Re: need for open-minded help with popularity-contest with systemd timer

On Thu, 30 Jul 2026 at 12:26:08 +0100, Richard Lewis wrote:
>Simon McVittie <[email protected]> writes:
>> SMTP over TCP is the lowest-common-denominator email submission
>> protocol, but any protocol over TCP has the big disadvantage that it's
>> necessary to be able to authenticate the sender in some way, if you
>> want any access control beyond "every process on this machine can send
>> email, and the system can't tell which process/user was responsible".
>
>(isnt the bit in quotes exactly what the current sendmail interface
>provides, perhaps changing it should be left for another day?)

No, if you exec /usr/sbin/sendmail, the MTA infrastructure has the 
option to log facts about that sendmail process, such as your uid (and 
in theory other facts like process ID, whether you're in a container or 
cgroup, AppArmor profile and so on), in mail.log or the message headers 
or whatever place its implementor thinks is most appropriate. For 
example:

% ssh people.debian.org
$ mail smcv

results in a mail that says:

     Received: from smcv by paradis.debian.org with local (Exim 4.96) ...

because the exim service on p.d.o is in a position to know who I am (it 
can get my uid from the sendmail process that I ran, and then my 
username from /etc/passwd or other NSS mechanisms).

Unlike the envelope-from (which is under my control), this is 
unforgeable, or at least, a MTA author can make it unforgeable - I don't 
know for sure that the authors of Exim did, but I hope so.

But if I had done SMTP to TCP port 25 or 587 then either I would 
have to authenticate (in practice with a username/password), or the mail 
would have to just say something like:

     Received: from localhost ([::1]:39364) by paradis.debian.org with smtp (Exim 4.96) ...

because the MTA doesn't actually know who I am, and has to believe my 
assertions about my identity (the envelope-from).

     smcv