Set IP preference

If you enable IPv6 then that will be the default.

You can override that and set IPv4 as the default protocol:

vim /etc/gai.conf

About line 54, un-comment the following line:

#
#    For sites which prefer IPv4 connections change the last line to
#
precedence ::ffff:0:0/96  100

Enable modules

First things first! Before writing any rule you have to enable the required modules in your current O.S.

- Note -

If your server is hosted over Internet then you're probably using a custom Kernel (such as OVH, Tripnet, ...). In that case most modules are already enabled.

MODPROBE=`which modprobe`


echo -e " "		
echo -e "-----------------------------"
echo -e " Enable networking modules"
echo -e "-----------------------------"

# IPv4
echo " ... IPv4"
$MODPROBE ip_tables
$MODPROBE iptable_filter
$MODPROBE iptable_mangle
# Allow to use state match
$MODPROBE ip_conntrack

# IPv6
echo " ... IPv6"
$MODPROBE ip6_tables
$MODPROBE ip6table_filter
$MODPROBE ip6table_mangle

# Allow NAT
echo " ... NAT"
$MODPROBE iptable_nat

# Allow active / passive FTP
echo " ... FTP"
$MODPROBE ip_conntrack_ftp
$MODPROBE ip_nat_ftp

# Allow log limits
echo " ... burst limit"
$MODPROBE ipt_limit

Network features

Now that you've enable some modules, you need to choose which Network's features you're gonna use or not.

IPTABLES=`which iptables`
IP6TABLES=`which ip6tables`


echo -e " "		
echo -e "------------------------"
echo -e " Set network features"
echo -e "------------------------"
echo " ... Enable common Linux protections"
# Avoid broadcast echo
echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
# avoid TCP SYN Cookie
echo 1 > /proc/sys/net/ipv4/tcp_syncookies
# protection against bogus responses
echo 1 > /proc/sys/net/ipv4/icmp_ignore_bogus_error_responses
# Avoid IP Spoofing (discard non routable IP@)
for f in /proc/sys/net/ipv4/conf/*/rp_filter; do echo 1 > $f; done
# Avoid ICMP redirect
echo 0 > /proc/sys/net/ipv4/conf/all/accept_redirects
echo 0 > /proc/sys/net/ipv4/conf/all/send_redirects
echo 0 > /proc/sys/net/ipv6/conf/all/accept_redirects
# Avoid Source Routed
echo 0 > /proc/sys/net/ipv4/conf/all/accept_source_route
echo 0 > /proc/sys/net/ipv6/conf/all/accept_source_route

## Check TCP window 
echo 1 > /proc/sys/net/ipv4/tcp_window_scaling
## Avoid DoS
echo 30 > /proc/sys/net/ipv4/tcp_fin_timeout
echo 1800 > /proc/sys/net/ipv4/tcp_keepalive_time
## Adjust TTL value
echo 64 > /proc/sys/net/ipv4/ip_default_ttl


# Port forwarding in general
echo " ... Enable forwarding"
echo 1 > /proc/sys/net/ipv4/ip_forward
echo 1 > /proc/sys/net/ipv6/conf/all/forwarding

Default policy

Attacks types

As a reminder, there are 3 different kind of attacks:

INPUT = intrusion. Someone want something from your computer, NOW.
OUTPUT = disclosure. Someone want something from your computer or network - and you might give it LATER.
FORWARD = get access. someone want something from your network. ... Or even worse: they will use your computer to perform attacks on your behalf!

You can read more about each attack over Internet.

Linux script

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`
IP6TABLES=`which ip6tables`


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

$IP6TABLES -F
$IP6TABLES -X	
$IPTABLES -F
$IPTABLES -X

$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
$IP6TABLES -t mangle -F
$IP6TABLES -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  reject all (each redirection manual needs configuration)"

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

$IP6TABLES -P INPUT DROP
$IP6TABLES -P FORWARD DROP
$IP6TABLES -P OUTPUT DROP

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

$IP6TABLES -A INPUT -p tcp -m state --state INVALID -j DROP
$IP6TABLES -A INPUT -p udp -m state --state INVALID -j DROP
$IP6TABLES -A INPUT -p icmp -m state --state INVALID -j DROP
$IP6TABLES -A OUTPUT -p tcp -m state --state INVALID -j DROP
$IP6TABLES -A OUTPUT -p udp -m state --state INVALID -j DROP
$IP6TABLES -A OUTPUT -p icmp -m state --state INVALID -j DROP
$IP6TABLES -A FORWARD -p tcp -m state --state INVALID -j DROP
$IP6TABLES -A FORWARD -p udp -m state --state INVALID -j DROP


echo " ... Avoid spoofing and local subnets"
# Reserved addresses. We shouldn't received any packets from them!
### TODO #### Adjust these values
$IPTABLES -A INPUT -s 10.0.0.0/8 -j DROP
#$IPTABLES -A INPUT -s 172.16.0.0/12 -j DROP
#$IPTABLES -A INPUT -s 192.168.0.0/16 -j DROP
$IPTABLES -A INPUT -s 169.254.0.0/16 -j DROP
	


## Localhost
echo -e " ... Allow localhost"
$IPTABLES -A INPUT ! -i lo -s 127.0.0.0/24 -j DROP	
$IPTABLES -A OUTPUT ! -o lo -d 127.0.0.0/24 -j DROP
$IPTABLES -A FORWARD -s 127.0.0.0/24 -j DROP

$IP6TABLES -A INPUT ! -i lo -s ::1/128 -j DROP
$IP6TABLES -A OUTPUT ! -o lo -d ::1/128 -j DROP
$IP6TABLES -A FORWARD -s ::1/128 -j DROP


## IPv6 security
# No IPv4 -> IPv6 tunneling
echo " ... Do not allow IPv4 @ tunnel in IPv6 !! Use native IPv6 instead !!"
$IP6TABLES -A INPUT -s 2002::/16 -j DROP		# 6to4 tunnels
$IP6TABLES -A FORWARD -s 2002::/16 -j DROP
$IP6TABLES -A INPUT -s 2001:0::/32 -j DROP		# Teredo tunnels
$IP6TABLES -A FORWARD -s 2001:0::/32 -j DROP
	
# Block IPv6 protocol in IPv4 frames
echo " ... Block IPv6 protocol in IPv4 frames"
$IPTABLES -A INPUT -p 41 -j DROP
$IPTABLES -A OUTPUT -p 41 -j DROP
$IPTABLES -A FORWARD -p 41 -j DROP


## Stateful connections
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
	
$IP6TABLES -A INPUT -m state --state ESTABLISHED -j ACCEPT
$IP6TABLES -A FORWARD -m state --state ESTABLISHED -j ACCEPT
$IP6TABLES -A OUTPUT -m state --state ESTABLISHED -j ACCEPT	


echo -e " ... Keep RELATED connections (required for FTP by example)"
$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

$IP6TABLES -A INPUT -m state --state RELATED -j ACCEPT
$IP6TABLES -A FORWARD -m state --state RELATED -j ACCEPT
$IP6TABLES -A OUTPUT -m state --state RELATED -j ACCEPT

Protocol(s) enforcement

IPTABLES=`which iptables`
IP6TABLES=`which ip6tables`


echo -e " "		
echo -e "------------------------"
echo -e " Protocols enforcement"
echo -e "------------------------"
echo -e " ... Layer 2: ICMP v4"
# ICMP packets should not be fragmented
$IPTABLES -A INPUT --fragment -p icmp -j DROP

# Limit ICMP Flood
$IPTABLES -A INPUT -p icmp -m limit --limit 1/s --limit-burst 1 -j ACCEPT
$IPTABLES -A OUTPUT -p icmp --icmp-type 0 -j ACCEPT
$IPTABLES -A OUTPUT -p icmp --icmp-type 3 -j ACCEPT
$IPTABLES -A OUTPUT -p icmp --icmp-type 8 -j ACCEPT
	
# Avoid common attacks ... but blocks ping :(
# [Network, Host, Protocol, Port] unreacheable + [Destination Host, Destination network] prohibited
#$IPTABLES -A OUTPUT -p icmp --icmp-type 3 -j DROP


echo -e " ... Layer 2: ICMP v6 "
# Feedback for problems
$IP6TABLES -A INPUT -p icmpv6 --icmpv6-type 1 -j ACCEPT
$IP6TABLES -A INPUT -p icmpv6 --icmpv6-type 2 -j ACCEPT
$IP6TABLES -A INPUT -p icmpv6 --icmpv6-type 3 -j ACCEPT
$IP6TABLES -A INPUT -p icmpv6 --icmpv6-type 4 -j ACCEPT
	
# Router and neighbor discovery 
$IP6TABLES -A INPUT -p icmpv6 --icmpv6-type 133 -j ACCEPT
$IP6TABLES -A INPUT -p icmpv6 --icmpv6-type 134 -j ACCEPT
$IP6TABLES -A INPUT -p icmpv6 --icmpv6-type 135 -j ACCEPT
$IP6TABLES -A INPUT -p icmpv6 --icmpv6-type 136 -j ACCEPT

$IP6TABLES -A OUTPUT -p icmpv6 --icmpv6-type 133 -j ACCEPT
$IP6TABLES -A OUTPUT -p icmpv6 --icmpv6-type 134 -j ACCEPT
$IP6TABLES -A OUTPUT -p icmpv6 --icmpv6-type 135 -j ACCEPT
$IP6TABLES -A OUTPUT -p icmpv6 --icmpv6-type 136 -j ACCEPT
	
# Ping requests
$IP6TABLES -A INPUT -p icmpv6 --icmpv6-type 128 -j ACCEPT
$IP6TABLES -A OUTPUT -p icmpv6 --icmpv6-type 128 -j ACCEPT

	
echo " ... Layer 4: TCP # check packets conformity"
# INCOMING packets check
# All new incoming TCP should be SYN first
$IPTABLES -A INPUT -p tcp ! --syn -m state --state NEW -j DROP
# Avoid SYN Flood (max 3 SYN packets / second. Then Drop all requests !!)
$IPTABLES -A INPUT -p tcp --syn -m limit --limit 1/s --limit-burst 3 -j ACCEPT
# Avoid fragment packets
$IPTABLES -A INPUT -f -j DROP
# Check TCP flags -- flag 64, 128 = bogues
$IPTABLES -A INPUT -p tcp --tcp-option 64 -j DROP
$IPTABLES -A INPUT -p tcp --tcp-option 128 -j DROP
echo " ... Layer 4: TCP # Avoid NMAP Scans"
# XMAS-NULL
$IPTABLES -A INPUT -p tcp --tcp-flags ALL NONE -j DROP
# XMAS-TREE
$IPTABLES -A INPUT -p tcp --tcp-flags ALL ALL -j DROP
# SYN/RST Scan
$IPTABLES -A INPUT -p tcp --tcp-flags SYN,RST SYN,RST -j DROP
# SYN/FIN Scan
$IPTABLES -A INPUT -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP
# SYN/ACK Scan
#$IPTABLES -A INPUT -p tcp --tcp-flags ALL ACK -j DROP
$IPTABLES -A INPUT -p tcp --tcp-flags SYN,ACK SYN,ACK -j DROP
# FIN/RST Scan
$IPTABLES -A INPUT -p tcp --tcp-flags FIN,RST FIN,RST -j DROP
# FIN/ACK Scan
$IPTABLES -A INPUT -p tcp -m tcp --tcp-flags FIN,ACK FIN -j DROP
# ACK/URG Scan
$IPTABLES -A INPUT -p tcp --tcp-flags ACK,URG URG -j DROP
# FIN/URG/PSH Scan
$IPTABLES -A INPUT -p tcp --tcp-flags FIN,URG,PSH FIN,URG,PSH -j DROP
# Stealth XMAS Scan
$IPTABLES -A INPUT -p tcp --tcp-flags ALL FIN,PSH,URG -j DROP
# XMAS-PSH Scan
$IPTABLES -A INPUT -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP
# End TCP connection
$IPTABLES -A INPUT -p tcp --tcp-flags ALL FIN -j DROP
# Ports scans
$IPTABLES -A INPUT -p tcp --tcp-flags FIN,SYN,RST,ACK SYN -j DROP
$IPTABLES -A INPUT -p tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG NONE -j DROP

Allow services and network protocols

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`
IP6TABLES=`which ip6tables`


$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


$IP6TABLES -A OUTPUT -p udp --dport 53 -m limit --limit 100/s -j ACCEPT
$IP6TABLES -A OUTPUT -p udp --sport 53 -m limit --limit 100/s -j ACCEPT
$IP6TABLES -A INPUT -p udp --dport 53 -m limit --limit 100/s -j ACCEPT
$IP6TABLES -A INPUT -p udp --sport 53 -m limit --limit 100/s -j ACCEPT

LAN communication

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

IPTABLES=`which iptables`
IP_LAN_V4="172.16.50.0/24"
IP_LAN_V6="2001:DB8:1::1"


# Allow LAN communication
if [ ! -z "$IP_LAN_V4" ] 
then
 	echo -e " ... Allow LAN communication - IP v4"
	$IPTABLES -A INPUT -s $IP_LAN_V4 -d $IP_LAN_V4 -j ACCEPT
	$IPTABLES -A OUTPUT -s $IP_LAN_V4 -d $IP_LAN_V4 -j ACCEPT
        # Allow forwarding within the LAN
        $IPTABLES -A FORWARD -s $IP_LAN_V4 -j ACCEPT
fi

if [ ! -z "$IP_LAN_V6" ] 
then
	echo -e " ... Allow LAN communication - IP v6"
	$IP6TABLES -A INPUT -s $IP_LAN_V6 -d $IP_LAN_V6 -j ACCEPT
	$IP6TABLES -A OUTPUT -s $IP_LAN_V6 -d $IP_LAN_V6 -j ACCEPT
        # Allow forwarding within the LAN
        $IP6TABLES -A FORWARD -s $IP_LAN_V6 -j ACCEPT
fi

Note: thanks to the ! -z operator if the variable is not set or "" then the rule will be skipped.

NTP (time syncronization) client

IPTABLES=`which iptables`


# NTP client
echo -e " ... Allow NTP time sync"
$IPTABLES -A OUTPUT -p udp --dport 123 -j ACCEPT
$IPTABLES -A INPUT -p udp --sport 123 -j ACCEPT

$IP6TABLES -A OUTPUT -p udp --dport 123 -j ACCEPT
$IP6TABLES -A INPUT -p udp --sport 123 -j ACCEPT

Samba file-share

IPTABLES=`which iptables`


# SAMBA share
# Access filtering is done in /etc/samba/smb.conf
$IPTABLES -A INPUT -p udp --dport 137 -j ACCEPT                 # NetBios Name Service
$IPTABLES -A INPUT -p udp --dport 138 -j ACCEPT                 # NetBios Data Exchange
$IPTABLES -A INPUT -p tcp --dport 139 -j ACCEPT                 # NetBios Session + Samba
$IPTABLES -A INPUT -p tcp --dport 445 -j ACCEPT                 # CIFS - Partage Win2K and more
$IPTABLES -A INPUT -p tcp --dport 548 -j ACCEPT                 # Apple File Sharing Protocol

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

Anonymous

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Navigation

Navigation

Categories

Wiki tools

Wiki tools

Firewall basics

Namespaces

Page actions

Contents

Set IP preference

Enable modules

Network features

Default policy

Attacks types

Linux script

Protocol(s) enforcement

Allow services and network protocols

DHCP

DNS

LAN communication

NTP (time syncronization) client

Samba file-share

FTP client

Anonymous

Search

Navigation

Wiki tools

Page tools

Categories

Firewall basics

Contents

Set IP preference

Enable modules

Network features

Default policy

Attacks types

Linux script

Protocol(s) enforcement

Allow services and network protocols

DHCP

DNS

LAN communication

NTP (time syncronization) client

Samba file-share

FTP client