/*
        Address reporting hack v1.0 by nim at nimlabs dot org
	based on: Transparent IP bind v1.0 by nathan 0x00 org

	Description: 	Allows faking an address in getsockname to help
                        with NAT traversal.

	Compiling:
			gcc -fPIC -shared -o munge_getsockname.so \
                          munge_getsockname.c -ldl

	Usage:		(To replace interal 10.1.2.3 IP with external
                         18.4.5.6 address)
			export LD_PRELOAD=/path/to/munge_getsockname.so
			export FROMIP="10.1.2.3"
			export TOIP="18.4.5.6"
			command to be munged
*/

#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <dlfcn.h>

#define DEBUGP(format, ...)
/*#define DEBUGP(format, ...) fprintf(stderr, format, __VA_ARGS__)*/

static char version[]="munge_getsockname.c v1.0 by nim at nimlabs dot org";

/* static data gets initialized as zero by default. */
static int (*real_getsockname)(int, __SOCKADDR_ARG, socklen_t*);
static int init_latch;
static in_addr_t from_ip;
static in_addr_t to_ip;
static int ip_valid;

static void latch_init()
{
        if (init_latch)
                return;

        real_getsockname = dlsym(RTLD_NEXT, "getsockname");

        char *from_ip_str = getenv("FROMIP");
        char *to_ip_str = getenv("TOIP");
        if (from_ip_str != NULL && to_ip_str != NULL)
        {
                from_ip = inet_addr(from_ip_str);
                to_ip = inet_addr(to_ip_str);
                ip_valid = 1;
                DEBUGP("denat: from_ip %x to_ip %x\n", from_ip, to_ip);
        }

        init_latch = 1;
}


int getsockname(int __fd, __SOCKADDR_ARG __addr, socklen_t* __len);

int getsockname(int __fd, __SOCKADDR_ARG __addr, socklen_t* __len)
{
	latch_init();

	int ret = real_getsockname(__fd, __addr, __len);

        if (0 != ret || 0 == *__len)
        {
                DEBUGP("munge: getsockname failed: ret %d len %d\n",
                       ret, *__len);
                return ret;
        }

        if (AF_INET != __addr.__sockaddr__->sa_family)
        {
                DEBUGP("munge: getsockname returned non IPv4: family %d\n",
                       __addr.__sockaddr__->sa_family);
                return ret;
        }

        in_addr_t retaddr = __addr.__sockaddr_in__->sin_addr.s_addr;

        DEBUGP("munge: intercepted getsockname got: len %d retaddr %x\n",
                *__len, retaddr);

        if (ip_valid && from_ip == retaddr)
        {
                DEBUGP("munge: REPLACED getsockname output: retaddr %x toaddr %x\n",
                       retaddr, to_ip);
                __addr.__sockaddr_in__->sin_addr.s_addr = to_ip;
        }

        return ret;
}