The last byte of a cache entry for Deadwood is a single “type” byte (also called “is_nxdomain” in the source code). This is an unsigned 8-bit value; right now only two values are used for this:
- 0, which indicates that it is a complete answer to our DNS question, and the NXDOMAIN bit in the header is not set (NXDOMAIN means “thisf host does not exist for any record type”)
- 1, which indicates that it is a complete answer to our DNS question, with the NXDOMAIN bit in the header set to 1 (which means, but Deadwood won’t use this, that no other host names exist for this query)
With eight bits to play with there, we can use this type to indicate various records which, while not complete DNS answers, are useful for a recursive DNS server
- Have type 2, which is a NS referral. A NS referral can be any combination of glued and glueless NS records. NS records with A or AAAA (IPv4 or IPv6) glue that we can use is converted in to the A and AAAA addresses we get from the glue records; NS records without glue are kept only as names (speaking of glueless, I am a pretty hard critic of DJB, but I am glad we got A6 and DNAME killed)
- Have type 3, which is an incomplete CNAME referral; this thing can be a CNAME chain if the upstream server gave us a CNAME chain
I will discuss the binary format of type 2 (NS referral) and type 3 (CNAME referral) records in a future blog.
