异步的libpcap：丢包？ [英] Asynchronous libpcap: losing packets?

问题描述

我有发送一套TCP SYN数据包的到主机（使用原始套接字）中的程序并使用 libpcap的（有过滤器），得到的反应。我想在异步I / O框架来实现这一点，但似乎的libpcap 缺少一些响应（即一个系列的第一个数据包时，它需要小于 100微秒的TCP SYN和响应）之间。该PCAP手柄的设置是这样的：

I have a program that sends a set of TCP SYN packets to a host (using raw sockets) and uses libpcap (with a filter) to obtain the responses. I'm trying to implement this in an asynchronous I/O framework, but it seems that libpcap is missing some of the responses (namely the first packets of a series when it takes less than 100 microseconds between the TCP SYN and the response). The pcap handle is setup like this:

pcap_t* pcap = pcap_open_live(NULL, -1, false, -1, errorBuffer);
pcap_setnonblock(pcap, true, errorBuffer);

然后我添加过滤器（包含在filterEx pression字符串）：

Then I add a filter (contained on the filterExpression string):

struct bpf_program filter;
pcap_compile(pcap, &filter, filterExpression.c_str(), false, 0);
pcap_setfilter(pcap, &filter);
pcap_freecode(&filter);

和上一个循环，发送每个数据包后，我用select知道，如果我可以从libpcap的读取：

And on a loop, after sending each packet, I use select to know if I can read from libpcap:

int pcapFd = pcap_get_selectable_fd(pcap);
fd_set fdRead;
FD_ZERO(&fdRead);
FD_SET(pcapFd, &fdRead);
select(pcapFd + 1, &fdRead, NULL, NULL, &selectTimeout);

和阅读：

if (FD_ISSET(pcapFd, &fdRead)) {
     struct pcap_pkthdr* pktHeader;
     const u_char* pktData;
     if (pcap_next_ex(pcap, &pktHeader, &pktData) > 0) {
         // Process received response.
     }
     else {
         // Nothing to receive (or error).
     }
}

正如我以前说过，某些数据包会丢失（落入没有领取其他人）。我知道这些数据包都是存在的，因为我可以（使用同步方式捕捉它们的tcpdump 或正在运行的线程 pcap_loop ）。我失去了一些细节吗？或者，这是一个问题，的libpcap ？

As I said before, some of the packets are missed (falling into the "nothing to receive" else). I know these packets are there, because I can capture them on a synchronous fashion (using tcpdump or a thread running pcap_loop). Am I missing some detail here? Or is this an issue with libpcap?

推荐答案

这似乎是使用libpcap的Linux下的内存映射的问题。请参阅我的其他问题了解详情。

This seem to be an issue with libpcap using memory mapping under Linux. Please see my other question for details.

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