Aikido, programming, system administration, and other things I find interesting

Generating EUID IPv6 address from a MAC

When you set up radvd (router advertisement daemon) on a network, all the computers that support IPv6 will adopt addresses in the advertised address space – except the machine that actually advertises the address prefix (thr router, if you will).

You can assign an address to the interface yourself, any address will do. For instance, you could make it ::1 so that you always know where the router is, or you could compose an amusing string out of letters and numbers (a-f, 0 for o, 1 for i or l, etc). Or you can assign it the UID address based on the interface’s MAC (like the other interfaces get). The algorithm to calculate the automatic address from the MAC is implemented in this Perl script:

#!/usr/bin/perl -w
use Term::ReadLine;
sub mac_euid {
   my $mac = shift;
   my ($b1,$b2,$b3,$b4,$b5,$b6) =
   $b1 = hex($b1) ^ 2;
   return sprintf "%x%x:%xff:fe%x:%x%x\n",$b1,hex($b2),hex($b3),hex($b4),
my $term = Term::ReadLine->new('MAC to EUID');
my $prompt = "Enter MAC:";
my $OUT = $term->OUT || \*STDOUT;
while ( defined ($_ = $term->readline($prompt)) ) {
   print "Invalid MAC $_\n"
   unless ($_=~/^([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2})$/i);
   my $res = mac_euid($_);
   print sprintf "EUID = %s\n",$res;

If the address is being configured in /etc/network/interfaces (Debian and Ubuntu based computers), you need four lines of configuration (prefix is the prefix for the subnet you are working for, and euid is the EUID part generated from the MAC):

up ip -6 addr add (prefix):(euid)/128 dev ethX
up ip -6 route add (prefix)::0/64 dev ethX
down ip -6 addr del (prefix):(euid)/128 dev ethX
down ip -6 route del (prefix)::0/64 dev ethX


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