High order two bits of every length octet must be zero, and the The root, a domain name is terminated by a length byte of zero. Since every domain name ends with the null label of
RFC 1035 - DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATIONĭomain names in messages are expressed in terms of a sequence of labels.Įach label is represented as a one octet length field followed by that = Answer authenticated: Answer/authority portion was not = Recursion available: Server can do recursive queries = Recursion desired: Do query recursively = Authoritative: Server is not an authority for domain Look for response with no errors Flags: 0x8180 Standard query response, No errorġ. (udp & 0x0f = 0) 8 bytes (0-7) of UDP header + 4th byte in to UDP data = DNS flags low byte (udp & 0x80 != 0) 8 bytes (0-7) of UDP header + 3rd byte in to UDP data = DNS flags high byte (udp port 53) - DNS typically responds from port 53 With a little math, you can do the same thing for UDP.
There is a Wireshark tool for making TCP capture filters: String-Matching Capture Filter Generator
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