Linux网络接口操作之ioctl-1_接口参数

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Linux提供了一系列网络接口操作相关的命令集，其中，一些传统的工具，如net-tools软件包中的ifconfig(8)，arp(8)，route(8)等都是通过ioctl(2)系统调用实现
本篇介绍使用ioctl(2)进行网络接口参数的获取与设置

函数原型

#include <sys/ioctl.h>
int ioctl(int d, int request, ...);

第一个参数fd指定一个由open(2)/socket(2)创建的文件描述符
第二个参数request指定操作的类型，即对该文件描述符执行何种操作
第三个参数为一块内存区域，通常依赖于request指定的操作类型

接口参数与操作的相关定义

内核版本：2.6.32.5

  • ioctl(2)使用struct ifreq与/或struct ifconf结构执行网络接口相关的操作，这两个结构的地址作为ioctl(2)的第三个参数

/* include/linux/if.h */

#define    IFNAMSIZ    16
#define    IFALIASZ    256

struct ifreq 
{
#define IFHWADDRLEN    6
    union
    {
        char    ifrn_name[IFNAMSIZ];
    } ifr_ifrn;
    
    union {
        struct    sockaddr ifru_addr;
        struct    sockaddr ifru_dstaddr;
        struct    sockaddr ifru_broadaddr;
        struct    sockaddr ifru_netmask;
        struct  sockaddr ifru_hwaddr;
        short    ifru_flags;
        int        ifru_ivalue;
        int        ifru_mtu;
        struct  ifmap ifru_map;
        char    ifru_slave[IFNAMSIZ];
        char    ifru_newname[IFNAMSIZ];
        void __user *    ifru_data;
        struct    if_settings ifru_settings;
    } ifr_ifru;
};

#define ifr_name    ifr_ifrn.ifrn_name        /* interface name        */
#define ifr_hwaddr    ifr_ifru.ifru_hwaddr    /* MAC address            */
#define    ifr_addr    ifr_ifru.ifru_addr        /* address                */
#define    ifr_dstaddr    ifr_ifru.ifru_dstaddr    /* other end of p-p lnk    */
#define    ifr_broadaddr    ifr_ifru.ifru_broadaddr    /* broadcast address    */
#define    ifr_netmask        ifr_ifru.ifru_netmask    /* interface net mask    */
#define    ifr_flags    ifr_ifru.ifru_flags        /* flags        */
#define    ifr_metric    ifr_ifru.ifru_ivalue    /* metric        */
#define    ifr_mtu        ifr_ifru.ifru_mtu        /* mtu            */
#define ifr_map        ifr_ifru.ifru_map        /* device map    */
#define ifr_slave    ifr_ifru.ifru_slave        /* slave device            */
#define    ifr_data    ifr_ifru.ifru_data        /* for use by interface    */
#define ifr_ifindex    ifr_ifru.ifru_ivalue    /* interface index        */
#define ifr_bandwidth    ifr_ifru.ifru_ivalue    /* link bandwidth    */
#define ifr_qlen        ifr_ifru.ifru_ivalue    /* Queue length     */
#define ifr_newname        ifr_ifru.ifru_newname    /* New name            */
#define ifr_settings    ifr_ifru.ifru_settings    /* Device/proto settings*/

struct ifconf 
{
    int    ifc_len;
    union 
    {
        char __user *ifcu_buf;
        struct ifreq __user *ifcu_req;
    } ifc_ifcu;
};

#define    ifc_buf    ifc_ifcu.ifcu_buf        /* buffer address        */
#define    ifc_req    ifc_ifcu.ifcu_req        /* array of structures    */

  • 操作类型，ioctl(2)的第二个参数

/* include/linux/sockios.h */

/* Socket configuration controls. */
#define SIOCGIFNAME        0x8910        /* get iface name        */
#define SIOCSIFLINK        0x8911        /* set iface channel    */
#define SIOCGIFCONF        0x8912        /* get iface list        */
#define SIOCGIFFLAGS    0x8913        /* get flags            */
#define SIOCSIFFLAGS    0x8914        /* set flags            */
#define SIOCGIFADDR        0x8915        /* get PA address        */
#define SIOCSIFADDR        0x8916        /* set PA address        */
#define SIOCGIFDSTADDR    0x8917        /* get remote PA address    */
#define SIOCSIFDSTADDR    0x8918        /* set remote PA address    */
#define SIOCGIFBRDADDR    0x8919        /* get broadcast PA address    */
#define SIOCSIFBRDADDR    0x891a        /* set broadcast PA address    */
#define SIOCGIFNETMASK    0x891b        /* get network PA mask        */
#define SIOCSIFNETMASK    0x891c        /* set network PA mask        */
#define SIOCGIFMETRIC    0x891d        /* get metric            */
#define SIOCSIFMETRIC    0x891e        /* set metric            */
#define SIOCGIFMEM        0x891f        /* get memory address (BSD)    */
#define SIOCSIFMEM        0x8920        /* set memory address (BSD)    */
#define SIOCGIFMTU        0x8921        /* get MTU size        */
#define SIOCSIFMTU        0x8922        /* set MTU size        */
#define SIOCSIFNAME        0x8923        /* set interface name */
#define    SIOCSIFHWADDR    0x8924        /* set hardware address */
#define SIOCGIFENCAP    0x8925        /* get/set encapsulations */
#define SIOCSIFENCAP    0x8926        
#define SIOCGIFHWADDR    0x8927        /* Get hardware address        */
#define SIOCGIFSLAVE    0x8929        /* Driver slaving support    */
#define SIOCSIFSLAVE    0x8930
#define SIOCADDMULTI    0x8931        /* Multicast address lists    */
#define SIOCDELMULTI    0x8932
#define SIOCGIFINDEX    0x8933        /* name -> if_index mapping    */
#define SIOGIFINDEX    SIOCGIFINDEX    /* misprint compatibility :-)    */
#define SIOCSIFPFLAGS    0x8934        /* set/get extended flags set    */
#define SIOCGIFPFLAGS    0x8935
#define SIOCDIFADDR        0x8936        /* delete PA address        */
#define    SIOCSIFHWBROADCAST    0x8937    /* set hardware broadcast addr    */
#define SIOCGIFCOUNT    0x8938        /* get number of devices */
...
#define SIOCETHTOOL     0x8946      /* Ethtool interface     */

结构字段与操作类型的含义在大都在注释中已注明

一般步骤

通过ioctl(2)执行网络接口参数的获取/设置的一般步骤为：

  1. 通过socket(2)创建IP套接字；由于ioctl(2)此时是与内核通信，因此对套接字的通信域与类型没有强制要求，通信域可以为AF_INET/AF_LOCAL，类型可以为SOCK_DGRAM/SOCK_STREAM/SOCK_RAW等

  2. 初始化struct ifconf与/或struct ifreq结构

  3. 对套接字描述符调用ioctl(2)，执行相应类型的SIO操作

  4. 获取返回至truct ifconf与/或struct ifreq结构中的相关信息

示例程序

本地网络接口信息：eth0网线已连接且已配置IPv4地址，eth1网线未连接且未配置IPv4地址

# ip l
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 00:0c:29:ed:9d:28 brd ff:ff:ff:ff:ff:ff
3: eth1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN qlen 1000
    link/ether 00:0c:29:ed:9d:32 brd ff:ff:ff:ff:ff:ff
    
# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 00:0c:29:ed:9d:28 brd ff:ff:ff:ff:ff:ff
    inet 192.168.56.139/24 brd 192.168.56.255 scope global eth0
    inet6 fe80::20c:29ff:feed:9d28/64 scope link 
       valid_lft forever preferred_lft forever
3: eth1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN qlen 1000
    link/ether 00:0c:29:ed:9d:32 brd ff:ff:ff:ff:ff:ff

1 . 通过SIOCGIFCONF操作获取系统中所有的网络接口

/* list_network_interfaces_ioctl.c */

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <string.h>

#define BUFSIZE 1024

int main(int argc, char *argv[])
{
    int sfd, if_count, i;
    struct ifconf ifc;
    struct ifreq ifr[10];
    char ipaddr[INET_ADDRSTRLEN] = {'\0'};

    memset(&ifc, 0, sizeof(struct ifconf));

    sfd = socket(AF_INET, SOCK_DGRAM, 0);

    ifc.ifc_len = 10 * sizeof(struct ifreq);
    ifc.ifc_buf = (char *)ifr;

    /* SIOCGIFCONF is IP specific. see netdevice(7) */
    ioctl(sfd, SIOCGIFCONF, (char *)&ifc);

    if_count = ifc.ifc_len / (sizeof(struct ifreq));
    for (i = 0; i < if_count; i++) {
        printf("Interface %s : ", ifr[i].ifr_name);    
        inet_ntop(AF_INET, 
        &(((struct sockaddr_in *)&(ifr[i].ifr_addr))->sin_addr),
        ipaddr, INET_ADDRSTRLEN);
        printf("%s\n", ipaddr);
    }
    
    close(sfd);

    exit(EXIT_SUCCESS);
}

编译并运行

# gcc list_network_interfaces_ioctl.c -g -o list_network_interfaces_ioctl

# ./list_network_interfaces_ioctl
Interface lo : 127.0.0.1
Interface eth0 : 192.168.56.139

SIOCGIFCONF操作需要同时使用struct ifconf与struct ifreq
初始化struct ifconf时，ifc_ifcu指定一个缓冲区的首地址，缓冲区中保存若干个连续的struct ifreq，ifc_len指定该缓冲区的长度；ioctl(2)返回后，内核将为每一个配置了IPv4地址的本地网络接口分配一个struct ifreq并保存到初始化的缓冲区中，struct ifreq中保存单个网络接口的名称及IPv4地址，struct ifconf的ifc_len被更新为初始化缓冲区中实际返回的数据长度

2 . 通过SIOCGIFADDR操作获取指定网络接口的IPv4地址

/* get_interface_ip_address_ioctl.c */

#include <stdio.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>

static char *get_ipaddr(const char *);

int main(int argc, char *argv[])
{

    if (argc != 2) {
        fprintf(stderr, "Usage: %s [network interface name]\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    char ifname[IFNAMSIZ] = {'\0'};
    strncpy(ifname, argv[1], IFNAMSIZ-1);

    char *ip = get_ipaddr(ifname);

    printf("Interface %s : %s\n", ifname, ip);
    
    return 0;
}

static char *get_ipaddr(const char *dev)
{
    int sfd, saved_errno, ret;
    struct ifreq ifr;
    char *ipaddr;

    ipaddr = (char *)malloc(INET_ADDRSTRLEN);
    memset(&ifr, 0, sizeof(ifr));
    ifr.ifr_addr.sa_family = AF_INET;
    strncpy(ifr.ifr_name, dev, IFNAMSIZ);

    sfd = socket(AF_INET, SOCK_DGRAM, 0);

    errno = saved_errno;
    ret = ioctl(sfd, SIOCGIFADDR, &ifr);
    if (ret == -1) {
        if (errno == 19) {
            fprintf(stderr, "Interface %s : No such device.\n", dev);
            exit(EXIT_FAILURE);
        }
        if (errno == 99) {
            fprintf(stderr, "Interface %s : No IPv4 address assigned.\n", dev);
            exit(EXIT_FAILURE);
        }
    }
    saved_errno = errno;

    inet_ntop(AF_INET, &(((struct sockaddr_in *)&ifr.ifr_addr)->sin_addr), ipaddr, INET_ADDRSTRLEN);
    
    close(sfd);
    return ipaddr;
}

编译并运行

# gcc get_interface_ip_address_ioctl.c -g -o get_interface_ip_address_ioctl
#
# ./get_interface_ip_address_ioctl eth0
Interface eth0 : 192.168.56.139
#
# ./get_interface_ip_address_ioctl eth1
Interface eth1 : No IPv4 address assigned.
#
# ./get_interface_ip_address_ioctl eth2
Interface eth2 : No such device.

SIOCGIFADDR操作使用struct ifreq中的ifr_ifru.ifru_addr字段；ifr_ifrn.ifrn_name指定为网络接口名称并调用ioctl(SIOCGIFADDR)，返回后将ifr_ifru.ifru_addr转换为IPv4套接字地址结构，IPv4地址保存在该结构中的sin_addr字段中

SIOCGIFCONF与SIOCGIFADDR属于IPv4特定的操作，对于未配置IPv4地址的网络接口，ioctl(SIOCGIFCONF)返回时不会分配struct ifreq结构，因而不会返回该接口的名称，而ioctl(SIOCGIFADDR)将以errno值99(Cannot assign requested address)而调用失败
若指定了系统中不存在的网络接口，则errno的值为19(No such device)

SIOCGIFCONF与SIOCGIFADDR无法获取网络接口的IPv6地址，ioctl的内核源码中通过读取/proc/net/if_inet6获取

3 . 通过SIOCGIFHWADDR操作获取指定网络接口的mac地址

/* get_interface_mac_address_ioctl.c */

#include <stdio.h>
#include <stdlib.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <errno.h>
#include <string.h>
#include <netinet/if_ether.h>
#include <net/if_arp.h>

static unsigned char *get_if_mac(const char *);

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

    char ifname[IFNAMSIZ] = {'\0'};
    strncpy(ifname, argv[1], IFNAMSIZ-1);

    unsigned char *mac = get_if_mac(ifname);
    
    printf("Interface %s : %.2x:%.2x:%.2x:%.2x:%.2x:%.2x\n",
    ifname, *mac, *(mac+1), *(mac+2), *(mac+3), *(mac+4), *(mac+5));

    return 0;
}

static unsigned char *get_if_mac(const char *dev)
{
    int sfd, ret, saved_errno, i;
    unsigned char *mac_addr;
    struct ifreq ifr;

    mac_addr = (unsigned char *)malloc(ETH_ALEN);

    sfd = socket(AF_INET, SOCK_DGRAM, 0);
    memset(&ifr, 0, sizeof(ifr));
    strncpy(ifr.ifr_name, dev, IFNAMSIZ);

    saved_errno = errno;
    ret = ioctl(sfd, SIOCGIFHWADDR, &ifr);
    if (ret == -1 && errno == 19) {
        fprintf(stderr, "Interface %s : No such device.\n", dev);
        exit(EXIT_FAILURE);
    }
    errno = saved_errno;

    if (ifr.ifr_addr.sa_family == ARPHRD_LOOPBACK) {
        printf("Interface %s : A Loopback device.\n", dev);
        printf("MAC address is always 00:00:00:00:00:00\n");
        exit(EXIT_SUCCESS);
    }

    if (ifr.ifr_addr.sa_family != ARPHRD_ETHER) {
        fprintf(stderr, "Interface %s : Not an Ethernet device.\n", dev);
        exit(EXIT_FAILURE);
    }

    memcpy(mac_addr, ifr.ifr_hwaddr.sa_data, ETH_ALEN);

    return (unsigned char *)mac_addr;
}

编译并运行

# gcc get_interface_mac_address_ioctl.c -g -o get_interface_mac_address_ioctl
#
# ./get_interface_mac_address_ioctl lo
Interface lo : A Loopback device.
MAC address is always 00:00:00:00:00:00
#
# ./get_interface_mac_address_ioctl eth0
Interface eth0 : 00:0c:29:ed:9d:28
#
# ./get_interface_mac_address_ioctl eth1
Interface eth1 : 00:0c:29:ed:9d:32
#
# ./get_interface_mac_address_ioctl eth2
Interface eth2 : No such device.

SIOCGIFHWADDR操作使用struct ifreq中的ifr_ifru.ifru_hwaddr字段，在ifr_ifrn.ifrn_name中填充指定的网络接口名称后，该接口的mac地址按顺序返回到ifr_ifru.ifru_hwaddr.sa_data数组的前6个字节中

4 . 通过SIOCGIFFLAGS操作获取指定网络接口的标志

/* get_interface_flags_ioctl.c */

#include <stdio.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <netinet/if_ether.h>
#include <net/if_arp.h>

static short get_if_flags(int, char *);

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

    int sfd;
    short flags;
    char ifname[IFNAMSIZ] = {'\0'};
    strncpy(ifname, argv[1], IFNAMSIZ-1);

    sfd = socket(AF_INET, SOCK_DGRAM, 0);
    flags = get_if_flags(sfd, ifname);

    printf("Interface %s : ", ifname);
    if (flags & IFF_UP)
        printf("UP ");

    if (flags & IFF_RUNNING)
        printf("RUNNING ");

    if (flags & IFF_LOOPBACK)
        printf("LOOPBACK ");

    if (flags & IFF_BROADCAST)
        printf("BROADCAST ");

    if (flags & IFF_MULTICAST)
        printf("MULTICAST ");

    if (flags & IFF_PROMISC)
        printf("PROMISC");

#ifndef IFF_LOWER_UP
#define IFF_LOWER_UP 0x10000
    if (flags & IFF_LOWER_UP)
        printf("LOWER_UP");
#endif

    printf("\n");

    close(sfd);
    exit(EXIT_SUCCESS);
}

static short get_if_flags(int s, char *dev)
{
    int saved_errno, ret;
    short if_flags;
    struct ifreq ifr;

    memset(&ifr, 0, sizeof(ifr));
    strncpy(ifr.ifr_name, dev, IFNAMSIZ);

    saved_errno = errno;
    ret = ioctl(s, SIOCGIFFLAGS, &ifr);
    if (ret == -1 && errno == 19) {
        fprintf(stderr, "Interface %s : No such device.\n", dev);
        exit(EXIT_FAILURE);
    }
    errno = saved_errno;
    if_flags = ifr.ifr_flags;

    return if_flags;
}

编译并运行

# gcc get_interface_flags_ioctl.c -g -o get_interface_flags_ioctl
#
# ./get_interface_flags_ioctl lo
Interface lo : UP RUNNING LOOPBACK
#
# ./get_interface_flags_ioctl eth0
Interface eth0 : UP RUNNING BROADCAST MULTICAST
#
# ./get_interface_flags_ioctl eth1
Interface eth1 : UP BROADCAST MULTICAST
#
# ./get_interface_flags_ioctl eth2
Interface eth2 : No such device.

ifr_ifrn.ifrn_name指定为网络接口名称后，ioctl(SIOCGIFFLAGS)调用将标志返回到ifr_ifru.ifru_flags字段

IFF_RUNNING表示该接口已被激活，且可以正常传输数据

IFF_UP表示giant接口已被激活，但可能无法正常传输数据，如网线未连接的情况

IFF_LOWER_UP表示网络的物理连接已就绪，即网线连接正常；由于struct ifreq的ifr_ifru.ifru_flags类型为short，用16进制表示仅为4位，因而无法获取与设置5位16进制的IFF_LOWER_UP标志(0x10000)

5 .通过SIOCSIFADDR操作设置指定网络接口的IPv4地址

/* set_interface_ip_address_ioctl.c */

#include <stdio.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>

static void set_ipaddr(const char *, const char *);

int main(int argc, char *argv[])
{

    if (argc != 3) {
        fprintf(stderr, "Usage: %s [network interface name] [ip address]\n",
        argv[0]);
        exit(EXIT_FAILURE);
    }

    char ifname[IFNAMSIZ] = {'\0'};
    strncpy(ifname, argv[1], IFNAMSIZ-1);
    char ipaddr[INET_ADDRSTRLEN] = {'\0'};
    strncpy(ipaddr, argv[2], INET_ADDRSTRLEN);

    set_ipaddr(ifname, ipaddr);

    printf("Interface %s : ip address is set to %s\n", ifname, ipaddr);
    
    return 0;
}

static void set_ipaddr(const char *dev, const char *ip)
{
    int sfd, saved_errno, ret;
    struct ifreq ifr;
    struct sockaddr_in sin;

    sfd = socket(AF_INET, SOCK_DGRAM, 0);

    memset(&ifr, 0, sizeof(ifr));
    strncpy(ifr.ifr_name, dev, IFNAMSIZ);

    memset(&sin, 0, sizeof(sin));
    sin.sin_family = AF_INET;
    inet_pton(AF_INET, ip, &(sin.sin_addr));

    memcpy(&ifr.ifr_addr, &sin, sizeof(struct sockaddr));

    errno = saved_errno;
    ret = ioctl(sfd, SIOCSIFADDR, &ifr);
    if (ret == -1) {
        if (errno == 19) {
            fprintf(stderr, "Interface %s : No such device.\n", dev);
            exit(EXIT_FAILURE);
        }
        if (errno == 99) {
            fprintf(stderr, "Interface %s : No IPv4 address assigned.\n", dev);
            exit(EXIT_FAILURE);
        }
    }
    saved_errno = errno;

    close(sfd);
}

编译并运行

# gcc set_interface_ip_address_ioctl.c -g -o set_interface_ip_address_ioctl
#
# ./set_interface_ip_address_ioctl eth1 10.0.0.1
Interface eth1 : ip address is set to 10.0.0.1
#
# ./get_interface_ip_address_ioctl eth1
Interface eth1 : 10.0.0.1
#
# ./set_interface_ip_address_ioctl eth1 10.0.0.2
Interface eth1 : ip address is set to 10.0.0.2
#
# ./get_interface_ip_address_ioctl eth1
Interface eth1 : 10.0.0.2

与ifconfig(8)相同，多次指定同一网络接口名称设置IP地址时，最后的设置将覆盖先前的设置而生效

6 . 通过SIOCGIFFLAGS操作设置指定网络接口的标志
使用ifconfig(8)将eth1设置为混杂模式，并关闭该接口，然后在程序中关闭混杂模式，并开启该接口

# ifconfig eth1 promisc
#
# ifconfig eth1 down
#
# ./get_interface_flags_ioctl eth1
Interface eth1 : BROADCAST MULTICAST PROMISC

/* set_interface_flags_ioctl.c */

#include <stdio.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>

static short get_if_flags(int, struct ifreq*);
static void set_if_flags(int, struct ifreq*);

int main(int argc, char *argv[])
{

    if (argc != 2) {
        fprintf(stderr, "Usage: %s [network interface name]\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    int sfd;
    short flags;
    struct ifreq ifr;

    char ifname[IFNAMSIZ] = {'\0'};
    strncpy(ifname, argv[1], IFNAMSIZ-1);

    sfd = socket(AF_INET, SOCK_DGRAM, 0);
    memset(&ifr, 0, sizeof(ifr));
    strncpy(ifr.ifr_name, ifname, IFNAMSIZ);
    flags = get_if_flags(sfd, &ifr);

    ifr.ifr_flags = flags;

    /* set IFF_UP if cleared */
    if (!(flags & IFF_UP)) {
        ifr.ifr_flags |= IFF_UP;
        set_if_flags(sfd, &ifr);
        printf("Interface %s : UP set.\n", ifname);
    }

    flags = ifr.ifr_flags;

    /* clear IFF_PROMISC if set */
    if (flags & IFF_PROMISC) {
        ifr.ifr_flags &= ~IFF_PROMISC;
        set_if_flags(sfd, &ifr);
        printf("Interface %s : PROMISC cleared.\n", ifname);
    }

    close(sfd);

    exit(EXIT_SUCCESS);
}

static short get_if_flags(int s, struct ifreq *ifr)
{
    int ret, saved_errno;
    short if_flags;

    saved_errno = errno;
    ret = ioctl(s, SIOCGIFFLAGS, ifr);
    if (ret == -1 && errno == 19) {
        fprintf(stderr, "Interface %s : No such device.\n", ifr->ifr_name);
        exit(EXIT_FAILURE);
    }
    errno = saved_errno;

    if_flags = ifr->ifr_flags;
    return if_flags;
}

static void set_if_flags(int s, struct ifreq *ifr)
{
    int ret, saved_errno;
    saved_errno = errno;
    ret = ioctl(s, SIOCSIFFLAGS, ifr);
    if (ret == -1) {
        fprintf(stderr, "Interface %s : %s\n", ifr->ifr_name, strerror(errno));
        exit(EXIT_FAILURE);
    }
    errno = saved_errno;
}

编译并运行

# gcc set_interface_flags_ioctl.c -g -o set_interface_flags_ioctl
#
# ./set_interface_flags_ioctl eth1
Interface eth1 : UP set.
Interface eth1 : PROMISC cleared.
#
# ./get_interface_flags_ioctl eth1
Interface eth1 : UP BROADCAST MULTICAST

7 .通过SIOCSIFNAME操作更改网络接口的名称

/* change_ifname_ioctl.c */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/socket.h>

static void change_ifname(char *, char *);
static void shutdown_if_up(char *);

int main(int argc, char *argv[])
{
    if (argc != 3) {
        fprintf(stderr, "%s [old ifname] [new ifname]\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    char old_ifname[IFNAMSIZ] = {'\0'};
    strncpy(old_ifname, argv[1], IFNAMSIZ);

    char new_ifname[IFNAMSIZ] = {'\0'};
    strncpy(new_ifname, argv[2], IFNAMSIZ);

    change_ifname(old_ifname, new_ifname);
    printf("Interface name %s has been changed to %s\n", old_ifname, new_ifname);

    return 0;
}

void change_ifname(char *old_dev, char *new_dev)
{
    int sfd, ret, saved_errno;
    struct ifreq ifr;

    shutdown_if_up(old_dev);

    sfd = socket(AF_INET, SOCK_DGRAM, 0);

    memset(&ifr, 0, sizeof(ifr));
    strncpy(ifr.ifr_name, old_dev, IFNAMSIZ);
    strncpy(ifr.ifr_newname, new_dev, IFNAMSIZ);

    saved_errno = errno;
    ret = ioctl(sfd, SIOCSIFNAME, &ifr);
    if (ret == -1) {
        fprintf(stderr, "Interface %s : %s\n", dev, strerror(errno));
        exit(EXIT_FAILURE);
    }
    errno = saved_errno;
}

static void shutdown_if_up(char *dev)
{
    int sfd, ret, saved_errno;
    short flags;
    struct ifreq ifr;

    sfd = socket(AF_INET, SOCK_DGRAM, 0);

    memset(&ifr, 0, sizeof(ifr));
    strncpy(ifr.ifr_name, dev, IFNAMSIZ);

    saved_errno = errno;
    ret = ioctl(sfd, SIOCGIFFLAGS, &ifr);
    if (ret == -1) {
        fprintf(stderr, "Interface %s : %s\n", dev, strerror(errno));
        exit(EXIT_FAILURE);
    }
    errno = saved_errno;

    flags = ifr.ifr_flags;
    if (flags & IFF_UP) {
        ifr.ifr_flags &= ~IFF_UP;
        saved_errno = errno;
        ret = ioctl(sfd, SIOCSIFFLAGS, &ifr);
        if (ret == -1) {
            fprintf(stderr, "Interface %s : %s\n",dev, strerror(errno));
            exit(EXIT_FAILURE);
        }
        errno = saved_errno;
    }
}

将struct ifreq的ifr_ifrn.ifrn_name指定为网络接口名称后，ioctl(SIOCSIFNAME)将指定的新名称写入到ifr_ifru.ifru_newname中；该操作要求网络接口为关闭状态，即(~IFF_UP)


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