Makes mDNS ddos amplification attack possible

Apparently mDNS can be used for ddos amplification, see for instance https://mdns.shadowserver.org/ and https://www.us-cert.gov/ncas/alerts/TA14-017A

Steps to reproduce:

dig @rusk.hpc2n.umu.se -p 5353 -t ptr _services._dns-sd._udp.local

The response is supposedly 2-10 times the size of the query, making for a moderate but noticeable amplification.

Workarounds are easy, but not responding outside localnet by default is probably reasonable for mDNS.

Reproduced at at least trusty and precise, would be very surprised if it didn't also apply to xenial but I left my xenial laptop at home today. :)

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: avahi-daemon 0.6.30-5ubuntu2.1
ProcVersionSignature: Ubuntu 3.13.0-83.127~precise1-generic 3.13.11-ckt35
Uname: Linux 3.13.0-83-generic x86_64
NonfreeKernelModules: openafs
ApportVersion: 2.0.1-0ubuntu17.13
Architecture: amd64
Date: Fri Apr 15 12:12:22 2016
MarkForUpload: True
ProcEnviron:
 LANGUAGE=en_US:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: avahi
UpgradeStatus: No upgrade log present (probably fresh install)