Randomizing srcip PoC

source_random.c

/*
   Copyright (C) 2000  Daniel Ryde
   Copyright (C) 2017  deseven <de7@deseven.info>

   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 connect() to use a set of IP addresses
   specified via the enviroment variable SRCADDRS.

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

   Example in bash:
   SRCADDRS="127.0.0.1,127.0.0.2,127.0.0.3" LD_PRELOAD=./srcrand.so curl url url url url

*/

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <dlfcn.h>
#include <errno.h>
#include <time.h>
#include <arpa/inet.h>

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

char *src_addrs_env;
int num_addrs = 0;
char* src_addrs[254];

unsigned long int src_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 (src_addrs_env = getenv ("SRCADDRS")) {
		const char s[3] = ",; ";
		char *token;
		token = strtok(src_addrs_env,s);
		while(token != NULL) {
			src_addrs[num_addrs] = token;
			token = strtok(NULL,s);
			num_addrs++;
		}
		srand(time(NULL));
	} else {
		printf("no source addrs are defined\nset env variable SRCADDRS first\n");
		exit(1);
	}
}

int connect (int fd, const struct sockaddr *sk, socklen_t sl) {
	static struct sockaddr_in *rsk_in;
	int randomAddr = rand() % num_addrs;

	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));*/
	src_addr_saddr = inet_addr (src_addrs[randomAddr]);
	local_sockaddr_in->sin_family = AF_INET;
	local_sockaddr_in->sin_addr.s_addr = src_addr_saddr;
	local_sockaddr_in->sin_port = htons (0);
        if ((rsk_in->sin_family == AF_INET)
		&& (src_addrs)) {
		real_bind (fd, (struct sockaddr *)local_sockaddr_in, sizeof (struct sockaddr));
	}
	return real_connect (fd, sk, sl);
}