Firewall principle
The firewall (FW) is a key component of your server's security.
Contents
Principle
This is how a firewall works:
You can search Internet to get all the related theory. My aim is not to explain that over here but just to give you HOW to do that.
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 !!
Advanced feature
Port forwarding
Principle
The aim is to reach a server located behind the actual server we are working on.
To do a port forwarding you have to:
- Open the incoming port
- Register the target server and allow POST-ROUTING operations on it
- Route the incoming port to the target server + port number
Requirements:
- Enable port forwading
- The current server must be able to reach the target {server,port}
IpTables script
IPTABLES=`which iptables`
# Requirement: enable port forwarding in general
echo 1 > /proc/sys/net/ipv4/conf/eth0/forwarding
# 1. Open target port
$IPTABLES -A INPUT -p tcp --dport 3389 -j ACCEPT
# 2. Enable Port forwarding to Windows server 192.168.100.10
$IPTABLES -A POSTROUTING -d 192.168.100.10 -t nat -j MASQUERADE
# 3. Redirect services to target server
$IPTABLES -A PREROUTING -t nat -p tcp --dport 3389 -j DNAT --to 192.168.100.10:3389
$IPTABLES -A PREROUTING -t nat -p tcp --dport 81 -j DNAT --to 192.168.100.10:80
