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szatox
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Joined: 27 Aug 2013
Posts: 3468
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 Posted: Fri Nov 08, 2024 8:35 pm Post subject: [solved] Postfix/How do I reject mail From: sender |
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Long story short I want to make postfix reject incoming mail immediately rather than accept the transfer, filter, and then send a bounce to a possibly spoofed address.
This one is _almost_ there:
| Code: | | smtpd_relay_restrictions = check_sender_access mysql:/etc/postfix/mysql-blacklist.cf |
Almost, because this directive executes my query using the envelope FROM rather than the headers' From:.
Envelope from and header's from don't have to match. In case of the particular entity which prompted me to look into this, they don't, because spam comes from a 3rd party service authorized by the domain owner.
I don't really mind the nuclear option of blacklisting the entire mailing service, but not being able to aim properly is a shame, so give me a hint.
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Last edited by szatox on Sun Nov 10, 2024 12:14 pm; edited 1 time in total |
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grknight
Retired Dev
Joined: 20 Feb 2015
Posts: 1935
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| Posted: Fri Nov 08, 2024 9:12 pm Post subject: |
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man 5 header_checks but be warned. This is not as easy as it appears.
A From header is not required to be a valid message. The From header may be SMTPUTF8 encoded.
Also, header checks examine all headers one at a time and each will reexamine the lookup for an action.
It really should be a pcre: or regexp: table type for speed. You can use anything of course, but traditional databases may bog down mail processing. |
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szatox
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Joined: 27 Aug 2013
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| Posted: Fri Nov 08, 2024 10:02 pm Post subject: |
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Thanks, looks promising indeed.
I'm not too worried about performance there, I don't have that many emails. Still, it is worth a note.
Can SMTPUTF8 mangle pure ASCII addresses? I'll be fine as long as the input is predictable, but having multiple possible forms of the same value would really be a problem.
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szatox
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| Posted: Sun Nov 10, 2024 12:10 pm Post subject: |
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I came up with this:
| Code: | query = WITH RECURSIVE
rxp AS (SELECT REGEXP_REPLACE('%s', 'From: *(.*<(.*)>.*)?','\\2') as sender WHERE '%s' LIKE 'From:%%'),
list AS ( SELECT sender FROM rxp UNION SELECT REGEXP_REPLACE( sender , '[^@.]*[@.](.*)', '\\1') FROM list )
SELECT 'REJECT Sender banned for: SPAM' FROM header_blacklist WHERE header_blacklist.sender IN ( SELECT sender FROM list ) AND header_blacklist.active = TRUE;
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Looks complicated, but it's only going to touch a few rows from a single, indexed table, and only when processing the From: header; otherwise it calculates an empty set so there's nothing to look up in the table.
I've been considering turning it into a stored function, to let the query optimizer do its part too, but then again I already have a bunch of plaintext bindings and they are easier to modify should I need to do that, while performance is not a problem I have to worry about.
That initial regexp, the one extracting email address is really wild though. I'm not even sure it should work, but I had a crazy idea to extract a substring which may or may not be surrounded by text to ignore, and it does in fact work for both formats:
From: sender@address
From: sender name <sender@address>
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