Using dig as a LLMNR or mDNS CLI Lookup utility

I was looking for a LLMNR commandline lookup utility. Actually, dig can do the job quite fine.

TL:DR

LLMNR lookup:

dig +noedns -p 5355 @224.0.0.252 wpad
resolvectl query -p llmnr wpad

mDNS lookup:

dig -p 5353 @224.0.0.251 example.local
resolvectl query -p mdns example.local
avahi-resolve-host-name example.local

Choosing the network interface:

• When dig, we can control the network interface we are querying by binding the socket (-b 192.168.1.42)¹. Without this flag, the request will use the default route to choose which network interface to use.
• With resolvectl we can choose the interface with -i.

LLMNR usage

LLMNR (Link-Local Multicast Name Resolution), RFC4795, is a Microsoft-centric DNS-based protocol for resolving names using multicast on the local network. It is expected to be used by default for single label names only:

By default, an LLMNR sender SHOULD send LLMNR queries only for single-label names. Stub resolvers supporting both DNS and LLMNR SHOULD avoid sending DNS queries for single-label names, in order to reduce unnecessary DNS queries.

In the Windows world, it is used alongside with other protocols² such as NBNS/NBT-NS (NetBios/TCP Name Service, RFC1001, RFC1002), WINS and DNS. Apparently, LLMNR is tried before NBNS but it isnot really documented as far as I know.

LLMNR is quite similar in spirit to mDNS (multicast DNS, RFC6762). mDNS is originated from Apple Bonjour where it is used with DNS-SD (DNS Service Discovery, RFC6783) and has been available on Linux systemd for a long time through Avahi. It is normally used for domain names in the local domain (eg. foo.local, foo.bar.local, etc.).

On Linux systems, in addition to Avahi, system-resolved supports both LLMNR and mDNS/DNS-SD.

LLMNR lookup with dig

I was looking for a CLI tool for resolving names with LLMNR but could not find any. In fact, dig can be the job well. LLMNR is really DNS with a few changes:

• queries requests are sent to multicast address 224.0.0.252 or ff02:0:0:0:0:0:1:3 with UDP port 5355;

• responses are sent to the requester IP address and port using the IP and port the request was sent to;

• a few flags have a different meaning.

LLMNR header:

1  1  1  1  1  1
0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                      ID                       |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR|   Opcode  | C|TC| T| Z| Z| Z| Z|   RCODE   |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    QDCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    ANCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    NSCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    ARCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

DNS header:

                                1  1  1  1  1  1
  0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                      ID                       |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR|   Opcode  |AA|TC|RD|RA|   Z    |   RCODE   |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    QDCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    ANCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    NSCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    ARCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

This means we can actually do LLMNR requests with dig. Dor example, we can do a DNS WPAD 😨 request:

dig +noedns -p 5355 @224.0.0.252 wpad

EDNS0 is disabled because the Microsoft LLMNR profile does not support EDNS0:

The Link Local Multicast Name Resolution (LLMNR) Profile [...] does not support Extension Mechanisms for DNS (EDNS0) [RFC2671].

mDNS lookup with dig

Similarly for mDNS, requests are sent to 224.0.0.251 or ff02::fb on UDP port 5353. Full mDNS queries are expected to send queries from this same IP/port tuple (and receive the answer on this same tuple):

A compliant Multicast DNS querier, which implements the rules specified in this document, MUST send its Multicast DNS queries from UDP source port 5353 (the well-known port assigned to mDNS), and MUST listen for Multicast DNS replies sent to UDP destination port 5353 at the mDNS link-local multicast address (224.0.0.251 and/or its IPv6 equivalent FF02::FB).

This cannot be done with dig: it cannot listen for answers sent to a multicast address.

However, the mDNS protocol allows for a simpler client implementation, One-Shot Multicast DNS Queries:

The most basic kind of Multicast DNS client may simply send standard DNS queries blindly to 224.0.0.251:5353, without necessarily even being aware of what a multicast address is. [...] these queries MUST NOT be sent using UDP source port 5353, since using UDP source port 5353 signals the presence of a fully compliant Multicast DNS querier, as described below.

This means we can use dig as well:

dig -p 5353 @224.0.0.251 example.local

Alternatives

Using resolvectl

Alternatively if systemd-resolved is running, resolvectl can be used:

resolvectl query -p llmnr example
resolvectl query -p mdns example.local

Using avahi

mDNS request can be done through the Avahi damone using:

avahi-resolve-host-name example.local