Iptables

Iptables is a packet filter. It can be used in the same fashion as routers and firewalls.

Usage

We show the firewall rules

 iptables -L

Show blocked articles

 iptables -L -n

A more in-detail list

 iptables -L -v

You can add rules to Iptables with commands or by writing in a file. It is recommended to write the rules in a file, since when Linux shuts down it will clear all the rules made with commands, but not the ones written in a file. Saving command rules can be made possible with the iptables-persistent packet.

Adding rules with commands. For instance, allow local traffic:

 iptables -A INPUT -i lo -j ACCEPT

Basic firewall rules

This firewall is useful for general application. Copy this into a file and run the restoration command.

  *filter
  :INPUT DROP [0:0]
  :FORWARD ACCEPT [0:0]
  :OUTPUT ACCEPT [0:0]
  
  # Allow local traffic
  -A INPUT -i lo -j ACCEPT
  # First packet is a SYN bit. If not, block traffic.
  -A INPUT -p tcp ! --syn -m state --state NEW -j DROP
  
  # Block broken packets
  -A INPUT -f -j DROP
  
  # Block XMAS packets
  -A INPUT -p tcp --tcp-flags ALL ALL -j DROP
  
  # Block "NULL" packets
  -A INPUT -p tcp --tcp-flags ALL NONE -j DROP
  
  # Throttle TCP RST (TCP RESET). 
  -A INPUT -p tcp -m tcp --tcp-flags RST RST -m limit --limit 2/second --limit-burst 2 -j ACCEPT
  
  # Block all invalid packets
  -A INPUT -m state --state INVALID -j DROP
  -A FORWARD -m state --state INVALID -j DROP
  -A OUTPUT -m state --state INVALID -j DROP
  
  # Block forged packets
  -A INPUT -s 169.254.0.0/16 -j DROP
  -A INPUT -s 127.0.0.0/8 -j DROP
  -A INPUT -s 224.0.0.0/4 -j DROP
  -A INPUT -d 224.0.0.0/4 -j DROP
  -A INPUT -s 240.0.0.0/5 -j DROP
  -A INPUT -d 240.0.0.0/5 -j DROP
  -A INPUT -s 0.0.0.0/8 -j DROP
  -A INPUT -d 0.0.0.0/8 -j DROP
  -A INPUT -d 239.255.255.0/24 -j DROP
  -A INPUT -d 255.255.255.255 -j DROP
  
  # Blocking an ICMP attack, limited amount of requests
  -A INPUT -p icmp --icmp-type address-mask-request -j DROP
  -A INPUT -p icmp --icmp-type timestamp-request -j DROP
  -A INPUT -p icmp --icmp-type router-solicitation -j DROP
  -A INPUT -p icmp -m limit --limit 2/second -j ACCEPT
  
  # Allow SSH
  -A INPUT -p tcp --dport 22 -m state --state NEW -j ACCEPT
 
  # Block SYN-FLOOD packets
  -A INPUT -p tcp -m state --state NEW -m limit --limit 50/second --limit-burst 50 -j ACCEPT
  -A INPUT -p tcp -m state --state NEW -j DROP
  
  # Allow trusted connections and block INVALID packets
  -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
  -A INPUT -m state --state INVALID -j DROP
  
  COMMIT

Saving and applying firewall configurations in a file

Save firewall configurations

 iptables-save > /etc/iptables/rules.v4

Restore firewall settings from file

 iptables-restore < /etc/iptables/rules.v4

Save IPv6 firewall settings in file

 ip6tables-save > /etc/iptables/rules.v6

Automatic firewall settings restoration from file on startup

Iptables-persistent packet can be found in Wheezy, in Jessie it can be found by the name netfilter-persistent.

Install from packet manager.

 apt-get install iptables-persistent

You can configure the firewall here. Only the changes made here will be loaded on bootup.

 nano /etc/iptables/rules.v4

 nano /etc/iptables/rules.v6

Save currently used firewall to file

 service iptables-persistent save

Restore from file

 service iptables-persistent reload

IPv6

Ip6tables is the firewall for an IPv6 network.

Here is a simple firewall configuration for general use.

#Allow local traffic
-A INPUT -i lo -j ACCEPT
-A OUTPUT -o lo -j ACCEPT

#Block RH0 packet traffic
-A INPUT -m rt --rt-type 0 -j DROP
-A FORWARD -m rt --rt-type 0 -j DROP
-A OUTPUT -m rt --rt-type 0 -j DROP

#Allow local network traffic
-A INPUT -s fe80::/10 -j ACCEPT
-A OUTPUT -s fe80::/10 -j ACCEPT
-A INPUT -d ff00::/8 -j ACCEPT
-A OUTPUT -d ff00::/8 -j ACCEPT

-A INPUT -p icmpv6 -j ACCEPT
-A OUTPUT -p icmpv6 -j ACCEPT

#Block the SSH port when using IPv6. It is recommended to make exceptions for certain addresses or networks, though.
-A INPUT -p tcp -m tcp --dport 22 -j DROP

-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A OUTPUT -m state --state INVALID -j DROP

NAT

Define two NICs for Linux both with IP-addresses. The internal NIC (eth0) must have a static IP address. It functions as the networks gateway. The WAN NIC (eth1) can have a dynamic IP-address.

Allow IPv4 forwarders:

 nano /etc/sysctl.conf

Remove the number sign:

 net.ipv4.ip_forward=1

Enable modifications with the command

 sysctl -p /etc/sysctl.conf

Write the following line to your iptables rules. We suggest using the iptables-persistent packet.

-A POSTROUTING -o eth1 -j MASQUERADE

Internal interface allows forwarding from eth0 to eth1 when the state is related / established. Otherwise traffic is blocked.

-A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i eth0 -i eth1 -m state --state INVALID -j DROP

Sources

http://www.thomas-krenn.com/en/wiki/Saving_Iptables_Firewall_Rules_Permanently

http://linux.fi/wiki/Iptables (Finnish)