Firewall principle

The firewall (FW) is a key component of your server's security.

Key points

Default policy

This is how you defined a default policy.

Note:

• You have to adjust the policy to your own settings
• You should NOT set the INPUT in ACCEPT mode. That's risky!

IPTABLES=`which iptables`

echo -e " "		
echo -e "------------------------"
echo -e " Flush existing rules "
echo -e "------------------------"

$IPTABLES -t filter -F
$IPTABLES -t filter -X

# delete NAT rules
$IPTABLES -t nat -F
$IPTABLES -t nat -X

# delete MANGLE rules (packets modifications)
$IPTABLES -t mangle -F
$IPTABLES -t mangle -X

echo -e " "		
echo -e "------------------------"
echo -e " Default policy"
echo -e "------------------------"
echo -e "              || --> OUTGOING    reject all "
echo -e "          --> ||     INCOMING    reject all "
echo -e "          --> || --> FORWARDING  accept all (each redirection needs configuration)"

# INCOMING = avoid intrusions
# OUTGOING = avoid disclosure of sensitive / private data
$IPTABLES -P INPUT DROP
$IPTABLES -P FORWARD ACCEPT
$IPTABLES -P OUTPUT ACCEPT			

echo -e " ... Reject invalid packets"
$IPTABLES -A INPUT -p tcp -m state --state INVALID -j DROP
$IPTABLES -A INPUT -p udp -m state --state INVALID -j DROP
$IPTABLES -A INPUT -p icmp -m state --state INVALID -j DROP
$IPTABLES -A OUTPUT -p tcp -m state --state INVALID -j DROP
$IPTABLES -A OUTPUT -p udp -m state --state INVALID -j DROP
$IPTABLES -A OUTPUT -p icmp -m state --state INVALID -j DROP
$IPTABLES -A FORWARD -p tcp -m state --state INVALID -j DROP
$IPTABLES -A FORWARD -p udp -m state --state INVALID -j DROP

echo -e " ... Keep ESTABLISHED connections "
$IPTABLES -A INPUT -m state --state ESTABLISHED -j ACCEPT
$IPTABLES -A FORWARD -m state --state ESTABLISHED -j ACCEPT
$IPTABLES -A OUTPUT -m state --state ESTABLISHED -j ACCEPT

echo -e " ... Keep RELATED connections (required for FTP)"
$IPTABLES -A INPUT -m state --state RELATED -j ACCEPT
$IPTABLES -A FORWARD -m state --state RELATED -j ACCEPT
$IPTABLES -A OUTPUT -m state --state RELATED -j ACCEPT

# Allow localhost communication
echo -e " ... Allow localhost"
$IPTABLES -A INPUT -i lo -s 127.0.0.0/24 -d 127.0.0.0/24 -j ACCEPT
$IPTABLES -A OUTPUT -o lo -s 127.0.0.0/24 -d 127.0.0.0/24 -j ACCEPT

DHCP

DHCP client:

IPTABLES=`which iptables`

# DHCP client >> Broadcast IP request 
$IPTABLES -A OUTPUT -p udp -d 255.255.255.255 --sport 68 --dport 67 -j ACCEPT
$IPTABLES -A INPUT -p udp -s 255.255.255.255 --sport 67 --dport 68 -j ACCEPT
$IPTABLES -A OUTPUT -p udp --dport 67 -j ACCEPT 
$IPTABLES -A OUTPUT -p udp --dport 68 -j ACCEPT

DNS

This will allow your computer to perform DNS requests:

IPTABLES=`which iptables`

$IPTABLES -A OUTPUT -p udp --dport 53 -m limit --limit 100/s -j ACCEPT
$IPTABLES -A OUTPUT -p udp --sport 53 -m limit --limit 100/s -j ACCEPT
$IPTABLES -A INPUT -p udp --sport 53 -m limit --limit 100/s -j ACCEPT
$IPTABLES -A INPUT -p udp --dport 53 -m limit --limit 100/s -j ACCEPT

LAN communication

To allow communication in the local network, without any restrictions:

IPTABLES=`which iptables`
LAN_ADDRESS="172.16.50.0/24"

$IPTABLES -A INPUT -s $LAN_ADDRESS -d $LAN_ADDRESS -j ACCEPT
$IPTABLES -A OUTPUT -s $LAN_ADDRESS -d $LAN_ADDRESS -j ACCEPT

FTP client

IPTABLES=`which iptables`

# FTP client - base rules
$IPTABLES -A INPUT -p tcp --sport 21 -m state --state ESTABLISHED -j ACCEPT
$IPTABLES -A OUTPUT -p tcp --dport 21 -m state --state NEW,ESTABLISHED -j ACCEPT

# Active FTP
$IPTABLES -A INPUT -p tcp --sport 20 -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPTABLES -A OUTPUT -p tcp --dport 20 -m state --state ESTABLISHED -j ACCEPT

# Passive FTP
$IPTABLES -A INPUT -p tcp --sport 1024: --dport 1024: -m state --state ESTABLISHED -j ACCEPT
$IPTABLES -A OUTPUT -p tcp --sport 1024: --dport 1024: -m state --state ESTABLISHED,RELATED -j ACCEPT

VPN

Adjust the following to your own port, network ID and protocol:

IPTABLES=`which iptables`

INT_ETH=eth0
IP_LAN_ETH=`/sbin/ifconfig $INT_ETH | grep 'inet addr:' | cut -d: -f2 | awk '{ print $1}'`

INT_VPN=tun0
VPN_PORT="8080"
VPN_PROTOCOL="udp"
LAN_ADDRESS_VPN="172.16.60.0/24"

echo -e " "		
echo -e "------------------------"
echo -e " VPN configuration"
echo -e "------------------------"

echo " " 
echo -e "# VPN interface  : $INT_VPN"
echo -e "# VPN IP @       : $LAN_ADDRESS_VPN"
echo -e "# VPN port       : $VPN_PORT"
echo -e "# VPN protocol   : $VPN_PROTOCOL"
echo -e "-------------------------------------- "

# Allow devices communication $ETH0 <--> tun0
$IPTABLES -t nat -A POSTROUTING -s $LAN_ADDRESS_VPN -o $INT_ETH -j MASQUERADE
$IPTABLES -A FORWARD -s $LAN_ADDRESS_VPN -j ACCEPT

echo -e " ... Allow VPN connections"
$IPTABLES -A INPUT -p $VPN_PROTOCOL --dport $VPN_PORT -j ACCEPT
$IPTABLES -A OUTPUT -p tcp --dport $VPN_PORT -j ACCEPT

echo -e " ... Allow everything to go through VPN - all INPUT,OUTPUT,FORWARD"
$IPTABLES -A INPUT -i $INT_VPN -m state ! --state INVALID -j ACCEPT
$IPTABLES -A OUTPUT -o $INT_VPN -m state ! --state INVALID -j ACCEPT
$IPTABLES -A FORWARD -o $INT_VPN -m state ! --state INVALID -j ACCEPT

echo -e " ... Allow VPN network communication (required for client <> client comm.)"
$IPTABLES -A INPUT -s $LAN_ADDRESS_VPN -d $LAN_ADDRESS_VPN -j ACCEPT
$IPTABLES -A OUTPUT -s $LAN_ADDRESS_VPN -d $LAN_ADDRESS_VPN -j ACCEPT

Servers

SSH

# SSH - max 3 connection request per minute
$IPTABLES -A INPUT -p tcp -m limit 3/min --limit-burst 3 --dport 22 -j ACCEPT

DHCP

This is how you enable a DHCP server with TFTP (netBoot) :

IPTABLES=`which iptables`

# Allow LAN communication
# ... Required for NFS and the NetBoot ...
$IPTABLES -A INPUT -s $LAN_ADDRESS -d $LAN_ADDRESS -m state ! --state INVALID -j ACCEPT
$IPTABLES -A OUTPUT -s $LAN_ADDRESS -d $LAN_ADDRESS -m state ! --state INVALID -j ACCEPT
 
########################
# INPUT filters
########################
 
##### DHCP client ######
# Broadcast IP request 
$IPTABLES -A OUTPUT -p udp -d 255.255.255.255 --sport 68 --dport 67 -j ACCEPT
# Send / reply to IPs requests
$IPTABLES -A INPUT -p udp -s 255.255.255.255 --sport 67 --dport 68 -j ACCEPT
 
###### DHCP server ######
# Received client's requests [udp + tcp]
$IPTABLES -A INPUT -p udp --sport 68 --dport 67 -j ACCEPT
$IPTABLES -A INPUT -p tcp --sport 68 --dport 67 -j ACCEPT

# NetBoot - TFTP server
$IPTABLES -A INPUT -p udp -s $LAN_ADDRESS --dport 69 -j ACCEPT
 
 
########################
# OUTPUT filters
########################
# DHCP [udp + tcp]
$IPTABLES -A OUTPUT -p udp --dport 67 -j ACCEPT
$IPTABLES -A OUTPUT -p tcp --dport 67 -j ACCEPT
$IPTABLES -A OUTPUT -p udp --dport 68 -j ACCEPT
$IPTABLES -A OUTPUT -p tcp --dport 68 -j ACCEPT
 
# TFTP NetBoot 
$IPTABLES -A OUTPUT -p udp --dport 69 -j ACCEPT

Note the difference between the broadcast request that every computer should allow and the plain OUTPUT allow on ports 67,68 for the DHCP server !!