C Program sending a TCP RST to the Server

Question

I need to simulate a TCP RST connection issue. For this purpose, I'd need to write a C Client that connects to a TCP Server and send a TCP RST connection. I have found this sample code but on the Server side I don't see any Connection Reset traced. I'm not a expert on C so I wonder if anything is missing to send a RST Socket.

Any help? Thanks

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <netinet/tcp.h>

int main() {
  int sock = socket(AF_INET, SOCK_STREAM, 0);
  if (sock == -1) {
    perror("Socket creation failed");
    exit(EXIT_FAILURE);
  }

  struct sockaddr_in server_addr;
  server_addr.sin_family = AF_INET;
  server_addr.sin_addr.s_addr = inet_addr("127.0.0.1"); // Replace with client IP
  server_addr.sin_port = htons(8080); // Replace with client port

  if (connect(sock, (struct sockaddr *)&server_addr, sizeof(server_addr)) != 0) {
    perror("Connection failed");
    exit(EXIT_FAILURE);
  }

  // Close the socket without graceful termination to send RST
  close(sock);

  return 0;
}

Answer 1

Between socket() & connect() you need to set the linger option. If the linger option is set and you exit a reset is send. close() usually starts an orderly shutdown with a “FIN” packet.

    struct linger sl;
    bzero(&sl, sizeof(sl));
    sl.l_onoff = 1;
    sl.l_linger = 0;
    if (setsockopt(sockfd, SOL_SOCKET, SO_LINGER, &sl, sizeof(sl)) < 0)
        perror("setsockopt");