Ubuntu – perror: ": Connection refused" | Socket programming in C

I have to do an assignment for my one lecture where i have to create a connection between a client and a server program in C. The server program and the client program have to run on different PC’s. After starting the server.c with:

1. gcc server.c -o server
2. ./server // number is part of the packet.h
   The server stops at "Listening…" which is correct.

After that i started the client.c with:

1. gcc client.c -o client
2. ./client
   The program trows a perror ": Connection refused" after checking the connect()-function. There has to be the problem but I can’t fix it.

I tried to figure out the exact problem. Here are a few things i already tried:

1. The server is 100% in my local internet. I tried with "netcat " to search for the server and the server is there.
2. I always used a port bigger than 5000 in my home network and in my uni network 50000 (the only one that can be used, my professor told me that).
3. I was working on ubuntu and i always wrote "%eth0" behind the ipv6.
4. I checked out a bunch of other codes and mine looked nearly the same.
5. I tried to debug the "client.c" file and the connect()-function always gave back -1. I think there has to be the problem but I could not fix that.

Here is my code:
Server.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <string.h>
// #include <libc.h>

#include "packet.h"

int main(int argc, char *argv[]){
    if(argc != 4){  // Test ob 4 Programmargumente mitgegeben wurden
        fprintf(stderr, "- Error: %s <IPv6 address> <port> <sNummer>n", argv[0]);
        exit(1);
    }

    struct packet received_packet, send_packet;
    const char *ip = argv[1];

    char* endptr;

    long port_long = strtol(argv[2], &endptr, 10);
    if(*endptr != '' || port_long < 0 || port_long > UINT16_MAX){
        fprintf(stderr, "- invalid port number: %sn", argv[2]);
        exit(1);
    }
    uint16_t port =(uint16_t)port_long;

    long sNummer_long = strtol(argv[3], &endptr, 10);
    if(*endptr != '' || sNummer_long < 0 || sNummer_long > UINT32_MAX){
        fprintf(stderr, "- invalid sNummer: %sn", argv[3]);
        exit(1);
    }
    send_packet.sNummer = (uint32_t)sNummer_long; 

    int csocket;                
    struct sockaddr_in6 addr;   
    socklen_t addrsize;          
    char test[1024];            

    csocket = socket(AF_INET6, SOCK_STREAM, 0);
    
    if(csocket < 0 ){ // Test ob TCP-Socket erstellt wurde
        perror("- socket errorn");
        exit(1);
    }
    printf("+  TCP server socket createdn");

    memset(&addr, 0, sizeof(addr)); 
    addr.sin6_family = AF_INET6;      
    addr.sin6_port = htons(port);            
    inet_pton(AF_INET6, ip, &addr.sin6_addr); 

    if( connect(csocket, (struct sockaddr *) &addr, sizeof(addr)) < 0 ){
        perror("- could not connect to servern");
        exit(1);
    }
    printf("+ Connected to server %s on port %dn",ip, port);

    printf("n ----- message -----n");
    printf("text: ");
    scanf("%s",send_packet.text);

    printf("s%d > %sn",send_packet.sNummer, send_packet.text);
    send(csocket, &send_packet, sizeof(send_packet), 0); 

    ssize_t bytesRead = recv(csocket, &received_packet, sizeof(received_packet), 0); 
    if( bytesRead <= 0 ){
        perror("- error receiving data from servern");
    }
    test[bytesRead] = '';
    printf("n ----- Server -----n");
    printf("s%d > %sn",received_packet.sNummer,received_packet.text);

    close(csocket);
    printf("n+ disconnected from servern");

    return 0;
}

Client.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <string.h>
// #include <libc.h>

#include "packet.h"

#define BACKLOG     5

int main(int argc, char *argv[]){
    if(argc != 3){  // Test ob auch 3 Programmargumente mitgegeben wurden
        fprintf(stderr, "- Error: %s <port> <sNummer>n",argv[0]);
        exit(1);
    }

    struct packet received_packet, response_packet;
    char* endptr;

    long port_long = strtol(argv[1], &endptr, 10);
    if(*endptr != '' || port_long < 0 || port_long > UINT16_MAX){
        fprintf(stderr, "- invalid port number: %sn", argv[1]);
        exit(1);
    }
    uint16_t port =(uint16_t)port_long;

    long sNummer_long = strtol(argv[2], &endptr, 10);
    if(*endptr != '' || sNummer_long < 0 || sNummer_long > UINT32_MAX){
        fprintf(stderr, "- invalid sNummer: %sn", argv[2]);
        exit(1);
    }
    response_packet.sNummer = (uint32_t)sNummer_long;

    int server_socket, client_socket;   
    struct sockaddr_in6 s_addr, c_addr; 
    socklen_t c_addrsize;               
    char test[1024];                    

    server_socket = socket(AF_INET6, SOCK_STREAM, 0); 
    
    if(server_socket < 0 ){ 
        perror("- socket errorn");
        exit(1);
    }
    printf("+  TCP server socket createdn");

    memset(&s_addr, 0, sizeof(s_addr)); 
    s_addr.sin6_family = AF_INET6;      
    s_addr.sin6_port = htons(port);    
    s_addr.sin6_addr = in6addr_any;     

    if(bind(server_socket, (struct sockaddr *) &s_addr, sizeof(s_addr)) < 0){ 
        perror("- bind errorn");
        exit(1);
    }
    printf("+ bind to port number: %dn", port);

    if(listen(server_socket, BACKLOG) < 0){ 
        perror("- listening error, can't hear somethingn");
        exit(1);
    }
    printf("Listening...n");

    while(1){
        c_addrsize = sizeof(c_addr);
        client_socket = accept(server_socket, (struct sockaddr*)&c_addr, &c_addrsize);
        
        if(client_socket > 0){
            printf("+ Client connectedn");
        }else {
            printf("- No Client connectedn");
            sleep(1);
            continue;
        }

        ssize_t bytesRead = recv(client_socket, &received_packet, sizeof(received_packet), 0);
        if( bytesRead <= 0 ){
            perror("- error receiving data from clientn");
            continue;
        }
        test[bytesRead] = '';
        printf("n ----- Client -----n");
        printf("s%d > %sn",received_packet.sNummer, received_packet.text);

        printf("n ----- response -----n");
        printf("text: ");
        scanf("%s",response_packet.text);

        send(client_socket, &response_packet, sizeof(response_packet), 0);

        close(client_socket);
        printf("n+ Client disconnectedn");
    }

    return 0;
}

and finally the packet.h. This packet is part of my assignment. It hast to be send from the client to the server and back.

#ifndef PACKET_H
#define PACKET_H

#include <stdint.h>

struct packet {
    char text[1024];
    uint32_t sNummer;
};

#endif