void rcv_encoded(uint16_t *ids, uint16_t len, int sd, char *buf,
struct sockaddr_in *client)
{
int i;
int dgram_num;
uint16_t id;
struct ip_header *iph = (struct ip_header*) buf;
struct udp_header *udph = (struct udp_header *)
(buf + sizeof(struct ip_header));
uint16_t initid = ntohs(len);
printf("Message length: %d\n", len);
for (i = 0; i < len * 2; i++)
{
recv_dgram(sd, buf, client);
id = ntohs(iph->id);
if (ntohs(udph->srcport) != port_from_date()
|| IP_FLAGS(iph) == IP_DONTFRAG
|| !in_range(initid, id))
{
/* not one of our packets */
#ifdef DEBUG
printf("A message we don't care about...\n");
printf(" ntohs(udph->srcport): %d\n", ntohs(udph->srcport));
printf(" port_from_date(): %d\n", port_from_date());
printf(" IP_FLAGS(iph): %d\n", IP_FLAGS(iph));
printf(" IP_DONTFRAG: %d\n", IP_DONTFRAG);
printf(" id: %d\n", id);
printf(" in_range(initid, id): %d\n", in_range(initid, id));
#endif
i--;
continue;
}
/* len is the initial ID */
dgram_num = (id - initid) / 0x10;
ids[dgram_num] = id;
printf("#%d\tID: %d\n", dgram_num + 1, ntohs(iph->id));
}
}
char *IPPacket2Str(IPHeader *packet)
{
const int BUFFER_LENGTH = 1024;
char *str = (char*)malloc(BUFFER_LENGTH);
ZeroMemory(str, BUFFER_LENGTH);
char *t1 = Int2Bin(packet->iph_tos, 8);
char *t2 = Int2Bin(IP_FLAGS(ntohs(packet->iph_flags)), 3);
char *src = Nethost2Str(packet->iph_sourceip);
char *dest = Nethost2Str(packet->iph_destip);
sprintf(str, "ver=%d hlen=%d tos=%s len=%-4d id=%-5d flags=%s offset=%d ttl=%3dms prot=%-2d xsum=%-5X src=%-15s dest=%-15s\n",
BYTE_H(packet->iph_ver), BYTE_L(packet->iph_ver)*4, t1, ntohs(packet->iph_len), ntohs(packet->iph_ident),
t2, IP_OFFSET(ntohs(packet->iph_offset)), packet->iph_ttl, packet->iph_protocol,
ntohs(packet->iph_xsum), src, dest);
free(t1); free(t2); free(src); free(dest);
return str;
}
void server()
{
struct sockaddr_in server;
struct sockaddr_in client;
int sd;
char buf[RECV_BUFLEN];
uint16_t len;
uint16_t *ids;
char *decoded;
struct ip_header *iph = (struct ip_header*) buf;
struct udp_header *udph = (struct udp_header *)
(buf + sizeof(struct ip_header));
if ((sd = socket(AF_INET, SOCK_RAW, IPPROTO_UDP)) == -1)
{
exit(sock_error("socket()", 0));
}
memset((char *) &server, 0, sizeof(server));
server.sin_family = AF_INET;
server.sin_port = htons(DST_PORT);
server.sin_addr.s_addr = htonl(INADDR_ANY);
if (bind(sd, (struct sockaddr *) &server, sizeof(server)) == -1)
{
exit(sock_error("bind()", 0));
}
while (1)
{
recv_dgram(sd, buf, &client);
if (ntohs(udph->srcport) == port_from_date()
&& IP_FLAGS(iph) == IP_DONTFRAG)
{
printf("Covert message arriving...\n");
len = iph->id;
ids = malloc(sizeof(uint16_t) * len * 2);
decoded = malloc(sizeof(char) * len + 1);
rcv_encoded(ids, len, sd, buf, &client);
printf("Message received. Decoding...\n\n");
decode(decoded, ids, len);
printf("%s\n", decoded);
free(ids);
free(decoded);
}
#ifdef DEBUG
else
{
printf("A message we don't care about...\n");
printf(" ntohs(udph->srcport): %d\n", ntohs(udph->srcport));
printf(" port_from_date(): %d\n", port_from_date());
printf(" IP_FLAGS(iph): %d\n", IP_FLAGS(iph));
printf(" IP_DONTFRAG: %d\n", IP_DONTFRAG);
}
#endif
}
}