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C++ libnetfilter_queue

TobRein

New Coder
Hello community,

I'm trying to intercept an IP packet with libnetfilter_queue and change the payload (append additional data or delete some data) of the packet.
For example, if the payload length is 2 bytes, I only can change the two bytes, not append any additional data.
I tried it with netcat.
When I send an "a" from the client to the server , on the server side "12" appears,
Wehn I send "ab", on the other side "123" appears.

How I can add additional data to the payload or delete some data ?

Can anybody help me please.

here my code:
C++:
//#######################################################################
#include <dirent.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <stdio.h>
#include <string>
#include <algorithm>
#include <iomanip>
#include <iostream>
#include <cstdlib>
#include <net/if.h>
#include <libnfnetlink/libnfnetlink.h>
#include <libmnl/libmnl.h>

extern "C"
{
    #include <libnetfilter_queue/libnetfilter_queue.h>
    #include <libnetfilter_queue/libnetfilter_queue_ipv4.h>
    #include <libnetfilter_queue/libnetfilter_queue_tcp.h>
    #include <libnetfilter_queue/libnetfilter_queue_udp.h>
    #include <libnetfilter_queue/pktbuff.h>
    #include <libnetfilter_queue/libnetfilter_queue.h>

}

using namespace std;

int call_back(struct nfq_q_handle *queue_handle, struct nfgenmsg *nfmsg, struct nfq_data *paket_daten, void *data)
{
   
    int rc = -1;
    u_int32_t id = -1;
   
    struct nfqnl_msg_packet_hdr *paket_header;
   
    paket_header = nfq_get_msg_packet_hdr(paket_daten);   
    id = ntohl(paket_header->packet_id);
   
    char *paket_inhalt;
    int paket_laenge_2 = nfq_get_payload(paket_daten, &paket_inhalt);
   
    struct pkt_buff * pkBuff = pktb_alloc(AF_INET, paket_inhalt, paket_laenge_2, 8192);
   
    struct iphdr *ip = nfq_ip_get_hdr(pkBuff);
   
    nfq_ip_set_transport_header(pkBuff, ip);
   
    struct tcphdr *tcp = nfq_tcp_get_hdr(pkBuff);
    char *payload = nfq_tcp_get_payload(tcp, pkBuff);

    unsigned int payloadLen = nfq_tcp_get_payload_len(tcp, pkBuff);
    int payloadLen_alt = payloadLen;
    unsigned short tcphdrlen = tcp->doff * 4;
   

   
    if (payloadLen > 0)
    {
        char dest[4096];
        strcpy(new_payload,"1234567890");
        memcpy(payload, new_payload, strlen(new_payload) + 1);
        int payload_len_neu = - 1;
        payload_len_neu = strlen(payload);
       
        rc = pktb_mangle(pkBuff, tcphdrlen + iphl_bytes , 0, payloadLen_alt, payload, payload_len_neu);
       
        nfq_tcp_compute_checksum_ipv4(tcp, ip);
    }

       
    return nfq_set_verdict(queue_handle, ntohl(paket_header->packet_id), NF_ACCEPT, pktb_len(pkBuff), pktb_data(pkBuff));
   
   
    pktb_free(pkBuff);
}


int main (void)
{
   
   
    struct nfq_handle *nfq_lib_handle;
    nfq_lib_handle = nfq_open();
   
    int rc = -1;
    rc = -1;
    rc = nfq_unbind_pf(nfq_lib_handle, AF_INET);
   
    rc = -1;
    rc = nfq_bind_pf(nfq_lib_handle, AF_INET);
   
    struct nfq_q_handle *queue_handle;
    queue_handle = nfq_create_queue(nfq_lib_handle,  0, &call_back, NULL);
   
    rc = -1;
    rc = nfq_set_mode(queue_handle, NFQNL_COPY_PACKET, 0xffff);
    int file_descriptor_nfq = -1;
    file_descriptor_nfq = nfq_fd(nfq_lib_handle);
   
    int paket_laenge = -1;
    char paket_puffer[8192];
       
    while (( paket_laenge = recv(file_descriptor_nfq, paket_puffer, sizeof(paket_puffer), 0)) >= 0)
    {

        nfq_handle_packet(nfq_lib_handle, paket_puffer, paket_laenge);
       

    }
    nfq_destroy_queue(queue_handle);
   
    nfq_close(nfq_lib_handle);
       
    return 0;
}

//#######################################################################
 
Last edited by a moderator:
Hello community,

I'm trying to intercept an IP packet with libnetfilter_queue and change the payload (append additional data or delete some data) of the packet.
For example, if the payload length is 2 bytes, I only can change the two bytes, not append any additional data.
I tried it with netcat.
When I send an "a" from the client to the server , on the server side "12" appears,
Wehn I send "ab", on the other side "123" appears.

How I can add additional data to the payload or delete some data ?

Can anybody help me please.

here my code:
C++:
//#######################################################################
#include <dirent.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <stdio.h>
#include <string>
#include <algorithm>
#include <iomanip>
#include <iostream>
#include <cstdlib>
#include <net/if.h>
#include <libnfnetlink/libnfnetlink.h>
#include <libmnl/libmnl.h>

extern "C"
{
    #include <libnetfilter_queue/libnetfilter_queue.h>
    #include <libnetfilter_queue/libnetfilter_queue_ipv4.h>
    #include <libnetfilter_queue/libnetfilter_queue_tcp.h>
    #include <libnetfilter_queue/libnetfilter_queue_udp.h>
    #include <libnetfilter_queue/pktbuff.h>
    #include <libnetfilter_queue/libnetfilter_queue.h>

}

using namespace std;

int call_back(struct nfq_q_handle *queue_handle, struct nfgenmsg *nfmsg, struct nfq_data *paket_daten, void *data)
{
  
    int rc = -1;
    u_int32_t id = -1;
  
    struct nfqnl_msg_packet_hdr *paket_header;
  
    paket_header = nfq_get_msg_packet_hdr(paket_daten);  
    id = ntohl(paket_header->packet_id);
  
    char *paket_inhalt;
    int paket_laenge_2 = nfq_get_payload(paket_daten, &paket_inhalt);
  
    struct pkt_buff * pkBuff = pktb_alloc(AF_INET, paket_inhalt, paket_laenge_2, 8192);
  
    struct iphdr *ip = nfq_ip_get_hdr(pkBuff);
  
    nfq_ip_set_transport_header(pkBuff, ip);
  
    struct tcphdr *tcp = nfq_tcp_get_hdr(pkBuff);
    char *payload = nfq_tcp_get_payload(tcp, pkBuff);

    unsigned int payloadLen = nfq_tcp_get_payload_len(tcp, pkBuff);
    int payloadLen_alt = payloadLen;
    unsigned short tcphdrlen = tcp->doff * 4;
  

  
    if (payloadLen > 0)
    {
        char dest[4096];
        strcpy(new_payload,"1234567890");
        memcpy(payload, new_payload, strlen(new_payload) + 1);
        int payload_len_neu = - 1;
        payload_len_neu = strlen(payload);
      
        rc = pktb_mangle(pkBuff, tcphdrlen + iphl_bytes , 0, payloadLen_alt, payload, payload_len_neu);
      
        nfq_tcp_compute_checksum_ipv4(tcp, ip);
    }

      
    return nfq_set_verdict(queue_handle, ntohl(paket_header->packet_id), NF_ACCEPT, pktb_len(pkBuff), pktb_data(pkBuff));
  
  
    pktb_free(pkBuff);
}


int main (void)
{
  
  
    struct nfq_handle *nfq_lib_handle;
    nfq_lib_handle = nfq_open();
  
    int rc = -1;
    rc = -1;
    rc = nfq_unbind_pf(nfq_lib_handle, AF_INET);
  
    rc = -1;
    rc = nfq_bind_pf(nfq_lib_handle, AF_INET);
  
    struct nfq_q_handle *queue_handle;
    queue_handle = nfq_create_queue(nfq_lib_handle,  0, &call_back, NULL);
  
    rc = -1;
    rc = nfq_set_mode(queue_handle, NFQNL_COPY_PACKET, 0xffff);
    int file_descriptor_nfq = -1;
    file_descriptor_nfq = nfq_fd(nfq_lib_handle);
  
    int paket_laenge = -1;
    char paket_puffer[8192];
      
    while (( paket_laenge = recv(file_descriptor_nfq, paket_puffer, sizeof(paket_puffer), 0)) >= 0)
    {

        nfq_handle_packet(nfq_lib_handle, paket_puffer, paket_laenge);
      

    }
    nfq_destroy_queue(queue_handle);
  
    nfq_close(nfq_lib_handle);
      
    return 0;
}

//#######################################################################
Hi there,

Can you provide a bit of context as to your issue? What is the purpose for this application?
 

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