Linux网络接口操作之if_nameindex

摘要：系统信息操作系统内核版本版本版本函数定义指定网络接口名称字符串作为参数若该接口存在，则返回相应的索引，否则返回指定网络接口索引以及一块长度至少为字节的内存区域作为参数若索引对应的网络接口存在，则在内存区域中

操作系统：

# lsb_release -ir
Distributor ID:    CentOS
Release:           6.7

内核版本：

# uname -r
2.6.32-573.26.1.el6.x86_64

gcc版本：

# gcc --version
gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-16)
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

glibc版本：

# ldd $(which ls)
    linux-vdso.so.1 =>  (0x00007ffc14d08000)
    libselinux.so.1 => /lib64/libselinux.so.1 (0x0000003b2ec00000)
    librt.so.1 => /lib64/librt.so.1 (0x0000003b2e400000)
    libcap.so.2 => /lib64/libcap.so.2 (0x0000003b2fc00000)
    libacl.so.1 => /lib64/libacl.so.1 (0x0000003b36000000)
    libc.so.6 => /lib64/libc.so.6 (0x0000003b2d400000)
    libdl.so.2 => /lib64/libdl.so.2 (0x0000003b2dc00000)
    /lib64/ld-linux-x86-64.so.2 (0x0000003b2d000000)
    libpthread.so.0 => /lib64/libpthread.so.0 (0x0000003b2d800000)
    libattr.so.1 => /lib64/libattr.so.1 (0x0000003b2f800000)
    
# /lib64/libc.so.6
GNU C Library stable release version 2.12, by Roland McGrath et al.
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Compiled by GNU CC version 4.4.7 20120313 (Red Hat 4.4.7-16).
Compiled on a Linux 2.6.32 system on 2016-02-16.
Available extensions:
    The C stubs add-on version 2.1.2.
    crypt add-on version 2.1 by Michael Glad and others
    GNU Libidn by Simon Josefsson
    Native POSIX Threads Library by Ulrich Drepper et al
    BIND-8.2.3-T5B
    RT using linux kernel aio
libc ABIs: UNIQUE IFUNC
For bug reporting instructions, please see:
.

#include 

unsigned if_nametoindex(const char *ifname);
char *if_indextoname(unsigned ifindex, char *ifname);
struct if_nameindex *if_nameindex(void);
void if_freenameindex(struct if_nameindex *ptr);

if_nametoindex()：指定网络接口名称字符串作为参数；若该接口存在，则返回相应的索引，否则返回0

if_indextoname()：指定网络接口索引以及一块长度至少为IF_NAMESIZE(16)字节的内存区域作为参数；若索引对应的网络接口存在，则在内存区域中返回该接口的名称字符串，否则返回NULL，并将errno设置为相应的值

if_nameindex()：返回动态分配的struct if_nameindex结构数组，数组中的每一个元素分别对应一个本地网络接口；struct if_nameindex结构的if_index字段为接口索引，if_name字段为接口名称字符串；索引为0且名称字符串为NULL表示结构数组的末尾；调用出错时，返回NULL，并将errno设置为相应的值

if_freenameindex()：通过if_nameindex()获取完毕接口名称与索引后，调用该函数以释放动态分配的内存区域

以上4个函数在系统的man文档中都可以查看相应的描述，且都是POSIX标准支持的，Linux内核可能未实现这些函数，或已实现但不同于POSIX标准
这些函数的原型声明与定义并未出现在CentOS 6.7的定制内核2.6.32-573.26.1.el6.x86_64以及原版内核2.6.32.5中，而是由系统的glibc-2.12实现：在glibc-2.12.2源码树中，函数的原型声明位于sysdeps/gnu/net/if.h与sysdeps/generic/net/if.h，函数的定义位于sysdeps/unix/sysv/linux/if_index.c中，本质上是对ioctl(2)的SIOCGIFNAME，SIOCGIFCONF，SIOCGIFINDEX等操作以及netlink套接字进行了封装

1. if_name_index.c

#include 
#include 
#include 
    
int main(void)
{
    struct if_nameindex *head, *ifni;
    ifni = if_nameindex();
    head = ifni;
    
    if (head == NULL) {
        perror("if_nameindex()");
        exit(EXIT_FAILURE);
    }   
    
    while (ifni->if_index != 0) {
        printf("Interfece %d : %s
", ifni->if_index, ifni->if_name);
        ifni++;
    }   
    
    if_freenameindex(head);
    head = NULL;
    ifni = NULL;
    
    exit(EXIT_SUCCESS);
}

编译并运行

# gcc if_name_index.c -o if_name_index
# ./if_name_index 
Interfece 1 : lo
Interfece 2 : eth0
Interfece 3 : eth1    

2. if_index_to_name.c

#include 
#include 
#include 
#include 

int main(int argc, char *argv[])
{
    if (argc != 2) {
        fprintf(stderr, "Usage: %s [interface index]
", argv[0]);
        exit(EXIT_FAILURE);
    }   

    int saved_errno = errno;
    char if_name[IFNAMSIZ] = {""};
    unsigned int if_index = (unsigned int )atoi(argv[1]);

    char *name = if_indextoname(if_index, if_name);
    if (name == NULL && errno == ENXIO) {
        fprintf(stderr, "Index %d : No such device
", if_index);
        exit(EXIT_FAILURE); 
    } 
    
    errno = saved_errno;  

    printf("Index %d : %s
", if_index, if_name);

    exit(EXIT_SUCCESS);
}

编译并运行

# gcc if_index_to_name.c -o if_index_to_name
# ./if_index_to_name
Usage: ./if_index_to_name [interface index]
# ./if_index_to_name 1
Index 1 : lo
# ./if_index_to_name 2
Index 2 : eth0
# ./if_index_to_name 3
Index 3 : eth1
# ./if_index_to_name 4
Index 4 : No such device

3. if_name_to_index.c

#include 
#include 
#include 

int main(int argc, char *argv[])
{
    if (argc != 2) {
        fprintf(stderr, "Usage: %s [interface name]
", argv[0]);
        exit(EXIT_FAILURE);
    }   

    unsigned int if_index;
    if_index = if_nametoindex(argv[1]);
    if (if_index == 0) {
        fprintf(stderr, "Interface %s : No such device
", argv[1]);
        exit(EXIT_FAILURE);
    }   

    printf("Interface %s : %d
", argv[1], if_index);   

    exit(EXIT_SUCCESS);
}

编译并运行

# gcc if_name_to_index.c -o if_name_to_index
# ./if_name_to_index eth0
Interface eth0 : 2
# ./if_name_to_index eth3
Interface eth3 : No such device