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/*
This file is part of ICMPTX
Original code copyright date unknown, edi/teso.
Copyright (C) 2006 Thomer M. Gil <thomer@thomer.com>
Copyright (C) 2008 John Plaxco <john@johnplaxco.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 ICMPTX. If not, see <http://www.gnu.org/licenses/>.
*/
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/socket.h>
#include <netinet/in_systm.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include "tun_dev.h"
/*
* standard ICMP header
*/
struct icmp {
u_int8_t type;
u_int8_t code;
u_int16_t cksum;
u_int16_t id;
u_int16_t seq;
};
/*
* standard IP header
*/
struct ip {
unsigned int ip_hl:4,
ip_v:4;
unsigned char ip_tos;
unsigned short ip_len;
unsigned short ip_id;
unsigned short ip_off;
unsigned char ip_ttl;
unsigned char ip_p;
unsigned short ip_sum;
struct in_addr ip_src,ip_dst;
};
unsigned short in_cksum(unsigned short *, int);
/* int sock - ICMP socket used to communicate
int proxy - 0 means send echo requests, 1 means send echo replies
struct sockaddr_in *target - For the client, points to the server. For the server, the default place to send replies
int tun_fd - Input/output file descriptor
int packetsize - maximum size of ICMP data payload
u_int16_t id - tunnel id field
*/
int icmp_tunnel(int sock, int proxy, struct sockaddr_in *target, int tun_fd, int packetsize, u_int16_t id) {
int len, result, fromlen, num;
char* packet;
fd_set fs;
unsigned char didSend, didReceive;
struct icmp *icmp, *icmpr;
struct timeval tv;
struct sockaddr_in from;
len = sizeof (struct icmp);
if ((packet = malloc (len+packetsize)) == NULL) {
fprintf(stderr, "Error allocating packet buffer");
exit(1);
}
memset (packet, 0, len+packetsize);
icmp = (struct icmp*)(packet);
icmpr = (struct icmp*)(packet+sizeof(struct ip));
/* here's the infinite loop that shovels packets back and forth while the tunnel's up */
while (1) {
FD_ZERO (&fs);
FD_SET (tun_fd, &fs);
FD_SET (sock, &fs);
didSend = didReceive = 0;
tv.tv_sec = 1;
tv.tv_usec = 0;
select (tun_fd>sock?tun_fd+1:sock+1, &fs, NULL, NULL, &tv);/* block until data's available in one direction or the other, or it's time to poll */
/* data available on tunnel device, need to transmit over icmp */
if (FD_ISSET(tun_fd, &fs)) {
result = tun_read(tun_fd, packet+len, packetsize);
if (!result) {/*eof*/
return 0;
} else if (result==-1) {
perror("read");
return -1;
}
icmp->type = proxy ? 0 : 8;/*echo response or echo request*/
icmp->code = 0;
icmp->id = id;/* mark the packet so the other end knows it's a tunnel packet */
icmp->seq = 0;
icmp->cksum = 0;
icmp->cksum = in_cksum((unsigned short*)packet, len+result);
result = sendto(sock, (char*)packet, len+result, 0, (struct sockaddr*)target, sizeof (struct sockaddr_in));
if (result==-1) {
perror ("sendto");
return -1;
}
didSend = 1;
}
/* data available on socket from icmp, need to pass along to tunnel device */
if (FD_ISSET(sock, &fs)) {
fromlen = sizeof (struct sockaddr_in);
num = recvfrom(sock, packet, len+packetsize, 0, (struct sockaddr*)&from, (socklen_t*) &fromlen);
/* make the destination be the source of the most recently received packet (this can be dangerous) */
memcpy(&(target->sin_addr.s_addr), &(from.sin_addr.s_addr), 4*sizeof(char));
if (icmpr->id == id) {/*this filters out all of the other ICMP packets I don't care about*/
tun_write(tun_fd, packet+sizeof(struct ip)+sizeof(struct icmp), num-sizeof(struct ip)-sizeof(struct icmp));
didReceive = 1;
} else if (icmpr->type == 8) {/* some normal ping request */
icmpr->type = 0;/*echo response*/
icmpr->code = 0;
icmpr->id = icmpr->id;
icmpr->seq = icmpr->seq;
icmpr->cksum = 0;
icmpr->cksum = in_cksum((unsigned short*)packet, num);
result = sendto(sock, (char*)packet+sizeof(struct ip), len+num-sizeof(struct ip)-sizeof(struct icmp), 0, (struct sockaddr*)target, sizeof (struct sockaddr_in));
}
} /* end of data available */
/*
* if we didn't send or receive anything, the select timed out
* so lets send an echo request poll to the server (helps with
* stateful firewalls)
*/
if (!proxy && !didSend && !didReceive) {
icmp->type = 8;/*echo request*/
icmp->code = 0;
icmp->id = id;/*mark the packet so the other end knows it's a tunnel packet*/
icmp->seq = 0;
icmp->cksum = 0;
icmp->cksum = in_cksum((unsigned short*)packet, len);
result = sendto(sock, (char*)packet, len, 0, (struct sockaddr*)target, sizeof (struct sockaddr_in));
if (result==-1) {
perror ("sendto");
return -1;
}
}
} /* end of while(1) */
return 0;
}
/*
* this is the function that starts it all rolling
* id - the id value for the icmp stream, to distinguish it from any other incoming ICMP packets
* packetsize - I think this is the mtu value for the packets going across the tunnel, seems to be used in buffer allocations
* isServer - 0 for client mode, 1 for server mode
* serverNameOrIP - the server's host name or IP address
* tun_fd - the file descriptor of the socket we read and write from
*/
int run_icmp_tunnel (int id, int packetsize, int isServer, char *serverNameOrIP, int tun_fd) {
struct sockaddr_in target;
struct in_addr inp;
int s;
if (!inet_aton(serverNameOrIP, &inp)) {
struct hostent* he;
if (!(he = gethostbyname (serverNameOrIP))) {
herror ("gethostbyname");
return -1;
}
memcpy (&target.sin_addr.s_addr, he->h_addr_list[0], he->h_length);
} else {
target.sin_addr.s_addr = inp.s_addr;
}
target.sin_family = AF_INET;
if ( (s = socket (AF_INET, SOCK_RAW, IPPROTO_ICMP)) == -1) {
perror ("socket");
return -1;
}
icmp_tunnel(s, isServer, &target, tun_fd, packetsize, (u_int16_t) id);
close(s);
return 0;
}
/*
* calculate the icmp checksum for the packet, including data
*/
unsigned short in_cksum (unsigned short *addr, int len) {
int nleft = len;
unsigned short *w = addr;
int sum = 0;
unsigned short answer = 0;
while (nleft > 1) {
sum += *w++;
nleft -= 2;
}
if (nleft == 1) {
*(unsigned char *) (&answer) = *(unsigned char *) w;
sum += answer;
}
sum = (sum >> 16) + (sum & 0xffff); /* add hi 16 to low 16 */
sum += (sum >> 16); /* add carry */
answer = ~(sum & 0xffff); /* truncate to 16 bits */
return (answer);
}
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