ufw: added ufw and way to block specific ips

wiki/ufw.md

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+# UFW
+
+[UFW (Uncomplicated Firewall)](https://code.launchpad.net/ufw) is a program to
+manage a netfilter firewall.
+
+## Block specific IPs
+
+To block the whole traffic from a specific IP to your computer run:
+`ufw deny from <ip> to any`
+This will not send a reject response, so the target of the block won't know he
+was blocked.
+If you don't care if the target knows you can also run:
+`sudo ufw reject from <ip> to any`
+
+## List all rules
+
+To show all rules you can run:
+`ufw status`
+or:
+`ufw status numbered`
+if you want to have a list of all rules with according numbers.
+
+## Remove rules
+
+Locate the rule you want to remove by showing all rules (numbered).
+Then run:
+`ufw delete <number of rule>`