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How to Capture and Modify TCP Packets Using Npcap Library in C

email:  tcpfast@gmail.com 

In this blog post, I will show you how to use the pcap library in C to capture and modify TCP packets on the fly. The pcap library is a powerful tool for network analysis and manipulation, which allows you to access raw packets from various network interfaces. You can use the pcap library to implement your own network applications, such as firewalls, proxies, sniffers, etc.

The Code

The code I will use as an example is as follows:

#include <stdio.h>
#include <stdlib.h>
#include <pcap.h>
#include <Packet32.h>
#include <ntddndis.h>

#define MAX_PACKET_SIZE 65536
#define KEYWORD "keyword1"
#define IP1 "192.168.0.1"
#define PORT1 12345

void packet_handler(u_char* user_data, const struct pcap_pkthdr* pkthdr, const u_char* packet);

int main()
{
    pcap_t* handle;
    char errbuf[PCAP_ERRBUF_SIZE];

    // Open the network adapter
    handle = pcap_open_live("\\Device\\NPF_{YOUR_ADAPTER_GUID}", MAX_PACKET_SIZE, 1, 1000, errbuf);
    if (handle == NULL) {
        printf("Cannot open adapter: %s\n", errbuf);
        return 1;
    }

    // Set the filter to capture only TCP packets
    struct bpf_program fp;
    char filter_exp[] = "tcp";
    if (pcap_compile(handle, &fp, filter_exp, 0, PCAP_NETMASK_UNKNOWN) == -1) {
        printf("Cannot compile filter: %s\n", pcap_geterr(handle));
        return 1;
    }
    if (pcap_setfilter(handle, &fp) == -1) {
        printf("Cannot set filter: %s\n", pcap_geterr(handle));
        return 1;
    }

    // Start capturing packets
    pcap_loop(handle, 0, packet_handler, NULL);

    // Close the network adapter
    pcap_close(handle);

    return 0;
}

void packet_handler(u_char* user_data, const struct pcap_pkthdr* pkthdr, const u_char* packet)
{
    // Parse the Ethernet header
    struct ether_header* eth_header = (struct ether_header*)packet;
    int eth_header_size = sizeof(struct ether_header);

    // Parse the IP header
    struct ip* ip_header = (struct ip*)(packet + eth_header_size);
    int ip_header_size = ip_header->ip_hl * 4;

    // Parse the TCP header
    struct tcphdr* tcp_header = (struct tcphdr*)(packet + eth_header_size + ip_header_size);
    int tcp_header_size = tcp_header->th_off * 4;

    // Calculate the TCP data offset
    int tcp_data_offset = eth_header_size + ip_header_size + tcp_header_size;

    // Get the TCP data
    const u_char* tcp_data = packet + tcp_data_offset;
    int tcp_data_size = pkthdr->caplen - tcp_data_offset;

    // Check if the first TCP packet contains the keyword
    if (strstr(tcp_data, KEYWORD) != NULL) {
        // Create a new packet
        u_char new_packet[MAX_PACKET_SIZE];
        int new_packet_size = 0;

        // Copy the Ethernet header
        memcpy(new_packet, packet, eth_header_size);
        new_packet_size += eth_header_size;

        // Change the destination IP to IP1
        struct ip* new_ip_header = (struct ip*)(new_packet + eth_header_size);
        new_ip_header->ip_dst.s_addr = inet_addr(IP1);

        // Change the destination port to Port1
        struct tcphdr* new_tcp_header = (struct tcphdr*)(new_packet + eth_header_size + ip_header_size);
        new_tcp_header->th_dport = htons(PORT1);

        // Copy the TCP data
        memcpy(new_packet + new_packet_size, tcp_data, tcp_data_size);
        new_packet_size += tcp_data_size;

        // Send the new packet
        pcap_sendpacket(handle, new_packet, new_packet_size);
    }
}

The Explanation

The code consists of two main parts: the main function and the packet handler function. The main function is responsible for opening the network adapter, setting the filter, and starting the packet capture loop. The packet handler function is called for each captured packet, and it performs the packet analysis and modification.

Opening the Network Adapter

The first step is to open the network adapter that you want to capture packets from. You need to specify the name of the adapter, which is usually in the form of “\Device\NPF_{GUID}”, where GUID is a unique identifier for the adapter. You can use the pcap_findalldevs function to get a list of all available adapters and their names. You also need to specify the maximum packet size, the promiscuous mode, the read timeout, and the error buffer. The promiscuous mode determines whether the adapter will capture all packets on the network or only those addressed to it. The read timeout determines how long the adapter will wait for packets before returning. The error buffer is used to store any error messages that may occur.

The function pcap_open_live returns a pointer to a pcap_t structure, which represents the opened adapter. If the function fails, it returns NULL and sets the error buffer accordingly. You should always check the return value and handle any errors.

Setting the Filter

The next step is to set the filter that will determine which packets will be captured and which will be ignored. You can use a filter expression that follows the syntax of the Berkeley Packet Filter (BPF) language, which is a simple and powerful way to specify packet criteria. For example, the filter expression “tcp” means to capture only TCP packets, while the filter expression “tcp and port 80” means to capture only TCP packets with port 80 as either the source or the destination port.

The function pcap_compile takes the filter expression and compiles it into a bpf_program structure, which can be used by the adapter. You need to pass the pointer to the pcap_t structure, the pointer to the bpf_program structure, the filter expression, the optimize flag, and the netmask. The optimize flag determines whether the filter will be optimized for speed or not. The netmask is used to determine the broadcast address of the network. If you don’t know the netmask, you can use the constant PCAP_NETMASK_UNKNOWN.

The function pcap_setfilter takes the pointer to the pcap_t structure and the pointer to the bpf_program structure, and sets the filter for the adapter. If the function fails, it returns -1 and sets the error message accordingly. You should always check the return value and handle any errors.

Starting the Packet Capture Loop

The final step is to start the packet capture loop, which will call the packet handler function for each captured packet. The function pcap_loop takes the pointer to the pcap_t structure, the number of packets to capture, the pointer to the packet handler function, and the user data. The number of packets to capture can be 0, which means to capture packets indefinitely, or a positive integer, which means to capture that many packets and then stop. The user data is a pointer to any data that you want to pass to the packet handler function. You can use it to store any information that you need for your application.

The function pcap_loop will block until the specified number of packets are captured, or until an error occurs, or until the capture is stopped by another thread. You can use the function pcap_breakloop to stop the capture from another thread. You should always check the return value and handle any errors.

Closing the Network Adapter

When you are done with the packet capture, you should close the network adapter by using the function pcap_close, which takes the pointer to the pcap_t structure and frees any resources associated with it.

Parsing the Packet Headers

The packet handler function is called for each captured packet, and it receives three parameters: the user data, the packet header, and the packet data. The user data is the same pointer that you passed to the pcap_loop function. The packet header is a pointer to a pcap_pkthdr structure, which contains the timestamp, the length, and the captured length of the packet. The packet data is a pointer to a u_char array, which contains the raw bytes of the packet.

The packet data consists of several headers, followed by the payload. The headers are the Ethernet header, the IP header, and the TCP header. The payload is the TCP data. You need to parse the headers to get the information that you need for your application, such as the source and destination IP addresses and ports, the TCP flags, the TCP sequence and acknowledgment numbers, etc.

The Ethernet header is 14 bytes long, and it contains the source and destination MAC addresses and the type of the next header. You can use the ether_header structure to access the fields of the Ethernet header. You need to cast the packet data pointer to a ether_header pointer, and then use the dot operator to access the fields. For example, eth_header->ether_type gives you the type of the next header.

The IP header is variable in length, depending on the value of the IP header length field, which is the first four bits of the first byte of the IP header. You can use the ip structure to access the fields of the IP header. You need to cast the packet data pointer plus the Ethernet header size to an ip pointer, and then use the dot operator to access the fields. For example, ip_header->ip_src.s_addr gives you the source IP address.

The TCP header is also variable in length, depending on the value of the TCP header length field, which is the first four bits of the fourth byte of the TCP header. You can use the tcphdr structure to access the fields of the TCP header. You need to cast the packet data pointer plus the Ethernet header size plus the IP header size to a tcphdr pointer, and then use the dot operator to access the fields. For example, tcp_header->th_flags gives you the TCP flags.

The TCP data is the payload of the TCP packet, and it contains the application data that is being transmitted. You can access the TCP data by adding the Ethernet header size, the IP header size, and the TCP header size to the packet data pointer. You can also get the TCP data size by subtracting the same values from the captured length of the packet.

Modifying the Packet Data

The packet handler function also performs the packet modification, which is the main purpose of this example. The modification consists of checking if the first TCP packet contains a certain keyword, and if so, creating a new packet with a different destination IP and port, and sending it to the network.

The first step is to check if the TCP data contains the keyword, which is defined as a macro at the beginning of the code. You can use the strstr function to search for the keyword in the TCP data. The function returns a pointer to the first occurrence of the keyword, or NULL if the keyword is not found. You can use an if statement to perform the check.

The second step is to create a new packet, which will have the same Ethernet header, IP header, and TCP header as the original packet, except for the destination IP and port, which will be changed to the values defined as macros at the beginning of the code. You also need to copy the TCP data to the new packet.

The third step is to send the new packet to the network, using the function pcap_sendpacket, which takes the pointer to the pcap_t structure, the pointer to the new packet data, and the size of the new packet. If the function fails, it returns -1 and sets the error message accordingly. You should always check the return value and handle any errors.

The Result

The result of running this code is that you will capture all TCP packets on the network, and if the first TCP packet contains the keyword “keyword1”, you will send a new packet with the same TCP data, but with a different destination IP and port. This can be useful for various purposes, such as redirecting traffic, spoofing packets, testing network applications, etc.

The Conclusion

In this blog post, I have shown you how to use the pcap library in C to capture and modify TCP packets on the fly. I have explained the code in detail, and I have demonstrated the result of running it. I hope you have learned something useful from this post, and I encourage you to experiment with the pcap library and create your own network applications. Thank you for reading.


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