-A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

    # Create a chain for outgoing proxy traffic
    -N PROXY_OUT
    -A OUTPUT -m owner --uid-owner proxy -j PROXY_OUT

    # Allow replies to the requestor
    -A PROXY_OUT -p tcp --sport 8080 -j ACCEPT

    # Prevent proxy from talking to anything non HTTP{,s} (and DNS)
    -A PROXY_OUT -p tcp -m multiport ! --dports 80,443,53 -j DROP
    # Allow DNS udp
    -A PROXY_OUT -p udp --dport 53 -j ACCEPT

    # Allow the proxy to specific private ips (demos servers)
    -A PROXY_OUT -d xxx.xxx.xxx.xxx/32 -j ACCEPT

    # Prevent proxy from talking to anything private
    -A PROXY_OUT ! -o <%= @public_iface %> -j DROP
    -A PROXY_OUT -d 10.0.0.0/8 -j DROP
    -A PROXY_OUT -d 172.16.0.0/12 -j DROP
    -A PROXY_OUT -d 192.168.0.0/16 -j DROP

    # Prevent proxy from talking to services via public ips
    <% @aws_public_ips.each do |name, facts| %>
    # <%= name %>
    -A PROXY_OUT -d <%= facts['ec2_public_ipv4'] %>/32 -j DROP
    <% end %>

Anything I missed? Blocking outgoing ports is to taste.

I would add:

-A PROXY_OUT -d 127.0.0.0/8 -j DROP

-A PROXY_OUT -d 169.254.0.0/16 -j DROP

That seems to cover everything in IPv4, but if you have IPv6 enabled, you probably want fd00::/8, fe80::/10 and ::1/128 as well (disclaimer: my knowledge of IPv6 is rather limited.)

For those who don't know about uid-owner. Here's how you would block any process running as the "mike" user from accessing anything on localhost:

iptables -A OUTPUT -m owner --uid-owner mike -d 127.0.0.0/8 -j REJECT

ip6tables -A OUTPUT -m owner --uid-owner mike -d ::1 -j REJECT

Also, it can be expressed without the dots. e.g. http://2130706433

that is how I got around all the internet filters in our high school. nothing makes you feel more like a hacker than punching some numbers on a calculator while all your friends watch, and bam, you're browsing forbidden websites while they all say "whoaa!"

Same thing happened to me in 1998 but in tech school. Good times. ;)

For those who are curious how the conversion was done: http://www.aboutmyip.com/AboutMyXApp/IP2Integer.jsp

Also:

  $ ping 2130706433
  PING 2130706433 (127.0.0.1): 56 data bytes
  64 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.043 ms
  64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.106 ms
  ^C

Dotted quad is just a convenient representation of an unsigned int.

localhost.fbi.gov is a great example. (Also a great way to exfiltrate fbi.gov cookies on a multi-user system, but that's another discussion completely.)