OpCode is a Word property and contains an Integer value that specifies the kind of query in a DNS request packet. OpCode is specified in the DNS query, and returned in the DNS response. OpCode may contain one of the follwing values and meanings:
- 0 - A standard query (QUERY)
- 1 - An inverse query (IQUERY)
- 2 - A server status request (STATUS)
- 3 - Reserved for future use
- 4 - Reserved for future use
- 5 - Reserved for future use
- 6 - Reserved for future use
- 7 - Reserved for future use
- 8 - Reserved for future use
- 9 - Reserved for future use
- 10 - Reserved for future use
- 11 - Reserved for future use
- 12 - Reserved for future use
- 13 - Reserved for future use
- 14 - Reserved for future use
- 15 - Reserved for future use
Note: Currently, the TIdDNSResolver implementation does not support all aspects of the DNS protocol, and includes the following limitations:
- The DNS query type is limited to standard queries, where the DNS Header OpCode is 0. Inverse queries (OpCode=1) and Status queries (OpCode=2) are not supported at this time.
OpCode is represented in a DNS header as a 4-bit field. Changing the value of OpCode insures that the appropriate bit field is updated in the DNS header for the message packet.