#include <sys/types.h>
#include <sys/socket.h>
#include <sys/select.h>
#include <netinet/in.h>
#include <err.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define	SENDSIZE	2000
#define	SENDPORT	1234
int
main(int argc, char **argv)
{
	struct addrinfo hints, *res;
	struct sockaddr_in6 sa6;
	struct msghdr msg;
	struct iovec iov;
	struct timeval tv;
	struct ip6_mtuinfo *minfo;
	struct cmsghdr *cmsg;
	fd_set fd;
	int s6, ret, v;
	char *buf, ctrl[CMSG_SPACE(sizeof(struct ip6_mtuinfo))];

	if (argc < 2)
		errx(1, "Usage: %s <IPv6 destination address>\n", argv[0]);

	memset(&hints, 0, sizeof(hints));
	hints.ai_family = AF_INET6;
	hints.ai_flags = AI_NUMERICHOST;
	ret = getaddrinfo(argv[1], NULL, &hints, &res);
	if (ret != 0)
		errx(1, "getaddrinfo(%s): %s", argv[1], gai_strerror(ret));
	s6 = socket(AF_INET6, SOCK_DGRAM, IPPROTO_UDP);
	if (s6 < 0)
		err(1, "socket()");
	v = 1;
	ret = setsockopt(s6, IPPROTO_IPV6, IPV6_RECVPATHMTU, &v, sizeof(v));
	if (ret < 0)
		err(1, "setsockopt(IPV6_RECVPATHMTU)");
	v = 1;
	ret = setsockopt(s6, IPPROTO_IPV6, IPV6_DONTFRAG, &v, sizeof(v));
	if (ret < 0)
		err(1, "setsockopt(IPV6_DONTFRAG)");
	buf = calloc(1, SENDSIZE);
	if (buf == NULL)
		err(1, "calloc()");
	sa6 = *(struct sockaddr_in6 *)res->ai_addr;
	sa6.sin6_port = htons(SENDPORT);
	ret = sendto(s6, buf, SENDSIZE, 0,
	    (struct sockaddr *)&sa6, sizeof(sa6));
	if (ret != SENDSIZE)
		warn("sendto()");

	FD_ZERO(&fd);
	FD_SET(s6, &fd);
	tv.tv_sec = 2;
	tv.tv_usec = 0;
	ret = select(s6 + 1, &fd, NULL, NULL, &tv);
	if (ret <= 0)
		printf("No ancillary data received\n");
	else {
		memset(&msg, 0, sizeof(msg));
		msg.msg_name = &sa6;
		msg.msg_namelen = sizeof(sa6);
		msg.msg_control = ctrl;
		msg.msg_controllen = sizeof(ctrl);
		msg.msg_iov = &iov;
		msg.msg_iovlen = 1;
		iov.iov_base = buf;
		iov.iov_len = SENDSIZE;
		cmsg = (struct cmsghdr *)ctrl;
		ret = recvmsg(s6, &msg, 0);
		if (ret < 0)
			warn("recvmsg()");
		else if (cmsg->cmsg_level == IPPROTO_IPV6 &&
		    cmsg->cmsg_type == IPV6_PATHMTU &&
		    cmsg->cmsg_len == CMSG_LEN(sizeof(struct ip6_mtuinfo))) {
			minfo = (struct ip6_mtuinfo *)CMSG_DATA(cmsg);
			ret = getnameinfo((struct sockaddr *)&minfo->ip6m_addr,
			    sizeof(minfo->ip6m_addr), buf, SENDSIZE, NULL, 0,
			    NI_NUMERICHOST);
			if (ret == 0)
				printf("IPV6_PATHMTU ancillary "
				    "data received:\nIPv6 address: %s\n"
				    "MTU: %u\n", buf, minfo->ip6m_mtu);
			else
				warnx("getnameinfo(): %s", gai_strerror(ret));

		}
	}
	free(buf);
	freeaddrinfo(res);
	close(s6);
	return (0);
}