Another sendmail relaying problem.

[Cowles, Steve]

Franco wrote:
> Hi, in the /etc/mail/access i have nothing
> this is all relay blocked.
> But if someone send an e-mail to a local user and
> in his from address put other local user e-mail it
> relay without problem.
> 

Based on my understanding of your post, the behavior you describe does NOT
sound like your system is an "open relay". It's just accepting e-mail from
an external source where the "from:" address is being forged to appear like
its coming from someone in your domain space and delivering to a local user.
Although this can be annoying, think of the case where one of your 20 local
users wants to send an e-mail to another local user. You want sednmail to
accept and deliver e-mail addressed in this way.

Whether or not sendmail can be configured to block e-mail addressed in this
form, from "just" external users?... I don't know. In postfix, I block
external users from addressing e-mail in this form (at least the MAIL FROM
part, not the header from) by first testing if the e-mail was submitted from
someone in my local network. If so, ACCEPT and deliver it. If the e-mail was
submitted from an external source -and- the from address (mail from) is set
to someone in my domain, then reject. By testing in this precise order,
local users can still send e-mail to other local users, but exteternal ones
cannot. Again, I'm only testing the "MAIL FROM", not the header from. i.e.

C:\> telnet mail.mydomain.com 25

220 mail.mydomain.com ESMTP Postfix (2.1.1)
helo external.otherdomain.com
250 mail.mydomain.com
mail from: <scowles at mydomain.com>
250 Ok
rcpt to: <scowles at mydomain.com>
554 <scowles at mydomain.com>: Sender address rejected: You are NOT from
mydomain.com
DATA
554 Error: no valid recipients
quit
221 Bye
Connection to host lost.

NOTE: The above test generates a 554 (do not pass go, do not collect $200.00
so to speak) and does NOT generate a bounce. Nice! Also, this test is done
prior to the Data portion of the e-mail being read.

But if an e-mail is submitted from a system on my local network...

C:\> telnet mail.mydomain.com 25

220 mail.mydomain.com ESMTP Postfix (2.1.1)
helo enterprise.mydomain.com
250 mail.mydomain.com
mail from: <scowles at mydomain.com>
250 Ok
rcpt to: <scowles at mydomain.com>
250 Ok                           <----- From/To accepted
DATA
354 End data with <CR><LF>.<CR><LF>
From: Steve Cowles <scowles at mydomain.com>
To: Steve Cowles <scowles at mydomain.com>
Subject: This is a test. Please ignore!
            <----- Delimiter between message header/body.
This is line one of the e-mail body.