TDNSHeader = class

  +------------+
  |   Header   |
  +------------+
  |  Question  |  question for the name server
  +------------+
  |   Answer   |  resource records answering the question
  +------------+
  | Authority  |  resource records pointing toward an authority
  +------------+
  | Additional |  resource records holding additional information
  +------------+

TDNSHeader is always present in the message packet, and includes values that specify which of the remaining sections are present. TDNSHeader also provides properties specific to the header section or the entire message packet, including:

• ID - Identifier assigned to a DNS query.

• Qr - Query (0) or Response (1) flag.

• OpCode - Indicates the kind of query to be performed.

• AA - Indicates the responsing DNS server is an authority for the domain.

• TC - Indicates the message was truncated.

• RD - Indicates recurive queries are desired on the DNS server.

• RA - Indicates recurive queries are available on the DNS server.

• RCode - Response code from the DNS server.

• QDCount - Number of entries in the DNS Question section.

• ANCount - Number of resource records in the DNS Answer section.

• NSCount - Number of resource records in the DNS Authority section.

• ARCount - Number of resource records in the DNS Additional section.

TDNSHeader is used in the message packet for both a DNS query and a DNS response, although some values are only appropriate in one context.

property AA: Word;

AA is used in a DNS response that may contain multiple resource records in the answer section of the DNS response, and each resource record may have a different owner. AA corresponds to the name which matches the Omain name in the query, or the owner name for the first resource record in the answer section.

AA is represented in a DNS header as a bit flag. Changing the value of AA insures that the appropriate bit field is updated in the DNS header for the message packet.

property ANCount: Word;

Use NSCount to determine the number of name server resource records in the DNS response.

Use ARCount to determine the number of additional resource records in in the DNS response.

property ARCount: Word;

Use ANCount to determine the number of Answer resource records in the DNS response.

Use NSCount to determine the number of name server resource records in the DNS response.

property BitCode: Word;

BitCode is used when updating values for bit fields while creating the DNS query, and when reading the values from bit fields in the DNS response. BitCode is ANDed with the appropriate value to access a particular bit field in the DNS header.

Use ClearByteCode to reset the value of all bit fields in BitCode.

property ID: Word;

The value in ID is assigned in the Create constructor, and contains a random value in the range 0 to 65535.

ID is read from the DNS Header returned in a DNS response packet, when the results are parsed after a call to TIdDNSResolver.Resolve.

property NSCount: Word;

Use ARCount to determine the number of Additional resource records in the DNS response.

Use ANCount to determine the number of resource records in the Answer scetion of the DNS response.

property OpCode: Word;

• 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.

property QDCount: Word;

QDCount is returned in the DNS response to indicate the number of Questions handled by the DNS server.

Use ANCount to determine the number of resource records returned in the Answer section of the DNS response.

Use NSCount to determine the number of name server resource records returned in the Authoruty section of the DNS response.

Use ARCount to determine the number of resource records returned in the Additional section of the DNS response.

property Qr: Word;

Qr is represented in a DNS header as a bit field. Changing the value of Qr insures that the appropriate bit field is updated in the DNS header for the message packet.

property RA: Word;

Use RD to indicate if recursive query support is desired in a DNS request.

RA is represented in a DNS header as a bit field. Changing the value of RA insures that the appropriate bit field is updated in the DNS header for the message packet.

property RCode: Word;

• 0 - No error condition.

• 1 - Query Format error. The name server was unable to interpret the query.

• 2 - Server failure. The name server was unable to process this query due to a problem with the name server.

• 3 - Name Error. Meaningful only for responses from an authoritative name server, this code signifies that the domain name referenced in the query does not exist.

• 4 - Not Implemented. The name server does not support the requested kind of query.

• 5 - Operation Refused. The name server refuses to perform the specified operation for policy reasons. For example, a name server may not wish to provide the information to the particular requester, or a name server may not wish to perform a particular operation (e.g., zone transfer) for particular data.

• 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.

RCode is represented in a DNS header as a 4-bit field. Changing the value of RCode insures that the appropriate bit field is updated in the DNS header for the message packet.

property RD: Word;

Use RA to detgermine if recursive query support is available on a DNS server issuing a response.

RD is represented in a DNS header as a bit field. Changing the value of RD insures that the appropriate bit field is updated in the DNS header for the message packet.

property TC: Word;

Messages handled by the UDP-based DNS Resolver are restricted to 512 bytes (not counting the UDP headers). Longer messages are truncated and TC is set to 1 in the header. For this reason, the UDP-based TIdDNSResolver does not implement DNS zone transfers, but is the recommended method for standard queries in the Internet.

TC is represented in a DNS header as a bit field. Changing the value of TC insures that the appropriate bit field is updated in the DNS header for the message packet.

procedure ClearByteCode;

Use BitCode to access the value for an individual bit field in the DNS Header.

constructor Create;