DNS/返答/referral: DNS問合せに対する５種類の返答DNS/返答/DJBのうちのひとつ、 DNS/1/委譲/delegation を示すためにも使われる。

A referral (a list of two or more name servers and IP addresses that are closer the next level down in the name hierarchy to the requested domain ...

リゾルバーを使うためのインタフェースは定義されないらしい。

1. rfc1034.txt より

%grep -2 -n referral rfc1034.txt 
187-   - Clients of the domain system should be able to identify
188-     trusted name servers they prefer to use before accepting
189:     referrals to name servers outside of this "trusted" set.


248-The domain system defines procedures for accessing the data and for
249:referrals to other name servers.  The domain system also defines
250-procedures for caching retrieved data and for periodic refreshing of
251-data defined by the system administrator.


312-     able to access at least one name server and use that name
313-     server's information to answer a query directly, or pursue the
314:     query using referrals to other name servers.  A resolver will
315-     typically be a system routine that is directly accessible to
316-     user programs; hence no protocol is necessary between the


846-In general, the user does not generate queries directly, but instead
847-makes a request to a resolver which in turn sends one or more queries to
848:name servers and deals with the error conditions and referrals that may
849-result.  Of course, the possible questions which can be asked in a query
850-does shape the kind of service a resolver can provide.


1009-name servers can answer queries in a simple manner; the response can
1010-always be generated using only local data, and either contains the
1011:answer to the question or a referral to other name servers "closer" to
1012-the desired information.

--
1134-part of the authoritative data, and are address RRs for the servers.
1135-These RRs are only necessary if the name server's name is "below" the
1136:cut, and are only used as part of a referral response.

https://moin.qmail.jp/DNS/RFC/1034/4.3#A14 

1188-   - The simplest mode for the server is non-recursive, since it
1189-     can answer queries using only local information: the response
1190:     contains an error, the answer, or a referral to some other
1191-     server "closer" to the answer.  All name servers must
1192-     implement non-recursive queries.

1194-   - The simplest mode for the client is recursive, since in this
1195-     mode the name server acts in the role of a resolver and
1196:     returns either an error or the answer, but never referrals.
1197-     This service is optional in a name server, and the name server
1198-     may also choose to restrict the clients which can use


1212-Non-recursive service is appropriate if the requester is capable of
1213:pursuing referrals and interested in information which will aid future
1214-requests.

1271-     whether the data comes from a zone or is cached.
1272-
1273:     A referral to name servers which have zones which are closer
1274-     ancestors to the name than the server sending the reply.

--
1320-         b. If a match would take us out of the authoritative data,
1321:            we have a referral.  This happens when we encounter a
1322-            node with NS RRs marking cuts along the bottom of a
1323-            zone.
--

2450-This response has an empty answer section, but is not authoritative, so
2451:it is a referral.  The name server on C.ISI.EDU, realizing that it is
2452-not authoritative for the MIL domain, has referred the requester to
2453-servers on A.ISI.EDU and SRI-NIC.ARPA, which it knows are authoritative
--

2539-This reply contains an authoritative reply for the alias USC-ISIC.ARPA,
2540:plus a referral to the name servers for ISI.EDU.  This sort of reply
2541-isn't very likely given that the query is for the host name of the name
2542-server being asked, but would be common for other aliases.