lsof看到sock fd但是netstat查看不到socket连接的场景

原创

cdh

修改于 2025-09-12 20:39:59

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文章被收录于专栏：笔记+笔记+

场景1：

应用程序创建了socket，但是当作为client端时没有调用connect去连接server端：

模拟构造问题现象程序：

#include <stdio.h>
#include <sys/socket.h>
#include <unistd.h>

int main(int argc,char *argv[])
{
int sockfd[16] = {0};
int i=0;
for(i=0;i<16;i++)
{
sockfd[i] = socket(AF_INET,SOCK_STREAM,0);
if (sockfd[i] == -1){
printf("Fail to create a socket.");
}

}
while(1)
sleep(10);

}

场景二：

client端连接server端后，三次握手连接已经建立了，然后client端发起关闭连接，server端也不做关闭连接处理，最后连接被client端rst掉，导致fd还在，但是netstat看到连接不存在。

模拟构造问题现象：

1 先运行server端程序./server

2 client端发起连接前状态

3. client端发起连接后状态：

4. 连接被关闭后netstat查看tcp连接已经不存在，但是fd还在

构造问题现象代码实例：

server端：

# cat server.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/types.h>

#define PORT 8080
#define BUFFER_SIZE 1024

int main() {
int server_fd, new_socket;
struct sockaddr_in address;
int addrlen = sizeof(address);
char buffer[BUFFER_SIZE] = {0};

server_fd = socket(AF_INET, SOCK_STREAM, 0);
if (server_fd == 0) {
perror("socket creation failed");
exit(EXIT_FAILURE);
}

address.sin_family = AF_INET;
address.sin_addr.s_addr = INADDR_ANY;
address.sin_port = htons(PORT);

if (bind(server_fd, (struct sockaddr *)&address, sizeof(address)) < 0) {
perror("bind failed");
exit(EXIT_FAILURE);
}

if (listen(server_fd, 5) < 0) {
perror("listen failed");
exit(EXIT_FAILURE);
}

new_socket = accept(server_fd, (struct sockaddr *)&address, (socklen_t *)&addrlen);
if (new_socket < 0) {
perror("accept failed");
exit(EXIT_FAILURE);
}

read(new_socket, buffer, BUFFER_SIZE);
printf("Message received: %s\n", buffer);
sleep(120);
write(new_socket, buffer, BUFFER_SIZE);
while(1)
  sleep(10);
return 0;
}

client端：

# cat client.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/types.h>

#define PORT 8080
#define BUFFER_SIZE 1024

int main() {
int sock = 0;
struct sockaddr_in serv_addr;
char *message = "Hello from client";
char buffer[BUFFER_SIZE] = {0};

if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
perror("socket creation failed");
exit(EXIT_FAILURE);
}

serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(PORT);

if (inet_pton(AF_INET, "127.0.0.1", &serv_addr.sin_addr) <= 0) {
perror("invalid address");
exit(EXIT_FAILURE);
}

if (connect(sock, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
perror("connection failed");
exit(EXIT_FAILURE);
}

send(sock, message, strlen(message), 0);
printf("Message sent\n");
sleep(60);
// Abnormal connection termination
shutdown(sock, SHUT_RDWR);
close(sock);
return 0;
}

场景3：

client端connect失败后未主动关闭连接导致fd泄露

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

#define MAX_ATTEMPTS 100  // 尝试创建 100 个 socket
#define UNREACHABLE_IP "192.0.2.1"  // 测试用不可达 IP (RFC 5737)
#define UNREACHABLE_PORT 12345      // 测试用不可达端口

int main() {
    int sockfd;
    struct sockaddr_in serv_addr;

    printf("PID = %d\n", getpid());
    printf("Run `ls -l /proc/%d/fd` to check leaked file descriptors.\n", getpid());
    printf("Press Enter to start...");
    getchar();

    for (int i = 0; i < MAX_ATTEMPTS; i++) {
        // 创建 TCP socket
        sockfd = socket(AF_INET, SOCK_STREAM, 0);
        if (sockfd < 0) {
            perror("socket() failed");
            exit(EXIT_FAILURE);
        }

        // 设置目标地址（不可达）
        memset(&serv_addr, 0, sizeof(serv_addr));
        serv_addr.sin_family = AF_INET;
        serv_addr.sin_port = htons(UNREACHABLE_PORT);
        if (inet_pton(AF_INET, UNREACHABLE_IP, &serv_addr.sin_addr) <= 0) {
            perror("inet_pton() failed");
            exit(EXIT_FAILURE);
        }

        // 尝试连接（预期失败）
        if (connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
            printf("[%d] connect() failed: %s\n", i, strerror(errno));
            // 故意不调用 close(sockfd)，模拟 fd 泄漏！
        } else {
            printf("[%d] connect() succeeded (unexpected)\n", i);
            close(sockfd);  // 如果意外成功，关闭 socket
        }

     
    }

    printf("Done. Check `ls -l /proc/%d/fd` for leaked file descriptors.\n", getpid());
    printf("Press Enter to exit...");
    getchar();

    return 0;
}