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Re: ml.org's dynamic DNS

On Tue, Mar 03, 1998 at 08:15:50AM -0500, Avery Pennarun wrote:

> SMTP servers should not be validating the envelope-from (if by that, you
> mean the MAIL FROM command in SMTP).

Yes they should.

> That would seriously confuse a lot of things.  As a total.net user, I
> should be able to (and I can) send mail as if I were apenwarr@bond.net,
> apenwarr@worldvisions.ca, apenwarr@debian.org, and
> avery.pennarun@uwaterloo.ca.  I am all four.

But all those are valid addresses, so you don't have to worry about them
validating it.

> Please let me know if this isn't what's happening in general.

Believe it or not, some ISPs do check the from line, and insist that you use
an address on their system if you want to use their smarthost.

> My understanding of all this new (and important, but frustrating) SMTP
> anti-forwarding stuff is that modern sites will do the following:
>  - forward any mail from any host (as validated by the DNS, not the
>    MAIL FROM or HELO) to anyone on the "local" network.
>  - forward any mail from anyone on the "local" network to anyone on any
>    host.

Well, that's a slight simplification, but it's about right.

They will forward mail from any host to any domain they are
prepared to relay for (typically ones server on the machine itself, but also
often others in the same institution or just random others on the net that
they have agreed to be a backup mailserver for: it's configurable, anyway).
They will not necessary forward mail _to_ any host on the local network

And they will forward mail to anywhere from any host whose IP is in a
certain set of networks.

> In my opinion you should always HELO with a DNS-reversal of your IP address
> -- whatever it may be.

No, you should use your hostname. There's some debate over whether dynamic
IP users should use a fixed hostname or change it to match their IP; there
are disadvantages to both. I would suggest using your mail domain, eg if you
are user@node.isp.com you should use node.isp.com (if your ISP still doesn't
give you your own mail domain, change ISP; you might as well go for static
IP when you're at it).

> (smail uses gethostbyname(), I think.) Then you MAIL
> FROM with your _desired_ From: address -- the one you want your mail bounces
> sent to.  If MAIL FROM does not match the From: header, the MTA should add
> the Envelope-From: header.

No it won't. The envelope from is always sent onwards as an envelope from by
any relaying MTAs, so there's no point in including it in the headers too.
What happens at the delivering MTA is not set by any RFC, but many will add
a Return-Path header with the envelope from (as it is at that point: in the
majority of cases the same as it has been all along) to all mails, whether
or not it's the same as the From: header.

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