The Link Local Multicast Name Resolution (LLMNR) protocol


   In this article I’ll talk about the Link Local Multicast Name Resolution protocol used by Windows in discovering neighboring devices. If you don’t know by now, Windows uses several name resolution mechanisms such as NetBIOS and DNS. LLMNR is one of these naming protocols and is used when there is no DNS infrastructure. You have to know that Windows will use LLMNR instead of NetBIOS but, there are two main conditions that have to be met: devices must have IPv6 and Network Discovery enabled. LLMNR can also be configured to use the 224.0.0.252 IPv4 multicast address but, it will not use it by default.
   LLMNR relies on the IPv6 FF02::1:3 multicast address. The operating mechanism of this protocol is pretty simple: suppose you have a home environment with no DNS infrastructure implemented. Windows operating machines can still discover and communicate with each other using LLMNR. When typing in a UNC path of a network computer, LLMNR will first attempt to resolve the host’s name by using it’s cached information. If a coresponding entry is found in the memory, the computer’s name is resolved automatically. If the desired information is not found, the Link Local Multicast Name Resolution protocol will request the name of the machine using a multicast packet sent at FF02:0:0:0:0:0:1:3 (FF02::1:3) address. All computers with Network Discovery enabled will respond with a unicast packet containing their IPv6 address. At layer 2, LLMNR communicates using the 01-00-5E-00-00-FC MAC address for IPv4 and 33-33-00-01-00-03 MAC address for IPv6. LLMNR uses UDP port 5355 for multicast communications and 5455 TCP port for unicast communications.
   Remember that LLMNR can only be used between computers that are on the same subnet. Also note that this protocol is compatible with Windows Vista and Windows Server 2008 or newer versions so, if there are XP or Windows 2000 machines on your network, LLMNR cannot be used. One important aspect of this protocol is that it does not require configuration by default. Of course, you will have to enable the Network Discovery feature from Control Panel\Network and Internet\Network and Sharing Center\Advanced sharing settings:
   I hope you’ve made a general idea of this protocol. In the following articles I will talk about NetBIOS and DNS and you will better understand the naming resolution protocols used by Windows machines. I hope this article will serve you well, stay tuned for the following posts from IT training day.
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