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/* TODO: act proper if interface is promisc (only reply to packets
send to local host */
#include <assert.h>
#include <linux/if_ether.h>
#include <linux/ip.h>
#include <linux/tcp.h>
#include <malloc.h>
#include <net/ethernet.h>
#include <net/if.h>
#include <netpacket/packet.h>
#include <stdio.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <sys/types.h>
#define BLEN 16384
#define PORT 80
const char * raw_payload =
"HTTP/1.0 301 Not enough crypto to process request\r\n"
"Location: %s\r\n"
"Connection: close\r\n"
"\r\n"
"<html><body>bye bye</body></html>";
unsigned chksum(unsigned short *buf, int length) {
unsigned sum = 0;
for (; length > 1; length -= 2)
sum += *buf++;
return sum;
}
unsigned short ipchksum(struct iphdr *ip) {
unsigned sum = chksum((unsigned short *) ip, sizeof(struct iphdr));
sum = (sum >> 16) + (sum & 0xFFFF);
sum += (sum >> 16);
return ~sum;
}
unsigned short tcpchksum(struct iphdr *ip, struct tcphdr *tcp, int length) {
unsigned sum =
chksum((unsigned short*) &(ip->saddr), sizeof(ip->saddr)) +
chksum((unsigned short*) &(ip->daddr), sizeof(ip->daddr)) +
((length + ip->protocol) << 8) +
chksum((unsigned short*) tcp, length);
if (length % 2)
sum += *(((char*) tcp) + length - 1);
sum = (sum >> 16) + (sum & 0xFFFF);
sum += (sum >> 16);
return ~sum;
}
int main(int argc, char **argv) {
if (argc != 2) {
printf("Usage: %s target-url\n", argv[0]);
return 1;
}
static unsigned ack_src;
/* prepare input buffer */
char *ibuf = malloc(BLEN);
assert(ibuf != NULL);
struct ethhdr *i_eth = (struct ethhdr*) ibuf;
struct iphdr *i_ip = (struct iphdr*) (i_eth + 1);
struct tcphdr *i_tcp = (struct tcphdr*) (i_ip + 1);
char *i_pld = (char*) (i_tcp + 1);
/* prepare output buffer */
const size_t os =
sizeof(struct ethhdr) +
sizeof(struct iphdr) +
sizeof(struct tcphdr) +
strlen(raw_payload) + strlen(argv[1]) - 1;
char *obuf = malloc(os);
assert(obuf != NULL);
struct ethhdr *o_eth = (struct ethhdr*) obuf;
struct iphdr *o_ip = (struct iphdr*) (o_eth + 1);
struct tcphdr *o_tcp = (struct tcphdr*) (o_ip + 1);
char *o_pld = (char*) (o_tcp + 1);
/* setup sock */
int fd = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_IP));
assert(fd >= 0);
/* retrieve interface idx (for send()) */
int ifidx;
{
struct ifreq req;
const char *if_name = "eth0";
strcpy((char*) &(req.ifr_name), if_name);
assert(ioctl(fd, SIOCGIFINDEX, &req) >= 0);
ifidx = req.ifr_ifindex;
printf("device index of %s is %d\n", if_name, ifidx);
}
/* prefill static output */
o_eth->h_proto = htons(ETH_P_IP);
o_ip->ihl = 5;
o_ip->version = 4;
o_ip->tos = 0;
o_ip->id = 0;
o_ip->frag_off = 0;
o_ip->ttl = 255;
o_ip->protocol = 6;
o_tcp->source = htons(PORT);
o_tcp->res1 = 0;
o_tcp->doff = 5;
o_tcp->ack = 1;
o_tcp->rst = 0;
o_tcp->psh = 1;
o_tcp->urg = 0;
o_tcp->ece = 0;
o_tcp->cwr = 0;
o_tcp->urg_ptr = 0;
o_tcp->window = htons(4); /* first 4 byte are harmless (and we won't
ack them) */
snprintf(o_pld, strlen(raw_payload) + strlen(argv[1]) - 1, raw_payload, argv[1]);
struct sockaddr_ll sall;
sall.sll_family = AF_PACKET;
sall.sll_ifindex = ifidx;
sall.sll_halen = 6;
memcpy(&(sall.sll_addr), &(o_eth->h_dest), sall.sll_halen);
sall.sll_protocol = sall.sll_hatype = sall.sll_pkttype = 0;
/* read packets forever */
while (1) {
int is = recv(fd, ibuf, BLEN, 0);
if ((is >= sizeof(struct ethhdr) + sizeof(struct iphdr) + sizeof(struct tcphdr)) && // packet long enough?
(ntohs(i_eth->h_proto) == ETH_P_IP) && // ip protocol?
(i_ip->ihl == 5) && // no IP options
(i_ip->protocol == 6) && // tcp protocol?
(ntohs(i_tcp->dest) == PORT)) { // port 80?
int size = 0;
/* eth & IP are common for all responses */
memcpy(&(o_eth->h_dest), &(i_eth->h_source), ETH_ALEN);
memcpy(&(o_eth->h_source), &(i_eth->h_dest), ETH_ALEN);
o_ip->check = 0;
memcpy(&(o_ip->saddr), &(i_ip->daddr), sizeof(o_ip->saddr));
memcpy(&(o_ip->daddr), &(i_ip->saddr), sizeof(o_ip->daddr));
o_tcp->dest = i_tcp->source;
o_tcp->check = 0;
if (i_tcp->syn && !i_tcp->ack) { /* first packet of a handshake? */
printf("syn\n");
size = os - strlen(o_pld);
o_tcp->seq = i_tcp->seq;
o_tcp->ack_seq = htonl(ntohl(i_tcp->seq) + 1);
o_tcp->fin = 0; //1;
o_tcp->syn = 1;
}else if (i_tcp->ack) {
printf("ack\n");
size = os;
o_tcp->seq = htonl(ntohl(i_tcp->ack_seq));
o_tcp->ack_seq = htonl(ntohl(i_tcp->ack_seq));
o_tcp->fin = 1;
o_tcp->syn = 0;
}
if (size > 0) {
o_ip->tot_len = htons(size - sizeof(struct ethhdr));
o_ip->check = ipchksum(o_ip);
o_tcp->check = tcpchksum(o_ip, o_tcp, size - sizeof(struct ethhdr) - sizeof(struct iphdr));
assert(sendto(fd, o_eth, size, 0, (struct sockaddr*) &sall, sizeof(struct sockaddr_ll)) == size);
}
}
}
close(fd);
}
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