Difference between revisions of "Firewall principle"
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[[Category:Linux]]
 
The firewall (FW) is a key component of your server's security.
 
The firewall (FW) is a key component of your server's security.
  
 +
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 setup a Firewall with IpTables.
  
 +
The following codes and examples are based '''iptables''' and '''ip6tables'''.
  
−
=Key points=
 
  
  
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==Default policy==
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=Principle=
  
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This is how you defined a default policy.
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This is how a firewall works:
  
 +
[[File:FW_principle.png|none|FW principle]]
  
−
Note:
 
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* You have to adjust the policy to your own settings
 
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* You should NOT set the INPUT in ACCEPT mode. That's risky!
 
  
  
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<syntaxhighlight lang="bash">
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This is how you can use a firewall at home | work:
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IPTABLES=`which iptables`
 
  
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echo -e " "
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[[File:FW_usage.png|none|FW usage]]
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echo -e "------------------------"
 
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echo -e " Flush existing rules "
 
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echo -e "------------------------"
 
  
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$IPTABLES -t filter -F
 
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$IPTABLES -t filter -X
 
  
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# delete NAT rules
 
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$IPTABLES -t nat -F
 
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$IPTABLES -t nat -X
 
  
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# delete MANGLE rules (packets modifications)
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The firewall performs:
−
$IPTABLES -t mangle -F
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* Basic security check
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$IPTABLES -t mangle -X
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* Security policy's enforcement
 +
* IP, port filtering
 +
* Port forwarding and NAT
  
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echo -e " "
 
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echo -e "------------------------"
 
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echo -e " Default policy"
 
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echo -e "------------------------"
 
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echo -e "              || --> OUTGOING    reject all "
 
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echo -e "          --> ||    INCOMING    reject all "
 
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echo -e "          --> || --> FORWARDING  accept all (each redirection needs configuration)"
 
  
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# INCOMING = avoid intrusions
 
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# OUTGOING = avoid disclosure of sensitive / private data
 
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$IPTABLES -P INPUT DROP
 
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$IPTABLES -P FORWARD ACCEPT
 
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$IPTABLES -P OUTPUT ACCEPT
 
  
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echo -e " ... Reject invalid packets"
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=Next steps=
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$IPTABLES -A INPUT -p tcp -m state --state INVALID -j DROP
 
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$IPTABLES -A INPUT -p udp -m state --state INVALID -j DROP
 
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$IPTABLES -A INPUT -p icmp -m state --state INVALID -j DROP
 
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$IPTABLES -A OUTPUT -p tcp -m state --state INVALID -j DROP
 
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$IPTABLES -A OUTPUT -p udp -m state --state INVALID -j DROP
 
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$IPTABLES -A OUTPUT -p icmp -m state --state INVALID -j DROP
 
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$IPTABLES -A FORWARD -p tcp -m state --state INVALID -j DROP
 
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$IPTABLES -A FORWARD -p udp -m state --state INVALID -j DROP
 
  
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echo -e " ... Keep ESTABLISHED connections "
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See [[Template:Menu_content_security#Firewall]]
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$IPTABLES -A INPUT -m state --state ESTABLISHED -j ACCEPT
 
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$IPTABLES -A FORWARD -m state --state ESTABLISHED -j ACCEPT
 
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$IPTABLES -A OUTPUT -m state --state ESTABLISHED -j ACCEPT
 
  
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echo -e " ... Keep RELATED connections (required for FTP)"
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=Note=
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$IPTABLES -A INPUT -m state --state RELATED -j ACCEPT
 
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$IPTABLES -A FORWARD -m state --state RELATED -j ACCEPT
 
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$IPTABLES -A OUTPUT -m state --state RELATED -j ACCEPT
 
  
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# Allow localhost communication
+
All this knowledge is the result of years of experiments and trials. Don't hesitate to search over Internet for more advanced || simpler rules.
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echo -e " ... Allow localhost"
 
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$IPTABLES -A INPUT -i lo -s 127.0.0.0/24 -d 127.0.0.0/24 -j ACCEPT
 
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$IPTABLES -A OUTPUT -o lo -s 127.0.0.0/24 -d 127.0.0.0/24 -j ACCEPT
 
  
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</syntaxhighlight>
 
  
  
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==DHCP==
 
  
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DHCP client:
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=References=
  
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<syntaxhighlight lang="bash">
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* My education at Telecom Bretagne [http://www.telecom-bretagne.eu/] && Chalmers [http://www.chalmers.se/en/]
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IPTABLES=`which iptables`
 
  
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# DHCP client >> Broadcast IP request
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* How to set IPv4 as default: http://bruteforce.gr/make-apt-get-use-ipv4-instead-ipv6.html
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$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
 
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$IPTABLES -A OUTPUT -p udp --dport 67 -j ACCEPT
 
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$IPTABLES -A OUTPUT -p udp --dport 68 -j ACCEPT
 
−
</syntaxhighlight>
 
  
−
 
+
* Working years and some co-workers help:  
−
==DNS==
+
** Julien Rialland
−
 
 
−
This will allow your computer to perform DNS requests:
 
−
 
 
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<syntaxhighlight lang="bash">
 
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IPTABLES=`which iptables`
 
−
 
 
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$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
 
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$IPTABLES -A INPUT -p udp --dport 53 -m limit --limit 100/s -j ACCEPT
 
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</syntaxhighlight>
 
−
 
 
−
 
 
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==LAN communication==
 
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To allow communication in the local network, without any restrictions:
 
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<syntaxhighlight lang="bash">
 
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IPTABLES=`which iptables`
 
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LAN_ADDRESS="172.16.50.0/24"
 
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$IPTABLES -A INPUT -s $LAN_ADDRESS -d $LAN_ADDRESS -j ACCEPT
 
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$IPTABLES -A OUTPUT -s $LAN_ADDRESS -d $LAN_ADDRESS -j ACCEPT
 
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</syntaxhighlight>
 
−
 
 
−
 
 
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==FTP client==
 
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<syntaxhighlight lang="bash">
 
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IPTABLES=`which iptables`
 
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# FTP client - base rules
 
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$IPTABLES -A INPUT -p tcp --sport 21 -m state --state ESTABLISHED -j ACCEPT
 
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$IPTABLES -A OUTPUT -p tcp --dport 21 -m state --state NEW,ESTABLISHED -j ACCEPT
 
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# Active FTP
 
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$IPTABLES -A INPUT -p tcp --sport 20 -m state --state ESTABLISHED,RELATED -j ACCEPT
 
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$IPTABLES -A OUTPUT -p tcp --dport 20 -m state --state ESTABLISHED -j ACCEPT
 
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# Passive FTP
 
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$IPTABLES -A INPUT -p tcp --sport 1024: --dport 1024: -m state --state ESTABLISHED -j ACCEPT
 
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$IPTABLES -A OUTPUT -p tcp --sport 1024: --dport 1024: -m state --state ESTABLISHED,RELATED -j ACCEPT
 
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</syntaxhighlight>
 
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−
 
 
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==VPN==
 
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Adjust the following to your own port, network ID and protocol:
 
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<syntaxhighlight lang="bash">
 
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IPTABLES=`which iptables`
 
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INT_ETH=eth0
 
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IP_LAN_ETH=`/sbin/ifconfig $INT_ETH | grep 'inet addr:' | cut -d: -f2 | awk '{ print $1}'`
 
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INT_VPN=tun0
 
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VPN_PORT="8080"
 
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VPN_PROTOCOL="udp"
 
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LAN_ADDRESS_VPN="172.16.60.0/24"
 
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echo -e " "
 
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echo -e "------------------------"
 
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echo -e " VPN configuration"
 
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echo -e "------------------------"
 
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echo " "
 
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echo -e "# VPN interface  : $INT_VPN"
 
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echo -e "# VPN IP @      : $LAN_ADDRESS_VPN"
 
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echo -e "# VPN port      : $VPN_PORT"
 
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echo -e "# VPN protocol  : $VPN_PROTOCOL"
 
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echo -e "-------------------------------------- "
 
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# Allow devices communication $ETH0 <--> tun0
 
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$IPTABLES -t nat -A POSTROUTING -s $LAN_ADDRESS_VPN -o $INT_ETH -j MASQUERADE
 
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$IPTABLES -A FORWARD -s $LAN_ADDRESS_VPN -j ACCEPT
 
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echo -e " ... Allow VPN connections"
 
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$IPTABLES -A INPUT -p $VPN_PROTOCOL --dport $VPN_PORT -j ACCEPT
 
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$IPTABLES -A OUTPUT -p tcp --dport $VPN_PORT -j ACCEPT
 
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echo -e " ... Allow everything to go through VPN - all INPUT,OUTPUT,FORWARD"
 
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$IPTABLES -A INPUT -i $INT_VPN -m state ! --state INVALID -j ACCEPT
 
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$IPTABLES -A OUTPUT -o $INT_VPN -m state ! --state INVALID -j ACCEPT
 
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$IPTABLES -A FORWARD -o $INT_VPN -m state ! --state INVALID -j ACCEPT
 
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echo -e " ... Allow VPN network communication (required for client <> client comm.)"
 
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$IPTABLES -A INPUT -s $LAN_ADDRESS_VPN -d $LAN_ADDRESS_VPN -j ACCEPT
 
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$IPTABLES -A OUTPUT -s $LAN_ADDRESS_VPN -d $LAN_ADDRESS_VPN -j ACCEPT
 
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</syntaxhighlight>
 
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−
 
 
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=Servers=
 
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==SSH==
 
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<syntaxhighlight lang="bash">
 
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# SSH - max 3 connection request per minute
 
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$IPTABLES -A INPUT -p tcp -m limit 3/min --limit-burst 3 --dport 22 -j ACCEPT
 
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</syntaxhighlight>
 
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−
 
 
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==DHCP==
 
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This is how you enable a DHCP server with TFTP (netBoot) :
 
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<syntaxhighlight lang="bash">
 
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IPTABLES=`which iptables`
 
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# Allow LAN communication
 
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# ... Required for NFS and the NetBoot ...
 
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$IPTABLES -A INPUT -s $LAN_ADDRESS -d $LAN_ADDRESS -m state ! --state INVALID -j ACCEPT
 
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$IPTABLES -A OUTPUT -s $LAN_ADDRESS -d $LAN_ADDRESS -m state ! --state INVALID -j ACCEPT
 
−
 
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########################
 
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# INPUT filters
 
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########################
 
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##### DHCP client ######
 
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# Broadcast IP request
 
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$IPTABLES -A OUTPUT -p udp -d 255.255.255.255 --sport 68 --dport 67 -j ACCEPT
 
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# Send / reply to IPs requests
 
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$IPTABLES -A INPUT -p udp -s 255.255.255.255 --sport 67 --dport 68 -j ACCEPT
 
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###### DHCP server ######
 
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# Received client's requests [udp + tcp]
 
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$IPTABLES -A INPUT -p udp --sport 68 --dport 67 -j ACCEPT
 
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$IPTABLES -A INPUT -p tcp --sport 68 --dport 67 -j ACCEPT
 
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# NetBoot - TFTP server
 
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$IPTABLES -A INPUT -p udp -s $LAN_ADDRESS --dport 69 -j ACCEPT
 
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########################
 
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# OUTPUT filters
 
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########################
 
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# DHCP [udp + tcp]
 
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$IPTABLES -A OUTPUT -p udp --dport 67 -j ACCEPT
 
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$IPTABLES -A OUTPUT -p tcp --dport 67 -j ACCEPT
 
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$IPTABLES -A OUTPUT -p udp --dport 68 -j ACCEPT
 
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$IPTABLES -A OUTPUT -p tcp --dport 68 -j ACCEPT
 
−
 
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# TFTP NetBoot
 
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$IPTABLES -A OUTPUT -p udp --dport 69 -j ACCEPT
 
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</syntaxhighlight>
 
−
 
 
−
 
 
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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 !!
 
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=Advanced feature=
 
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==Port forwarding==
 
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===Principle===
 
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The aim is to reach a server located behind the actual server we are working on.
 
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[[File:FW_port_forwarding.png|none|FW port forwarding - without NAT]]
 
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[[File:FW_port_forwarding_with_NAT.png|none|FW port forwarding - with NAT]]
 
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To do a port forwarding you have to:
 
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* Open the incoming port
 
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* Register the target server and allow POST-ROUTING operations on it
 
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* Route the incoming port to the target server + port number
 
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Requirements:
 
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* Enable port forwading
 
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* The current server must be able to reach the target {server,port}
 
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−
 
 
−
 
 
−
===IpTables script===
 
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−
<syntaxhighlight lang="bash">
 
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IPTABLES=`which iptables`
 
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# Requirement: enable port forwarding in general
 
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echo 1 > /proc/sys/net/ipv4/conf/eth0/forwarding
 
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# 1. Open target port
 
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$IPTABLES -A INPUT -p tcp --dport 3389 -j ACCEPT
 
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# 2. Enable Port forwarding to Windows server 192.168.100.10
 
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$IPTABLES -A POSTROUTING -d 192.168.100.10 -t nat -j MASQUERADE
 
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# 3. Redirect services to target server
 
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$IPTABLES -A PREROUTING -t nat -p tcp --dport 3389 -j DNAT --to 192.168.100.10:3389
 
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$IPTABLES -A PREROUTING -t nat -p tcp --dport 81 -j DNAT --to 192.168.100.10:80
 
−
</syntaxhighlight>
 

Latest revision as of 10:34, 23 May 2015

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

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 setup a Firewall with IpTables.

The following codes and examples are based iptables and ip6tables.


Principle

This is how a firewall works:

FW principle


This is how you can use a firewall at home | work:

FW usage


The firewall performs:

  • Basic security check
  • Security policy's enforcement
  • IP, port filtering
  • Port forwarding and NAT


Next steps

See Template:Menu_content_security#Firewall

Note

All this knowledge is the result of years of experiments and trials. Don't hesitate to search over Internet for more advanced || simpler rules.



References

  • My education at Telecom Bretagne [1] && Chalmers [2]
  • Working years and some co-workers help:
    • Julien Rialland
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