hello-dns/tdns/dns.hh

#pragma once
#include <stdint.h>
// note - some platforms are confused over these #defines. Specifically, BYTE_ORDER without __ is a false prophet and may lie!
struct dnsheader {
        unsigned        id :16;         /* query identification number */
#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
                        /* fields in third byte */
        unsigned        qr: 1;          /* response flag */
        unsigned        opcode: 4;      /* purpose of message */
        unsigned        aa: 1;          /* authoritative answer */
        unsigned        tc: 1;          /* truncated message */
        unsigned        rd: 1;          /* recursion desired */
                        /* fields in fourth byte */
        unsigned        ra: 1;          /* recursion available */
        unsigned        unused :1;      /* unused bits (MBZ as of 4.9.3a3) */
        unsigned        ad: 1;          /* authentic data from named */
        unsigned        cd: 1;          /* checking disabled by resolver */
        unsigned        rcode :4;       /* response code */
#elif __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ 
                        /* fields in third byte */
        unsigned        rd :1;          /* recursion desired */
        unsigned        tc :1;          /* truncated message */
        unsigned        aa :1;          /* authoritative answer */
        unsigned        opcode :4;      /* purpose of message */
        unsigned        qr :1;          /* response flag */
                        /* fields in fourth byte */
        unsigned        rcode :4;       /* response code */
        unsigned        cd: 1;          /* checking disabled by resolver */
        unsigned        ad: 1;          /* authentic data from named */
        unsigned        unused :1;      /* unused bits (MBZ as of 4.9.3a3) */
        unsigned        ra :1;          /* recursion available */
#endif
                        /* remaining bytes */
        uint16_t        qdcount;    /* number of question entries */
        uint16_t        ancount;    /* number of answer entries */
        uint16_t        nscount;    /* number of authority entries */
        uint16_t        arcount;    /* number of resource entries */
};

static_assert(sizeof(dnsheader) == 12, "dnsheader size must be 12");