BSD socket: why `send` still return success, even when I drop the packets at the server?

Question

Here is my test code:

#include <arpa/inet.h>
#include <sys/socket.h>
#include <netdb.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
int set_keep_alive(int sockfd) {
    int optval = 1;
    if (setsockopt(sockfd, SOL_SOCKET, SO_KEEPALIVE, &optval, sizeof(optval)) == -1) {
        perror("setsockopt");
        close(sockfd);
        return -1;
    }
    int flags = 6;
         if (setsockopt(sockfd , IPPROTO_TCP, TCP_KEEPIDLE, (void *)&flags, sizeof(flags)))
        {
            return -1;
         };

         flags = 3;
         if (setsockopt(sockfd , IPPROTO_TCP,  TCP_KEEPINTVL, (void *)&flags, sizeof(flags)))
         {
            return -1;
         };

         flags = 3;
         if (setsockopt(sockfd , IPPROTO_TCP, TCP_KEEPCNT, (void *)&flags, sizeof(flags)))
         {
            return -1;
        };
    return 0;
}
int main(int argc, char **argv) {
    if (argc != 3) {
        fprintf(stderr, "usage: %s <ip> <port>\n", argv[0]);
        return -1;
    }
    char const *IP = argv[1];
    int PORT = atoi(argv[2]);

    int fd = socket(AF_INET, SOCK_STREAM, 0);
    set_keep_alive(fd);
    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(PORT);
    inet_pton(AF_INET, IP, &addr.sin_addr);
    int ret;
    ret = connect(fd, (struct sockaddr *)&addr, sizeof(addr));
    if (ret < 0) {
        printf("connect failed, ret = %d, errno=%d\n", ret, errno);
        perror("");
        return 1;
    }

    for(int i = 0;;i++, sleep(1)) {
        char buf[100];
        sprintf(buf, "hello%d", i%10);
        int ret = send(fd, buf, strlen(buf), MSG_CONFIRM);
        printf("%d,write ret = %d\n", i, ret);
    }
    close(fd);
    return 0;
}

Assume its called client.c

1. gcc client.c got a.out
2. In the server, use nc -l -p <PORT>
3. ./a.out <Server IP> <PORT>: it will print

0,write ret = 6
1,write ret = 6
2,write ret = 6
3,write ret = 6
4,write ret = 6
5,write ret = 6
6,write ret = 6
....

4. In the server, there is a shell script called port-close.sh which drops newly incoming packets to the port

#!/bin/sh
iptables -I INPUT -p tcp --dport $1 -j DROP

if I run ./port-close.sh <PORT>, in the client, after I wait for 15 seconds (the keep-alive time), still print ret = 6 which means success.

Please note replace <PORT> and <Server IP> to your configuration

Answer 1

Sending data successfully does not mean that the data must be received. You prevented the data from getting from the TCP layer to the network. You didn't prevent it from being sent from the application to the TCP layer, which is the sending that succeeded.