Disable LLMNR and NBT-NS

Link-Local Multicast Name Resolution (LLMNR) (UDP/5355)

Netbios Name Service (NBT-NS) (UDP/137)

LLMNR and NBT-NS are unnecessary Windows protocols and therefore unnecessary Windows attack surfaces.

LLMNR and NBT-NS are local subnet broadcast name resolution mechanisms. If DNS is working, you don’t need these protocols. If DNS isn’t working, fix it. Since they are local subnet mechanisms they are not scalable. There is no reason to fall back upon these protocols. You would not want to learn that you were relying upon these protocols.

Additionally, there are malware kits which exploit these protocols to capture user credentials (a compromised machine on the subnet responds to a name resolution request and receives user name and the NTLMv2 hash of the credentials; use Jack the Ripper and you have the password).

How do you disable LLMNR and NBT-NS? You want to use group policies. While a registry modification would work, you want group policy to enforce the change on any systems which may have been missed, any new systems, and any unsupported maintenance.



There is a feature that will stop working when LLMNR is disabled: ad hoc networking will fail.




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