udp_dns_reply_v4.c

static __always_inline
int udp_dns_reply_v4(struct cursor *c, uint32_t key)
{
	struct udphdr  *udp;
	struct dnshdr  *dns;
 
	// check that we have a DNS packet
	if (!(udp = parse_udphdr(c)) || udp->dest != __bpf_htons(DNS_PORT)
	||  !(dns = parse_dnshdr(c)))
		return 1;
 
	// get the rrl bucket from the map by IPv4 address
	struct bucket *b = bpf_map_lookup_elem(&state_map, &key);
 
	// did we see this IPv4 address before?
	if (b)
		return do_rate_limit(udp, dns, b);
 
	// create new starting bucket for this IPv4 address
	struct bucket new_bucket;
	new_bucket.start_time = bpf_ktime_get_ns();
	new_bucket.n_packets = 0;
 
	// store the bucket and pass the packet
	bpf_map_update_elem(&state_map, &key, &new_bucket, BPF_ANY);
	return 1;
}