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Date: Tue, 12 May 2026 22:47:23 -0300
Message-ID: <CALLMduBAgnJe45s3jEwH_qEBGj6WQOBymHzvx3QMc6iaEazxvw@mail.gmail.com>
Subject: [PHP-DEV] Pre-RFC discussion about adding a DNS function for Null MX handling
To: internals@lists.php.net
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From: samuel.txd@gmail.com (Samuel Fontebasso)

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Hi internals,

I would like to gather feedback before drafting an RFC for a small addition
to PHP's DNS functions:

    dns_check_mx(string $hostname): bool

The function would check whether a hostname has at least one non-null MX
record.

Today, checkdnsrr($domain, "MX") correctly returns true for a domain
publishing a Null MX record:

    example.com. IN MX 0 .

This is correct, since an MX record exists. However, RFC 7505 (
https://www.rfc-editor.org/rfc/rfc7505.html) defines this as an explicit
signal that the domain does not accept email.

I am not proposing any change to checkdnsrr(). It should remain a low-level
DNS record existence check.

Proposed behavior:
- false when there are no MX records;
- false when all MX records are Null MX records;
- true when at least one MX record has an exchange target other than ".".

This would not validate email deliverability or perform SMTP-level checks.
It would only provide a DNS-level distinction between "has an MX record"
and "has a non-null MX record".

If there is interest in this direction, I am willing to work on the RFC and
implementation.

I would appreciate feedback on the scope, naming, and edge cases before
preparing an RFC.

Regards,
Samuel Fontebasso

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<div dir=3D"ltr"><p class=3D"gmail-isSelectedEnd">Hi internals,<br><br>I wo=
uld like to gather feedback before drafting an RFC for a small addition to =
PHP&#39;s DNS functions:<br><br>=C2=A0 =C2=A0 dns_check_mx(string $hostname=
): bool<br><br>The function would check whether a hostname has at least one=
 non-null MX record.<br><br>Today, checkdnsrr($domain, &quot;MX&quot;) corr=
ectly returns true for a domain publishing a Null MX record:<br><br>=C2=A0 =
=C2=A0 <a href=3D"http://example.com">example.com</a>. IN MX 0 .<br><br>Thi=
s is correct, since an MX record exists. However, RFC 7505 (<a href=3D"http=
s://www.rfc-editor.org/rfc/rfc7505.html">https://www.rfc-editor.org/rfc/rfc=
7505.html</a>) defines this as an explicit signal that the domain does not =
accept email.<br><br>I am not proposing any change to checkdnsrr(). It shou=
ld remain a low-level DNS record existence check.<br><br>Proposed behavior:=
<br>- false when there are no MX records;<br>- false when all MX records ar=
e Null MX records;<br>- true when at least one MX record has an exchange ta=
rget other than &quot;.&quot;.<br><br>This would not validate email deliver=
ability or perform SMTP-level checks. It would only provide a DNS-level dis=
tinction between &quot;has an MX record&quot; and &quot;has a non-null MX r=
ecord&quot;.<br><br>If there is interest in this direction, I am willing to=
 work on the RFC and implementation.<br><br>I would appreciate feedback on =
the scope, naming, and edge cases before preparing an RFC.<br><br>Regards,<=
br>Samuel Fontebasso<br></p></div>

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