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Gill-Bates
Asked: 2022-08-23 00:02:41 +0800 CST

MX-Record: IP or FQDN?

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I have not found any reference to this in the RFCs and hope to find a reliable answer here.

Domain mail.example.com runs a mail server. I want to set the MX record for the domain awesomeexample.com pointing to mail.example.com. Can I use the FQDN or do I have to use the static IP address?

domain-name-system

0 Answers

  1. Best Answer

    AFAIK The MX record data format is formalised in RFC 1035 which defines that MX records should point to host names (FQDN's) rather than IP-addresses.

    MX RDATA format

    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                  PREFERENCE                   |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                   EXCHANGE                    /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

    where:

    PREFERENCE
    A 16 bit integer which specifies the preference given to this RR among others at the same owner. Lower values are preferred.

    EXCHANGE
    A <domain-name> which specifies a host willing to act as a mail exchange for the owner name.

    MX records cause type A additional section processing for the host specified by EXCHANGE. The use of MX RRs is explained in detail in [RFC-974].

    (of course IPv6 wasn't conceived then yet, but an IPv6 host will (also) do a lookup for an AAAA record instead.)

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