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It is however possible for most record types to exist with the same label, class and type, but with different data.
Such a group of records is hereby defined to be a Resource Record Set (RRSet).
Eight separate issues are considered: IP packet header address usage from multi-homed servers, TTLs in sets of records with the same name, class, and type, correct handling of zone cuts, three minor issues concerning SOA records and their use, the precise definition of the Time to Live (TTL) Use of the TC (truncated) header bit the issue of what is an authoritative, or canonical, name, and the issue of what makes a valid DNS label.
The first six of these are areas where the correct behaviour has been somewhat unclear, we seek to rectify that.
This memo does not use the oft used expressions MUST, SHOULD, MAY, or their negative forms.
This document addresses several additional problem areas. Those issues are the question of which source address a multi-homed DNS server should use when replying to a query, the issue of differing TTLs for DNS records with the same label, class and type, and the issue of canonical names, what they are, how CNAME records relate, what names are legal in what parts of the DNS, and what is the valid syntax of a DNS name.
Clarifications to the DNS specification to avoid these problems are made in this memo.
Some multi-homed hosts running DNS servers generate a reply using a source address that is not the same as the destination address from the client's request packet.
Such replies will be discarded by the client because the source address of the reply does not match that of a host to which the client sent the original request. To avoid these problems, servers when responding to queries using UDP must cause the reply to be sent with the source address field in the IP header set to the address that was in the destination address field of the IP header of the packet containing the query causing the response.