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Postfix Documentation
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Dangerous use of smtpd_recipient_restrictions

By now the reader may wonder why we need smtpd client, helo or sender restrictions, when their evaluation is postponed until the RCPT TO or ETRN command. Some people recommend placing ALL the access restrictions in the smtpd_recipient_restrictions list. Unfortunately, this can result in too permissive access. How is this possible?

The purpose of the smtpd_recipient_restrictions feature is to control how Postfix replies to the RCPT TO command. If the restriction list evaluates to REJECT or DEFER, the recipient address is rejected; no surprises here. If the result is PERMIT, then the recipient address is accepted. And this is where surprises can happen.

Here is an example that shows when a PERMIT result can result in too much access permission:

1 /etc/postfix/
main.cf:
2     
smtpd_recipient_restrictions = 
3         
permit_mynetworks
4         
check_helo_access hash:/etc/postfix/helo_access
5         
reject_unknown_helo_hostname
6         
reject_unauth_destination
7 
8 /etc/postfix/helo_access:
9     localhost.localdomain PERMIT

Line 5 rejects mail from hosts that don't specify a proper hostname in the HELO command (with Postfix < 2.3, specify reject_unknown_hostname). Lines 4 and 9 make an exception to allow mail from some machine that announces itself with "HELO localhost.localdomain".

The problem with this configuration is that smtpd_recipient_restrictions evaluates to PERMIT for EVERY host that announces itself as "localhost.localdomain", making Postfix an open relay for all such hosts.

In order to avoid surprises like these with smtpd_recipient_restrictions, you should place non-recipient restrictions AFTER the reject_unauth_destination restriction, not before. In the above example, the HELO based restrictions should be placed AFTER reject_unauth_destination, or better, the HELO based restrictions should be placed under smtpd_helo_restrictions where they can do no harm.

Postfix Documentation
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