如何從原始套接字獲取IP src地址

Question

我想通過原始套接字發送TCP數據包，然後卡住了tcp報頭的校驗和字段。由於我不知道如何找出原始套接字使用的實際IP地址，因此爲校驗和計算構建TCP僞標頭似乎是不可能的。

Answer 1

你知道ip地址。

因爲：在通過此套接字發送任何數據之前，您應該使用bind()它，爲此，您需要指定您正在查找的IP地址。

---

更新：

綁定到INADDR_ANY綁定到任何本地接口。要獲得有關所有本地接口（包括每個IP地址）的信息，您可以使用getifaddrs()。

Answer 2

看看下面的代碼是否有幫助。我用這個代碼來創建我自己的send_raw_tcp（）函數，所以我知道它的工作原理。

//---cat rawtcp.c--- 
// Run as root or SUID 0, just datagram no data/payload 
#include <unistd.h>mm 
#include <netinet/tcp.h> 
// Packet length 
#define PCKT_LEN 8192 

// May create separate header file (.h) for all 
// headers' structures 
// IP header's structure 
struct ipheader { 
unsigned char  iph_ihl:5, /* Little-endian */ 
        iph_ver:4; 
unsigned char  iph_tos; 
unsigned short int iph_len; 
unsigned short int iph_ident; 
unsigned char  iph_flags; 
unsigned short int iph_offset; 
unsigned char  iph_ttl; 
unsigned char  iph_protocol; 
unsigned short int iph_chksum; 
unsigned int  iph_sourceip; 
unsigned int  iph_destip; 
}; 

/* Structure of a TCP header */ 
struct tcpheader { 
unsigned short int tcph_srcport; 
unsigned short int tcph_destport; 
unsigned int  tcph_seqnum; 
unsigned int  tcph_acknum; 
unsigned char  tcph_reserved:4, tcph_offset:4; 
// unsigned char tcph_flags; 
    unsigned int 
     tcp_res1:4,  /*little-endian*/ 
     tcph_hlen:4,  /*length of tcp header in 32-bit words*/ 
     tcph_fin:1,  /*Finish flag "fin"*/ 
     tcph_syn:1,  /*Synchronize sequence numbers to start a connection*/ 
     tcph_rst:1,  /*Reset flag */ 
     tcph_psh:1,  /*Push, sends data to the application*/ 
     tcph_ack:1,  /*acknowledge*/ 
     tcph_urg:1,  /*urgent pointer*/ 
     tcph_res2:2; 
unsigned short int tcph_win; 
unsigned short int tcph_chksum; 
unsigned short int tcph_urgptr; 
}; 

// Simple checksum function, may use others such as Cyclic Redundancy Check, CRC 
unsigned short csum(unsigned short *buf, int len) 
{ 
     unsigned long sum; 
     for(sum=0; len>0; len--) 
       sum += *buf++; 
     sum = (sum >> 16) + (sum &0xffff); 
     sum += (sum >> 16); 
     return (unsigned short)(~sum); 
} 

int main(int argc, char *argv[]) 
{ 
int sd; 
// No data, just datagram 
char buffer[PCKT_LEN]; 
// The size of the headers 
struct ipheader *ip = (struct ipheader *) buffer; 
struct tcpheader *tcp = (struct tcpheader *) (buffer + sizeof(struct ipheader)); 
struct sockaddr_in sin, din; 
int one = 1; 
const int *val = &one; 

memset(buffer, 0, PCKT_LEN); 

if(argc != 5) 
{ 
printf("- Invalid parameters!!!\n"); 
printf("- Usage: %s <source hostname/IP> <source port> <target hostname/IP> <target port>\n", argv[0]); 
exit(-1); 
} 

sd = socket(PF_INET, SOCK_RAW, IPPROTO_TCP); 
if(sd < 0) 
{ 
    perror("socket() error"); 
    exit(-1); 
} 
else 
printf("socket()-SOCK_RAW and tcp protocol is OK.\n"); 

// The source is redundant, may be used later if needed 
// Address family 
sin.sin_family = AF_INET; 
din.sin_family = AF_INET; 
// Source port, can be any, modify as needed 
sin.sin_port = htons(atoi(argv[2])); 
din.sin_port = htons(atoi(argv[4])); 
// Source IP, can be any, modify as needed 
sin.sin_addr.s_addr = inet_addr(argv[1]); 
din.sin_addr.s_addr = inet_addr(argv[3]); 
// IP structure 
ip->iph_ihl = 5; 
ip->iph_ver = 4; 
ip->iph_tos = 16; 
ip->iph_len = sizeof(struct ipheader) + sizeof(struct tcpheader); 
ip->iph_ident = htons(54321); 
ip->iph_offset = 0; 
ip->iph_ttl = 64; 
ip->iph_protocol = 6; // TCP 
ip->iph_chksum = 0; // Done by kernel 

// Source IP, modify as needed, spoofed, we accept through command line argument 
ip->iph_sourceip = inet_addr(argv[1]); 
// Destination IP, modify as needed, but here we accept through command line argument 
ip->iph_destip = inet_addr(argv[3]); 

// The TCP structure. The source port, spoofed, we accept through the command line 
tcp->tcph_srcport = htons(atoi(argv[2])); 
// The destination port, we accept through command line 
tcp->tcph_destport = htons(atoi(argv[4])); 
tcp->tcph_seqnum = htonl(1); 
tcp->tcph_acknum = 0; 
tcp->tcph_offset = 5; 
tcp->tcph_syn = 1; 
tcp->tcph_ack = 0; 
tcp->tcph_win = htons(32767); 
tcp->tcph_chksum = 0; // Done by kernel 
tcp->tcph_urgptr = 0; 
// IP checksum calculation 
ip->iph_chksum = csum((unsigned short *) buffer, (sizeof(struct ipheader) + sizeof(struct tcpheader))); 

// Inform the kernel do not fill up the headers' structure, we fabricated our own 
if(setsockopt(sd, IPPROTO_IP, IP_HDRINCL, val, sizeof(one)) < 0) 
{ 
    perror("setsockopt() error"); 
    exit(-1); 
} 
else 
    printf("setsockopt() is OK\n"); 

printf("Using:::::Source IP: %s port: %u, Target IP: %s port: %u.\n", argv[1], atoi(argv[2]), argv[3], atoi(argv[4])); 

// sendto() loop, send every 2 second for 50 counts 
unsigned int count; 
for(count = 0; count < 20; count++) 
{ 
if(sendto(sd, buffer, ip->iph_len, 0, (struct sockaddr *)&sin, sizeof(sin)) < 0) 
// Verify 
{ 
    perror("sendto() error"); 
    exit(-1); 
} 
else 
    printf("Count #%u - sendto() is OK\n", count); 
sleep(2); 
} 
close(sd); 
return 0; 
}