#include <netinet/tcp.h>
#include <netlink/netlink.h>
#include <netlink/msg.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <linux/inet_diag.h>

// see <netinet/tcp.h>
#define TCP_ALL ((1 << (TCP_CLOSING + 1)) - 1)

/**
 * report `netstat -nat` or `ss -an` data using netlink method with libnl library
 * author: Elan Ruusamäe <glen@pld-linux.org>
 *
 */

int main() {
	struct nl_handle *nl = nl_handle_alloc();

	if (nl_connect(nl, NETLINK_INET_DIAG) < 0) {
		fprintf(stderr, "Couldn't connect to netlink: %s", nl_geterror());
		return -1;
	}

	struct inet_diag_req req = {
		.idiag_states = TCP_ALL,
	};

	struct nl_msg *nm = nlmsg_alloc_simple(TCPDIAG_GETSOCK, NLM_F_ROOT|NLM_F_MATCH|NLM_F_REQUEST);
	nlmsg_append(nm, &req, sizeof(struct inet_diag_req), 0);

	if (nl_send_auto_complete(nl, nm) < 0) {
		fprintf(stderr, "nl_send(): %s", nl_geterror());
		return -1;
	}
	nl_msg_dump(nm, stderr);
	nlmsg_free(nm);

	struct sockaddr_nl peer;
	unsigned char *buf = NULL;
	int len;

	while (1) {
		if ((len = nl_recv(nl, &peer, &buf, NULL)) <= 0) {
			fprintf(stderr, "nl_recv(): %s", nl_geterror());
			return -1;
		}

		struct nlmsghdr *h = (struct nlmsghdr*)buf;
		while (nlmsg_ok(h, len)) {
			if (h->nlmsg_type == NLMSG_DONE) {
				return 0;
			}

			struct inet_diag_msg *r = nlmsg_data(h);

			struct nl_addr *src = nl_addr_build(r->idiag_family, r->id.idiag_src, r->idiag_family == AF_INET ? 4 : 6);
			struct nl_addr *dst = nl_addr_build(r->idiag_family, r->id.idiag_dst, r->idiag_family == AF_INET ? 4 : 6);

			char saddr[64], daddr[64];
			nl_addr2str(src, saddr, sizeof(saddr));
			nl_addr2str(dst, daddr, sizeof(daddr));

			printf("%16s:%d\t%16s:%d\n", saddr, ntohs(r->id.idiag_sport), daddr, ntohs(r->id.idiag_dport));

			h = nlmsg_next(h, &len);
		}
		free(buf);
	}

	nl_handle_destroy(nl);
}