Bind programs to specific network interface. (Linux)

bind.c

/*
https://gist.github.com/stefansundin/e4fce00d97f7694c7a27
http://www.ryde.net/code/bind.c.txt
http://daniel-lange.com/archives/53-Binding-applications-to-a-specific-IP.html
*/

/*
   Copyright (C) 2000  Daniel Ryde

   This library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   This library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.
*/

/*
   LD_PRELOAD library to make bind and connect to use a virtual
   IP address as localaddress. Specified via the enviroment
   variable BIND_ADDR.

   Compile on Linux with:
   gcc -nostartfiles -fpic -shared bind.c -o bind.so -ldl -D_GNU_SOURCE


   Example in bash to make inetd only listen to the localhost
   lo interface, thus disabling remote connections and only
   enable to/from localhost:

   BIND_ADDR="127.0.0.1" LD_PRELOAD=./bind.so /sbin/inetd


   Example in bash to use your virtual IP as your outgoing
   sourceaddress for ircII:

   BIND_ADDR="your-virt-ip" LD_PRELOAD=./bind.so ircII

   Note that you have to set up your servers virtual IP first.


   This program was made by Daniel Ryde
   email: [email protected]
   web:   http://www.ryde.net/

   TODO: I would like to extend it to the accept calls too, like a
   general tcp-wrapper. Also like an junkbuster for web-banners.
   For libc5 you need to replace socklen_t with int.
*/



#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>
#include <errno.h>

int (*real_bind)(int, const struct sockaddr *, socklen_t);
int (*real_connect)(int, const struct sockaddr *, socklen_t);

char *bind_addr_env;
unsigned long int bind_addr_saddr;
unsigned long int inaddr_any_saddr;
struct sockaddr_in local_sockaddr_in[] = { 0 };

void _init (void)
{
	const char *err;

	real_bind = dlsym (RTLD_NEXT, "bind");
	if ((err = dlerror ()) != NULL) {
		fprintf (stderr, "dlsym (bind): %s\n", err);
	}

	real_connect = dlsym (RTLD_NEXT, "connect");
	if ((err = dlerror ()) != NULL) {
		fprintf (stderr, "dlsym (connect): %s\n", err);
	}

	inaddr_any_saddr = htonl (INADDR_ANY);
	if ((bind_addr_env = getenv ("BIND_ADDR"))) {
		bind_addr_saddr = inet_addr (bind_addr_env);
		local_sockaddr_in->sin_family = AF_INET;
		local_sockaddr_in->sin_addr.s_addr = bind_addr_saddr;
		local_sockaddr_in->sin_port = htons (0);
	}
}

int bind (int fd, const struct sockaddr *sk, socklen_t sl)
{
	static struct sockaddr_in *lsk_in;

	lsk_in = (struct sockaddr_in *)sk;
	// printf("bind: %d %s:%d\n", fd, inet_ntoa (lsk_in->sin_addr.s_addr), ntohs (lsk_in->sin_port));
	if ((lsk_in->sin_family == AF_INET)
		&& (lsk_in->sin_addr.s_addr == inaddr_any_saddr)
		&& (bind_addr_env)) {
		lsk_in->sin_addr.s_addr = bind_addr_saddr;
	}
	return real_bind (fd, sk, sl);
}

int connect (int fd, const struct sockaddr *sk, socklen_t sl)
{
	static struct sockaddr_in *rsk_in;
	
	rsk_in = (struct sockaddr_in *)sk;
	// printf("connect: %d %s:%d\n", fd, inet_ntoa (rsk_in->sin_addr.s_addr), ntohs (rsk_in->sin_port));
	if ((rsk_in->sin_family == AF_INET)
		&& (bind_addr_env)) {
		real_bind (fd, (struct sockaddr *)local_sockaddr_in, sizeof (struct sockaddr));
	}
	return real_connect (fd, sk, sl);
}