网络编程之RoutingSockets(二)

1. sysctl Operations

#include 
#include 
int sysctl(int *name, u_int namelen, void *oldp, size_t *oldlenp,
void *newp, size_t newlen);
Returns: 0 if OK, −1 on error

　　对于最基本的API操作，并不需要额外关注，注意name参数可能的选项搭配方式：

　　UNP着重介绍了NET_RT_IFLIST，这里也看看该选项的解释：

• returns information on all configured interfaces. If name[5] is nonzero, it is an interface index number, and only information on that interface is returned.
• All the addresses assigned to each interface are also returned, and if name[3] is nonzero, only addresses for that address family are returned.
• For each interface, one RTM_IFINFO message is returned, followed by one RTM_NEWADDR message for each address assigned to the interface. The RTM_IFINFO message is followed by one datalink socket address structure, and each RTM_NEWADDR message is followed by up to three socket address structures: the interface address, the network mask, and the broadcast address.

　　上面的话总结一下就是如果sysctl都采用默认的选项，那么返回每个接口上每个地址的相关信息：网络地址，子网掩码，广播地址；因为可能某些接口绑定的不只一个地址，这种现象叫地址别名。如下图所示一种返回的数据结构情况：

2. 相对应的API修改

char * net_rt_iflist(int family, int flags, size_t *lenp)
{
    int mib[6];
    char *buf;
    mib[0] = CTL_NET;
    mib[1] = AF_ROUTE;
    mib[2] = 0;
    mib[3] = family; /* only addresses of this family */
    mib[4] = NET_RT_IFLIST;
    mib[5] = flags; /* interface index or 0 */
    if (sysctl(mib, 6, NULL, lenp, NULL, 0) <0)
        return (NULL);
    if ( (buf = malloc(*lenp)) == NULL)
        return (NULL);
    if (sysctl(mib, 6, buf, lenp, NULL, 0) <0) {
        free(buf);
        return (NULL);
    }
    //第一次的调用sysctl是为了得到返回信息的大小
    return (buf);
}


struct ifi_info * get_ifi_info(int family, int doaliases)
{
    int flags;
    char *buf, *next, *lim;
    size_t len;
    struct if_msghdr *ifm;
    struct ifa_msghdr *ifam;
    struct sockaddr *sa, *rti_info[RTAX_MAX];
    struct sockaddr_dl *sdl;
    struct ifi_info *ifi, *ifisave, *ifihead, **ifipnext;
    buf = Net_rt_iflist(family, 0, &len);
    ifihead = NULL;
    ifipnext = &ifihead;
    lim = buf + len;
    //所有的返回信息都存储在buf中，且大小为len
    for (next = buf; next ifm_msglen) {
        ifm = (struct if_msghdr *) next;
        if (ifm->ifm_type == RTM_IFINFO) {
                //ioctl(sockfd, SIOCGIFFLAGS, &ifrcopy)中获取接口的标志位，与这里进行对比下
            if (((flags = ifm->ifm_flags) & IFF_UP) == 0)
                continue; /* ignore if interface not up */
            sa = (struct sockaddr *) (ifm + 1);
            //ifm->ifm_addrs置位的相应信息填充rti_info数组
            get_rtaddrs(ifm->ifm_addrs, sa, rti_info);
            if ( (sa = rti_info[RTAX_IFP]) != NULL) {
                ifi = Calloc(1, sizeof(struct ifi_info));
                *ifipnext = ifi; /* prev points to this new one */
                ifipnext = &ifi->ifi_next; /* ptr to next one goes here */
                ifi->ifi_flags = flags;
                if (sa->sa_family == AF_LINK) {
                    sdl = (struct sockaddr_dl *) sa;
                    ifi->ifi_index = sdl->sdl_index;
                    if (sdl->sdl_nlen > 0)
                        snprintf(ifi->ifi_name, IFI_NAME, "%*s",sdl->sdl_nlen, &sdl->sdl_data[0]);
                    else
                        snprintf(ifi->ifi_name, IFI_NAME, "index %d",sdl->sdl_index);
                    if ( (ifi->ifi_hlen = sdl->sdl_alen) > 0)
                        memcpy(ifi->ifi_haddr, LLADDR(sdl), min(IFI_HADDR, sdl->sdl_alen));
                }
            }
        } else if (ifm->ifm_type == RTM_NEWADDR) {
            //每次填充ifi结构体之前，先检查是否该地址是别名
            if (ifi->ifi_addr) { /* already have an IP addr for i/f */
                if (doaliases == 0)
                    continue;
                /* we have a new IP addr for existing interface */
                //直接copy相同的接口的其他成员信息，除了该地址
                ifisave = ifi;
                ifi = Calloc(1, sizeof(struct ifi_info));
                *ifipnext = ifi; /* prev points to this new one */
                ifipnext = &ifi->ifi_next; /* ptr to next one goes here */
                ifi->ifi_flags = ifisave->ifi_flags;
                ifi->ifi_index = ifisave->ifi_index;
                ifi->ifi_hlen = ifisave->ifi_hlen;
                memcpy(ifi->ifi_name, ifisave->ifi_name, IFI_NAME);
                memcpy(ifi->ifi_haddr, ifisave->ifi_haddr, IFI_HADDR);
            }
            ifam = (struct ifa_msghdr *) next;
            sa = (struct sockaddr *) (ifam + 1);
            get_rtaddrs(ifam->ifam_addrs, sa, rti_info);
            if ( (sa = rti_info[RTAX_IFA]) != NULL) {
                ifi->ifi_addr = Calloc(1, sa->sa_len);
                memcpy(ifi->ifi_addr, sa, sa->sa_len);
            }
            if ((flags & IFF_BROADCAST) && (sa = rti_info[RTAX_BRD]) != NULL) {
                ifi->ifi_brdaddr = Calloc(1, sa->sa_len);
                memcpy(ifi->ifi_brdaddr, sa, sa->sa_len);
            }
            if ((flags & IFF_POINTOPOINT) &&(sa = rti_info[RTAX_BRD]) != NULL) {
                ifi->ifi_dstaddr = Calloc(1, sa->sa_len);
                memcpy(ifi->ifi_dstaddr, sa, sa->sa_len);
            }
        } else
            err_quit("unexpected message type %d", ifm->ifm_type);
    }
/* "ifihead" points to the first structure in the linked list */
    return (ifihead); /* ptr to first structure in linked list */
}

　　特别强调上面程序一点：只有不同的接口或者该接口有多个地址才会调用修改链表的语句，来使当前ifi结构体添加到链表尾端。

3. Interface Name and Index Functions

#include 
unsigned int if_nametoindex(const char *ifname);
Returns: positive interface index if OK, 0 on error
char *if_indextoname(unsigned int ifindex, char *ifname);
Returns: pointer to interface name if OK, NULL on error
struct if_nameindex *if_nameindex(void);
Returns: non-null pointer if OK, NULL on error
void if_freenameindex(struct if_nameindex *ptr);

struct if_nameindex {
    unsigned int if_index; /* 1, 2, ... */
    char *if_name; /* null-terminated name: "le0", ... */
};

　　从函数名字也能看出来该组API的用法，特别强调参数的用法：

• The ifname argument points to a buffer of size IFNAMSIZ that the caller must allocate to hold the result, and this pointer is also the return value upon success.
• if_nameindex returns a pointer to an array of if_nameindex structures. The final entry in this array contains a structure with an if_index of 0 and an if_name that is a null pointer. The memory for this array, along with the names pointed to by the array members, is dynamically obtained and is returned by calling if_freenameindex.

4. 最后

　　该部分内容包括网络编程之ioctl Operations，并不是重点，但是也应该稍微了解下。系统提供了这样的接口及route或者arp等程序的作用机理。