How do I get the MAC/physical/hardware address? (using C)

[sfp-a7x]

Hi all,

I'm stumped on this one. I'm trying to write a C program for *nix (specifically Linux and BSD) that grabs the MAC address (a.k.a. physical or hardware address) for each network adapter on the machine.

I can get a pointer to the bytes that contain the hardware address, but the problem is I don't know how long the hardware address is. For Ethernet, I know that it's 6 bytes long, but other kinds of adapters have different hardware address lengths. I have the ARPHRD_* value (see <net/if_arp.h> ), which tells me what kind of adapter it is. How can I use this value to determine the number of bytes in the hardware address? Unfortunately, "man 7 netdevice" doesn't tell me what I need to know.

Here's some minimal code:

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#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <string.h>

int main(int argc, char** argv) {
     int sd;
     struct ifreq ifr;
     unsigned char* hw_address;
     int hw_address_len;
     int i;

     /* make socket */
     sd = socket(PF_INET, SOCK_DGRAM, 0);
     if (sd == -1) {
          /* insert freak-out code here */
     }

     /* set interface name (lo, eth0, eth1,..) */
     memset(&ifr, 0, sizeof(ifr));
     strncpy(ifr.ifr_name, "eth0", IFNAMSIZ);

     /* get interface hardware address */
     ioctl(sd, SIOCGIFHWADDR, &ifr);
     
     close(sd);
     
     /* get hardware address and length */
     hw_address = ifr.ifr_hwaddr.sa_data;
     hw_address_len = 6; /* <- what goes here to make it
                            non-Ethernet-specific? */

     /* print out the type of hardware */
     printf("ARPHRD_* value of device (see <net/if_arp.h>):  %i\n",
            ifr.ifr_hwaddr.sa_family);

     /* print out hardware address */
     printf("Hardware address:  ");
     for (i = 0; i < hw_address_len; i++) {
          printf("%.2x%s", hw_address[i], i==hw_address_len-1 ? "" : ":");
     }
     printf("\n");

     return(0);
}

Thanks!

[lorenb]

ifr_hwaddr is a sockaddr structure:

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struct sockaddr {
   u_char sa_len;                        /* total length */
   u_char sa_family;                   /* address family */
   char sa_data[14];                   /* address data */
}

Try the ifr.ifr_hwaddr.sa_len variable.

[sfp-a7x]

Try the ifr.ifr_hwaddr.sa_len variable.

Thanks for the suggestion. I tried it out and got:

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error: structure has no member named `sa_len'

Looking in sys/socket.h (which leads me to bits/socket.h and bits/sockaddr.h), I see:

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struct sockaddr
  { 
    unsigned short int sa_family;
    char sa_data[14];
  };

I peeked at the two bytes in sa_family by doing this:

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fprintf(stderr, "Contents of sa_data:  ");
for (i = -2; i < 14; i++) {
    fprintf(stderr, "%2.2hhx%s", ifr.ifr_hwaddr.sa_data[i], i==13 ? "" : ":");
}
fprintf(stderr, "\n");

and got 01:00 for the first two bytes (the two before sa_data). Neither one can be length... Maybe I should have looked at the two bytes after sa_data? I'm not sure how it's stored in memory... ARPHRD_ETHER equals 1, so I imagine I got the correct two bytes (I'm on a little-endian machine with an Ethernet device).

Thoughts?

Thanks!

[lorenb]

Hmmm. Looking at net-tools-1.60 specifically at the ifconfig code it looks like they keep a table of known hardware types and the lengths. Then they reference it as needed. So maybe you can't determine it at runtime.

Take a look at:

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net-tools-1.60/lib/interface.c -- see the function ife_print_long
net-tools-1.60/lib/hw.c
net-tools-1.60/lib/net-support.h

[sfp-a7x]

Hmmm. Looking at net-tools-1.60 specifically at the ifconfig code it looks like they keep a table of known hardware types and the lengths. Then they reference it as needed. So maybe you can't determine it at runtime.

<snip>

Thanks for the pointer! It's good to know that such a standard tool doesn't have a way to determine length at runtime because that means I can stop searching.

I'll examine the code and see if it's something I can easily incorporate into my code.

Thanks again!