Disabling NetBIOS for Windows via DHCP option

coreybrett · May 31, 2017, 3:35 PM

Does anyone know how to disable NetBIOS for Windows clients via DHCP?

I've tried creating an "Additional BOOTP/DHCP Option" via the GUI of number 46, type Unsigned 8-bit int and value 2.

With the above, there doesn't seem to be any difference, Windows is still using NetBIOS after a DHCP renewal.

johnpoz · May 31, 2017, 4:19 PM

number 46 would be used to set your netbios node type, not disable it. To disable it you would need to use option 43 would you not? Vendor specific stuff!

The format is a bit odd. And you need to set the first part vendor option code to 0x01 and length to 0x04 and then the actual value of 0x00000002, so with pfsense dhcp you could set that via options and string and then 01:04:00:00:00:02

So did a quick test and yup disables.

So set option, then looked and yup enabled - did a ipconfig /renew and then shows disabled.

I then removed the setting, another /renew and its back. Keep in mind your client has to be set to use dhcp option which should be the default, but if its hard coded enabled on the client this setting wont do anything

edit: here even grabbed sniff of the dhcp ack when I did a renew and you can see the 43 specific option being sent - 3rd pic

coreybrett · May 31, 2017, 5:40 PM

Thanks for the help!

Setting Option 46 (NetBIOS Node Type) to a value of 0x2 (P-node) was my objective.

I guess option 46 vs 43 is where I am lacking understanding.

Are your settings for 43 changing the node type to P-node, and if so, what's the point of 46?

johnpoz · May 31, 2017, 6:02 PM

43 disables netbios over tcp.. This is what your post stated

"Disabling NetBIOS for Windows via DHCP option"

Yes option 46 would be used to set the node type, P, B, M or Hybrid.. So what exactly is your question, are you trying to disable it - ie the old ports 139, etc. and just use over 445.. Or are you trying to set the node type of netbios?

So you want to use P node.. You do understand P node points to a WINS server only - you are still running wins? Wow ;)

If what your wanting is to set node type to P.. Give me second.

edit:
here you go… You see mine was hybrid, set dhcp option 46 with string 02, and then /renew and shazam p-node (peer-to-peer).. But for that do do any good you would need to be running a wins server.. Which really has little use any more..

coreybrett · May 31, 2017, 6:01 PM

Ok, sorry, my brain was stuck on something, I thought I had read somewhere that setting the node type to 0x2 was what disabled NetBIOS. Obviously not.

43 with your settings is exactly what I needed. Thank you.

johnpoz · May 31, 2017, 6:09 PM

Great glad could be of help.. I just set mine to b-node.. Little use of any of the other nodes.. But broadcast still comes in handy now and then. Normally default is H-node.. Unless your talking old school which use to default to b if I recall correctly - lot of years ago playing with this stuff.. Kind of walk down memory lane ;)