Hello everybody,
this is a somewhat theoretical topic, but it seems to come up every few
years in usenet discussions, so I'll ask for clarification/opinions.
Consider this DNS configuration:
A.example.com. CNAME M.example.com.
M.example.com. MX 10 S.example.com.
S.example.com. A 192.0.2.1
Now an MTA receives a message addressed to
<user(_at_)A(_dot_)example(_dot_)com>. This
MTA is a relay system and has no special configuration with regard to
A.example.com.
RFC 5321, section 5.1 specifies how to resolve "A.example.com", so by
following the chain A.example.com -> M.example.com -> 192.0.2.1 the MTA
will end up connecting to 192.0.2.1. We now have an SMTP connection and
I will call this MTA the client and 192.0.2.1 the server.
Which address should the client use in the RCPT TO command?
1) <user(_at_)A(_dot_)example(_dot_)com>, because a relay must pass on the
message
unmodified.
2) <user(_at_)M(_dot_)example(_dot_)com>, because M.example.com is the
canonical name
of the recipient domain.
3) Undefined behaviour. The MTA could even have rejected the message.
4) Other.
If the client sends "RCPT TO:<user(_at_)A(_dot_)example(_dot_)com>", how should
the
server treat this?
1) Exactly the same as "RCPT TO:<user(_at_)M(_dot_)example(_dot_)com>", because
A.example.com is an alias for M.example.com, so both must behave the
same.
2) <user(_at_)A(_dot_)example(_dot_)com> and
<user(_at_)M(_dot_)example(_dot_)com> may designate different
mail boxes (and only one of them may exist).
3) Other.
I do have an opinion and I think I can back it up with quotes from the
RFC, but I've tried to state the question neutrally.
hp
--
_ | Peter J. Holzer | Der eigene Verstand bleibt gefühlt messer-
|_|_) | Sysadmin WSR | scharf. Aber die restliche Welt blickt's
| | | hjp(_at_)hjp(_dot_)at | immer weniger.
__/ | http://www.hjp.at/ | -- Matthias Kohrs in desd
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