Topic Path: Symbol Reference > Classes > TDNSHeader Class > TDNSHeader Properties > OpCode Property
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TDNSHeader.OpCode Property

Indicates the kind of query to be performed on the DNS server.

property OpCode: Word;

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 following values and associated meanings: 

 

Value 
Meaning 
A standard query (QUERY) 
An inverse query (IQUERY) 
A server status request (STATUS) 
Reserved for future use 
Reserved for future use 
Reserved for future use 
Reserved for future use 
Reserved for future use 
Reserved for future use 
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 

 

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 ensures that the appropriate bit field is updated in the DNS header for the message packet.

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