UNP_18_路由套接字

struct sockaddr_dl {
  uint8_t      sdl_len;
  sa_family_t  sdl_family; /* AF_LINK */ 
  uint16_t     sdl_index; /* system assigned index, if > 0 */ 
  uint8_t      sdl_type; /* IFT_ETHER, etc. from  */ 
  uint8_t      sdl_nlen; /* name length, starting in sdl_data[0] */           
  uint8_t      sdl_alen; /* link-layer address length */ 
  uint8_t      sdl_slen; /* link-layer selector length */ 
  char         sdl_data[12]; /* minimum work area, can be larger; 
                      contains i/f name and link-layer address */ 
};

#include<sys/param.h>
#include<sys/sysctl.h>
int sysctl(int* name, u_int namelen, void* oldp, size_t* oldlenp, void * newp, size_t newlen);
// 返回：若成功返回 0，出错返回 -1

#include	"unproute.h"
#include	<netinet/udp.h>
#include	<netinet/ip_var.h>
#include	<netinet/udp_var.h>		/* for UDPCTL_xxx constants */

int main(int argc, char **argv)
{
	int		mib[4], val;
	size_t	len;

	mib[0] = CTL_NET;
	mib[1] = AF_INET;
	mib[2] = IPPROTO_UDP;
	mib[3] = UDPCTL_CHECKSUM;

	len = sizeof(val);
	Sysctl(mib, 4, &val, &len, NULL, 0);
	printf("udp checksum flag: %d\n", val);

	exit(0);
}

#include<net/if.h>
unsigned int if_nametoindex(const char * ifname);
// 返回：成功返回为正值的接口索引，出错返回 0
char* if_indextoname(unisgned int ifindex, char* ifname);
// 返回：成功返回指向接口名字的指针，出错返回 NULL
struct if_nameindex* if_nameindex(void);
// 返回：成功为非空指针，出错返回 NULL
void if_freenameindex(struct if_nameindex* ptr)

unsigned int if_nametoindex(const char *name)
{
	unsigned int		idx, namelen;
	char				*buf, *next, *lim;
	size_t				len;
	struct if_msghdr	*ifm;
	struct sockaddr		*sa, *rti_info[RTAX_MAX];
	struct sockaddr_dl	*sdl;
  // net_rt_iflist 函数用来返回接口列表
	if ( (buf = net_rt_iflist(0, 0, &len)) == NULL)
		return(0);

	namelen = strlen(name);
	lim = buf + len;
	for (next = buf; next < lim; next += ifm->ifm_msglen) {
    // 处理缓冲区中的消息，仅仅查找 RTM_IFINFO 消息
    // 找到后调用 get_rtaddrs 函数设置指向各个套接字地址结构的指针
    // 如果存在一个接口名字，那就比较其中的接口名字（rti_info[RTAX_IFP]）和调用者参数中要查找的接口名字
		ifm = (struct if_msghdr *) next;
		if (ifm->ifm_type == RTM_IFINFO) {
			sa = (struct sockaddr *) (ifm + 1);
			get_rtaddrs(ifm->ifm_addrs, sa, rti_info);
			if ( (sa = rti_info[RTAX_IFP]) != NULL) {
				if (sa->sa_family == AF_LINK) {
					sdl = (struct sockaddr_dl *) sa;
					if (sdl->sdl_nlen == namelen && strncmp(&sdl->sdl_data[0], name, sdl->sdl_nlen) == 0) {
						idx = sdl->sdl_index;	/* save before free() */
						free(buf);
						return(idx);
					}
				}
			}

		}
	}
	free(buf);
	return(0);		/* no match for name */
}

// 该函数和上一个函数类似，仅仅是比较的类型不一样而已
char * if_indextoname(unsigned int idx, char *name)
{
	char				*buf, *next, *lim;
	size_t				len;
	struct if_msghdr	*ifm;
	struct sockaddr		*sa, *rti_info[RTAX_MAX];
	struct sockaddr_dl	*sdl;

	if ( (buf = net_rt_iflist(0, idx, &len)) == NULL)
		return(NULL);

	lim = buf + len;
	for (next = buf; next < lim; next += ifm->ifm_msglen) {
		ifm = (struct if_msghdr *) next;
		if (ifm->ifm_type == RTM_IFINFO) {
			sa = (struct sockaddr *) (ifm + 1);
			get_rtaddrs(ifm->ifm_addrs, sa, rti_info);
			if ( (sa = rti_info[RTAX_IFP]) != NULL) {
				if (sa->sa_family == AF_LINK) {
					sdl = (struct sockaddr_dl *) sa;
					if (sdl->sdl_index == idx) {
						int slen = min(IFNAMSIZ - 1, sdl->sdl_nlen);
						strncpy(name, sdl->sdl_data, slen);
						name[slen] = 0;	/* null terminate */
						free(buf);
						return(name);
					}
				}
			}

		}
	}
	free(buf);
	return(NULL);		/* no match for index */
}

// 返回一个 if_nameindex 结构数组，包含所有接口名字和索引对
struct if_nameindex * if_nameindex(void)
{
	char				*buf, *next, *lim;
	size_t				len;
	struct if_msghdr	*ifm;
	struct sockaddr		*sa, *rti_info[RTAX_MAX];
	struct sockaddr_dl	*sdl;
	struct if_nameindex	*result, *ifptr;
	char				*namptr;
  // 构建返回数组，确定大小
	if ( (buf = net_rt_iflist(0, 0, &len)) == NULL)
		return(NULL);

	if ( (result = malloc(len)) == NULL)	/* overestimate */
		return(NULL);
	ifptr = result;
	namptr = (char *) result + len;	/* names start at end of buffer */

	lim = buf + len;
	for (next = buf; next < lim; next += ifm->ifm_msglen) {
    // 遍历所有消息，从中查找 RTM_IFINFO 消息以及后面的数据链路套接字地址结构。把接口名字和索引放到返回数组中
		ifm = (struct if_msghdr *) next;
		if (ifm->ifm_type == RTM_IFINFO) {
			sa = (struct sockaddr *) (ifm + 1);
			get_rtaddrs(ifm->ifm_addrs, sa, rti_info);
			if ( (sa = rti_info[RTAX_IFP]) != NULL) {
				if (sa->sa_family == AF_LINK) {
					sdl = (struct sockaddr_dl *) sa;
					namptr -= sdl->sdl_nlen + 1;
					strncpy(namptr, &sdl->sdl_data[0], sdl->sdl_nlen);
					namptr[sdl->sdl_nlen] = 0;	/* null terminate */
					ifptr->if_name = namptr;
					ifptr->if_index = sdl->sdl_index;
					ifptr++;
				}
			}

		}
	}
  // 把返回数组的最后一个元素置空
	ifptr->if_name = NULL;	/* mark end of array of structs */
	ifptr->if_index = 0;
	free(buf);
	return(result);			/* caller must free() this when done */
}

// 释放 if_nameindex 结构内存即可，因为是一整块内存，不需要遍历
void if_freenameindex(struct if_nameindex *ptr)
{
	free(ptr);
}