Howdie,
On Monday 26 January 2004 13:49, C Jagadish wrote:
> Hi,
>
> How do I route packets to destination "A" through an interface
> differnet from the entry in the routing table? ( In RedHat
> Linux 7.2)
I don't think you can do this exactly.
However, you can limit a packet to go through a specific interface assuming
you have a route through it even though normally this will not be the route
chosen, by using the SO_BINDTODEVICE socket options.
Attached is small example that creates a shared library that if preloaded
(using LD_PRELOAD) will hijack all the socket() system calls and set this
option on all sockets created, thus enabling to limit any executable to use a
specific interface in a similar fashion to way the ping program -r option
does for ping.
It's a rather crude example, if you plan to make real use of it make sure to
handle error conditions in saner fashion. A better way to choose the
interface to route packets to is also a good idea :-)
Cheers,
Gilad
--
Gilad Ben-Yossef <gilad@codefidence.com>
Codefidence. A name you can trust (tm)
http://www.codefidence.com
"The future is here, it's just not evenly distributed yet."
- William Gibson
/************************************************************
* Copyright (C) 2004 Codefidence ltd.
* Author: Gilad Ben-Yossef <gilad@codefidence.com>
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version 2
* of the License, or (at your option) any later version.
*
* This program 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 General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
* 02111-1307, USA.
*/
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <net/if.h>
#define __USE_GNU // This is needed for the RTLD_NEXT definition
#include <dlfcn.h>
/* Change this to route the packet via a different interface. */
#define IFNAME "eth3"
int socket(int domain, int type, int protocol) {
int (*origsock)(int domain, int type, int protocol);
char * error;
int sock;
origsock = dlsym(RTLD_NEXT, "socket");
if ((error = dlerror()) != NULL) {
fprintf (stderr, "%s\n", error);
exit(1);
}
sock = origsock(domain, type, protocol);
if(sock != -1) {
struct ifreq interface;
strncpy(interface.ifr_ifrn.ifrn_name, IFNAME, IFNAMSIZ);
if (setsockopt(sock, SOL_SOCKET, SO_BINDTODEVICE, (char *)&interface, sizeof(interface)) < 0) {
perror("sendpacket: setting SO_BINDTODEVICE");
exit(1);
}
}
return sock;
}
/*
Compile with:
gcc sendto.c -o sendto.so -ldl -shared
Use with:
LD_PRELOAD=sendto.so ./program
*/
