make postfix reject invalid TLS certs on receiving

muekno · Nov 21, 2024 at 6:11 PM

Sorry may be a silli question but im searching the internet now for nearly two hours without success
I like to configure my postfix so that incoming mails are rejected if the TLS cert is invalid i.e. selfsinged etc.

Thanks for any hint

Regards
Rainer

till · Nov 21, 2024 at 7:15 PM

From ChatGPT:

To configure Postfix on Linux to reject incoming connections when the SSL certificate presented by the client is invalid or self-signed, you need to set up mutual TLS authentication (also known as TLS client authentication). This involves configuring Postfix's SMTP server (smtpd) to request and require a valid client certificate during the TLS handshake.

Here's how to achieve this:

Enable TLS for Incoming Connections: Ensure that TLS is enabled for incoming connections.

smtpd_tls_security_level = encrypt

This setting in your main.cf file forces all SMTP connections to use TLS encryption.

Request Client Certificates: Configure Postfix to ask for client certificates.

smtpd_tls_ask_ccert = yes

This setting tells Postfix to request a certificate from the client during the TLS handshake.

Require Valid Client Certificates: Instruct Postfix to require a valid client certificate, rejecting connections without one.

smtpd_tls_req_ccert = yes

With this setting, Postfix will only accept connections from clients that present a certificate signed by a trusted Certificate Authority (CA).

Specify Trusted CAs: Provide the path to the CA certificates that Postfix should use to verify client certificates.

smtpd_tls_CAfile = /etc/ssl/certs/ca-certificates.crt

Replace /etc/ssl/certs/ca-certificates.crt with the path to your trusted CA bundle. This file should contain all the CA certificates that you trust to sign client certificates.

Optional - Verify Client Hostnames: You can enforce hostname verification to ensure the client's certificate matches its hostname.

smtpd_tls_verify_cert_match = hostname

Reload Postfix Configuration: After making these changes, reload Postfix to apply the new settings.

sudo systemctl reload postfix

Example Configuration Snippet in main.cf:

smtpd_tls_security_level = encrypt
smtpd_tls_ask_ccert = yes
smtpd_tls_req_ccert = yes
smtpd_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
smtpd_tls_verify_cert_match = hostname

Notes:

Trusted CAs: Ensure that the CA certificates specified are up-to-date and include all the authorities you trust. Self-signed certificates or certificates signed by untrusted CAs will be rejected.

Client Configuration: Clients connecting to your Postfix server must present a valid certificate signed by a trusted CA. Ensure that legitimate clients are configured accordingly.

Testing: Before deploying these settings in a production environment, test the configuration with a client presenting both valid and invalid certificates to confirm that Postfix behaves as expected.

Additional Resources:

Postfix TLS Support

Postfix Configuration Parameters

By following these steps, Postfix will reject incoming connections when the client presents an invalid or self-signed SSL certificate.
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muekno · Nov 21, 2024 at 9:26 PM

Thank you
Regards
Rainer

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make postfix reject invalid TLS certs on receiving

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