pupdns/include/dns_packets.h

#pragma once
#include <stddef.h>

#define DNS_HEADER_BYTES 12

struct DNSPacketHeader {
    /*
     * ID (Packet Identifier)
     * a random ID is assigned to query packets.
     * Response packets must contain the same ID as the query.
     */
    unsigned int packet_id : 16;
    
    /*
     * RD (Recursion Desired)
     * if set by the client, the server should
     * attempt to resolve the query recursively
     * 
     * the server may not allow recursive resolution.
     * (see RA / Recursion Available)
     */
    unsigned int recursion_desired : 1;

    /*
     * TC (Truncated)
     * if true, this message was truncated.
     * usually happens when the response payload
     * is larger than the max UDP packet size
     *
     * if set, the query may be re-issued over TCP for the full payload.
     */
    unsigned int is_truncated : 1;

    /*
     * AA (Authoritative Answer)
     * 
     * 1 if the responding server is authoritative for the queried domain
     */
    unsigned int is_authoritative : 1;

    /*
     * OPCODE (Operation Code)
     * 
     * 0    - standard query (QUERY)
     * 1    - inverse query (IQUERY)
     * 2    - server status request (STATUS)
     * 3-15 - reserved
     *
     * almost always will be 0
     */
    unsigned int opcode : 4;

    /*
     * QR (Query/Response)
     * 0 if the packet is a query
     * 1 if the packet is a response
     */
    unsigned int is_response : 1;

    /*
     * RCODE (Response Code)
     * Set by the server to indicate success or failure of the query.
     * Provides details about the cause of any failure.
     *
     * 0    - No error
     * 1    - Format error - the nameserver was unable to interpret the query
     * 2    - Server failure - the nameserver was unable to process the query due to an internal problem
     * 3    - Name error - Meaningful only from authoritative nameserver -- the domain name specified in the query does not exist.
     * 4    - Not implemented - the nameserver does not support the requested query type
     * 5    - Refused - the nameserver refuses to perform the requested operation. for example, it may refuse a zone transfer.
     * 6-15 - Reserved for future use
     */
    unsigned int response_code : 4;

    /*
     * Z (Reserved)
     * in the original spec, this is reserved for future use.
     *
     * in later RFCs, it is used for DNSSEC queries.
     * (to be implemented)
     */
    unsigned int z : 3;

    /*
     * RA (Recursion Available)
     * if set by the server, incicates that the server allows
     * recursive queries.
     */
    unsigned int recursion_available : 1;   

    /*
     * QDCOUNT (Question Count)
     * the number of entries in the question section
     */
    unsigned int question_count : 16;

    /*
     * ANCOUNT (Answer Count)
     * the number of entries in the answer section
     */
    unsigned int answer_count : 16;

    /*
     * NSCOUNT (Nameserver Count)
     * the number of entries in the authority records section
     */
    unsigned int ns_count : 16;

    /*
     * ARCOUNT (Additional Records Count)
     * the number of entries in the additional records section
     */
    unsigned int ar_count : 16;
};


/*
 * RecordPreamble
 * shared fields that all DNS record types use
 */
struct RecordPreamble {
    // TODO: add label sequence
    
    /*
     * TYPE
     * the record type
     */
    unsigned int type : 16;

    /*
     * CLASS
     * specifies the class of the data in the RDATA field.
     * in practice, always set to 1
     */
    unsigned int class : 16;

    /*
     * TTL (Time-To-Live)
     * how long a record can be cached before it should be requeried.
     */
    unsigned int ttl : 32;

    /*
     * RDLENGTH (Record Data Length)
     * length of the record-type-specific data.
     */
    unsigned int rd_data_length: 16;
};

/*
 *  A Record
 *  represents the mapping of a domain name to an IPv4 address
 */
struct ARecord {
    struct RecordPreamble preamble;
    unsigned int ip : 32;
};