#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <netinet/in_systm.h>
#include <netinet/ip_var.h>
#include <netinet/tcpip.h>
#include <openssl/rc4.h>
#include <openssl/md5.h>
#define MAX_PACKET_SIZE 65536
#define IP_HDR_LEN 20
struct ip_auth_packet {
struct ip ip_header;
char data[MAX_PACKET_SIZE];
};
void print_error(const char *msg) {
fprintf(stderr, "%s\n", msg);
}
int main(int argc, char *argv[]) {
if (argc != 4) {
print_error("Usage: ip_cryptAuthAll <divert port> <remote IP> <key>");
exit(EXIT_FAILURE);
}
int divert_port = atoi(argv[1]);
char *remote_ip_str = argv[2];
char *key = argv[3];
// Create a divert socket
int sock = socket(AF_INET, SOCK_RAW, IPPROTO_DIVERT);
if (sock < 0) {
perror("Socket creation failed");
exit(EXIT_FAILURE);
}
// Bind the socket to the divert port
struct sockaddr_in addr;
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_port = htons(divert_port);
addr.sin_addr.s_addr = htonl(INADDR_ANY);
if (bind(sock, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
perror("Bind failed");
close(sock);
exit(EXIT_FAILURE);
}
while (1) {
struct ip_auth_packet packet;
struct sockaddr_in src_addr;
socklen_t src_addr_len = sizeof(src_addr);
ssize_t recv_len = recvfrom(sock, &packet, sizeof(packet), 0,
(struct sockaddr *)&src_addr, &src_addr_len);
if (recv_len < 0) {
perror("Receive error");
continue;
}
char *src_ip_str = inet_ntoa(packet.ip_header.ip_src);
char *dst_ip_str = inet_ntoa(packet.ip_header.ip_dst);
if (strcmp(dst_ip_str, remote_ip_str) == 0) {
// Outgoing packet to the specified <remote IP>
// Encrypt the payload using RC4
RC4_KEY rc4_key;
RC4_set_key(&rc4_key, strlen(key), (const unsigned char *)key);
RC4(&rc4_key, recv_len - IP_HDR_LEN, (unsigned char *)&packet.data[IP_HDR_LEN],
(unsigned char *)&packet.data[IP_HDR_LEN]);
// Calculate MD5 hash of (encrypted payload + key)
unsigned char md5_result[MD5_DIGEST_LENGTH];
MD5_CTX md5_ctx;
MD5_Init(&md5_ctx);
MD5_Update(&md5_ctx, &packet.data[IP_HDR_LEN], recv_len - IP_HDR_LEN);
MD5_Update(&md5_ctx, key, strlen(key));
MD5_Final(md5_result, &md5_ctx);
// Concatenate encrypted payload with MD5 hash
memcpy(&packet.data[IP_HDR_LEN + recv_len - IP_HDR_LEN], md5_result, MD5_DIGEST_LENGTH);
// Adjust IP header
packet.ip_header.ip_len = htons(recv_len + MD5_DIGEST_LENGTH);
packet.ip_header.ip_sum = 0;
packet.ip_header.ip_sum = checksum((unsigned short *)&packet.ip_header, IP_HDR_LEN);
// Send the modified packet
sendto(sock, &packet, recv_len + MD5_DIGEST_LENGTH, 0, (struct sockaddr *)&src_addr, src_addr_len);
} else if (strcmp(src_ip_str, remote_ip_str) == 0) {
// Incoming packet from specified <remote IP>
// Check packet payload for MD5 authentication
unsigned char md5_check[MD5_DIGEST_LENGTH];
MD5_CTX md5_ctx_check;
MD5_Init(&md5_ctx_check);
MD5_Update(&md5_ctx_check, &packet.data[IP_HDR_LEN], recv_len - IP_HDR_LEN - MD5_DIGEST_LENGTH);
MD5_Update(&md5_ctx_check, key, strlen(key));
MD5_Final(md5_check, &md5_ctx_check);
if (memcmp(md5_check, &packet.data[recv_len - MD5_DIGEST_LENGTH], MD5_DIGEST_LENGTH) == 0) {
// Decrypt payload using RC4
RC4_KEY rc4_key;
RC4_set_key(&rc4_key, strlen(key), (const unsigned char *)key);
RC4(&rc4_key, recv_len - IP_HDR_LEN - MD5_DIGEST_LENGTH,
(unsigned char *)&packet.data[IP_HDR_LEN], (unsigned char *)&packet.data[IP_HDR_LEN]);
// Adjust IP header
packet.ip_header.ip_len = htons(recv_len - MD5_DIGEST_LENGTH);
packet.ip_header.ip_sum = 0;
packet.ip_header.ip_sum = checksum((unsigned short *)&packet.ip_header, IP_HDR_LEN);
// Send the modified packet
sendto(sock, &packet, recv_len - MD5_DIGEST_LENGTH, 0, (struct sockaddr *)&src_addr, src_addr_len);
} else {
print_error("Packet authentication failed");
continue;
}
} else {
// Pass the packet without modification
sendto(sock, &packet, recv_len, 0, (struct sockaddr *)&src_addr, src_addr_len);
}
}
close(sock);
return 0;
}
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C language is one of the most popular general-purpose programming language developed by Dennis Ritchie at Bell laboratories for UNIX operating system. The initial release of C Language was in the year 1972. Most of the desktop operating systems are written in C Language.
When ever you want to perform a set of operations based on a condition if-else is used.
if(conditional-expression) {
// code
} else {
// code
}
You can also use if-else for nested Ifs and if-else-if ladder when multiple conditions are to be performed on a single variable.
Switch is an alternative to if-else-if ladder.
switch(conditional-expression) {
case value1:
// code
break; // optional
case value2:
// code
break; // optional
...
default:
// code to be executed when all the above cases are not matched;
}
For loop is used to iterate a set of statements based on a condition.
for(Initialization; Condition; Increment/decrement){
// code
}
While is also used to iterate a set of statements based on a condition. Usually while is preferred when number of iterations are not known in advance.
while(condition) {
// code
}
Do-while is also used to iterate a set of statements based on a condition. It is mostly used when you need to execute the statements atleast once.
do {
// code
} while (condition);
Array is a collection of similar data which is stored in continuous memory addresses. Array values can be fetched using index. Index starts from 0 to size-1.
data-type array-name[size];
data-type array-name[size][size];
Function is a sub-routine which contains set of statements. Usually functions are written when multiple calls are required to same set of statements which increases re-usuability and modularity.
Two types of functions are present in C
Library functions are the in-built functions which are declared in header files like printf(),scanf(),puts(),gets() etc.,
User defined functions are the ones which are written by the programmer based on the requirement.
return_type function_name(parameters);
function_name (parameters)
return_type function_name(parameters) {
//code
}