Stuff

#!/bin/sh

IPTABLES=/sbin/iptables
AWK=/usr/bin/awk
IFCONFIG=/sbin/ifconfig


# External (Internet-facing) interface
EXTIF="eth0"

# External IP address (autmatically detected)
EXTIP="`$IFCONFIG $EXTIF | $AWK /$EXTIF/'{next}//{split($0,a,":");split(a[2],a," ");print a[1];exit}'`"
 
# Internal interface
INTIF="br0"

# Internal IP address (in CIDR notation)
INTIP="10.0.0.1/32"

# Internal network address (in CIDR notation)
INTNET="10.0.0.0/24"

# The address of anything/everything (in CIDR notation)
UNIVERSE="0.0.0.0/0"


echo "External Interface: $EXTIF"
echo "       External IP: $EXTIP"
echo "Internal Interface: $INTIF"
echo "  Internal Network: $INTNET"
echo "       Internal IP: $INTIP"

echo
echo "Loading rules..."

# Enabling IP forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward


# Clear any existing rules and set the default policy to DROP
$IPTABLES -P INPUT DROP
$IPTABLES -F INPUT 
$IPTABLES -P OUTPUT DROP
$IPTABLES -F OUTPUT 
$IPTABLES -P FORWARD DROP
$IPTABLES -F FORWARD 
$IPTABLES -F -t nat

# Delete all User-specified chains
$IPTABLES -X

# Reset all IPTABLES counters
$IPTABLES -Z

###################################################
# INPUT: Incoming traffic from various interfaces #
###################################################

# Loopback interface is valid
$IPTABLES -A INPUT -i lo -s $UNIVERSE -d $UNIVERSE -j ACCEPT


# Local interface, local machines, going anywhere is valid
$IPTABLES -A INPUT -i $INTIF -s $INTNET -d $UNIVERSE -j ACCEPT


# Remote interface, claiming to be local machines, IP spoofing, get lost
$IPTABLES -A INPUT -i $EXTIF -s $INTNET -d $UNIVERSE -j REJECT


# External interface, from any source, for ICMP traffic is valid
$IPTABLES -A INPUT -i $EXTIF -p ICMP -s $UNIVERSE -d $EXTIP -j ACCEPT


# Allow any related traffic coming back to the MASQ server in.
$IPTABLES -A INPUT -i $EXTIF -s $UNIVERSE -d $EXTIP -m state --state ESTABLISHED,RELATED -j ACCEPT


# Internal interface, DHCP traffic accepted
$IPTABLES -A INPUT -i $INTIF -p tcp --sport 68 --dport 67 -j ACCEPT
$IPTABLES -A INPUT -i $INTIF -p udp --sport 68 --dport 67 -j ACCEPT


# External interface, HTTP/HTTPS traffic allowed
$IPTABLES -A INPUT -i $EXTIF -m state --state NEW,ESTABLISHED,RELATED -p tcp -s $UNIVERSE -d $EXTIP --dport 80 -j ACCEPT
$IPTABLES -A INPUT -i $EXTIF -m state --state NEW,ESTABLISHED,RELATED -p tcp -s $UNIVERSE -d $EXTIP --dport 443 -j ACCEPT

# External interface, SSH traffic allowed
$IPTABLES -A INPUT -i $EXTIF -m state --state NEW,ESTABLISHED,RELATED -p tcp -s $UNIVERSE -d $EXTIP --dport 22 -j ACCEPT


# Catch-all rule, reject anything else
$IPTABLES -A INPUT -s $UNIVERSE -d $UNIVERSE -j REJECT


####################################################
# OUTPUT: Outgoing traffic from various interfaces #
####################################################

# Workaround bug in netfilter
$IPTABLES -A OUTPUT -m state -p icmp --state INVALID -j DROP

# Loopback interface is valid.
$IPTABLES -A OUTPUT -o lo -s $UNIVERSE -d $UNIVERSE -j ACCEPT


# Local interfaces, any source going to local net is valid
$IPTABLES -A OUTPUT -o $INTIF -s $EXTIP -d $INTNET -j ACCEPT


# local interface, MASQ server source going to the local net is valid
$IPTABLES -A OUTPUT -o $INTIF -s $INTIP -d $INTNET -j ACCEPT


# outgoing to local net on remote interface, stuffed routing, deny
$IPTABLES -A OUTPUT -o $EXTIF -s $UNIVERSE -d $INTNET -j REJECT


# anything else outgoing on remote interface is valid
$IPTABLES -A OUTPUT -o $EXTIF -s $EXTIP -d $UNIVERSE -j ACCEPT


# Internal interface, DHCP traffic accepted
$IPTABLES -A OUTPUT -o $INTIF -p tcp -s $INTIP --sport 67 -d 255.255.255.255 --dport 68 -j ACCEPT
$IPTABLES -A OUTPUT -o $INTIF -p udp -s $INTIP --sport 67 -d 255.255.255.255 --dport 68 -j ACCEPT


# Catch all rule, all other outgoing is denied and logged. 
$IPTABLES -A OUTPUT -s $UNIVERSE -d $UNIVERSE -j REJECT


# ----- Begin OPTIONAL FORWARD Section -----


# ----- End OPTIONAL FORWARD Section -----


$IPTABLES -A FORWARD -i $EXTIF -o $INTIF -m state --state ESTABLISHED,RELATED  -j ACCEPT
$IPTABLES -A FORWARD -i $INTIF -o $EXTIF -j ACCEPT

$IPTABLES -A FORWARD -j REJECT

$IPTABLES -t nat -A POSTROUTING -o $EXTIF -j SNAT --to $EXTIP


echo "Done."