Contact

Subscribe via Email

Subscribe via RSS/JSON

Categories

Creative Commons Attribution 4.0 International License
© Rakhesh Sasidharan

Elsewhere

Resolving names using Link-local Multicast Name Resolution (LLMNR)

A neat thing about IPv6 is that IPv6 nodes can be up and running and talking to each other without much infrastructure support.

For instance, IPv6 nodes can self configure their address via SLAAC. If a network prefix is published, well and good, hosts can use that; but if not hosts will have link-local addresses (fe80::/10). How will these stateless addresses talk to each other though? As in, say the name WIN-CLIENT02 has the link-local address fe80::e070:559a:8794:8c01, how will another client WIN-CLIENT01 know thus? These are stateless addresses, not published in DNS, so how will name resolution take place?

Similarly, even if the address were manually configured – a Unique-local or Global unicast address, for instance, set by the node user – how will name resolution take place? Organizations might have DNS to help with this, but home users can’t be expected to have DNS. With IPv4, one could hope to remember the IPv4 address, but IPv6 addresses are not human-friendly so some form of help is required.

This is where Link-Local Multicast Name Resolution (LLMNR) comes into the picture. It is a way of name resolution that works for the link-local scope only and uses multicast.

What is LLMNR?

All nodes on a link subscribe to a multicast address called ff02:1:3. From my previous post you’ll know what this means – ff00::/8 is the prefix for multicast addresses; 2 indicates the address has a link-local scope; and 1:3 is the group ID for LLMNR hosts. So the way things work is that when WIN-CLIENT01 wants to resolve the name of WIN-CLIENT02, it sends an LLMNR query (basically a UDP packet similar to a DNS query; remember multicast only works with UDP) to port 5355 of the ff02:1:3 multicast address. Since WIN-CLIENT02 is subscribed to this group it receives this query and responds with its addresses.

Here’s a Wireshark capture when one of my clients is trying to ping WIN-CLIENT02 and hence look up its name. First it queries DNS (step 5350), doesn’t get an answer (step 5353), so it sends a multicast query to ff02:1:3 asking for the IP address (step 5354). Note that the multicast query is sent to the IPv4 multicast address too (224.0.0.252) for the IPv6 AAAA record – this is because I had asked ping to only use the IPv6 address.

llmnr-reply

(The Wireshark output only shows LLMNR and DNS messages as I am filtering for traffic to ports 5355 and 53).

WIN-CLIENT02 replies (step 5357) and its reply has all its IPv6 addresses.

llmnr-reply2

Suppose I hadn’t insisted ping use the IPv6 address, the above queries would be for the A record instead (including the one to the IPv6 multicast address). And when no replies come from WIN-CLIENT02, NetBIOS over TCP/IP (NetBT) will be used to try and find the IPv4 address.

llmnr-no-answer

While playing with this I realized that by default Windows clients do not respond to LLMNR queries. This is because Network Discovery is turned off – even for Domain connected interfaces. So if you too find that LLMNR queries are being sent but not responded to, check the Network Discovery setting on your machine (go to “Network and Sharing Center” in the Control Panel > “Change Advances Sharing Settings” > and turn on Network Discovery for whichever network profile you are connected to) .

You can also enable Network Discovery via GPO for domain joined computers. The setting is at Computer Policy\Policies\Administrative Templates\Network\Link-Layer Topology Discovery – there are two settings there, the second one is what you want.

Multicast MAC address

On an unrelated note, just like Unicast IPs have a corresponding MAC address, so do Multicast IPs. They have a special MAC address that start with 33:33 and the remaining four octets (group of 8 bits or group of 2 Hex digits each) are the last four octets of the Multicast IP address. So for a multicast IPv6 address ff02::1:3 – which can be expanded as ff02::0001:0003 – who last four octets are 00, 01, 00, and 03, the MAC address would be 33:33:00:01:00:03. So all hosts subscribed to this multicast address will create a special MAC address on their interfaces so packets sent to this MAC are passed on to the upper layers for processing. That’s how multicast works behind the scenes.

On Windows machines, you can use the arp command to view IPv4 address to MAC address mappings. But arp doesn’t work for IPv6 as there’s no ARP protocol in IPv6. Instead we have the Neighbor Discovery Protocol (NDP) and to see the mappings discovered through that we have to use netsh. Here’s the full command:

In the output you can see many other MAC addresses starting with 33:33 for the various IPv6 multicast groups the node is automatically a member of.

mac-33