Standard DNS Error

DNS messages have a standard structure defined in RFC 1035

1. Standard DNS Request

A DNS request (or query) is sent from a client (resolver) to a DNS server to ask about a domain name.

Example:

Client asks: "What is the IP address of www.example.com?"

A request typically contains:

Header (with fields like ID, flags, opcode, etc.)

Question section (the query: name, type, class)

2. DNS Opcode

Opcode is a 4-bit field in the DNS header (bits 11–14) that specifies the kind of query.

Opcode Meaning

0 Standard query (QUERY) – most common (A, AAAA, MX, etc.)

1 Inverse query (IQUERY) – obsolete

2 Server status request (STATUS)

4 Notify (DNS NOTIFY, RFC 1996)

5 Update (DNS UPDATE, RFC 2136)

others Reserved / experimental

Most traffic uses opcode = 0.

3. DNS Rcode (Response Code)

Rcode is a 4-bit field in the DNS header that tells if the request succeeded or failed.

Rcode Meaning

0 NoError – success

1 FormErr – format error in query

2 ServFail – server failed to complete request

3 NXDomain – name does not exist

4 NotImp – operation not implemented

5 Refused – server refused to answer

6 YXDomain – name exists when it should not

7 YXRRSet – RR set exists when it should not

8 NXRRSet – RR set does not exist

9 NotAuth – not authoritative for zone

10 NotZone – name not in zone

16 BADVERS / BADSIG (EDNS extended codes)

≥17 Other extended errors

4. DNS Response

A DNS response is sent back from the DNS server to the client with the answer (if available).

Structure:

Header (with matching ID, flags, opcode, rcode)

Question (copied from request)

Answer section (the records: e.g., www.example.com. IN A 93.184.216.34)

Authority section (nameservers)

Additional section (extra info, like glue records or OPT for EDNS)

In short:

Request = client asking a question.

Opcode = type of query (standard, update, etc.).

Rcode = result code in the response (success, NXDOMAIN, etc.).

Response = server’s answer (with answers/authority/additional records).