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Re: [ietf-smtp] Address literals

2016-08-01 05:24:58
Thanks a lot to all who replied!

On Sun 31/Jul/2016 14:22:56 +0200 Peter J. Holzer wrote:
On 2016-07-31 10:07:12 +0200, Alessandro Vesely wrote:
SMTP provides for:

    address-literal  = "[" ( IPv4-address-literal /
                     IPv6-address-literal /
                     General-address-literal ) "]"
                     ; See Section 4.1.3

    Mailbox        = Local-part "@" ( Domain / address-literal )

However, it is not clear how mailboxes containing an address literal are to
be treated by an MTA when they appear in the envelope.  Specifically, I have
three questions, assuming a recipient is indicated using an address literal:

I asked a similar question about CNAME records about 2 years ago, and
the consensus was that a relaying MTA must not change the recipients
address while the receiver MTA may treat the address any way it pleases.

I think it is safe to generalize that to address literals.

A difference is that the delivery MTA ("receiving server") pretty much has to know all of the domains under which it will receive mail, while it can miss DNAT numbers, especially in the misconfigurational barrier case that address literals are designed to bypass.

1. If the host can resolve the address and obtain a domain name, it can
replace it so as to obtain a regular mailbox.  Right?

No. If a relay receives an email for <alice@[]>, it cannot
rewrite it to <alice(_at_)example(_dot_)com>, just because it finds a PTR record
« PTR».

Of course the receiver MTA may deliver mail for <alice@[]> and
<alice(_at_)example(_dot_)com> to the same mailbox, and it might do that
canonicalizing the domain part first. But that's local policy.

Fine.  Ned's description of what happens inside of an ADMD is clear and 

I couldn't find any occurrence of the term "literal" in RFC 6409, but generalizing Section 8.7 "Resolve Aliases" yields that MSAs are allowed to rewrite them. Now, since one cannot lookup secondary MXes of an address literal, an MTA getting RCPT TO:<alice@[] must be either a submission or a delivery server. Rewriting is allowed in both cases.


Determining whether the MTA is the receiver is local policy. There is
probably no general rule like "does this IP address match one of this
host's interfaces?"

Relying on configuration settings to define a super-duper local policy doesn't seem to be a viable option, for a couple of reasons. First, to avoid the misconfigurational oxymoron above. Second, more flexibility brings more bugs, so it can hardly be afforded for a rarely used feature.

Of all the non-configured elements that a receive-or-relay decision can be based on, only two come to my mind:

* Does the client have relay privileges?

* Does the address literal have a reverse name?

3. If in case (2) the server relays, it can either drop or keep the domain
part when it issues RCPT TO to its peer.

No. It must not change the recipient address, and just dropping the
domain part would in any case result in a syntactically invalid address.

My bad. I was thinking of <postmaster@[]> and failed to bring into the cache that, in general, local names are not accepted in the envelope. Rewriting with the greeting name obtained on connecting to the server would probably create more problems than it might solve, wouldn't it?

Any other ideas?


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