mail server. This
seems kind of ugly, but at least results in looking up data about the
ISP the user is actually connected to. Even in the case of rent-a-pops,
the DNS servers specified are those of the company leasing the pops, not
those providing them.
While it'd be useful to add any of these solutions to mail client
software, it would require changes to protocols (in the case of PPP
IPCP) and system APIs (in the case of DHCP and PPP IPCP to get the SMTP
server info to the mail client application). Changes also would be
needed to the mail client application for any of the solutions listed
above, though the change would not require added interaction with the
local system for the MX lookup on DNS server approach.
Most present mail client software packages already have support for SMTP
AUTH. This provides a cleaner solution for the mobile user, and so
probably is the best path to continue along. The mail clients do need to
make it easier to select port 587 (presently possible with the major
packages by specifying the actual port number, should be automatic) for
SUBMISSION service. Geographic selection of outgoing mail servers is
really not very different from selecting among several HTTP servers, and
products and techniques are in the marketplace for that.
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Daniel Senie dts@senie.com
Amaranth Networks Inc. http://www.amaranthnetworks.com
