在Linux上使用C语言编程获取IPv4地址及子网掩码

在Linux上(如Ubuntu或CentOS), 获取某个Network Interface比如eth0的IP地址等信息，我们可以使用ifconfig或者ip addr show命令。

$ ifconfig eth0
eth0      Link encap:Ethernet  HWaddr :::ba:8d:be
          inet addr:192.168.1.102  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80:::64ff:feba:8dbe/ Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:  Metric:
          RX packets: errors: dropped: overruns: frame:
          TX packets: errors: dropped: overruns: carrier:
          collisions: txqueuelen:
          RX bytes: (29.1 MB)  TX bytes: (4.0 MB)
          Interrupt:

$ ip addr show eth0
: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu  qdisc mq state UP group default qlen
    link/ether :::ba:8d:be brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.102/ brd 192.168.1.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80:::64ff:feba:8dbe/ scope link
       valid_lft forever preferred_lft forever

那么用C语言编程怎么实现呢？ 先用strace工具观察一下ifconfig eth0的运行内幕。

$ strace ifconfig eth0
execve("/sbin/ifconfig", ["ifconfig", "eth0"], [/* 71 vars */]) = 0
brk(0)                                  = 0x9565000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb76fe000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=114965, ...}) = 0
mmap2(NULL, 114965, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb76e1000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/i386-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0P\234\1\0004\0\0\0"..., 512) = 512
fstat64(3, {st_mode=S_IFREG|0755, st_size=1763068, ...}) = 0
mmap2(NULL, 1772156, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xb7530000
mmap2(0xb76db000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1aa000) = 0xb76db000
mmap2(0xb76de000, 10876, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xb76de000
close(3)                                = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb752f000
set_thread_area({entry_number:-1 -> 6, base_addr:0xb752f940, limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1, seg_not_present:0, useable:1}) = 0
mprotect(0xb76db000, 8192, PROT_READ)   = 0
mprotect(0x8058000, 4096, PROT_READ)    = 0
mprotect(0xb7724000, 4096, PROT_READ)   = 0
munmap(0xb76e1000, 114965)              = 0
brk(0)                                  = 0x9565000
brk(0x9586000)                          = 0x9586000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=8752496, ...}) = 0
mmap2(NULL, 2097152, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb732f000
mmap2(NULL, 4096, PROT_READ, MAP_PRIVATE, 3, 0x855000) = 0xb76fd000
close(3)                                = 0
uname({sys="Linux", node="idorax", ...}) = 0
access("/proc/net", R_OK)               = 0
access("/proc/net/unix", R_OK)          = 0
socket(PF_LOCAL, SOCK_DGRAM, 0)         = 3
socket(PF_INET, SOCK_DGRAM, IPPROTO_IP) = 4
access("/proc/net/if_inet6", R_OK)      = 0
socket(PF_INET6, SOCK_DGRAM, IPPROTO_IP) = 5
access("/proc/net/ax25", R_OK)          = -1 ENOENT (No such file or directory)
access("/proc/net/nr", R_OK)            = -1 ENOENT (No such file or directory)
access("/proc/net/rose", R_OK)          = -1 ENOENT (No such file or directory)
access("/proc/net/ipx", R_OK)           = -1 ENOENT (No such file or directory)
access("/proc/net/appletalk", R_OK)     = -1 ENOENT (No such file or directory)
access("/proc/sys/net/econet", R_OK)    = -1 ENOENT (No such file or directory)
access("/proc/sys/net/ash", R_OK)       = -1 ENOENT (No such file or directory)
access("/proc/net/x25", R_OK)           = -1 ENOENT (No such file or directory)
open("/proc/net/dev", O_RDONLY)         = 6
fstat64(6, {st_mode=S_IFREG|0444, st_size=0, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb76fc000
read(6, "Inter-|   Receive               "..., 1024) = 447
close(6)                                = 0
munmap(0xb76fc000, 4096)                = 0
ioctl(5, SIOCGIFFLAGS, {ifr_name="eth0", ifr_flags=IFF_UP|IFF_BROADCAST|IFF_RUNNING|IFF_MULTICAST}) = 0
ioctl(5, SIOCGIFHWADDR, {ifr_name="eth0", ifr_hwaddr=00:25:64:ba:8d:be}) = 0
ioctl(5, SIOCGIFMETRIC, {ifr_name="eth0", ifr_metric=0}) = 0
ioctl(5, SIOCGIFMTU, {ifr_name="eth0", ifr_mtu=1500}) = 0
ioctl(5, SIOCGIFMAP, {ifr_name="eth0", ifr_map={mem_start=0, mem_end=0, base_addr=0, irq=16, dma=0, port=0}}) = 0
ioctl(5, SIOCGIFMAP, {ifr_name="eth0", ifr_map={mem_start=0, mem_end=0, base_addr=0, irq=16, dma=0, port=0}}) = 0
ioctl(5, SIOCGIFTXQLEN, {ifr_name="eth0", ifr_qlen=1000}) = 0
open("/usr/share/locale/locale.alias", O_RDONLY|O_CLOEXEC) = 6
fstat64(6, {st_mode=S_IFREG|0644, st_size=2570, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb76fc000
read(6, "# Locale name alias data base.\n#"..., 4096) = 2570
read(6, "", 4096)                       = 0
close(6)                                = 0
munmap(0xb76fc000, 4096)                = 0
open("/usr/share/locale/en_US/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/en/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale-langpack/en_US/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale-langpack/en/LC_MESSAGES/net-tools.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
ioctl(4, SIOCGIFADDR, {ifr_name="eth0", ifr_addr={AF_INET, inet_addr("192.168.1.102")}}) = 0
ioctl(4, SIOCGIFDSTADDR, {ifr_name="eth0", ifr_dstaddr={AF_INET, inet_addr("192.168.1.102")}}) = 0
ioctl(4, SIOCGIFBRDADDR, {ifr_name="eth0", ifr_broadaddr={AF_INET, inet_addr("192.168.1.255")}}) = 0
ioctl(4, SIOCGIFNETMASK, {ifr_name="eth0", ifr_netmask={AF_INET, inet_addr("255.255.255.0")}}) = 0
fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 6), ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb76fc000
write(1, "eth0      Link encap:Ethernet  H"..., 58eth0      Link encap:Ethernet  HWaddr 00:25:64:ba:8d:be
) = 58
write(1, "          inet addr:192.168.1.10"..., 75          inet addr:192.168.1.102  Bcast:192.168.1.255  Mask:255.255.255.0
) = 75
open("/proc/net/if_inet6", O_RDONLY)    = 6
fstat64(6, {st_mode=S_IFREG|0444, st_size=0, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb76fb000
read(6, "00000000000000000000000000000001"..., 1024) = 108
read(6, "", 1024)                       = 0
write(1, "          inet6 addr: fe80::225:"..., 61          inet6 addr: fe80::225:64ff:feba:8dbe/64 Scope:Link
) = 61
read(6, "", 1024)                       = 0
close(6)                                = 0
munmap(0xb76fb000, 4096)                = 0
write(1, "          UP BROADCAST RUNNING M"..., 61          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
) = 61
write(1, "          RX packets:31318 error"..., 65          RX packets:31318 errors:0 dropped:0 overruns:0 frame:0
) = 65
write(1, "          TX packets:29503 error"..., 67          TX packets:29503 errors:0 dropped:0 overruns:0 carrier:0
) = 67
write(1, "          collisions:0 txqueuele"..., 40          collisions:0 txqueuelen:1000
) = 40
write(1, "          RX bytes:29127563 (29."..., 65          RX bytes:29127563 (29.1 MB)  TX bytes:4104503 (4.1 MB)
) = 65
write(1, "          Interrupt:16 \n", 24          Interrupt:16
) = 24
write(1, "\n", 1
)                       = 1
close(5)                                = 0
exit_group(0)                           = ?
+++ exited with 0 +++

注意上面的socket()和ioctl(), 我们不难发现如下关键调用，

socket(PF_INET, SOCK_DGRAM, IPPROTO_IP) = 4

ioctl(4, SIOCGIFADDR, {ifr_name="eth0", ifr_addr={AF_INET, inet_addr("192.168.1.102")}}) = 0
ioctl(4, SIOCGIFNETMASK, {ifr_name="eth0", ifr_netmask={AF_INET, inet_addr("255.255.255.0")}}) = 0

好了，现在可以上C代码了，(foo_get.c)

 /**
  * get IPv4 address and subnet mask of a network interface
  */
 #include <stdio.h>
 #include <unistd.h>
 #include <string.h>
 #include <sys/socket.h>
 #include <sys/ioctl.h>
 #include <net/if.h>
 #include <arpa/inet.h>

 int
 main(int argc, char *argv[])
 {
         int rc = ;
         struct sockaddr_in *addr = NULL;

         if (argc != ) {
                 fprintf(stderr, "Usage: %s <ifname>\n", argv[]);
                 return -;
         }

         char *ifname = argv[];

         struct ifreq ifr;
         memset(&ifr, , sizeof(struct ifreq));

         /* 0. create a socket */
         int fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_IP);
         if (fd == -)
                 return -;

         /* 1. set type of address to retrieve : IPv4 */
         ifr.ifr_addr.sa_family = AF_INET;

         /* 2. copy interface name to ifreq structure */
         strncpy(ifr.ifr_name, ifname, IFNAMSIZ - );

         /* 3. get the IP address */
         if ((rc = ioctl(fd, SIOCGIFADDR, &ifr)) != )
                 goto done;

         char ipv4[] = {  };
         addr = (struct sockaddr_in *)&ifr.ifr_addr;
         strncpy(ipv4, inet_ntoa(addr->sin_addr), sizeof(ipv4));

         /* 4. get the mask */
         if ((rc = ioctl(fd, SIOCGIFNETMASK, &ifr)) != )
                 goto done;

         char mask[] = {  };
         addr = (struct sockaddr_in *)&ifr.ifr_addr;
         strncpy(mask, inet_ntoa(addr->sin_addr), sizeof(mask));

         /* 5. display */
         printf("IFNAME:IPv4:MASK\n");
         printf("%s:%s:%s\n", ifname, ipv4, mask);

         /* 6. close the socket */
 done:
         close(fd);

         return rc;
 }

编译并测试，

$ gcc -g -Wall -std=gnu99 -o foo_get foo_get.c

$ ./foo_get eth0
IFNAME:IPv4:MASK
eth0:192.168.1.102:255.255.255.0

$ ./foo_get lo
IFNAME:IPv4:MASK
lo:127.0.0.1:255.0.0.0

参考资料：

• NETDEVICE(7)
• INET(3)
• https://stackoverflow.com/questions/6652384/how-to-set-the-ip-address-from-c-in-linux