An LD_PRELOAD library to wrap socket() and apply a SO_MARK value

mark.c

/** Wrapper utility to mark all sockets of a program
 *
 * Compile:
 *  gcc -fPIC -c -o mark.o mark.c
 *  gcc -shared -o mark.so mark.o -ldl
 *
 * Usage example:
 *  MARK=10 LD_PRELOAD=./mark.so wget -qO- ifconfig.me
 *
 * @author András Korn <[email protected]>
 * @author Steffen Vogel <[email protected]>
 * @author D.J. Capelis
 * @copyright 2025, András Korn
 * @copyright 2014-2015, Steffen Vogel
 * @copyright 2007, Regents of the University of California
 * @license GPLv3
 *********************************************************************************/

#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>

#include <sys/types.h>
#include <sys/socket.h>

#define DEF_MARK 0xCD

typedef int (*socket_t)(int domain, int type, int protocol);

static socket_t _socket;

int socket(int domain, int type, int protocol)
{	
	if (!_socket)
		_socket = (socket_t) dlsym(RTLD_NEXT, "socket");

	int sd = _socket(domain, type, protocol);

	if (sd >= 0) {
		if (domain == AF_INET || domain == AF_INET6) {
			unsigned mark;
			char *endptr, *env = getenv("MARK"), *mark_debug = getenv("MARK_DEBUG");

			if (env != NULL) {
				mark = strtoul(env, &endptr, 0);
				
				if (env == endptr) /* skip if invalid mark given */
					return sd;
			}
			else
				mark = DEF_MARK;
			
			if (mark_debug != NULL) printf("mark: setting SO_MARK for fd=%u to %#x\n", sd, mark);
		
			setsockopt(sd, SOL_SOCKET, SO_MARK, &mark, sizeof(mark));
		}
	}
	
	return sd;
}