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portforward.c
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portforward.c
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/* Licensed under MIT
* Original source: https://github.com/antialize/utils/blob/master/portforward.c
* It was later licensed: https://github.com/antialize/utils/issues/1
*/
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>
#include <stdlib.h>
#include <netdb.h>
#include <signal.h>
#define DIE(msg) perror(msg); exit(1);
/**
* Send the traffic from src socket to dst socket
*
* When done or on any error, this dies and will kill the forked process.
*/
void com(int src, int dst) {
char buf[1024 * 4];
int r, i, j;
r = read(src, buf, 1024 * 4);
while (r > 0) {
i = 0;
while (i < r) {
j = write(dst, buf + i, r - i);
if (j == -1) {
DIE("write"); // TODO is errno EPIPE
}
i += j;
}
r = read(src, buf, 1024 * 4);
}
if (r == -1) {
DIE("read");
}
shutdown(src, SHUT_RD);
shutdown(dst, SHUT_WR);
close(src);
close(dst);
exit(0);
}
/**
* Opens a connection to the destination.
*
* On any error, this dies and will kill the forked process.
*/
int open_forwarding_socket(char *forward_name, int forward_port) {
int forward_socket;
struct hostent *forward;
struct sockaddr_in forward_address;
forward = gethostbyname(forward_name);
if (forward == NULL) {
DIE("gethostbyname");
}
bzero((char *) &forward_address, sizeof(forward_address));
forward_address.sin_family = AF_INET;
bcopy((char *)forward->h_addr, (char *) &forward_address.sin_addr.s_addr, forward->h_length);
forward_address.sin_port = htons(forward_port);
forward_socket = socket(AF_INET, SOCK_STREAM, 0);
if (forward_socket == -1) {
DIE("socket");
}
if (connect(forward_socket, (struct sockaddr *) &forward_address, sizeof(forward_address)) == -1) {
DIE("connect");
}
return forward_socket;
}
/**
* Forwards all traffic from the client's socket to the destination
* host/port. This also initiates the connection to the destination.
*/
void forward_traffic(int client_socket, char *forward_name, int forward_port) {
int forward_socket;
pid_t down_pid;
forward_socket = open_forwarding_socket(forward_name, forward_port);
// Fork - child forwards traffic back to client, parent sends from client
// to forwarded port
down_pid = fork();
if (down_pid == -1) {
DIE("fork");
}
if (down_pid == 0) {
com(forward_socket, client_socket);
} else {
com(client_socket, forward_socket);
}
}
/**
* Opens the listening port or dies trying.
*/
int open_listening_port(int server_port) {
struct sockaddr_in server_address;
int server_socket;
server_socket = socket(AF_INET, SOCK_STREAM, 0);
if (server_socket == -1) {
DIE("socket");
}
bzero((char *) &server_address, sizeof(server_address));
server_address.sin_family = AF_INET;
server_address.sin_addr.s_addr = INADDR_ANY;
server_address.sin_port = htons(server_port);
if (bind(server_socket, (struct sockaddr *) &server_address, sizeof(server_address)) == -1) {
DIE("bind");
}
if (listen(server_socket, 40) == -1) {
DIE("listen");
}
return server_socket;
}
/**
* Handles a single connection. Will fork in order to handle another
* connection while the child starts to forward traffic.
*/
void accept_connection(int server_socket, char *forward_name, int forward_port) {
int client_socket;
pid_t up_pid;
client_socket = accept(server_socket, NULL, NULL);
if (client_socket == -1) {
DIE("accept");
}
// Fork - Child handles this connection, parent listens for another
up_pid = fork();
if (up_pid == -1) {
DIE("fork");
}
if (up_pid == 0) {
forward_traffic(client_socket, forward_name, forward_port);
exit(1);
}
close(client_socket);
}
/**
* Argument parsing and validation
*/
void parse_arguments(int argc, char **argv, int *server_port, char **forward_name, int *forward_port) {
if (argc < 3) {
fprintf(stderr, "Not enough arguments\n");
fprintf(stderr, "Syntax: %s listen_port forward_host [forward_port]\n", argv[0]);
exit(1);
}
*server_port = atoi(argv[1]);
if (*server_port < 1) {
fprintf(stderr, "Listen port is invalid\n");
exit(1);
}
*forward_name = argv[2];
if (argc == 3) {
*forward_port = *server_port;
} else {
*forward_port = atoi(argv[3]);
if (*forward_port < 1) {
fprintf(stderr, "Forwarding port is invalid\n");
exit(1);
}
}
}
/**
* Coordinates the effort
*/
int main(int argc, char **argv) {
int server_port, forward_port, server_socket;
char *forward_name;
parse_arguments(argc, argv, &server_port, &forward_name, &forward_port);
signal(SIGCHLD, SIG_IGN);
server_socket = open_listening_port(server_port);
while (1) {
accept_connection(server_socket, forward_name, forward_port);
}
return 0;
}