Problem about dhcp option 125

[L'oiseau de mer]

Recently i need set a dhcp sever with option 125, i use the verion 4.2.3-P2
but i don't know how to configure.

I also see this men page about vendor identify,
it says that use option vivso, and later follow a string.

So i try to these tests:
send vivso "openbsd 5";
send vivso 6F:70:65:6E:62:73:64:20:35;
option vivso 6F:70:65:6E:62:73:64:20:35;
option space openbsd code width 2 length width 2;
option openbsd.test1-location code 1 = text;
option openbsd.test2-probability code 2 = integer 8;
option vsio.openbsd code 12345 = encapsulate openbsd;

option space BSD;
option BSD.option-125 code 125 = string;
option BSD.option-125 "openbsd";

But these tests is all invalid. When i see these packets in wireshark,
i always findn't this string "openbsd".

I don't know where is this problem, anyone can't help me?
Thanks very much.