ESMTP args truncated in RCPT TO stage

[hxdmp]

I'm using this library to build a Milter server connected to a Postfix instance. Doing that, I noticed that the esmtpArgs parameter at the RCPT TO stage is truncated. Precisely, only one substring after the first \0 byte is respected, althought there might be multiple ones. One example where this can happen is when the client requests a DSN (https://www.rfc-editor.org/rfc/rfc3461.html#section-4.1). But other cases are possible, see: https://www.rfc-editor.org/rfc/rfc3461.html#section-10.1

Let me explain this whole thing with an example to make it clearer.

1. Spin up a basic Milter server with the following RCPT TO hook:

func (m *myMilter) RcptTo(rcptTo, esmtpArgs string, _ *milter.Modifier) (*milter.Response, error) {
    fmt.Printf("RCPT TO: %q; esmtpArgs: %q\n", rcptTo, esmtpArgs)
    return milter.RespContinue, nil
}

2. Submit the following (valid) RCPT TO command to your mail server:

RCPT TO: <test@example.com> NOTIFY=SUCCESS ORCPT=rfc822;test@anotherexample.com

3. The following is received by the Milter and printed to the console:

RCPT TO: "test@example.com"; esmtpArgs: "NOTIFY=SUCCESS"

4. Inspecting the raw bytes, one can see that there was actually more data:

"test@example.com\x00NOTIFY=SUCCESS\x00ORCPT=rfc822;test@anotherexample.com\x00"

Conclusion

This behavior is probably caused by the following code:
https://github.com/d--j/go-milter/blob/30d4d62396d57db43116e28f8bb5c5b61ecf2478/session.go#L224C1-L224C1

Maybe it's already sufficient to replace all occurrences of 0x00 after the email address with a space and pass that to the Milter interface implementation. As this is not a real C string, there should be no harm in doing so (we don't need a zero-terminator).

I'm waiting for your feedback before starting to implement it.

[d--j]

Hi @hxdmp

thanks for poiting that out. I was under the impression that Postfix/Sendmail passes in all values as space seperated second string.

Your proposed solution sounds good to me. One can use DecodeCStrings to get a string slice and join it with space.

The MAIL ESMTP args need the same handling.

[hxdmp]

Thanks for that very fast response! I'll implement that and open a pull request.

[hxdmp]

I created #16 to fix this for both instances. Due to the implementation of DecodeCStrings, the trailing zero-byte in the input data is omitted - in other words: There is no trailing space.