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I've a Postfix server running for my own email and everything works fine. While I was upgrading the machine and decided to review the security settings / read and implement some best practices online and most online tutorials tell me to set smtpd_client_restrictions under main.cf similarly to this:

smtpd_client_restrictions = 
    permit_mynetworks,
    permit_sasl_authenticated,
    reject_invalid_hostname,
    reject_unknown_client_hostname,
    reject_rbl_client zen.spamhaus.org,
    reject_rbl_client bl.spamcop.net,
    reject_rbl_client cbl.abuseat.org

And at the same time I also see people setting the submission service under master.cf as:

submission inet  n       -       y       -       -       smtpd
 (...)
 -o smtpd_client_restrictions=reject_non_fqdn_recipient,reject_unknown_recipient_domain,permit_mynetworks,permit_sasl_authenticated,reject

My question: Why does the smtpd_client_restrictions under submission ends with reject and under main.cf nobody recommends ending the list with reject? Aren't they just the same thing according to the docs:

-o name=value (short form) Override the named main.cf configuration parameter. The parameter value can refer to other parameters as $name etc., just like in main.cf. See postconf(5) for syntax. http://www.postfix.org/master.5.html

Also,

Restrictions are applied in the order as specified; the first restriction that matches wins. http://www.postfix.org/postconf.5.html#smtpd_client_restrictions

If "the first restriction that matches wins" rule really applies won't ending it with reject would cause it to be impossible to send email at all?

Thank you.

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  • 1
    You have permit_ parameters. Those might match. – Michael Hampton Feb 11 '18 at 22:48
  • @MichaelHampton so If permit_sasl_authenticated checks no further restrictions will be tested? – TCB13 Feb 12 '18 at 0:41
  • That's correct. – Michael Hampton Feb 12 '18 at 0:44
  • @MichaelHampton so do you think that smtpd_client_restrictions, smtpd_sender_restrictions, smtpd_relay_restrictions should all have reject as the last check in main.cf? – TCB13 Feb 12 '18 at 0:47
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It's redundant. If you reach the end and haven't matched anything, the default is reject anyway. But having it there makes that explicit for people who don't know what the default is.

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  • Hmm... I was under the impression that the default is permit because of this: "The end of each list is equivalent to a PERMIT result" postfix.org/SMTPD_ACCESS_README.html – TCB13 Feb 12 '18 at 1:51
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    Well, that's confusing. Elsewhere in the docs it states that reject is the default. Maybe you'd better be explicit. I don't trust the docs anymore. :) – Michael Hampton Feb 12 '18 at 2:33

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