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The Postfix documentation says the following:

With Postfix ≥ 3.7, built with OpenSSL version is 3.0.0 or later, if the parameter value is either empty or "auto", then the DH parameter selection is delegated to the OpenSSL library, which selects appropriate parameters based on the TLS handshake.

I also found this from the OpenSSL documentation:

If "auto" DH parameters are switched on then the parameters will be selected to be consistent with the size of the key associated with the server's certificate. If there is no certificate (e.g. for PSK ciphersuites), then it it will be consistent with the size of the negotiated symmetric cipher key.

In previous versions, I believe the best practice (for example, as recommended by Mozilla for Postfix 3.4) was to use the ffdhe2048 DH parameters defined by RFC 7919. But now, both Postfix and OpenSSL recommend against changing this option from the default of "auto".

Because of that, I'm not sure if I should still be explicitly setting the DH parameters from RFC 7919 anymore. I'm unsure where exactly the "auto" DH parameters in OpenSSL come from. I found the RFC 7919 parameters in the OpenSSL source code, but I'm still unsure if this is what the "auto" option will use.

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