RFC 1912 (rfc1912) - Page 2 of 16

Common DNS Operational and Configuration Errors

RFC 1912                   Common DNS Errors               February 1996


   Make sure your PTR and A records match.  For every IP address, there
   should be a matching PTR record in the in-addr.arpa domain.  If a
   host is multi-homed, (more than one IP address) make sure that all IP
   addresses have a corresponding PTR record (not just the first one).
   Failure to have matching PTR and A records can cause loss of Internet
   services similar to not being registered in the DNS at all.  Also,
   PTR records must point back to a valid A record, not a alias defined
   by a CNAME.  It is highly recommended that you use some software
   which automates this checking, or generate your DNS data from a
   database which automatically creates consistent data.

   DNS domain names consist of "labels" separated by single dots.  The
   DNS is very liberal in its rules for the allowable characters in a
   domain name.  However, if a domain name is used to name a host, it
   should follow rules restricting host names.  Further if a name is
   used for mail, it must follow the naming rules for names in mail
   addresses.

   Allowable characters in a label for a host name are only ASCII
   letters, digits, and the `-' character.  Labels may not be all
   numbers, but may have a leading digit  (e.g., 3com.com).  Labels must
   end and begin only with a letter or digit.  See [RFC 1035] and [RFC
   1123].  (Labels were initially restricted in [RFC 1035] to start with
   a letter, and some older hosts still reportedly have problems with
   the relaxation in [RFC 1123].)  Note there are some Internet
   hostnames which violate this rule (411.org, 1776.com).  The presence
   of underscores in a label is allowed in [RFC 1033], except [RFC 1033]
   is informational only and was not defining a standard.  There is at
   least one popular TCP/IP implementation which currently refuses to
   talk to hosts named with underscores in them.  It must be noted that
   the language in [1035] is such that these rules are voluntary -- they
   are there for those who wish to minimize problems.  Note that the
   rules for Internet host names also apply to hosts and addresses used
   in SMTP (See RFC 821).

   If a domain name is to be used for mail (not involving SMTP), it must
   follow the rules for mail in [RFC 822], which is actually more
   liberal than the above rules.  Labels for mail can be any ASCII
   character except "specials", control characters, and whitespace
   characters.  "Specials" are specific symbols used in the parsing of
   addresses.  They are the characters "()@,;:\".[]".  (The "!"
   character wasn't in [RFC 822], however it also shouldn't be used due
   to the conflict with UUCP mail as defined in RFC 976)  However, since
   today almost all names which are used for mail on the Internet are
   also names used for hostnames, one rarely sees addresses using these
   relaxed standard, but mail software should be made liberal and robust
   enough to accept them.




Barr                         Informational