using C code to get same info as ifconfig

Question

Is there a way in Linux, using C code, to get the same information that "ifconfig eth0" would return? I'm interested in things like IP address, link status, and MAC address.

Here's sample output from ifconfig:

eth0      Link encap:Ethernet  HWaddr 00:0F:20:CF:8B:42
          inet addr:217.149.127.10  Bcast:217.149.127.63  Mask:255.255.255.192
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:2472694671 errors:1 dropped:0 overruns:0 frame:0
          TX packets:44641779 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:1761467179 (1679.8 Mb)  TX bytes:2870928587 (2737.9 Mb)
          Interrupt:28

**'ifconfig' hasn't been maintained on Linux for many years.** Use 'ip' instead. http://serverfault.com/questions/633087/where-is-the-statement-of-deprecation-of-ifconfig-on-linux — mikemaccana, Aug 28 '15 at 14:55

score 82 · Answer 1 · answered Feb 09 '11 at 22:43

82

One way to get to the bottom of problems like this, particularly in cases when you don't have source, is strace.

It gives you a list of all the system calls made by any program you pass it, along with their arguments and return values. If your program just dumps some info and quits rather than running for an extended time it can be pretty straightforward to just do a man on all the system calls you see that look like they might provide the info you're looking for.

When I run

strace ifconfig

Some of the interesting calls are:

open("/proc/net/dev", O_RDONLY)         = 6

followed by a bunch of ioctls, corroborating @payne's answer:

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=84:2b:2b:b7:9e:6d}) = 0
ioctl(5, SIOCGIFMETRIC, {ifr_name="eth0", ifr_metric=0}) = 0
ioctl(5, SIOCGIFMTU, {ifr_name="eth0", ifr_mtu=1500}) = 0

answered Feb 09 '11 at 22:43

Paul Rubel

24,802
7
54
76

13

+1 for an answer that shows how you found the answer, and how to apply the technique to other problems – R.. GitHub STOP HELPING ICE Feb 09 '11 at 23:48
2

Clever, of course it does ignores the fact that for the **Linux** `ifconfig` command, [source](http://sourceforge.net/projects/net-tools/) is readily available. – mctylr Jul 18 '14 at 16:56
2

This has to be one of the best tools I have ever seen for trying to emulate the functionality of commands and filters. Some times you are in a rush and need to know what sequence of system calls will get you what you need to do without calling popen() to shell out the command. One of the best tips I have been lucky to find concerning Linux programming. Thank you very much! – CCS Nov 25 '15 at 21:47

score 37 · Accepted Answer · answered Feb 09 '11 at 22:29

37

Yes, ifconfig itself is written in C. :) See: http://cvsweb.netbsd.org/bsdweb.cgi/src/sbin/ifconfig/ifconfig.c?rev=1.169&content-type=text/x-cvsweb-markup

Do man netdevice to see the details (on Linux). You use the ioctl() system call.

answered Feb 09 '11 at 22:29

payne

12,793
5
35
45

score 13 · Answer 3 · answered May 09 '16 at 13:21

There is simpler approach. copied from http://man7.org/linux/man-pages/man3/getifaddrs.3.html

   #include <arpa/inet.h>
   #include <sys/socket.h>
   #include <netdb.h>
   #include <ifaddrs.h>
   #include <stdio.h>
   #include <stdlib.h>
   #include <unistd.h>
   #include <linux/if_link.h>

   int main(int argc, char *argv[])
   {
       struct ifaddrs *ifaddr, *ifa;
       int family, s, n;
       char host[NI_MAXHOST];

       if (getifaddrs(&ifaddr) == -1) {
           perror("getifaddrs");
           exit(EXIT_FAILURE);
       }

       /* Walk through linked list, maintaining head pointer so we
          can free list later */

       for (ifa = ifaddr, n = 0; ifa != NULL; ifa = ifa->ifa_next, n++) {
           if (ifa->ifa_addr == NULL)
               continue;

           family = ifa->ifa_addr->sa_family;

           /* Display interface name and family (including symbolic
              form of the latter for the common families) */

           printf("%-8s %s (%d)\n",
                  ifa->ifa_name,
                  (family == AF_PACKET) ? "AF_PACKET" :
                  (family == AF_INET) ? "AF_INET" :
                  (family == AF_INET6) ? "AF_INET6" : "???",
                  family);

           /* For an AF_INET* interface address, display the address */

           if (family == AF_INET || family == AF_INET6) {
               s = getnameinfo(ifa->ifa_addr,
                       (family == AF_INET) ? sizeof(struct sockaddr_in) :
                                             sizeof(struct sockaddr_in6),
                       host, NI_MAXHOST,
                       NULL, 0, NI_NUMERICHOST);
               if (s != 0) {
                   printf("getnameinfo() failed: %s\n", gai_strerror(s));
                   exit(EXIT_FAILURE);
               }

               printf("\t\taddress: <%s>\n", host);

           } else if (family == AF_PACKET && ifa->ifa_data != NULL) {
               struct rtnl_link_stats *stats = (struct rtnl_link_stats *)ifa->ifa_data;

               printf("\t\ttx_packets = %10u; rx_packets = %10u\n"
                      "\t\ttx_bytes   = %10u; rx_bytes   = %10u\n",
                      stats->tx_packets, stats->rx_packets,
                      stats->tx_bytes, stats->rx_bytes);
           }
       }

       freeifaddrs(ifaddr);
       exit(EXIT_SUCCESS);
   }

score 9 · Answer 4 · edited Jun 26 '13 at 20:45

9

One simple way is to use the popen function see: http://pubs.opengroup.org/onlinepubs/009696899/functions/popen.html

Use something like:

FILE *fp;

char returnData[64];

fp = popen("/sbin/ifconfig eth0", "r");

while (fgets(returnData, 64, fp) != NULL)
{
    printf("%s", returnData);
}

pclose(fp);

edited Jun 26 '13 at 20:45

Camille Goudeseune

2,525
2
25
47

answered Mar 28 '13 at 09:49

Lewis R

354
3
7

2

People won't like it, but it's a great portable answer, and rolling your own using ioctls is not for the faint-hearted! – Nicholas Wilson Mar 28 '13 at 10:17
1

Fun answer. I would upvote if you had also piped it to `awk` for parsing out values. Since you've forked and invoked the shell already with `popen`, I would suggest using `awk` as the classic Unix choice for parsing data rather than `strtok`, `strstr`, etc. in C. – mctylr Jul 18 '14 at 16:49

score 9 · Answer 5 · answered Oct 20 '14 at 16:16

9

Here is how I get MAC and MTU in my code:

void getMACAddress(std::string _iface,unsigned char MAC[6]) {
        int fd = socket(AF_INET, SOCK_DGRAM, 0);
        struct ifreq ifr;
        ifr.ifr_addr.sa_family = AF_INET;
        strncpy(ifr.ifr_name , _iface.c_str() , IFNAMSIZ-1);
        ioctl(fd, SIOCGIFHWADDR, &ifr);
        for(unsigned int i=0;i<6;i++)
            MAC[i] = ifr.ifr_hwaddr.sa_data[i];
        ioctl(fd, SIOCGIFMTU, &ifr);
        close(fd);
        printf("MTU: %d\n",ifr.ifr_mtu);
        printf("MAC:%.2x:%.2x:%.2x:%.2x:%.2x:%.2x\n",MAC[0],MAC[1],MAC[2],MAC[3],MAC[4],MAC[5]);
    }

answered Oct 20 '14 at 16:16

Behrooz

458
5
13

What do I pass in to the `unsigned char MAC[6]` ?? – Piotr Kula Jun 08 '15 at 21:22
@ppumkin: MAC[6] is an output parameter, he uses it instead of return value. So enter there a pointer to your own array of unsigned chars. Like this: `unsigned char outMAC[6]; getMACAddress("eth0", outMac);` – Youda008 Dec 26 '15 at 19:10
But how to get the interface name when all you have is the bind address? – Carlo Wood Jul 10 '19 at 15:38

score 2 · Answer 6 · answered Mar 17 '18 at 10:35

void parse_ioctl(const char *ifname)
{
    printf("%s\n", "scarf rosari...");
    int sock;
    struct ifreq ifr;
    struct sockaddr_in *ipaddr;
    char address[INET_ADDRSTRLEN];
    size_t ifnamelen;

    /* copy ifname to ifr object */
    ifnamelen = strlen(ifname);
    if (ifnamelen >= sizeof(ifr.ifr_name)) {
        printf("error :%s\n", ifr.ifr_name);
        return ;
    }
    memcpy(ifr.ifr_name, ifname, ifnamelen);
    ifr.ifr_name[ifnamelen] = '\0';

    /* open socket */
    sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP);
    if (sock < 0) {
        printf("error :%s\n", "unable to open socket..");
        return;
    }

    /* process mac */
    if (ioctl(sock, SIOCGIFHWADDR, &ifr) != -1) {
        printf("Mac address: %02x:%02x:%02x:%02x:%02x:%02x\n",
                (unsigned char)ifr.ifr_hwaddr.sa_data[0],
                (unsigned char)ifr.ifr_hwaddr.sa_data[1],
                (unsigned char)ifr.ifr_hwaddr.sa_data[2],
                (unsigned char)ifr.ifr_hwaddr.sa_data[3],
                (unsigned char)ifr.ifr_hwaddr.sa_data[4],
                (unsigned char)ifr.ifr_hwaddr.sa_data[5]);
    }

    /* process mtu */
    if (ioctl(sock, SIOCGIFMTU, &ifr) != -1) {
        printf("MTU: %d\n", ifr.ifr_mtu);
    }

    /* die if cannot get address */
    if (ioctl(sock, SIOCGIFADDR, &ifr) == -1) {
        close(sock);
        return;
    }

    /* process ip */
    ipaddr = (struct sockaddr_in *)&ifr.ifr_addr;
    if (inet_ntop(AF_INET, &ipaddr->sin_addr, address, sizeof(address)) != NULL) {
        printf("Ip address: %s\n", address);
    }

    /* try to get broadcast */
    if (ioctl(sock, SIOCGIFBRDADDR, &ifr) != -1) {
        ipaddr = (struct sockaddr_in *)&ifr.ifr_broadaddr;
        if (inet_ntop(AF_INET, &ipaddr->sin_addr, address, sizeof(address)) != NULL) {
            printf("Broadcast: %s\n", address);
        }
    }

    /* try to get mask */
    if (ioctl(sock, SIOCGIFNETMASK, &ifr) != -1) {
        ipaddr = (struct sockaddr_in *)&ifr.ifr_netmask;
        if (inet_ntop(AF_INET, &ipaddr->sin_addr, address, sizeof(address)) != NULL) {
            printf("Netmask: %s\n", address);
        }
    }

    close(sock);
}

usage :

parse_ioctl("eth0");

using C code to get same info as ifconfig

6 Answers6

Linked

Related