Code1 #include <stdio.h>
2 #include <stdlib.h>
3 #include <unistd.h>
4 #include <pcap.h>
5 #include <sys/types.h>
6 #include <netinet/ip.h>
7 #include <netinet/if_ether.h>
8 #include <netinet/tcp.h>
9 #include <netinet/in.h>
10 #include <arpa/inet.h>
11 #include <string.h>
12
13 void handle_packet(const struct pcap_pkthdr* pkthdr,const u_char* packet,int sockfd);
14 unsigned short check_sum(unsigned short *addr,int len);
15
16 struct psd_header{
17 unsigned long saddr;
18 unsigned long daddr;
19 char mbz;
20 char ptcl;
21 unsigned short tcpl;
22 };
23
24 unsigned short port;
25
26 int main(int argc, char **argv)
27 {
28 char *dev = NULL;
29 char errbuf[PCAP_ERRBUF_SIZE];
30 pcap_t *handle = NULL;
31 struct bpf_program filter;
32 char filter_app[] = "tcp port 80";
33 bpf_u_int32 mask;
34 bpf_u_int32 net;
35 struct pcap_pkthdr header;
36 const u_char *packet;
37 int sockfd;
38 int on = 1;
39
40 sockfd = socket(PF_INET,SOCK_RAW,IPPROTO_TCP);
41 if(sockfd < 0){
42 fprintf(stderr,"Socket error:\n");
43 exit(-4);
44 }
45 if(setsockopt(sockfd,IPPROTO_IP,IP_HDRINCL,&on,sizeof(on)) < 0)
46 fprintf(stderr,"HDRINCL error\n");
47
48
49 if(NULL == (dev = pcap_lookupdev(errbuf))){
50 fprintf(stderr,"pcap_lookupdev() Error:%s\n",errbuf);
51 exit(-1);
52 }
53
54 fprintf(stdout,"sniffing on device: %s\n",dev);
55
56 pcap_lookupnet(dev,&net,&mask,errbuf);
57
58 if(NULL == (handle = pcap_open_live(dev,BUFSIZ,0,0,errbuf))){
59 fprintf(stderr,"pcap_open_live() error:%s\n",errbuf);
60 exit(-2);
61 }
62
63 if(-1 == pcap_compile(handle,&filter,filter_app,1,net)){
64 fprintf(stderr,"pcap_compile() error.\n");
65 exit(-3);
66 }
67
68 pcap_setfilter(handle,&filter);
69
70 while(1){
71 packet = pcap_next(handle,&header);
72 printf("captured a packet with length of [%d]\n",header.len);
73 handle_packet(&header,packet,sockfd);
74 }
75
76 pcap_close(handle);
77
78 return 0;
79 }
80
81 void handle_packet(const struct pcap_pkthdr* pkthdr,const u_char* packet,int sockfd){
82 u_char buffer[65535];
83 int len = pkthdr->len;
84 //struct ip* ip;
85 struct iphdr* ip;
86 struct tcphdr* tcp;
87 char srcbuf[16];
88 char dstbuf[16];
89 u_char temp[65535];
90 struct sockaddr_in addr;
91 struct psd_header psdhdr;
92 int ip_len;
93
94 memset(buffer,0,65535);
95 memset(temp,0,65535);
96 memcpy(buffer,packet,len);
97 memset(srcbuf,0,16);
98 memset(dstbuf,0,16);
99 memset(&addr,0,sizeof(struct sockaddr_in));
100 memset(&psdhdr,0,sizeof(struct psd_header));
101
102 ip = (struct iphdr *)(buffer + sizeof(struct ethhdr));
103 tcp = (struct tcphdr *)(buffer + sizeof(struct ethhdr) + sizeof(struct iphdr));
104
105 ip_len = ntohs(ip->tot_len);
106 strcpy(srcbuf,inet_ntoa(((struct ip *)(buffer + sizeof(struct ethhdr)))->ip_src));
107 strcpy(dstbuf,inet_ntoa(((struct ip *)(buffer + sizeof(struct ethhdr)))->ip_dst));
108 printf("source ip:%s\n",srcbuf);
109 printf("destination ip:%s\n",dstbuf);
110 printf("len:%d\n",ip_len);
111
112 // printf("%d\n",((struct iphdr *)(buffer + sizeof(struct ethhdr)))->daddr);
113 if(strcmp(dstbuf,"192.168.1.2") == 0){
114 // printf("source ip:%s\n",srcbuf);
115 // printf("destination ip:%s\n",dstbuf);
116 if(strcmp(srcbuf,"192.168.1.5") == 0){
117 addr.sin_family = AF_INET;
118 addr.sin_port = htons(80);
119 addr.sin_addr.s_addr = inet_addr("202.120.58.161");
120 ip->daddr = addr.sin_addr.s_addr;
121 ip->saddr = inet_addr("192.168.1.2");
122 ip->check = 0;
123 ip->check = check_sum((unsigned short *)ip,sizeof(struct iphdr));
124 tcp->dest = addr.sin_port;
125 port = tcp->source;
126 tcp->source = htons(80);
127 tcp->check = 0;
128 psdhdr.saddr = ip->saddr;
129 psdhdr.daddr = ip->daddr;
130 psdhdr.mbz = 0;
131 psdhdr.ptcl = IPPROTO_TCP;
132 psdhdr.tcpl = htons(ip_len - sizeof(struct iphdr));
133 memcpy(temp,&psdhdr,sizeof(struct psd_header));
134 memcpy(temp + sizeof(struct psd_header),tcp,ip_len - sizeof(struct iphdr));
135 tcp->check = check_sum((unsigned short *)temp,sizeof(struct psd_header) + ip_len - sizeof(struct iphdr));
136 sendto(sockfd,buffer + sizeof(struct ethhdr),ip_len,0,(struct sockaddr *)(&addr),sizeof(struct sockaddr));
137 }
138 else if(strcmp(srcbuf,"202.120.58.161") == 0){
139 addr.sin_family = AF_INET;
140 addr.sin_port = port;
141 addr.sin_addr.s_addr = inet_addr("192.168.1.5");
142 ip->daddr = addr.sin_addr.s_addr;
143 ip->saddr = inet_addr("192.168.1.2");
144 ip->check = 0;
145 ip->check = check_sum((unsigned short *)ip,sizeof(struct iphdr));
146 tcp->dest = addr.sin_port;
147 tcp->source = htons(80);
148 tcp->check = 0;
149 psdhdr.saddr = ip->saddr;
150 psdhdr.daddr = ip->daddr;
151 psdhdr.mbz = 0;
152 psdhdr.ptcl = IPPROTO_TCP;
153 psdhdr.tcpl = htons(ip_len - sizeof(struct iphdr));
154 memcpy(temp,&psdhdr,sizeof(struct psd_header));
155 memcpy(temp + sizeof(struct psd_header),tcp,ip_len - sizeof(struct iphdr));
156 tcp->check = check_sum((unsigned short *)temp,sizeof(struct psd_header) + ip_len - sizeof(struct iphdr));
157 sendto(sockfd,buffer + sizeof(struct ethhdr),ip_len,0,(struct sockaddr *)(&addr),sizeof(struct sockaddr));
158 }
159 }
160 }
161
162 unsigned short check_sum(unsigned short *addr,int len){
163 register int nleft = len;
164 register int sum = 0;
165 register u_short *w = addr;
166 u_short answer = 0;
167
168 while(nleft > 1){
169 sum += *w++;
170 nleft -= 2;
171 }
172 if(nleft == 1){
173 *(u_char *)(&answer) = *(u_char *)w;
174 sum += answer;
175 }
176 sum = (sum >> 16) + (sum & 0xffff);
177 sum += (sum >> 16);
178 answer = ~sum;
179 return (answer);
180 }
181
192.168.1.5的浏览器中输入http://192.168.1.2/，将会显示bbs.sjtu.edu.cn
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