Is it possible with Postfix to add a warning to all emails received over a cleartext (non-TLS) connection?
For example modifying the subject and/or adding a custom header.
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4I don't know if it's possible, but it would be pointless: you can only know if the connection from the next-to-last relay to the last relay used TLS, you can't know anything about previous hops. If you want email security, use PGP or S/MIME. – Gilles 'SO- stop being evil' Nov 14 '15 at 23:03
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2I agree end-to-end encryption is needed to keep emails private. It's been in the news recently that Gmail will be deploying something similar to what I described, and I wondered how hard it would be to do on one's own email server. I think it would be interesting to see. – Dr.Haribo Nov 16 '15 at 12:33
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The way to go here is to first tell Postfix to add the TLS connection status into the Received header. In /etc/postfix/main.cf add
smtpd_tls_received_header = yes
This will add something like (using TLSv1.2 with cipher … (256/256bits)) to the postfix generated received header.
This information can then be used to add a custom headers. E.g. one could add a X-Transport-Layer-Security header and set its value depending on whether the above snippet is present in the Received header. Possible methods to do so include
- writing a custom content filter which is called by Postfix,
- leveraging Sieve scripts to edit the message, or
- use some of the quadrillion other methods to process messages (proxy, milter, etc.).
I don't know of any ready-made solution to do this, but it shouldn't be much work either.
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1Yeah Sieve's
editheaderextension might be the easiest way to go. Info here: mvmf.org/docs/draft-degener-sieve-editheader-00.txt (section 5). To send your email over to the Sieve rules you might want to follow my answer here unix.stackexchange.com/a/252907/23085. Use the same procedure but just pipe the emails directly to Dovecot local delivery and requireeditheaderextension. The code should be easy to come with following the examples on the link. – TCB13 Jan 25 '16 at 13:55 -
1For Thunderbird there is an add-on called Paranoia which shows a happy, neutral or sad smiley face depending on whether Received headers indicate encryption or not. – Dr.Haribo Jan 25 '16 at 15:32
