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#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <limits.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netdb.h>
#include <netinet/tcp.h>
#include <arpa/inet.h>
 
#define PACKETSIZE 96
 
struct packet {
	struct tcphdr hdr;
	char buffer[PACKETSIZE];
};
 
struct pseudo_header {
	in_addr_t s_addr;
	in_addr_t d_addr;
	uint8_t zeroes;
	uint8_t protocol;
	uint16_t length;
};
 
int tcp_scan(struct addrinfo *addr);
uint16_t checksum(void *data, int length);
 
int main(int argc, char *argv[])
{
	struct addrinfo hints;
	struct addrinfo *result;
	int ret_getaddr;
 
	if (argc < 2) {
		fprintf(stderr, "Usage: %s host\n", argv[0]);
		exit(EXIT_FAILURE);
	}
 
	hints.ai_family = AF_INET;
	hints.ai_socktype = SOCK_STREAM;
	hints.ai_protocol = IPPROTO_TCP;
	hints.ai_flags = 0;
 
	ret_getaddr = getaddrinfo(argv[1], NULL, &hints, &result);
	if (ret_getaddr != 0) {
		fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(ret_getaddr));
		exit(EXIT_FAILURE);
	}
 
	for (struct addrinfo *rp = result; rp != NULL; rp = rp->ai_next) {
		tcp_scan(rp);
	}
 
	exit(EXIT_SUCCESS);
}
 
uint16_t checksum(void *data, int length) {
	uint16_t *p = (uint16_t *) data;
	uint32_t sum;
 
	for (sum = 0; length > 1; length -= 2) {
		sum += *p++;
	}
	if (length == 1) {
		sum += *(uint8_t *) p;
	}
 
	sum = (sum >> 16) + (sum & 0xFFFF);
	sum += (sum >> 16);
 
	return ~sum;
}
 
int tcp_scan(struct addrinfo *addr) {
	struct packet pckt;
	int pcktlen = sizeof(struct packet);
	int tcp_socket;
	ssize_t ret_send, ret_recv;
	struct pseudo_header pseudo;
 
	memset(&pckt, 0, sizeof(struct packet));
	pckt.hdr.th_sport = htons(rand() % (UINT16_MAX - 1024) + 1024);
	pckt.hdr.th_dport = htons(80);
	pckt.hdr.th_seq = rand() % UINT32_MAX;
	pckt.hdr.syn = 1;
	pckt.hdr.th_off = sizeof(struct tcphdr) / 4;
	pckt.hdr.th_win = PACKETSIZE;
 
	strcpy(pckt.buffer, "Hello, Example Domain!");
 
	pseudo.s_addr = inet_addr("192.168.1.2");
	pseudo.d_addr = inet_addr("93.184.216.34");
	pseudo.protocol = IPPROTO_TCP;
	pseudo.zeroes = 0;
	pseudo.length = htons( pcktlen);
 
	pckt.hdr.th_sum = checksum(&pckt, pcktlen) + checksum(&pseudo, sizeof(pseudo));
 
	tcp_socket = socket(AF_INET, SOCK_RAW, IPPROTO_TCP);
	if (tcp_socket == -1) {
		perror("tcp_socket");
		return 1;
	}
 
	ret_send = sendto(tcp_socket, &pckt, pcktlen, 0, addr->ai_addr, addr->ai_addrlen);
	if (ret_send == -1) {
		perror("sendto");
		return 1;
	}
 
	close(tcp_socket);
 
	return 0;
}

prog.c:15:16: error: field 'hdr' has incomplete type
  struct tcphdr hdr;
                ^
prog.c:27:21: warning: 'struct addrinfo' declared inside parameter list
 int tcp_scan(struct addrinfo *addr);
                     ^
prog.c:27:21: warning: its scope is only this definition or declaration, which is probably not what you want
prog.c: In function 'main':
prog.c:32:18: error: storage size of 'hints' isn't known
  struct addrinfo hints;
                  ^
prog.c:46:16: warning: implicit declaration of function 'getaddrinfo' [-Wimplicit-function-declaration]
  ret_getaddr = getaddrinfo(argv[1], NULL, &hints, &result);
                ^
prog.c:48:40: warning: implicit declaration of function 'gai_strerror' [-Wimplicit-function-declaration]
   fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(ret_getaddr));
                                        ^
prog.c:48:19: warning: format '%s' expects argument of type 'char *', but argument 3 has type 'int' [-Wformat=]
   fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(ret_getaddr));
                   ^
prog.c:52:56: error: dereferencing pointer to incomplete type 'struct addrinfo'
  for (struct addrinfo *rp = result; rp != NULL; rp = rp->ai_next) {
                                                        ^
prog.c:53:12: warning: passing argument 1 of 'tcp_scan' from incompatible pointer type [-Wincompatible-pointer-types]
   tcp_scan(rp);
            ^
prog.c:27:5: note: expected 'struct addrinfo *' but argument is of type 'struct addrinfo *'
 int tcp_scan(struct addrinfo *addr);
     ^
prog.c:32:18: warning: unused variable 'hints' [-Wunused-variable]
  struct addrinfo hints;
                  ^
prog.c: At top level:
prog.c:76:21: warning: 'struct addrinfo' declared inside parameter list
 int tcp_scan(struct addrinfo *addr) {
                     ^
prog.c:76:5: error: conflicting types for 'tcp_scan'
 int tcp_scan(struct addrinfo *addr) {
     ^
prog.c:27:5: note: previous declaration of 'tcp_scan' was here
 int tcp_scan(struct addrinfo *addr);
     ^
prog.c: In function 'tcp_scan':
prog.c:88:27: error: invalid application of 'sizeof' to incomplete type 'struct tcphdr'
  pckt.hdr.th_off = sizeof(struct tcphdr) / 4;
                           ^
prog.c:107:55: error: dereferencing pointer to incomplete type 'struct addrinfo'
  ret_send = sendto(tcp_socket, &pckt, pcktlen, 0, addr->ai_addr, addr->ai_addrlen);
                                                       ^
prog.c:80:20: warning: unused variable 'ret_recv' [-Wunused-variable]
  ssize_t ret_send, ret_recv;
                    ^

Standard output is empty