== DNS/RFC/6891 ==
<<TableOfContents()>>

https://tools.ietf.org/html/rfc6891

Extension Mechanisms for DNS (EDNS(0))
  EDNS0についての定義

OPT RRについての定義

One OPT pseudo-RR can be added to the additional data section of either a request or a response. 

An OPT is called a pseudo-RR
     because it pertains to a particular transport level message and not to any actual DNS data. 
OPT RRs shall never be cached, forwarded, or stored in or loaded from master files.

The quantity of OPT pseudo-RRs per message shall be either zero or one, but not greater.