Книга: Iptables Tutorial 1.2.2

Displacement of rules to different chains

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Displacement of rules to different chains

This section will briefly describe my choices within the tutorial regarding user specified chains and some choices specific to the rc.firewall.txt script. Some of the paths I have chosen to go here may be wrong from one or another aspect. I hope to point these aspects and possible problems out to you when and where they occur. Also, this section contains a brief look back to the Traversing of tables and chains chapter. Hopefully, this will remind you a little bit of how the specific tables and chains are traversed in a real live example.

I have displaced all the different user-chains in the fashion I have to save as much CPU as possible but at the same time put the main weight on security and readability. Instead of letting a TCP packet traverse ICMP, UDP and TCP rules, I simply match all TCP packets and then let the TCP packets traverse a user specified chain. This way we do not get too much overhead out of it all. The following picture will try to explain the basics of how an incoming packet traverses Netfilter. With these pictures and explanations, I wish to explain and clarify the goals of this script. We will not discuss any specific details yet, but instead further on in the chapter. This is a really trivial picture in comparison to the one in the Traversing of tables and chains chapter where we discussed the whole traversal of chains and tables in depth.


Based upon this picture, let us make clear what our goals are. This whole example script is based upon the assumption that we are looking at a scenario containing one local network, one firewall and an Internet connection connected to the firewall. This example is also based upon the assumption that we have a static IP to the Internet (as opposed to DHCP, PPP and SLIP and others). In this case, we also want to allow the firewall to act as a server for certain services on the Internet, and we trust our local network fully and hence we will not block any of the traffic from the local network. Also, this script has as a main priority to only allow traffic that we explicitly want to allow. To do this, we want to set default policies within the chains to DROP. This will effectively kill all connections and all packets that we do not explicitly allow inside our network or our firewall.

In the case of this scenario, we would also like to let our local network do connections to the Internet. Since the local network is fully trusted, we want to allow all kinds of traffic from the local network to the Internet. However, the Internet is most definitely not a trusted network and hence we want to block them from getting to our local network. Based upon these general assumptions, let's look at what we need to do and what we do not need and want to do.


First of all, we want the local network to be able to connect to the Internet, of course. To do this, we will need to SNAT all packets since none of the local computers have real IP addresses. All of this is done within the POSTROUTING chain, which is created last in this script. This means that we will also have to do some filtering within the FORWARD chain since we will otherwise allow outsiders full access to our local network. We trust our local network to the fullest, and because of that we specifically allow all traffic from our local network to the Internet. Since no one on the Internet should be allowed to contact our local network computers, we will want to block all traffic from the Internet to our local network except already established and related connections, which in turn will allow all return traffic from the Internet to our local network.


As for our firewall, we may be a bit low on funds perhaps, or we just want to offer a few services to people on the Internet. Therefore, we have decided to allow HTTP, FTP, SSH and IDENTD access to the actual firewall. All of these protocols are available on the actual firewall, and hence it should be allowed through the INPUT chain, and we need to allow the return traffic through the OUTPUT chain. However, we also trust the local network fully, and the loopback device and IP address are also trusted. Because of this, we want to add special rules to allow all traffic from the local network as well as the loopback network interface. Also, we do not want to allow specific packets or packet headers in specific conjunctions, nor do we want to allow some IP ranges to reach the firewall from the Internet. For instance, the 10.0.0.0/8 address range is reserved for local networks and hence we would normally not want to allow packets from such a address range since they would with 90% certainty be spoofed. However, before we implement this, we must note that certain Internet Service Providers actually use these address ranges within their own networks. For a closer discussion of this, read the Common problems and questions chapter.

Since we have an FTP server running on the server, as well as the fact we want to traverse as few rules as possible, we add a rule which lets all established and related traffic through at the top of the INPUT chain. For the same reason, we want to split the rules down into sub-chains. By doing this, our packets will hopefully only need to traverse as few rules as possible. By traversing less rules, we make the rule-set less time-consuming for each packet, and reduce latency within the network.

In this script, we choose to split the different packets down by their protocol family, for example TCP, UDP or ICMP. All TCP packets traverse a specific chain named tcp_packets, which will contain rules for all TCP ports and protocols that we want to allow. Also, we want to do some extra checking on the TCP packets, so we would like to create one more subchain for all packets that are accepted for using valid port numbers to the firewall. This chain we choose to call the allowed chain, and should contain a few extra checks before finally accepting the packet. As for ICMP packets, these will traverse the icmp_packets chain. When we decided on how to create this chain, we could not see any specific needs for extra checks before allowing the ICMP packets through if we agree with the type and code of the ICMP packet, and hence we accept them directly. Finally, we have the UDP packets which need to be dealt with. These packets, we send to the udp_packets chain which handles all incoming UDP packets. All incoming UDP packets should be sent to this chain, and if they are of an allowed type we should accept them immediately without any further checking.

Since we are running on a relatively small network, this box is also used as a secondary workstation and to give some extra leeway for this, we want to allow certain specific protocols to make contact with the firewall itself, such as speak freely and ICQ.


Finally, we have the firewalls OUTPUT chain. Since we actually trust the firewall quite a lot, we allow pretty much all traffic leaving the firewall. We do not do any specific user blocking, nor do we do any blocking of specific protocols. However, we do not want people to use this box to spoof packets leaving the firewall itself, and hence we only want to allow traffic from the IP addresses assigned to the firewall itself. We would most likely implement this by adding rules that ACCEPT all packets leaving the firewall in case they come from one of the IP addresses assigned to the firewall, and if not they will be dropped by the default policy in the OUTPUT chain.

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