Buffer Size Issue with Npcap in Windows 10 using pcap.h in C++

Question

I have encountered an issue while trying to set a large buffer size using Npcap in a C++ program on Windows 10. The code works correctly when using WinPcap, obtaining the desired 2GB buffer size, but fails to achieve the same result with Npcap. I am seeking guidance on resolving this issue.

Example:

#include <iostream>
#include <pcap.h>

void packet_handler(u_char *user, const struct pcap_pkthdr *pkthdr, const u_char *packet) {
    // Your packet handling logic goes here
    // This function will be called for each captured packet
    std::cout << "Packet captured!" << std::endl;
}

int main() {
    char errbuf[PCAP_ERRBUF_SIZE];

    // Change "eth0" to your network interface name
    const char *dev = "eth0";

    // Open the capture interface
    pcap_t *handle = pcap_open_live(dev, 65536, 1, 1000, errbuf);

    if (handle == nullptr) {
        std::cerr << "Couldn't open device " << dev << ": " << errbuf << std::endl;
        return 1;
    }

    // Set the buffer size
    int buffer_size = 2147483647; // 2 GB
    if (pcap_set_buffer_size(handle, buffer_size) != 0) {
        std::cerr << "Failed to set buffer size: " << pcap_geterr(handle) << std::endl;
        return 1;
    }

    // Start capturing packets
    if (pcap_loop(handle, 0, packet_handler, nullptr) < 0) {
        std::cerr << "Error during packet capture: " << pcap_geterr(handle) << std::endl;
        return 1;
    }

    // Close the capture handle
    pcap_close(handle);

    return 0;
}

My issue is that pcap_set_buffer_size didn't Get 2GB RAM while I used Npcap!

The provided code works correctly on Windows with WinPcap, achieving the desired 2GB buffer size. However, when using Npcap, the same code fails to obtain the expected buffer size from RAM.

Answer 1

In the current versions of the libpcap library, including Npcap on Windows, the buffer size must be set before the packet capture handle is activated. This means that you should set the buffer size right after creating the capture handle with pcap_create, but before activating it with pcap_activate.

If you use pcap_open_live, which is a convenience function that combines the steps of creating and activating a capture handle, you cannot set the buffer size before opening the handle. Instead, you should use pcap_create and pcap_activate for more control.

Example Code Using pcap_create and pcap_activate

// Create the capture handle
pcap_t *handle = pcap_create(device, errbuf);
if (handle == nullptr) {
    std::cerr << "Could not create capture handle: " << errbuf << std::endl;
    return 1;
}

// Set the buffer size to 2 MB (2,097,152 bytes)
if (pcap_set_buffer_size(handle, 2097152) != 0) {
    std::cerr << "Could not set buffer size: " << pcap_geterr(handle) << std::endl;
    pcap_close(handle);
    return 1;
}

// Set the snaplen to 2000 bytes
if (pcap_set_snaplen(handle, 2000) != 0) {
    std::cerr << "Could not set snaplen: " << pcap_geterr(handle) << std::endl;
    pcap_close(handle);
    return 1;
}

// Set promiscuous mode
if (pcap_set_promisc(handle, 1) != 0) {
    std::cerr << "Could not set promiscuous mode: " << pcap_geterr(handle) << std::endl;
    pcap_close(handle);
    return 1;
}

// Set the read timeout
if (pcap_set_timeout(handle, 1000) != 0) {
    std::cerr << "Could not set read timeout: " << pcap_geterr(handle) << std::endl;
    pcap_close(handle);
    return 1;
}

// Activate the capture handle
if (pcap_activate(handle) != 0) {
    std::cerr << "Could not activate capture handle: " << pcap_geterr(handle) << std::endl;
    pcap_close(handle);
    return 1;
}

// Start capturing packets
if (pcap_loop(handle, 0, packetHandler, nullptr) < 0) {
    std::cerr << "Error in pcap_loop: " << pcap_geterr(handle) << std::endl;
    pcap_close(handle);
    return 1;
}

// Close the capture handle
pcap_close(handle);
return 0;

}

This example is what i work with, buffer size of 2MB and snaplen if 2000 bytes.