偶尔在网上看到了这篇文章,
并结合中的例子,
然后修改了一下tcp计算checksum部分,在linux2.6.24上用netfilter_queue在用户态实现NAT
程序功能: 将输出端目的地为 220.181.37.55 的包,都改为目的地为 202.118.236.130,输入段反之,达到DNAT的一小半功能,完整的NAT要做状态记录的.
直接上代码:
nf_queue_test.c
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/*
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* =====================================================================================
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*
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* Filename: nf_queue_test.c
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*
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* Description: 用netfilter_queue 在用户态修改网络数据包的例子程序
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*
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* Version: 1.0
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* Created: 04/02/2010 09:49:48 AM
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* Revision: none
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* Compiler: gcc
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*
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* Author: LeiuX (xulei), xulei@pact518.hit.edu.cn
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* Company: HIT
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*
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* =====================================================================================
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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#include <netdb.h>
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#include <string.h>
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#include <netinet/in.h>
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#include <arpa/inet.h>
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#include <asm/byteorder.h>
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#include <linux/netfilter.h>
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#include <libnetfilter_queue/libnetfilter_queue.h>
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#include <linux/ip.h>
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#include <linux/tcp.h>
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#ifdef __LITTLE_ENDIAN
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#define IPQUAD(addr) \
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((unsigned char *)&addr)[0], \
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((unsigned char *)&addr)[1], \
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((unsigned char *)&addr)[2], \
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((unsigned char *)&addr)[3]
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#else
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#define IPQUAD(addr) \
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((unsigned char *)&addr)[3], \
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((unsigned char *)&addr)[2], \
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((unsigned char *)&addr)[1], \
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((unsigned char *)&addr)[0]
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#endif
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struct tcp_pseudo /*the tcp pseudo header*/
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{
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__u32 src_addr;
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__u32 dst_addr;
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__u8 zero;
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__u8 proto;
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__u16 length;
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} pseudohead;
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-
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long checksum(unsigned short *addr, unsigned int count) {
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/* Compute Internet Checksum for "count" bytes
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* beginning at location "addr".
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*/
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register long sum = 0;
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while( count > 1 ) {
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/* This is the inner loop */
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sum += * addr++;
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count -= 2;
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}
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/* Add left-over byte, if any */
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if( count > 0 )
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sum += * (unsigned char *) addr;
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/* Fold 32-bit sum to 16 bits */
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while (sum>>16)
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sum = (sum & 0xffff) + (sum >> 16);
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return ~sum;
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}
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/*************************tcp checksum**********************/
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long get_tcp_checksum(struct iphdr * myip, struct tcphdr * mytcp) {
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__u16 total_len = ntohs(myip->tot_len);
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int tcpopt_len = mytcp->doff*4 - 20;
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int tcpdatalen = total_len - (mytcp->doff*4) - (myip->ihl*4);
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pseudohead.src_addr=myip->saddr;
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pseudohead.dst_addr=myip->daddr;
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pseudohead.zero=0;
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pseudohead.proto=IPPROTO_TCP;
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pseudohead.length=htons(sizeof(struct tcphdr) + tcpopt_len + tcpdatalen);
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int totaltcp_len = sizeof(struct tcp_pseudo) + sizeof(struct tcphdr) + tcpopt_len +tcpdatalen;
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//unsigned short * tcp = new unsigned short[totaltcp_len];
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unsigned short * tcp = malloc(totaltcp_len);
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memcpy((unsigned char *)tcp,&pseudohead,sizeof(struct tcp_pseudo));
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memcpy((unsigned char *)tcp+sizeof(struct tcp_pseudo),(unsigned char*)mytcp,sizeof(struct tcphdr));
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memcpy((unsigned char *)tcp+sizeof(struct tcp_pseudo)+sizeof(struct tcphdr),(unsigned char *)myip+(myip->ihl*4)+(sizeof(struct tcphdr)), tcpopt_len);
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memcpy((unsigned char *)tcp+sizeof(struct tcp_pseudo)+sizeof(structtcphdr)+tcpopt_len, (unsigned char *)mytcp+(mytcp->doff*4), tcpdatalen);
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/* printf("pseud length: %d\n",pseudohead.length);
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printf("tcp hdr length: %d\n",mytcp->doff*4);
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printf("tcp hdr struct length: %d\n",sizeof(struct tcphdr));
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printf("tcp opt length: %d\n",tcpopt_len);
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printf("tcp total+psuedo length: %d\n",totaltcp_len);
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fflush(stdout);
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printf("tcp data len: %d, data start %u\n", tcpdatalen,mytcp + (mytcp->doff*4));
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*/
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-
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return checksum(tcp,totaltcp_len);
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}
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static u_int16_t tcp_checksum(struct iphdr* iphdrp){
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struct tcphdr *tcphdrp =
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(struct tcphdr*)((u_int8_t*)iphdrp + (iphdrp->ihl<<2));
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return get_tcp_checksum(iphdrp, tcphdrp);
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}
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static void set_tcp_checksum(struct iphdr* iphdrp){
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struct tcphdr *tcphdrp =
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(struct tcphdr*)((u_int8_t*)iphdrp + (iphdrp->ihl<<2));
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tcphdrp->check = 0;
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tcphdrp->check = get_tcp_checksum(iphdrp, tcphdrp);
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}
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/****************************tcp checksum end****************************/
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/********************************Ip checksum*****************************/
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static u_int16_t ip_checksum(struct iphdr* iphdrp){
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return checksum((unsigned short*)iphdrp, iphdrp->ihl<<2);
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}
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static void set_ip_checksum(struct iphdr* iphdrp){
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iphdrp->check = 0;
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iphdrp->check = checksum((unsigned short*)iphdrp, iphdrp->ihl<<2);
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}
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/****************************Ip checksum end******************************/
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static int cb(struct nfq_q_handle *qh, struct nfgenmsg *nfmsg,
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struct nfq_data *nfa, void *data){
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(void)nfmsg;
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(void)data;
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u_int32_t id = 0;
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struct nfqnl_msg_packet_hdr *ph;
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unsigned char *pdata = NULL;
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int pdata_len;
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ph = nfq_get_msg_packet_hdr(nfa);
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if (ph){
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id = ntohl(ph->packet_id);
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}
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pdata_len = nfq_get_payload(nfa, (char**)&pdata);
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if(pdata_len == -1){
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pdata_len = 0;
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}
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struct iphdr *iphdrp = (struct iphdr *)pdata;
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printf("len %d iphdr %d %u.%u.%u.%u ->",
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pdata_len,
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iphdrp->ihl<<2,
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IPQUAD(iphdrp->saddr));
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printf(" %u.%u.%u.%u %s",
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IPQUAD(iphdrp->daddr),
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getprotobynumber(iphdrp->protocol)->p_name);
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printf(" ipsum %hu", ip_checksum(iphdrp));
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if(iphdrp->protocol == IPPROTO_TCP){
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printf(" tcpsum %hu", tcp_checksum(iphdrp));
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}
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#define TO "220.181.37.55"
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#define DNAT_TO "202.118.236.130"
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if(iphdrp->daddr == inet_addr(TO)){
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printf(" !hacked!");
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iphdrp->daddr = inet_addr(DNAT_TO);
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set_ip_checksum(iphdrp);
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if(iphdrp->protocol == IPPROTO_TCP){
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set_tcp_checksum(iphdrp);
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printf(" ipsum+ %hu tcpsum+ %hu",
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ip_checksum(iphdrp), tcp_checksum(iphdrp));
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}
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}
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if(iphdrp->saddr == inet_addr(DNAT_TO)){
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iphdrp->saddr = inet_addr(TO);
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printf(" !hacked!");
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set_ip_checksum(iphdrp);
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if(iphdrp->protocol == IPPROTO_TCP){
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set_tcp_checksum(iphdrp);
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printf(" ipsum+ %hu tcpsum+ %hu",
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ip_checksum(iphdrp), tcp_checksum(iphdrp));
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}
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}
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printf("\n");
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return nfq_set_verdict_mark(qh, id, NF_REPEAT, 1,
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(u_int32_t)pdata_len, pdata);
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}
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int main(int argc, char **argv)
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{
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struct nfq_handle *h;
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struct nfq_q_handle *qh;
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struct nfnl_handle *nh;
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int fd;
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int rv;
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char buf[4096];
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h = nfq_open();
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if (!h) {
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exit(1);
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}
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nfq_unbind_pf(h, AF_INET);
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/*2.6.24 的内核有BUG, nfq_unbind_pf 返回值不正确,
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见:http://article.gmane.org/gmane.c ... ilter.general/33573*/
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/*
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if (nfq_unbind_pf(h, AF_INET) < 0){
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exit(1);
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}
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*/
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if (nfq_bind_pf(h, AF_INET) < 0) {
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exit(1);
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}
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int qid = 0;
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if(argc == 2){
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qid = atoi(argv[1]);
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}
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printf("binding this socket to queue %d\n", qid);
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qh = nfq_create_queue(h, qid, &cb, NULL);
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if (!qh) {
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exit(1);
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}
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if (nfq_set_mode(qh, NFQNL_COPY_PACKET, 0xffff) < 0) {
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exit(1);
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}
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nh = nfq_nfnlh(h);
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fd = nfnl_fd(nh);
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while ((rv = recv(fd, buf, sizeof(buf), 0)) && rv >= 0) {
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nfq_handle_packet(h, buf, rv);
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}
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/* never reached */
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nfq_destroy_queue(qh);
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nfq_close(h);
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exit(0);
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}
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