thx alot first.
but how should I recive a packet?
example manpage gived is:

 $a->ethnew("eth0");
  $a->ethset(source => 'my.target.lan',dest =>'my.target.lan');    
  $a->ethsend;
  $p = $a->pcapinit("eth0","dst port 21",1500,30);
  $f = dump_open($p,"/my/home/log");
  loop $p,10,\&dump,$f;
[download]

where can I get a packet? when dump_open() ? Does it log the recved pack to /my/home/log?
And always when I call
$a->ethset(source => '111.111.111.2',dest =>'111.111.111.2');
error says can't look up mac addr, but if I give the max addr, it says undefind vararibles.