1

The DNS zone name _domainkeys is used for email authentication through TXT records. The zone names _tcp and _udp are used to advertise application availability through SRV records.

Naturally, a webmaster would want to be sure not to use these zone names for anything else - only in accordance with the specs.

Are there any other such zone names with strict specs that a webmaster should take care not to use by accident?

1

Underscores are not allowed in host names so I doubt they can "accidentally" be chosen to name a website.

Using underscores as a selector is a debate in itself among relevant circles, because basically they can be seen as a cheap way to do something without having to create a new DNS resource record type.

Anyway, the following two documents currently being discussed may give more context and data:

Section 4.3 of the first one gives the following list of underscore cases:

        +------------+---------------------+-------------+
        | RR Type    | _NODE NAME          | REFERENCE   |
        +------------+---------------------+-------------+
        | *          | _example            | Section 4.5 |
        | NULL       | _ta-* {Section 4.4} | [RFC8145]   |
        | OPENPGPKEY | _openpgpkey         | [RFC7929]   |
        | SMIMEA     | _smimecert          | [RFC8162]   |
        | SRV        | _dccp               | [RFC2782]   |
        | SRV        | _http               | [RFC4386]   |
        | SRV        | _ipv6               | [RFC5026]   |
        | SRV        | _ldap               | [RFC4386]   |
        | SRV        | _ocsp               | [RFC4386]   |
        | SRV        | _sctp               | [RFC2782]   |
        | SRV        | _sip                | [RFC5509]   |
        | SRV        | _tcp                | [RFC2782]   |
        | SRV        | _udp                | [RFC2782]   |
        | SRV        | _xmpp               | [RFC3921]   |
        | TLSA       | _dane               | [RFC7671]   |
        | TLSA       | _sctp               | [RFC6698]   |
        | TLSA       | _tcp                | [RFC6698]   |
        | TLSA       | _udp                | [RFC6698]   |
        | TXT        | _acme-challenge     | [ACME]      |
        | TXT        | _dmarc              | [RFC7489]   |
        | TXT        | _domainkey          | [RFC6376]   |
        | TXT        | _mta-sts            | [MTA-STS]   |
        | TXT        | _spf                | [RFC7208]   |
        | TXT        | _tcp                | [RFC6763]   |
        | TXT        | _udp                | [RFC6763]   |
        | TXT        | _vouch              | [RFC5518]   |
        | URI        | _acct               | [RFC6118]   |
        | URI        | _dccp               | [RFC7566]   |
        | URI        | _email              | [RFC6118]   |
        | URI        | _ems                | [RFC6118]   |
        | URI        | _fax                | [RFC6118]   |
        | URI        | _ft                 | [RFC6118]   |
        | URI        | _h323               | [RFC6118]   |
        | URI        | _iax                | [RFC6118]   |
        | URI        | _ical-access        | [RFC6118]   |
        | URI        | _ical-sched         | [RFC6118]   |
        | URI        | _ifax               | [RFC6118]   |
        | URI        | _im                 | [RFC6118]   |
        | URI        | _mms                | [RFC6118]   |
        | URI        | _pres               | [RFC6118]   |
        | URI        | _pstn               | [RFC6118]   |
        | URI        | _sctp               | [RFC6118]   |
        | URI        | _sip                | [RFC6118]   |
        | URI        | _sms                | [RFC6118]   |
        | URI        | _tcp                | [RFC6118]   |
        | URI        | _udp                | [RFC6118]   |
        | URI        | _unifmsg            | [RFC6118]   |
        | URI        | _vcard              | [RFC6118]   |
        | URI        | _videomsg           | [RFC6118]   |
        | URI        | _voice              | [RFC6118]   |
        | URI        | _voicemsg           | [RFC6118]   |
        | URI        | _vpim               | [RFC6118]   |
        | URI        | _web                | [RFC6118]   |
        | URI        | _xmpp               | [RFC6118]   |
        +------------+---------------------+-------------+

The whole purposes of these new specifications is to create a "IANA registry" of all underscore cases like that. If they get approved (they should be but noone knows when), then at some point you will get a new entry below https://www.iana.org/protocols that lists all specified cases of DNS names using an underscore.

Also, separately, while not a zone and no underscores, there are now specific DNS names that may trigger specific behavior so one should refrain from using them. But they are constructed in such a way that it is "impossible" to reuse these names just by chance.

Look at https://www.rfc-editor.org/rfc/rfc8509.txt : names like root-key-sentinel-is-ta-<key-tag> or root-key-sentinel-not-ta-<key-tag> under specific circumstances will trigger specific behavior from a compliant resolver. This is used to help troubleshooting DNSSEC cases.

  • Do you know if there are any non-underscore zone names besides www and maybe m that have very strong semantics around their use? – Maximillian Laumeister Jan 19 at 17:42
  • 1
    Not sure to follow you but "in the past" thing like ftp.example.com was supposed to be an FTP server, mail.example.com an email receiver, etc. but those are all human conventions, you can call your website by https://mail.example.com/ and it will technically work the same way than https://www.example.com/ except that it may confuse visitors indeed. – Patrick Mevzek Jan 19 at 22:07

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