otherport.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <dlfcn.h>
#include <string.h>

ssize_t (*real_sendto)(int socket, const void *buffer, size_t length, int flags,
		       const struct sockaddr *dest_addr, socklen_t dest_len);
ssize_t (*real_recvfrom)(int socket, void *buffer, size_t length, int flags,
			 struct sockaddr *address, socklen_t *address_len);

unsigned short old_port, new_port;

void _init(void)
{
	char *old_port_env;
	if (!(old_port_env = getenv("OLD_PORT"))) {
		fprintf(stderr, "getenv(OLD_PORT) failed\n");
		exit(42);
	}
	old_port = htons(atoi(old_port_env));

	char *new_port_env;
	if (!(new_port_env = getenv("NEW_PORT"))) {
		fprintf(stderr, "getenv(NEW_PORT) failed\n");
		exit(42);
	}
	new_port = htons(atoi(new_port_env));

	const char *err;
	real_sendto = dlsym(RTLD_NEXT, "sendto");
	if ((err = dlerror()) != NULL) {
		fprintf(stderr, "dlsym(sendto) failed: %s\n", err);
		exit(42);
	}

	real_recvfrom = dlsym(RTLD_NEXT, "recvfrom");
	if ((err = dlerror()) != NULL) {
		fprintf(stderr, "dlsym(recvfrom): %s\n", err);
		exit(42);
	}
}

ssize_t sendto(int socket, const void *buffer, size_t length, int flags,
	       const struct sockaddr *dest_addr, socklen_t dest_len)
{
	static struct sockaddr_in *dest_addr_in;
	dest_addr_in = (struct sockaddr_in *)dest_addr;

	if (dest_addr_in->sin_port == old_port) {
		static struct sockaddr_storage new_dest_addr;
		memcpy(&new_dest_addr, dest_addr, dest_len);

		static struct sockaddr_in *new_dest_addr_in;
		new_dest_addr_in = (struct sockaddr_in *)&new_dest_addr;
		new_dest_addr_in->sin_port = new_port;

		return real_sendto(socket, buffer, length, flags,
				   (struct sockaddr *)&new_dest_addr, dest_len);
	}

	return real_sendto(socket, buffer, length, flags, dest_addr, dest_len);
}

ssize_t recvfrom(int socket, void *buffer, size_t length, int flags,
		 struct sockaddr *address, socklen_t *address_len)
{
	static struct sockaddr_in *address_in;
	address_in = (struct sockaddr_in *)address;

	if (address_in != NULL && address_in->sin_port == old_port) {
		address_in->sin_port = new_port;
	}

	ssize_t res;
	res = real_recvfrom(socket, buffer, length, flags, address,
			    address_len);

	if (address_in != NULL && address_in->sin_port == new_port) {
		address_in->sin_port = old_port;
	}

	return res;
}