Wireshark BPF 捕获过滤器

Wireshark

发布日期: 2023-04-15

更新日期: 2023-04-16

文章字数: 427

阅读时长: 2 分

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Wireshark BPF 捕获过滤器

Wireshark 中，如果要过滤出所有的 POST 或者是 GET 请求，我们可以用显示过滤器里的 method 过滤。但是如果流量过大的话，涌入的数据包会填满缓冲区，过滤也需要消耗大量的 CPU 资源。

Capture Filter

Capture filter is not a display filter

Capture filters (like tcp port 80) are not to be confused with display filters (like tcp.port == 80). The former are much more limited and are used to reduce the size of a raw packet capture. The latter are used to hide some packets from the packet list.

Capture filters are set before starting a packet capture and cannot be modified during the capture. Display filters on the other hand do not have this limitation and you can change them on the fly.

https://wiki.wireshark.org/CaptureFilters

捕获过滤器可以在从网卡接收数据的阶段就进行过滤，大大优化抓包性能。

但是需要我们生成一个 BPF 格式的过滤规则，主要是基于二进制与偏移的。

生成器

https://www.wireshark.org/tools/string-cf.html

https://www.wireshark.org/docs/wsug_html_chunked/ChCapCaptureFilterSection.html

这里为了性能和批量生成，使用 C++ 重构了代码。

https://github.com/sfc9982/wireshark-bpf-capfilter

auto stringToBpf(std::string str, int offset) -> std::string {
    std::vector<unsigned char> bytes   = utf8Convert(str);
    std::string                snippet = "";
    int                        slicelen;
    std::string                hexbytes;
    std::string                conj = "";
    std::string                offstr;


    while (bytes.size() > 0) {
        if (bytes.size() >= 4) {
            slicelen = 4;
        }
        else if (bytes.size() >= 2) {
            slicelen = 2;
        }
        else {
            slicelen = 1;
        }

        auto slicebytes = std::vector<unsigned char>(bytes.begin(), bytes.begin() + slicelen);
        bytes.erase(bytes.begin(), bytes.begin() + slicelen);

        hexbytes = "";
        for (auto i: slicebytes) {
            char buf[3];
            sprintf(buf, "%02x", i);
            hexbytes += buf;
        }

        if (offset) {
            offstr = " + " + std::to_string(offset);
        }
        else {
            offstr = "";
        }

        snippet += conj + "tcp[((tcp[12:1] & 0xf0) >> 2)" + offstr + ":" + std::to_string(slicelen) + "] = 0x" + hexbytes;

        conj = " && ";
        offset += slicelen;
    }

    return snippet;
}