XDP blog, post 1 gist 6

xdp1-6-udp_dns_reply-part2.c

 // continuing:

    if (dns->arcount) {
        struct dns_rr  *opt_rr;
        uint8_t        *opt_owner = c->pos;
 
        opt_owner = c->pos;
        if (++c->pos > c->end || *opt_owner
        || !(opt_rr = parse_dns_rr(c))
        ||   opt_rr->type != __bpf_htons(RR_TYPE_OPT))
            return -1;
 
        if (opt_rr->rdata_len == 0)
            ; /* pass */
 
        else if (c->pos + 1 > c->end)
            return -1;
 
        else if (((void *)&opt_rr->rdata_len - (void *)dns) % 2 == 1) {
            uint16_t old1 = *(uint16_t *)(c->pos - 3);
            uint16_t old2 = *(uint16_t *)(c->pos - 1);
            opt_rr->rdata_len = 0;
            update_checksum(&udp->check, old1,
                    *(uint16_t *)(c->pos - 3));
            update_checksum(&udp->check, old2,
                    *(uint16_t *)(c->pos - 1));
        } else {
            uint16_t old_val = opt_rr->rdata_len;
            opt_rr->rdata_len = 0;
            update_checksum(&udp->check, old_val, 0);
        }
    }
    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;
}