Editing CNAME record (section)

=== Restrictions ===
{{unordered list
|CNAME records must always be pointed to another domain name, never to an IP address.
|If a CNAME record is present at a node, no other data should be present; this ensures that the data for a canonical name and its aliases cannot be different. (RFC 1034 section 3.6.2, RFC 1912 section 2.4) The exception is when [[DNSSEC]] is being used, in which case there can be DNSSEC related records such as RRSIG, NSEC, etc. (RFC 2181 section 10.1)
|CNAME records that point to other CNAME records should be avoided due to their lack of efficiency, but are not an error.<ref name="RFC1034">{{cite web|url=https://tools.ietf.org/html/rfc1034|last=Mockapetris|first=P.|date=November 1987|title=RFC 1034 - Domain names - concepts and facilities|publisher=Internet Engineering Task Force|access-date=15 July 2019}}</ref> It is possible, then, to create unresolvable loops with CNAME records, as in:
{{sxhl|2=zone|foo.example.com.  CNAME  bar.example.com.
bar.example.com.  CNAME  foo.example.com.}}
|A CNAME record cannot be present at the zone apex. RFC 1034 section 4.2.1<ref>{{cite web|url=https://tools.ietf.org/html/rfc1034#section-4.2.1|last=Mockapetris|first=P.|date=November 1987|title=RFC 1034 section 4.2.1|access-date=15 July 2019}}</ref> demands a [[SOA record]] at the zone apex and RFC 1034 section 3.6.2<ref>{{cite web|url=https://tools.ietf.org/html/rfc1034#section-3.6.2|last=Mockapetris|first=P.|date=November 1987|title=RFC 1034 section 3.6.2|access-date=15 July 2019}}</ref> demands that are no other records be present if a CNAME record is present. Therefore a CNAME record cannot appear at the zone apex. 
|{{clarify|date=November 2017|reason=needs an example and a citation; this issue may also be irrelevant for WP.|text=CNAME records that are served by DNAME records may cause recursive loops in older resolvers.}}
|[[MX record|MX]] and [[NS record]]s must never point to a CNAME alias (RFC 2181 section 10.3). So, for example, a zone must '''not''' contain constructs such as:
{{sxhl|2=zone|example.com.      MX     0   foo.example.com.
foo.example.com.  CNAME  host.example.com.
host.example.com. A      192.0.2.1}}
|Domains that are used in the [[SMTP]] MAIL and RCPT commands may not have a CNAME record.<ref name="RFC1123">{{cite web|url=https://tools.ietf.org/html/rfc1123#section-5.2.2|last=Braden|first=R.|date=October 1989|title=RFC1123 - MAIL - SMTP & RFC-822|access-date=23 July 2020}}</ref> In practice this may work, but can have different behavior with different mail servers, and can have undesired effects.<ref name="DJB_CNAME">{{cite web|url=http://cr.yp.to/im/cname.html|title=CNAME records in mail|last=Bernstein|first=D. J.|access-date=3 June 2011}}</ref>}}