// SPDX-License-Identifier: GPL-2.0
/*
 * Needs something like:
 *
 * iptables -t nat -A POSTROUTING -o nomatch -j MASQUERADE
 *
 * so NAT engine attaches a NAT null-binding to each connection.
 *
 * With unmodified kernels, child or parent will exit with
 * "Port number changed" error, even though no port translation
 * was requested.
 */

#include <errno.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/wait.h>

#define LEN 512
#define PORT 56789
#define TEST_TIME 5

static void die(const char *e)
{
        perror(e);
        exit(111);
}

static void die_port(const struct sockaddr_in *sin, uint16_t want)
{
        uint16_t got = ntohs(sin->sin_port);
        char str[INET_ADDRSTRLEN];

        inet_ntop(AF_INET, &sin->sin_addr, str, sizeof(str));

        fprintf(stderr, "Port number changed, wanted %d got %d from %s\n", want, got, str);
        exit(1);
}

static int udp_socket(void)
{
        static const struct timeval tv = {
                .tv_sec = 1,
        };
        int fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);

        if (fd < 0)
                die("socket");

        setsockopt(fd, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv));
        return fd;
}

int main(int argc, char *argv[])
{
        struct sockaddr_in sa1 = {
                .sin_family = AF_INET,
        };
        struct sockaddr_in sa2 = {
                .sin_family = AF_INET,
        };
        int s1, s2, status;
        time_t end, now;
        socklen_t plen;
        char buf[LEN];
        bool child;

        sa1.sin_port = htons(PORT);
        sa2.sin_port = htons(PORT + 1);

        s1 = udp_socket();
        s2 = udp_socket();

        inet_pton(AF_INET, "127.0.0.11", &sa1.sin_addr);
        inet_pton(AF_INET, "127.0.0.12", &sa2.sin_addr);

        if (bind(s1, (struct sockaddr *)&sa1, sizeof(sa1)) < 0)
                die("bind 1");
        if (bind(s2, (struct sockaddr *)&sa2, sizeof(sa2)) < 0)
                die("bind 2");

        child = fork() == 0;

        now = time(NULL);
        end = now + TEST_TIME;

        while (now < end) {
                struct sockaddr_in peer;
                socklen_t plen = sizeof(peer);

                now = time(NULL);

                if (child) {
                        if (sendto(s1, buf, LEN, 0, (struct sockaddr *)&sa2, sizeof(sa2)) != LEN)
                                continue;

                        if (recvfrom(s2, buf, LEN, 0, (struct sockaddr *)&peer, &plen) < 0)
                                die("child recvfrom");

                        if (peer.sin_port != htons(PORT))
                                die_port(&peer, PORT);
                } else {
                        if (sendto(s2, buf, LEN, 0, (struct sockaddr *)&sa1, sizeof(sa1)) != LEN)
                                continue;

                        if (recvfrom(s1, buf, LEN, 0, (struct sockaddr *)&peer, &plen) < 0)
                                die("parent recvfrom");

                        if (peer.sin_port != htons((PORT + 1)))
                                die_port(&peer, PORT + 1);
                }
        }

        if (child)
                return 0;

        wait(&status);

        if (WIFEXITED(status))
                return WEXITSTATUS(status);

        return 1;
}