c - 如何从原始套接字获取 IP src 地址

Question

我想通过原始套接字发送 TCP 数据包，然后我被 tcp 标头的校验和字段卡住了。由于我不知道如何计算原始套接字使用的实际 IP 地址，因此似乎不可能构造 TCP 伪标头以进行校验和计算。

Answer 1

你知道IP地址。

因为：在通过此套接字发送任何数据之前，您应该bind()这样做，为此您需要指定您要查找的 IP 地址。

---

更新：

Binding toINADDR_ANY绑定到任何本地接口。要获取有关所有本地接口的信息（包括每个接口的 IP 地址），您可以使用getifaddrs().

Answer 2

如果有帮助，请查看以下代码。我用这段代码制作了我自己的 send_raw_tcp() 函数，所以我知道它可以工作。

//---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;
}