Multicast DNS (mDNS) is a way for devices on a local link network to automatically discover other services and devices. In some implementations of mDNS, the mDNS server replies to unicast queries from outside the link local network (e.g., the WAN). This mDNS response may result in information disclosure of devices on the network. Furthermore, the information returned in the response is greater in size than the query and may be used for denial-of-service (DoS) amplification.

RFC 6762 Section 5.5 states the following:

"In specialized applications there may be rare situations where it
  makes sense for a Multicast DNS querier to send its query via unicast
  to a specific machine.  When a Multicast DNS responder receives a
  query via direct unicast, it SHOULD respond as it would for "QU"
  questions, as described above in Section 5.4.  Since it is possible
  for a unicast query to be received from a machine outside the local
  link, responders SHOULD check that the source address in the query
  packet matches the local subnet for that link (or, in the case of
  IPv6, the source address has an on-link prefix) and silently ignore
  the packet if not.

   There may be specialized situations, outside the scope of this
  document, where it is intended and desirable to create a responder
  that does answer queries originating outside the local link."

While unicast queries originating from outside the local link are not specifically disallowed, RFC 6762 recommends to ignore any such packets. Some implementations of mDNS do however respond to unicast queries originating outside the local link, possibly for specialized use cases beyond the scope of RFC 6762. 

In these circumstances, the mDNS response to a query from outside the local link allows for information disclosure about devices on the network, such as model number and operating system.

Additionally, the mDNS response to a query from outside the local link may be used for denial of service amplification attacks, due to the larger response size compared to the query size.

More information can be found in security researcher's blog.

An mDNS response to a unicast query originating outside of the local link network may result in information disclosure, such as disclosing the device type/model that responds to the request or the operating system running such software. The mDNS response may also be used to amplify denial of service attacks against other networks.

Block inbound and outbound mDNS on the WAN

If such mDNS behavior is not a requirement for your organization, consider blocking the mDNS UDP port 5353 from entering or leaving your local link network.

Disable mDNS services

Some software and devices may allow disabling of the mDNS services. Please consult with the vendor of your product.

Despite attempts to analyze scan results, it is not entirely clear exactly which software responds to mDNS queries. Vendors have been alerted, but currently only a small number of devices have been confirmed to respond to unicast queries from the WAN. In Linux, the Avahi software is also known to allow unicast queries.

Listed below are vendors that are affected, in the sense that their software or devices by default can respond to unicast queries from outside the link local network. While this technically follows established RFCs and is not a vulnerability in the normal sense, for reasons outlined above this may be unwanted behavior. If you are aware of a software or device that responds to mDNS unicast queries from outside the local link, please contact us.