Protocol and Format

DNS Messages

DNS Message

The first 6 fields are 16-bits long, other fields are variable length. Maximum size for a DNS label is 64 Bytes, for a DNS name 255 bytes, for a DNS RDATA value 65535 bytes

• Identification field is used to match up replies and requests.
• Control field contains :

  QR

  OpCode

  AA

  TC

  RD

  RA

  Z

  AD

  CD

  RCODE

  • QR : 1 bit, request (0) or response (1)
  • OpCode : 4 bits, request type
    • QUERY Standard request
    • IQUERY Inverse request (obsoleted by RFC3425)
    • STATUS Server status query
    • NOTIFY Database update notification (RFC1996)
    • UPDATE Dynamic database update (RFC2136)
  • AA Authoritative Answer : 1 bit, reply from authoritative (1) or from cache (0)
  • TC Truncated : 1 bit, response too large for UDP (1).
  • RD Recursion Desired: 1bit, ask for recursive (1) or iterative (0) response
  • RA Recursion Available : 1bit, server manages recursive (1) or not (0)
  • 1 bit Zeros, reserved for extensions
  • 1 bit AD Authenticated data, used by DNSSEC
  • 1 bit CD Checking Disabled, used by DNSSEC
  • 4 bits Rcode, Error Codes : NOERROR, SERVFAIL, NXDOMAIN (no such domain), REFUSED...
• ... count fields give the number of entry in each following sections:
• the Question section contains incomplete RR {DNSname, TYPE, CLASS}.
• sections Response, Authority Additional contain complete RR {DNSname, TYPE, CLASS, TTL, (RDATA_LENGTH), RDATA} TYPE, CLASS and RDATA_LENGTH are 2-bytes long, TTL is 4-bytes long, DNSname follows some specific coding rules with compression (not described here)

Depending of the request, the 4 sections contain various RR, some required, some optional...

• Question count equals to 1 in general, but could be 0 or >1 in very special cases
• Response section contains the matching RRset w.r.t the question
• Authority section contains NS record for Authoritative zone servers. Sometime it also contains the SOA record.
• Additional contains any additional useful information. For instance Address Records (A, AAAA) for host names used in previous sections (Glue,...)

For an iterative response (RD=0 or RA=0),

• Response section is empty in general,
• Authority section contains NS records describing "a better zone server" for next iteration. (Worst case is root zone)
• Additional section contains if known addresses for servers described in authority section.

For a positive recursive response (RD=RA=1, Rcode=NOERROR, Response-Count>0),

• Response contains the searched RRset
• Authority contains the authoritative zone name for the response and the zone servers.

For a negative recursive response (RD=RA=1, NXDOMAIN or NO DATA),

• Response section is empty.
• Authority section contains an SOA record with the Negative Cache TTL as parameter