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Re: Selection of Appropriate Local SMTP Relay

  • From: Daniel Senie
  • Date: Mon Jan 10 19:42:49 2000

Joe Abley wrote:
> On Mon, Jan 10, 2000 at 12:16:25PM -0500, Daniel Senie wrote:
> > 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.
> I'm not convinced that I do.
> It's quite possible to specify a different mail relay for every individual
> address in an ISP's network. I might be using octet boundaries so that
> I can follow the same well-known zone structure as is
> currently used, but that's all.

Ny using octet boundaries, you're making an inference that the IP
addresses are being used in a classful fashion. It's doubtful this will
be the case in today's network.

> It seems to me that handing out mail relays in IPCP and DHCP are also
> very valid approaches, although they arguably require more software changes
> to be useful (e.g. in access-server firmware).

Agree, which is what I said as well.

> Here are a couple of points:
> 1. The use of MX records in the manner I suggested allows the SMTP
> relays to be used to be changed fairly dynamically -- using IPCP,
> for example, a caller would only get one set per call (and calls can
> be pretty long-held; I see calls lasting five days here).
> 2. The use of MX records also allows backup mail relays to be specified,
> with an associated priority, in a familiar way.

As others point out, the IP address given out for a particular dialup
will NOT be representative of the ISP. For example, I use a national
dialup provider for access to the 'net when I'm travelling. This
provider rents POPs from,, and probably plenty of
others. The same dialup POPs are rented by a LOT of other ISPs. Having
an MX lookup on the INADDR for the IP Address of the dialup port will be
quite problematic, since the desired effect is to get the name of the
SMTP server for the PROPER ISP, not for the POP vendor.

> 3. Using a network-layer protocol like PPP or DHCP to collect the data
> requires some consistent application-layer interface to retrieve the
> specified nameserver info by client software; this, ideally, would
> require standardisation across platforms, and practically does not sound
> trivial. However, there's already a well-defined mechanism in almost
> every platform for applications to perform DNS queries.

You didn't comment on my other suggestion, that of doing MX on the
assigned name servers. This has all of the advantages you want, without
the problems associated with trying to determine which ISP is in use
based on IP address. Now I'm not convinced the MX on DNS server approach
is a good solution, but it's probably better than doing MX on the IP
address assigned.

Daniel Senie                                        [email protected]
Amaranth Networks Inc.