DNS CNAME limitations cname 在哪些情况下不能配置
https://www.rfc-editor.org/rfc/rfc1912.html
https://www.rfc-editor.org/rfc/rfc2181.html
说明:
domain name 不能配置cname 记录
cname 记录不能和其它记录同时存在以及其它规则
2.4 CNAME records A CNAME record is not allowed to coexist with any other data. In
other words, if suzy.podunk.xx is an alias for sue.podunk.xx, you
can't also have an MX record for suzy.podunk.edu, or an A record, or
even a TXT record. Especially do not try to combine CNAMEs and NS
records like this!: podunk.xx. IN NS ns1
IN NS ns2
IN CNAME mary
mary IN A 1.2.3.4 This is often attempted by inexperienced administrators as an obvious
way to allow your domain name to also be a host. However, DNS
servers like BIND will see the CNAME and refuse to add any other
resources for that name. Since no other records are allowed to
coexist with a CNAME, the NS entries are ignored. Therefore all the
hosts in the podunk.xx domain are ignored as well! If you want to have your domain also be a host, do the following: podunk.xx. IN NS ns1
IN NS ns2
IN A 1.2.3.4
mary IN A 1.2.3.4 Don't go overboard with CNAMEs. Use them when renaming hosts, but
plan to get rid of them (and inform your users). However CNAMEs are
useful (and encouraged) for generalized names for servers -- `ftp'
for your ftp server, `www' for your Web server, `gopher' for your
Gopher server, `news' for your Usenet news server, etc. Don't forget to delete the CNAMEs associated with a host if you
delete the host it is an alias for. Such "stale CNAMEs" are a waste
of resources. Barr Informational [Page 6]
RFC 1912 Common DNS Errors February 1996 Don't use CNAMEs in combination with RRs which point to other names
like MX, CNAME, PTR and NS. (PTR is an exception if you want to
implement classless in-addr delegation.) For example, this is
strongly discouraged: podunk.xx. IN MX mailhost
mailhost IN CNAME mary
mary IN A 1.2.3.4 [RFC 1034] in section 3.6.2 says this should not be done, and [RFC
974] explicitly states that MX records shall not point to an alias
defined by a CNAME. This results in unnecessary indirection in
accessing the data, and DNS resolvers and servers need to work more
to get the answer. If you really want to do this, you can accomplish
the same thing by using a preprocessor such as m4 on your host files. Also, having chained records such as CNAMEs pointing to CNAMEs may
make administration issues easier, but is known to tickle bugs in
some resolvers that fail to check loops correctly. As a result some
hosts may not be able to resolve such names. Having NS records pointing to a CNAME is bad and may conflict badly
with current BIND servers. In fact, current BIND implementations
will ignore such records, possibly leading to a lame delegation.
There is a certain amount of security checking done in BIND to
prevent spoofing DNS NS records. Also, older BIND servers reportedly
will get caught in an infinite query loop trying to figure out the
address for the aliased nameserver, causing a continuous stream of
DNS requests to be sent.
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