Force any binary to use a specific network interface in Linux

.gitignore

*.so

README.md

setnif

Force any binary to use a specific network interface.

Build: $ make

Use: LD_PRELOAD=$(pwd)/setnif.so NIF=eth1 program args...

Linux only.

Makefile

all: setnif.so
.PHONY: all

setnif.so: setnif.c
	gcc -O2 -o $@ -fpic -shared $< -ldl -D_GNU_SOURCE

clean:
	rm -f setnif.so

setnif.c

#include <dlfcn.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>

#define CONST_STR_LEN(s) (s), (sizeof(s) - 1)

static int (*std_socket)(int, int, int);
static const char *nif;
static int nif_len;

static void __attribute__ ((constructor)) setnif_init(void) {
	const char *err;

	std_socket = dlsym(RTLD_NEXT, "socket");
	nif = getenv("NIF");
	nif_len = strlen(nif);
}

static int set_nif(int fd) {
	if (NULL == nif) return fd;

	if (setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, nif, strlen(nif)) < 0) {
		int e = errno;
		close(fd);
		errno = e;
		return -1;
	}

	return fd;
}

int socket(int domain, int type, int protocol) {
	int fd;

	if (NULL == std_socket) {
		errno = ENOSYS;
		return -1;
	}

	fd = std_socket(domain, type, protocol);
	if (fd < 0) return fd;

	switch (domain) {
	case AF_INET:
	case AF_INET6:
		return set_nif(fd);
	default:
		return fd;
	}
}