First thing to do after installing Ubuntu 15.04 Desktop is to block unsolicited incoming network traffic.

sudo iptables -A INPUT -i lo -j ACCEPT
sudo iptables -A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
sudo iptables -A INPUT -d 224.0.0.251/32 -p udp --dport 5353 -j ACCEPT
sudo iptables -A INPUT -j DROP

To reload these rules when you reboot follow these steps:

sudo iptables-save > /etc/iptables.rules

sudo nano /etc/network/if-pre-up.d/iptables-restore
#!/bin/sh
iptables-restore /etc/iptables.rules
exit 0

sudo chmod a+x /etc/network/if-pre-up.d/iptables-restore

Test it by flushing your rules and restoring them with the script:
sudo iptables -F
sudo iptables -L -v
sudo /etc/network/if-pre-up.d/iptables-restore
sudo iptables -L -v

Now you're good to do a reboot test.. (btw only use it on a desktop you have physical access to, on a remote server you'll cut yourself off unless you open ssh too)
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