DNS queries can include several flags that modify the behavior of the query and the response. Below is a list of common DNS query flags along with their explanations:
DNS Query Flags
| Flag | Description |
|---|---|
| QR | Query/Response: This flag indicates whether the message is a query (0) or a response (1). |
| OPCODE | Operation Code: This 4-bit field specifies the kind of query. Common values include: 0 – Standard Query 1 – Inverse Query 2 – Server Status Request |
| AA | Authoritative Answer: This flag is set in a response to indicate that the responding server is an authority for the domain queried. |
| TC | Truncated: This flag indicates that the message is truncated. This occurs when the message size exceeds the maximum allowed size for UDP (512 bytes). |
| RD | Recursion Desired: This flag is set by the client to indicate that it wants the server to perform recursive queries on its behalf. |
| RA | Recursion Available: This flag is set by the server in a response to indicate that it supports recursion. |
| Z | Reserved for Future Use: This 3-bit field is reserved for future use and should be set to zero in all queries and responses. |
| RCODE | Response Code: This 4-bit field indicates the response status. Common values include: 0 – No error 1 – Format error 2 – Server failure 3 – Name error 4 – Not implemented 5 – Refused |
| AD | Authenticated Data: This flag is set in a response to indicate that the data included in the response has been authenticated using DNSSEC. |
| CD | Checking Disabled: This flag is set by the client to indicate that it does not want the server to perform DNSSEC validation on the response. |
Explanation of Each Flag
QR (Query/Response):
- Indicates whether the message is a query (0) or a response (1).
OPCODE (Operation Code):
- Specifies the type of operation requested. Standard queries are typically represented by
0.
AA (Authoritative Answer):
- Indicates that the responding server is an authoritative source for the queried domain.
TC (Truncated):
- Indicates that the message was truncated due to size limitations, suggesting that the client may need to use TCP to obtain the full response.
RD (Recursion Desired):
- Signals to the server that the client is requesting recursive resolution of the query.
RA (Recursion Available):
- Indicates that the server is capable of performing recursive queries.
Z (Reserved for Future Use):
- Should always be set to zero, as it is reserved for future use.
RCODE (Response Code):
- Provides the status of the query response, indicating whether it was successful or if an error occurred.
AD (Authenticated Data):
- Indicates that the data in the response has been authenticated via DNSSEC, ensuring its integrity.
CD (Checking Disabled):
- Instructs the server to skip DNSSEC validation for this query, typically used in scenarios where the client does not trust the server’s validation process.
Conclusion
These flags play a crucial role in the operation of the DNS protocol, providing important information about the nature of the query and the response