DNS/RFC/3225について、ここに記述してください。

Indicating Resolver Support of DNSSEC https://tools.ietf.org/html/rfc3225

Do flagsの話

3. Protocol Changes

   The mechanism chosen for the explicit notification of the ability of
   the client to accept (if not understand) DNSSEC security RRs is using
   the most significant bit of the Z field on the EDNS0 OPT header in
   the query.  This bit is referred to as the "DNSSEC OK" (DO) bit.  In
   the context of the EDNS0 OPT meta-RR, the DO bit is the first bit of
   the third and fourth bytes of the "extended RCODE and flags" portion
   of the EDNS0 OPT meta-RR, structured as follows:

                +0 (MSB)                +1 (LSB)
         +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      0: |   EXTENDED-RCODE      |       VERSION         |
         +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      2: |DO|                    Z                       |
         +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

   Setting the DO bit to one in a query indicates to the server that the
   resolver is able to accept DNSSEC security RRs.  The DO bit cleared
   (set to zero) indicates the resolver is unprepared to handle DNSSEC
   security RRs and those RRs MUST NOT be returned in the response
   (unless DNSSEC security RRs are explicitly queried for).  The DO bit
   of the query MUST be copied in the response.

   More explicitly, DNSSEC-aware nameservers MUST NOT insert SIG, KEY,
   or NXT RRs to authenticate a response as specified in [RFC2535]
   unless the DO bit was set on the request.  Security records that
   match an explicit SIG, KEY, NXT, or ANY query, or are part of the
   zone data for an AXFR or IXFR query, are included whether or not the
   DO bit was set.

   A recursive DNSSEC-aware server MUST set the DO bit on recursive
   requests, regardless of the status of the DO bit on the initiating
   resolver request.  If the initiating resolver request does not have
   the DO bit set, the recursive DNSSEC-aware server MUST remove DNSSEC
   security RRs before returning the data to the client, however cached
   data MUST NOT be modified.

   In the event a server returns a NOTIMP, FORMERR or SERVFAIL response
   to a query that has the DO bit set, the resolver SHOULD NOT expect
   DNSSEC security RRs and SHOULD retry the query without EDNS0 in
   accordance with section 5.3 of [RFC2671].