XDP blog, post 1 gist 2

xdpblog1-2-dnshdr.c

struct dnshdr {
    uint16_t id;
    union {
        struct {
            uint8_t  rd     : 1;
            uint8_t  tc     : 1;
            uint8_t  aa     : 1;
            uint8_t  opcode : 4;
            uint8_t  qr     : 1;
 
            uint8_t  rcode  : 4;
            uint8_t  cd     : 1;
            uint8_t  ad     : 1;
            uint8_t  z      : 1;
            uint8_t  ra     : 1;
        }        as_bits_and_pieces;
        uint16_t as_value;
    } flags;
    uint16_t qdcount;
    uint16_t ancount;
    uint16_t nscount;
    uint16_t arcount;
};
 
static __always_inline
int udp_dns_reply(struct cursor *c)
{
    struct udphdr  *udp;
    struct dnshdr  *dns;
 
    if (!(udp = parse_udphdr(c))|| udp->dest != __bpf_htons(DNS_PORT)
    ||  !(dns = parse_dnshdr(c)))
        return -1;
 
    uint16_t old_val = dns->flags.as_value;
    dns->flags.as_bits_and_pieces.ad = 0;
    dns->flags.as_bits_and_pieces.qr = 1;
    dns->flags.as_bits_and_pieces.rcode = RCODE_REFUSED;
    update_checksum(&udp->check, old_val, dns->flags.as_value);
 
    udp->dest   = udp->source;
    udp->source = __bpf_htons(DNS_PORT);
 
    return 0;
}

@Wqrld, these are just snippets (embedded in blog posts). Please checkout https://github.com/NLnetLabs/XDPeriments/ for the full code.