7. SOA RRs

Three minor issues concerning the Start of Zone of Authority (SOA) Resource Record need some clarification.

7.1. Placement of SOA RRs in authoritative answers

RFC1034, in section 3.7, indicates that the authority section of an authoritative answer may contain the SOA record for the zone from which the answer was obtained. When discussing negative caching, RFC1034 section 4.3.4 refers to this technique but mentions the additional section of the response. The former is correct, as is implied by the example shown in section 6.2.5 of RFC1034. SOA records, if added, are to be placed in the authority section.

7.2. TTLs on SOA RRs

It may be observed that in section 3.2.1 of RFC1035, which defines the format of a Resource Record, that the definition of the TTL field contains a throw away line which states that the TTL of an SOA record should always be sent as zero to prevent caching. This is mentioned nowhere else, and has not generally been implemented. Implementations should not assume that SOA records will have a TTL of zero, nor are they required to send SOA records with a TTL of zero.

7.3. The SOA.MNAME field

It is quite clear in the specifications, yet seems to have been widely ignored, that the MNAME field of the SOA record should contain the name of the primary (master) server for the zone identified by the SOA. It should not contain the name of the zone itself. That information would be useless, as to discover it, one needs to start with the domain name of the SOA record - that is the name of the zone.