l2tp-ip_pktinfo-bad-ifindex: https://github.com/xelerance/xl2tpd/issues/147#issuecomment-387302089

enodev.c

#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <stdbool.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

struct buf_t {
	char *start;
	int len;
};

int fd;
int val = 1;
struct sockaddr_in addr;
struct buf_t buf;

void loop() {
	struct msghdr msgh;
	struct iovec iov;
	struct cmsghdr *cmsg;
	int rv;
	struct sockaddr_in from;
	struct in_pktinfo to;
	char cbuf[512];

	buf.len = 4096;
	buf.start = malloc(buf.len);

	while (true) {
		memset(&msgh, 0, sizeof(struct msghdr));
		iov.iov_base = buf.start;
		iov.iov_len  = buf.len;
		msgh.msg_control = cbuf;
		msgh.msg_controllen = sizeof(cbuf);
		msgh.msg_name = &from;
		msgh.msg_namelen = sizeof(from);
		msgh.msg_iov  = &iov;
		msgh.msg_iovlen = 1;
		msgh.msg_flags = 0;

		rv = recvmsg(fd, &msgh, 0);
		for (cmsg = CMSG_FIRSTHDR(&msgh);
				cmsg != NULL;
				cmsg = CMSG_NXTHDR(&msgh, cmsg)) {
			if (cmsg->cmsg_level == IPPROTO_IP && cmsg->cmsg_type == IP_PKTINFO) {
				to = *(struct in_pktinfo *)CMSG_DATA(cmsg);
			}
		}
		iov.iov_len = rv;
		rv = sendmsg(fd, &msgh, 0);
		if (rv < 0) {
			fprintf(stderr, "sendmsg: %s\n", strerror(errno));
		}
	}
}

int main() {

	fd = socket(AF_INET, SOCK_DGRAM, 0);
	setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &val, sizeof(val));
	setsockopt(fd, SOL_SOCKET, SO_NO_CHECK, &val, sizeof(val));

	addr.sin_family = AF_INET;
	addr.sin_addr.s_addr = 0;
	addr.sin_port = htons(1701);
	bind(fd, (struct sockaddr *)&addr, sizeof(addr));

	setsockopt(fd, IPPROTO_IP, IP_PKTINFO, &val, sizeof(val));
#define IP_IPSEC_REFINFO 30
	setsockopt(fd, IPPROTO_IP, IP_IPSEC_REFINFO, &val, sizeof(val));

	loop();
	return 0;
}