XDP blog, post 1 gist 5

xdp1-5-udp_dns_reply.c

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;
 
    uint8_t        *qname;
    struct dns_qrr *qrr;
 
    if (dns->flags.as_bits_and_pieces.qr
    ||  dns->qdcount != __bpf_htons(1)
    ||  dns->ancount || dns->nscount
    ||  dns->arcount >  __bpf_htons(1)
    ||  !(qname = parse_dname(c, (void *)dns))
    ||  !(qrr = parse_dns_qrr(c)))
        return -1;

// we continue below