Revision as of 12:03, 7 June 2014

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

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.

The following codes and examples are based iptables and ip6tables.

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

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`
IP6TABLES=`which ip6tables`

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

$IP6TABLES -F

$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  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 ACCEPT			

$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!
$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
	
# Only localhost on Loopback interface + no forward
$IPTABLES -A INPUT ! -i lo -s 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 FORWARD -s ::1/128 -j DROP


# 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


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)"
$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

# 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

$IP6TABLES -A INPUT -i lo -j ACCEPT
$IP6TABLES -A OUTPUT -o lo  -j ACCEPT

Protocol(s) enforcement

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


echo -e " "		
echo -e "------------------------"
echo -e " Protocols enforcement"
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



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
	
# 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`
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:

Allow forwarding for the target
Open the incoming port [input + output]
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

You have to declare the following only ONCE in all your FW script:

Enable module

#### Requirement: enable port forwarding in general
echo 1 > /proc/sys/net/ipv4/conf/eth0/forwarding
echo 1 > /proc/sys/net/ipv6/conf/all/forwarding

Allow forwarding for [all,some] requests

#### Requirement: allow forwarding
# Allow all forwarding
$IPTABLES -A FORWARD -J ACCEPT
# Allow forward for some particular network or IP @
$IPTABLES -A FORWARD -s 172.16.50.0/24 -J ACCEPT
$IPTABLES -A FORWARD -s 5.39.81.23 -J ACCEPT

Perform port forwarding

You have to do the following for each computer / server you plan to serve:

####
# 1. Open target port
$IPTABLES -A INPUT -p tcp --dport 3389 -j ACCEPT
$IPTABLES -A OUTPUT -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

Source address filtering

You can restricted the access of a particular service to a limited set of source networks, IP @.

Example:

# Only allow company's offices to access our Tomcat
$IPTABLES -A INPUT -p tcp --dport 8088 -s 192.168.1.0/24 -j ACCEPT           # Sweden LAN
$IPTABLES -A INPUT -p tcp --dport 8088 -s 90.83.80.64/27 -j ACCEPT           # FR remote
$IPTABLES -A INPUT -p tcp --dport 8088 -s 90.83.80.123/27 -j ACCEPT          # FR remote
$IPTABLES -A INPUT -p tcp --dport 8088 -s 77.68.140.115/24 -j ACCEPT         # DK remote
$IPTABLES -A INPUT -p tcp --dport 8088 -s 0.0.0.0/0 -j DROP             # DROP all the rest !

Don't forget to drop all the rest at the end !!

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Difference between revisions of "Firewall principle"

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Revision as of 12:03, 7 June 2014

Contents

Principle

Enable modules

Default policy

Protocol(s) enforcement

Allow services and network protocols

DHCP

DNS

LAN communication

FTP client

VPN

Servers

SSH

DHCP

Advanced feature

Port forwarding

Principle

IpTables script

Source address filtering

Anonymous

Search

Navigation

Wiki tools

Page tools

Categories

Difference between revisions of "Firewall principle"

Revision as of 12:03, 7 June 2014

Contents

Principle

Enable modules

Default policy

Protocol(s) enforcement

Allow services and network protocols

DHCP

DNS

LAN communication

FTP client

VPN

Servers

SSH

DHCP

Advanced feature

Port forwarding

Principle

IpTables script

Source address filtering