tcpdump 中未看到 tcpkeeplive 消息

Question

我正在客户端服务器程序下面运行，该程序通过本地环路地址进行通信，并尝试捕获 tcpdump 输出。虽然程序专门设置了定期发送保活消息，但我在 tcpdump 中没有看到它。知道为什么吗？

服务器.py

import socket

def start_server():
    server_host = '127.0.0.1'
    server_port = 12345
    backlog = 5  # Maximum number of queued connections

    # Create a TCP socket
    server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

    # Enable TCP keep-alive on the server socket
    server_socket.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)

    # Set TCP keep-alive options (optional, you can adjust these as needed)
    server_socket.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPIDLE, 60)
    server_socket.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPINTVL, 10)
    server_socket.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPCNT, 5)

    # Bind the socket to the server address and port
    server_socket.bind((server_host, server_port))

    # Listen for incoming connections
    server_socket.listen(backlog)
    print(f"Server is listening on {server_host}:{server_port}")

    while True:
        print("Waiting for a connection...")
        client_socket, client_address = server_socket.accept()
        print(f"Accepted connection from {client_address}")

        while True:
            data = client_socket.recv(1024)
            if not data:
                break

            print(f"Received: {data.decode('utf-8')}")
            client_socket.send(data)  # Echo the data back to the client

        client_socket.close()
        print(f"Connection with {client_address} closed")

if __name__ == "__main__":
    start_server()

客户端.py

import socket
import time

def start_client():
    server_host = '127.0.0.1'
    server_port = 12345

    # Create a TCP socket
    client_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

    # Enable TCP keep-alive on the client socket
    client_socket.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)

    # Set TCP keep-alive options (optional, you can adjust these as needed)
    client_socket.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPIDLE, 60)
    client_socket.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPINTVL, 10)
    client_socket.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPCNT, 5)

    try:
        # Connect to the server
        client_socket.connect((server_host, server_port))
        print(f"Connected to {server_host}:{server_port}")

        while True:
            message = input("Enter a message to send (or 'exit' to quit): ")
            if message == 'exit':
                break

            client_socket.send(message.encode('utf-8'))
            response = client_socket.recv(1024)
            print(f"Server response: {response.decode('utf-8')}")

    finally:
        client_socket.close()
        print("Connection closed")

if __name__ == "__main__":
    start_client()

我保持套接字通道打开 5 分钟，没有任何活动，但没有看到任何保持活动的 pkt，但是仍然有一些 pkt 传输定期发生，但似乎没有保持活动

/home/ravi> tcpdump -i lo -n -vvv 'port 12345'
tcpdump: listening on lo, link-type EN10MB (Ethernet), capture size 262144 bytes
....
....
03:41:48.073820 IP (tos 0x0, ttl 64, id 50758, offset 0, flags [DF], proto TCP (6), length 52)
    127.0.0.1.12345 > 127.0.0.1.49088: Flags [.], cksum 0xfe28 (incorrect -> 0x38ba), seq 4, ack 4, win 512, options [nop,nop,TS val 2055232093 ecr 2055170654], length 0
03:41:48.073832 IP (tos 0x0, ttl 64, id 27799, offset 0, flags [DF], proto TCP (6), length 52)
    127.0.0.1.49088 > 127.0.0.1.12345: Flags [.], cksum 0xfe28 (incorrect -> 0x38ba), seq 3, ack 5, win 512, options [nop,nop,TS val 2055232093 ecr 2055170654], length 0
03:41:48.073868 IP (tos 0x0, ttl 64, id 27800, offset 0, flags [DF], proto TCP (6), length 52)
    127.0.0.1.49088 > 127.0.0.1.12345: Flags [.], cksum 0xfe28 (incorrect -> 0x38b9), seq 4, ack 5, win 512, options [nop,nop,TS val 2055232093 ecr 2055170654], length 0
03:41:48.073876 IP (tos 0x0, ttl 64, id 50759, offset 0, flags [DF], proto TCP (6), length 52)
    127.0.0.1.12345 > 127.0.0.1.49088: Flags [.], cksum 0xfe28 (incorrect -> 0x38b9), seq 5, ack 4, win 512, options [nop,nop,TS val 2055232093 ecr 2055170654], length 0
....
....

Answer 1

进展顺利，但我们这里有一个小问题。 tcpdump 很容易记录应用程序级别的所有流量。

KEEP_ALIVE 是内核级配置。你通常做的是设置套接字选项，正如我在你的代码中看到的那样。发布此内容后，保持连接活动的责任是操作系统（具体来说是内核）之间发生的交互，因此在应用程序级别（因此转储）我们将不会看到任何数据包经过。

希望这有帮助。