我正在开发一个程序,我需要知道我的程序内的IP头字段的值。由于IP报头是20字节:
struct ipheader {
unsigned char ip_hl:4, ip_v:4; /* this means that each member is 4 bits */
unsigned char ip_tos;
unsigned short int ip_len;
unsigned short int ip_id;
unsigned short int ip_off;
unsigned char ip_ttl;
unsigned char ip_p;
unsigned short int ip_sum;
unsigned int ip_src;
unsigned int ip_dst;
};
有任何方法,我可以知道这些字段的值在我的C程序里?
如果编译器没有在该结构中引入任何对齐块(确保CHAR_BIT
为8,sizeof(struct ipheader)
为20),您应该能够将其原样包含在代码中,然后添加如下内容:
struct ipheader *iph = (struct ipheader *)blk;
printf ("TTL = %dn", iph->ip_ttl);
在该代码中,您将有一个由blk
指向的IP头,这可能是char*
。将其转换为正确的指针类型将允许您轻松访问字段。
下面的完整程序展示了这一点:
#include <stdio.h>
#include <limits.h>
struct ipheader {
/* 0 */ unsigned char ip_hl:4, ip_v:4;
/* 1 */ unsigned char ip_tos;
/* 2 */ unsigned short int ip_len;
/* 3 */ unsigned short int ip_id;
/* 4 */ unsigned short int ip_off;
/* 5 */ unsigned char ip_ttl;
/* 6 */ unsigned char ip_p;
/* 7 */ unsigned short int ip_sum;
/* 8 */ unsigned int ip_src;
/* 9 */ unsigned int ip_dst;
};
int main (void) {
char blk[] = {
'x00','x11','x22','x22','x33','x33','x44','x44',
'x55','x66','x77','x77','x88','x88','x88','x88',
'x99','x99','x99','x99'
};
struct ipheader *iph = (struct ipheader *)(&blk);
printf ("TTL = %xn", iph->ip_ttl);
printf ("sum = %xn", iph->ip_sum);
printf ("dst = %xn", iph->ip_dst);
return 0;
}
输出如下所示:
TTL = 55
sum = 7777
dst = 99999999
其中一些值可以通过SOL_IP/IPPROTO/IP
级别的setsockopt()/getsockopt()
调用来设置/检索。参考您的操作系统文档(例如:Linux man 7 ip
)
您应该使用setsockopt/getsockopt
例程与套接字机制进行接口。这些函数在不同的级别上运行(IPPROTO_IP
, IPPROTO_TCP
等),一些选项仅适用于特定的套接字类型(例如选项IP_TTL
仅适用于AF_INET
套接字)。
get TTL value:
int sock = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);
unsigned int opt_val;
unsigned int opt_len;
getsockopt(sock, IPPROTO_IP, IP_TTL, &opt_val, &opt_len);
printf("ttl %dn", opt_val);
set TTL value:
int sock = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);
unsigned char ttl_val = 32;
setsockopt(sock, IPPROTO_IP, IP_TTL, &ttl_val, sizeof(ttl_val));