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On some Linux computers, ifconfig displays the MAC address ("HWaddr") hex digits in uppercase. On other Linux computers, the [a-f] digits are lowercase.

Why the difference? Are there two competing versions of the program? Is there an option somewhere to control it?

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Beside ifconfig being obsolete, all relevant functions don't care if a hex digit is lower or upper case. What upsets you about lower case? – ott-- Feb 6 '13 at 21:21
just curious as to why it's sometimes different. Sure my tools can use… – OJW Feb 6 '13 at 21:46
Only see uppercase on my centos systems. Can you supply distro/version for the lowercase? – ckhan Feb 6 '13 at 22:36
@ckhan which type of systems prefer uppercase? My lowercase example was Ubuntu 12.04 (tested on desktop and server editions) – OJW Feb 6 '13 at 22:38
UC was centos, obviously. ignore previous. – OJW Feb 6 '13 at 23:07
up vote 5 down vote accepted

There are several possible reasons.

 Different interface types

One possibility: ifconfig delegates printing of the hardware address to the interface implementation. Actual printing is performed by a function in the interface struct for the specific interface in net-tools-1.60 (from here), called by lib/interface.c, line 678:

printf(_("HWaddr %s  "), hw->print(ptr->hwaddr));

See the source code for nettools-1.60 in lib/hw.c for a list of supported interface types: There are separate implementations for ethernet, fiber, token ring, etc.

Now to the actual printing function: Example: lib/ether.c:

/* Display an Ethernet address in readable format. */
static char *pr_ether(unsigned char *ptr)
    static char buff[64];

    snprintf(buff, sizeof(buff), "%02X:%02X:%02X:%02X:%02X:%02X",
         (ptr[0] & 0377), (ptr[1] & 0377), (ptr[2] & 0377),
         (ptr[3] & 0377), (ptr[4] & 0377), (ptr[5] & 0377)
    return (buff);

The different kinds of network interfaces can be implemented to display the same kind of information differently by using lowercase xs in the formatting string instead (i.e. %02x instead of %02X). For example, the IrDA interface uses lowercase hex digits (lib/irda.c):

 * Function irda_print (ptr)
 *    Print hardware address of interface
static char *irda_print(unsigned char *ptr)
    static char buff[8];

    sprintf(&buff[strlen(buff)], "%02x:%02x:%02x:%02x", ptr[3], ptr[2], 
        ptr[1], ptr[0]);

    return (buff);

Patches to ifconfig

Another possibility, and likely the reason here: Distributions often change the software packages. The above code snippet is from the original net-tools-1.60, let's look at Debian's, or rather, its patch net-tools-1.60-23.diff (from here):

+--- net-tools.orig/lib/ether.c
++++ net-tools/lib/ether.c
+@@ -39,7 +39,7 @@ static char *pr_ether(unsigned char *ptr
+ {
+     static char buff[64];
+-    snprintf(buff, sizeof(buff), "%02X:%02X:%02X:%02X:%02X:%02X",
++    snprintf(buff, sizeof(buff), "%02x:%02x:%02x:%02x:%02x:%02x",
+        (ptr[0] & 0377), (ptr[1] & 0377), (ptr[2] & 0377),
+        (ptr[3] & 0377), (ptr[4] & 0377), (ptr[5] & 0377)
+   );

So Debian patched the source code to ifconfig for their distribution, which might also get picked up by Ubuntu (Wikipedia: "Ubuntu packages are based on packages from Debian's unstable branch").

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