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When I began my career in network security, almost a decade back, the industry standard was stateful firewalls that even had the ability to inspect some application layer protocols. I had only heard that a packet filtering firewall was the previous generation of firewalls. I never got to work with and understand how a strict packet filtering, stateless firewall works. Now, I want to understand it better. I had an intern ask me this and I want to show him the difference between the 2 firewalls. I believe iptables can be used to set up a stateless packet filtering firewall. The thing is I am not sure how a stateless firewall would track incoming responses that originate from the behind the firewall. I tried my best to find some resources online on how to implement such a firewall. Any pointers to some article would be very helpful.

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For a stateless firewall, you can either accept or drop a packet based on its protocol, port number and origin ip address. For example, the rule below accepts all TCP packets from the 192.168.1.x subnet that are bound for port 80.

-A INPUT -p tcp -s 192.168.1.0/24 -m tcp --dport 80 -j ACCEPT

For output, the rules look similar. Therefore, outgoing(egress) packets are only accepted if there is a matching rule. The rule below allows only outgoing packets on port 80.

-A OUTPUT -o eth0 -p tcp --sport 80 -j ACCEPT

For a stateful firewall, you have the ability to monitor state. For example, the rule below only accepts packets to port 80 if it is initiating a new connection or is associated with an existing connection. The tracking is done by a kernel module "ip_conntrack". It keeps a table of all active connections.

-A INPUT -p tcp --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT
  • What about rule for the return traffic for egress packets? – user1720897 Dec 8 '14 at 15:13
  • The first rule. Since it is stateless, it cannot differentiate return traffic from new traffic so it can only decide based on protocol, port number and origin ip address. – limbenjamin Dec 8 '14 at 15:17
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    Suppose I go out to www.google.com from the LAN. This would mean something like 192.168.1.100:5400 -> www.google.com:80. Now the return traffic would be www.google.com:80 -> 192.168.1.100:5400. How did a stateless firewall handle this sort of return traffic? What rules did they have in place to tackle such traffic? – user1720897 Dec 8 '14 at 17:55
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    You would have to manually open port 513 to 65535 for it to happen. Have a look at serverfault.com/questions/253508/…. The first 2 answers use the ip_conntrack module to provide stateful capability. The last uses only stateless features. – limbenjamin Dec 9 '14 at 1:15

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