XDP blog, post 1 gist 1

xdpblog1-1_xdp_dns_says_no.c

SEC("xdp-dns-says-no-v1")
int xdp_dns_says_no(struct xdp_md *ctx)
{
    struct cursor   c;
    struct ethhdr  *eth;
    uint16_t        eth_proto;
    struct iphdr   *ipv4;
    struct ipv6hdr *ipv6;
 
    cursor_init(&c, ctx);
    if (!(eth = parse_eth(&c, &eth_proto)))
        return XDP_PASS;
 
    if (eth_proto == __bpf_htons(ETH_P_IP)) {
        if (!(ipv4 = parse_iphdr(&c))
        ||    ipv4->protocol != IPPROTO_UDP
        ||    udp_dns_reply(&c))
            return XDP_PASS;
 
        uint32_t swap_ipv4 = ipv4->daddr;
        ipv4->daddr = ipv4->saddr;
        ipv4->saddr = swap_ipv4;
 
    } else if (eth_proto == __bpf_htons(ETH_P_IPV6)) {
        if (!(ipv6 = parse_ipv6hdr(&c))
        ||    ipv6->nexthdr != IPPROTO_UDP
        ||    udp_dns_reply(&c))
            return XDP_PASS;
 
        struct in6_addr swap_ipv6 = ipv6->daddr;
        ipv6->daddr = ipv6->saddr;
        ipv6->saddr = swap_ipv6;
    } else
        return XDP_PASS;
 
    uint8_t swap_eth[ETH_ALEN];
    memcpy(swap_eth, eth->h_dest, ETH_ALEN);
    memcpy(eth->h_dest, eth->h_source, ETH_ALEN);
    memcpy(eth->h_source, swap_eth, ETH_ALEN);
    return XDP_TX;
}