原始TCP套接字在发送初始数据后接收常量PSH，ACK数据包

Question

我目前正在尝试仅使用原始套接字创建Linux echo客户端。我在一台计算机上运行普通的TCP服务器，在客户端我正在使用原始TCP套接字。我正在客户端运行Libpcap来接收数据包并在收到数据包时进行回复。我完成了三次握手，并在客户端和服务器之间建立了连接。

我能够发送我的第一个ACK，PSH，ACK（数据包3和4）和数据，并从服务器接收我最初发送的数据（数据包5和6）的回复。

任何想法如何停止PSH，ACK发送？我已经尝试发送一个ACK，但它不会停止发送。下面是一个wireshark捕获和我正在使用的一些代码。

这是wireshark Server（192.168.1.81）客户端（192.168.1.85）上的数据包的屏幕截图：Wireshark capture

客户端原始TCP包装器

void send_raw_tcp_packet(int src_port, int dst_port, struct ifreq interface, 
char* src_ip, char* dst_ip, int seq, int ack, char *data, int flags) {
  struct sockaddr_in sin;
  int *ip_flags, *tcp_flags, status, sending_socket;
  const int on = 1;
  struct tcp_packet packet;
  ip_flags = (int *)calloc(4, sizeof(int));
  tcp_flags = (int *)calloc(8, sizeof(int));
  int payloadlen = 0;
  memset(&packet, 0, sizeof(struct tcp_packet));

  // IPv4 header
  packet.iphdr.ip_hl = IP4_HDRLEN / sizeof(uint32_t); //header length = 5
  packet.iphdr.ip_v = 4; //version = 4
  packet.iphdr.ip_tos = 0; //TOS
  packet.iphdr.ip_len = htons(IP4_HDRLEN + TCP_HDRLEN); //length: IP header     + TCP header
  packet.iphdr.ip_id = htons(0); //ID
  ip_flags[0] = 0; //Zero
  ip_flags[1] = 0; //Don't frag
  ip_flags[2] = 0; //More frag
  ip_flags[3] = 0; //Frag offset
  packet.iphdr.ip_off = htons((ip_flags[0] << 15) + (ip_flags[1] << 14) +    (ip_flags[2] << 13) + ip_flags[3]);
  packet.iphdr.ip_ttl = 255; //TTL
  packet.iphdr.ip_p = IPPROTO_TCP; //Protocol
  printf("src_ip: %s\n", src_ip);
  printf("dst_ip: %s\n", dst_ip);
  // Source IPv4 address (32 bits)
  if ((status = inet_pton(AF_INET, src_ip, &(packet.iphdr.ip_src))) != 1) {
     perror("inet_pton, src_ip");
  exit(EXIT_FAILURE);
  }
  // Destination IPv4 address (32 bits)
  if ((status = inet_pton(AF_INET, dst_ip, &(packet.iphdr.ip_dst))) != 1) {
      perror("inet_pton, dst_ip");
    exit(EXIT_FAILURE);
  }

  packet.iphdr.ip_sum = 1;
  packet.iphdr.ip_sum = checksum((uint16_t *)&packet.iphdr, IP4_HDRLEN);

  // TCP header
  if (src_port == 0) {
    packet.tcphdr.th_sport = generate_rand(65535.0);
  } else {
    packet.tcphdr.th_sport = src_port;
  }
  if (dst_port == 0) {
    packet.tcphdr.th_dport = generate_rand(65535.0);
  } else {
    packet.tcphdr.th_dport = htons(dst_port);
  }
  packet.tcphdr.th_seq = htonl(seq); //SEQ
  packet.tcphdr.th_ack = htonl(ack); //ACK - 0 for first packet
  packet.tcphdr.th_x2 = 0; //Reserved
  packet.tcphdr.th_off = TCP_HDRLEN / 4; //Offset

  // Flags (8 bits)
  if(flags == PSHACK){
    tcp_flags[0] = 0; //FIN
    tcp_flags[1] = 0; //SYN
    tcp_flags[3] = 1; //PSH
    tcp_flags[4] = 1; //ACK
    tcp_flags[2] = 0; //RST
  }else if(flags == SYNACK){
    tcp_flags[0] = 0; //FIN
    tcp_flags[1] = 1; //SYN
    tcp_flags[3] = 0; //PSH
    tcp_flags[4] = 1; //ACK
    tcp_flags[2] = 0; //RST
  }else if(flags == FINACK){
    tcp_flags[0] = 1; //FIN
    tcp_flags[1] = 0; //SYN
    tcp_flags[3] = 0; //PSH
    tcp_flags[4] = 1; //ACK
    tcp_flags[2] = 0; //RST
  }else if(flags == FIN){
    tcp_flags[0] = 1; //FIN
    tcp_flags[1] = 0; //SYN
    tcp_flags[3] = 0; //PSH
    tcp_flags[4] = 0; //ACK
    tcp_flags[2] = 0; //RST
  } else if(flags == SYN){
    tcp_flags[0] = 0; //FIN
    tcp_flags[1] = 1; //SYN
    tcp_flags[3] = 0; //PSH
    tcp_flags[4] = 0; //ACK
    tcp_flags[2] = 0; //RST
  }else if(flags == ACK){
    tcp_flags[0] = 0; //FIN
    tcp_flags[1] = 0; //SYN
    tcp_flags[3] = 0; //PSH
    tcp_flags[4] = 1; //ACK
    tcp_flags[2] = 0; //RST
  }else if(flags == RST){
    tcp_flags[0] = 0; //FIN
    tcp_flags[1] = 0; //SYN
    tcp_flags[3] = 0; //PSH
    tcp_flags[4] = 0; //ACK
    tcp_flags[2] = 1; //RST
  }
  tcp_flags[5] = 0; //URG
  tcp_flags[6] = 0; //ECE
  tcp_flags[7] = 0; //CWR
  packet.tcphdr.th_flags = 0;
  for (int i = 0; i < 8; i++) {
    packet.tcphdr.th_flags += (tcp_flags[i] << i);
  }

  packet.tcphdr.th_win = htons(64240); //Window size
  packet.tcphdr.th_urp = htons(0); //Urgent Pointer
  //memset(packet.payload, 0, sizeof(packet.payload));
  if(data != NULL){
    sprintf (packet.payload, "%s", data);
    payloadlen = strlen(packet.payload);
  }
  //payloadlen = strlen(packet.payload);
  packet.tcphdr.th_sum = tcp4_checksum(packet.iphdr, packet.tcphdr, (uint8_t     *) packet.payload, payloadlen);

  memset(&sin, 0, sizeof(struct sockaddr_in));
  sin.sin_family = AF_INET;
  sin.sin_addr.s_addr = packet.iphdr.ip_dst.s_addr;

  if ((sending_socket = socket(AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0) {
    perror("socket() failed ");
    exit(EXIT_FAILURE);
  }

  if (setsockopt(sending_socket, IPPROTO_IP, IP_HDRINCL, &on, sizeof(on)) <     0) {
    perror("setsockopt() failed to set IP_HDRINCL ");
    exit(EXIT_FAILURE);
  }

  if (setsockopt(sending_socket, SOL_SOCKET, SO_BINDTODEVICE,     &interface,sizeof(interface)) < 0) {
    perror("setsockopt() failed to bind to interface ");
    exit(EXIT_FAILURE);
  }

  if (sendto(sending_socket, &packet, IP4_HDRLEN + TCP_HDRLEN + payloadlen,     0, (struct sockaddr *)&sin, sizeof(sin)) < 0) {
    perror("sendto() failed ");
    exit(EXIT_FAILURE);
  }

  close(sending_socket);
  // Free allocated memory.
  free(ip_flags);
  free(tcp_flags);
}

服务器代码

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <sys/wait.h>
#include <signal.h>

#define PORT "8045"
#define MAXCONNECTIONS 1024


int main(void){
    int sockfd, new_fd,rv, yes =1;
    struct addrinfo hints, *servinfo, *p;
    struct sockaddr_storage their_addr;
    socklen_t sin_size;
    struct sigaction sa;
    char s[INET6_ADDRSTRLEN];


    hints = set_hints(AF_UNSPEC, SOCK_STREAM, AI_PASSIVE);

    if ((rv = getaddrinfo(NULL, PORT, &hints, &servinfo)) != 0) {
    fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
    return 1;
}

for(p = servinfo; p != NULL; p = p->ai_next) {
    if ((sockfd = socket(p->ai_family, p->ai_socktype,p->ai_protocol)) == -1) {
        perror("server: socket");
        continue;
    }

    if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &yes,
            sizeof(int)) == -1) {
        perror("setsockopt");
        exit(1);
    }

    if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
        close(sockfd);
        perror("server: bind");
        continue;
    }

    break;
}

freeaddrinfo(servinfo);

if (listen(sockfd, MAXCONNECTIONS) == -1) {
    perror("listen");
    exit(1);
}

printf("server: waiting for connections...\n");
sin_size = sizeof their_addr;
new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);
if (new_fd == -1) {
    perror("accept");
}

inet_ntop(their_addr.ss_family,get_in_addr((struct sockaddr *)&their_addr), s, sizeof s);
printf("server: got connection from %s\n", s);
while(1) {
    char data[1024];
    //recv_normal_tcp_packet(new_fd, data, sizeof(data));
    int bytes_receieved = recv(new_fd, data, sizeof(data),0);
    printf("Received: %s from %s\n",data, s);
    send_normal_tcp_packet(new_fd, data, bytes_receieved);
}
close(new_fd);
close(sockfd);

return 0;
}

Answer 1

您的客户端未正确确认收到的数据（来自服务器）。

现在看来，数据包＃6包含来自服务器的数据。数据包＃7在200ms后重新传输，因为客户端没有足够快地确认。那是一个小问题。

主要问题在于数据包＃8：客户端尝试确认序列号321910546.因此服务器可能会忽略它。在这种情况下，正确的数字应为6（seq 1 + len 5）。

因此，服务器尝试多次重新发送回声，因为它没有收到有效的确认。