侦听 UDP 数据包的 Python 原始套接字；只收到一半的数据包 [英] Python raw socket listening for UDP packets; only half of the packets received

问题描述

我正在尝试在 Python 中创建一个仅侦听 UDP 数据包的 raw 套接字:

I am trying to create a raw socket in Python that listens for UDP packets only:

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_UDP)
s.bind(('0.0.0.0', 1337))
while True:
    print s.recvfrom(65535)

这需要以root身份运行，并在端口1337上创建一个原始套接字，它监听UDP数据包并在收到它们时打印它们；没有问题.

This needs to be run as root, and creates a raw socket on port 1337, which listens for UDP packets and prints them whenever they are received; no problems there.

现在让我们做一个小客户端来测试它是否有效:

Now let's make a little client to test if this works:

import socket
c = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
c.connect(('127.0.0.1', 1337))
c.send('message 1')
c.send('message 2')
c.send('message 3')
c.send('message 4')
c.send('message 5')
c.send('message 6')

始终如一地，只有第一条、第三条和第五条消息(message 1、message 3 和 message 5)会通过并被打印在服务器输出中.第二条、第四条和第六条消息没有显示在服务器输出上，而是客户端收到异常:

Consistently, only the first, third, and fifth message (message 1, message 3 and message 5) will get through and be printed in the server output. The second, fourth, and sixth messages do not show up on the server output, and instead the client gets an exception:

>>> c.send('message 2')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
socket.error: [Errno 111] Connection refused

在 Wireshark 中运行它表明它正在收到无法到达目标"的 ICMP 回复.我已经能够在 3 台不同的机器上重现这一点(尽管它们都运行 Linux).我错过了什么吗?这是 UDP 持续丢弃数据包的预期行为，因为使用它的协议应该能够容忍数据包丢失?即便如此，为什么在本地接口上发送的数据包会被丢弃?

Running this in Wireshark shows that it is getting an ICMP reply for "Destination unreachable". I have been able to reproduce this on 3 distinct machines (all running Linux though). Am I missing something? Is this expected behaviour for UDP to consistently drop packets, since protocols using it are supposed to be tolerant of packet loss? Even so, why would packets be dropped when sent on the local interface?

将服务器绑定到 127.0.0.1 而不是 0.0.0.0 具有相同的结果.

Binding the server to 127.0.0.1 instead of 0.0.0.0 has the same result.

推荐答案

以一种愚蠢的方式解决了它；如果有其他方法，请告诉我，我会更改接受的答案.

Solved it in kind of a silly manner; please let me know if there is another way, and I will change the accepted answer.

解决方法是简单地使用绑定在同一个端口上的两个套接字；一份生的，一份非生的:

The solution is simply to use two sockets bound on the same port; one raw, one not raw:

import socket, select
s1 = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s1.bind(('0.0.0.0', 1337))
s2 = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_UDP)
s2.bind(('0.0.0.0', 1337))
while True:
    r, w, x = select.select([s1, s2], [], [])
    for i in r:
        print i, i.recvfrom(131072)

这使得目标不可达"ICMP 数据包消失，并使所有数据包正常通过.我认为操作系统希望在端口上监听一个非原始套接字以使事情顺利进行，然后在同一端口上监听的任何原始套接字都将接收数据包的副本.

This makes the "Destination unreachable" ICMP packets go away and makes all packets go through fine. I think the operating system wants a non-raw socket listening on the port for things to go well, and then any raw sockets listening on that same port will receive copies of the packets.

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