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/*
* rebind: Intercept bind calls and bind to a different port
* Copyright 2010 Joel Martin
* Licensed under LGPL version 3 (see docs/LICENSE.LGPL-3)
*
* Overload (LD_PRELOAD) bind system call. If REBIND_PORT_OLD and
* REBIND_PORT_NEW environment variables are set then bind on the new
* port (of localhost) instead of the old port.
*
* This allows a bridge/proxy (such as websockify) to run on the old port and
* translate traffic to/from the new port.
*
* Usage:
* LD_PRELOAD=./rebind.so \
* REBIND_PORT_OLD=23 \
* REBIND_PORT_NEW=2023 \
* program
*/
//#define DO_DEBUG 1
#include <stdio.h>
#include <stdlib.h>
#define __USE_GNU 1 // Pull in RTLD_NEXT
#include <dlfcn.h>
#include <string.h>
#include <netinet/in.h>
#if defined(DO_DEBUG)
#define DEBUG(...) \
fprintf(stderr, "rebind: "); \
fprintf(stderr, __VA_ARGS__);
#else
#define DEBUG(...)
#endif
int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen)
{
static void * (*func)();
int do_move = 0;
struct sockaddr_in * addr_in = (struct sockaddr_in *)addr;
struct sockaddr_in addr_tmp;
socklen_t addrlen_tmp;
char * PORT_OLD, * PORT_NEW, * end1, * end2;
int ret, oldport, newport, askport = htons(addr_in->sin_port);
uint32_t askaddr = htons(addr_in->sin_addr.s_addr);
if (!func) func = (void *(*)()) dlsym(RTLD_NEXT, "bind");
DEBUG(">> bind(%d, _, %d), askaddr %d, askport %d\n",
sockfd, addrlen, askaddr, askport);
/* Determine if we should move this socket */
if (addr_in->sin_family == AF_INET) {
// TODO: support IPv6
PORT_OLD = getenv("REBIND_OLD_PORT");
PORT_NEW = getenv("REBIND_NEW_PORT");
if (PORT_OLD && (*PORT_OLD != '\0') &&
PORT_NEW && (*PORT_NEW != '\0')) {
oldport = strtol(PORT_OLD, &end1, 10);
newport = strtol(PORT_NEW, &end2, 10);
if (oldport && (*end1 == '\0') &&
newport && (*end2 == '\0') &&
(oldport == askport)) {
do_move = 1;
}
}
}
if (! do_move) {
/* Just pass everything right through to the real bind */
ret = (long) func(sockfd, addr, addrlen);
DEBUG("<< bind(%d, _, %d) ret %d\n", sockfd, addrlen, ret);
return ret;
}
DEBUG("binding fd %d on localhost:%d instead of 0x%x:%d\n",
sockfd, newport, ntohl(addr_in->sin_addr.s_addr), oldport);
/* Use a temporary location for the new address information */
addrlen_tmp = sizeof(addr_tmp);
memcpy(&addr_tmp, addr, addrlen_tmp);
/* Bind to other port on the loopback instead */
addr_tmp.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
addr_tmp.sin_port = htons(newport);
ret = (long) func(sockfd, &addr_tmp, addrlen_tmp);
DEBUG("<< bind(%d, _, %d) ret %d\n", sockfd, addrlen, ret);
return ret;
}
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