Oskar Andreasson - Iptables Tutorial 1.2.2

Next is the option to load ipt_owner module, which could for example be used to only allow certain users to make certain connections, etc. I will not use that module in this example but basically, you could allow only root to do FTP and HTTP connections to redhat.com and DROP all the others. You could also disallow all users but your own user and root to connect from your box to the Internet. Might be boring for others, but you will be a bit more secure to bouncing hacker attacks and attacks where the hacker will only use your host as an intermediate host. For more information about the ipt_owner match, look at the Owner match section within the How a rule is built chapter.

We may also load extra modules for the state matching code here. All modules that extend the state matching code and connection tracking code are called ip_conntrack_* and ip_nat_*. Connection tracking helpers are special modules that tell the kernel how to properly track the specific connections. Without these so called helpers, the kernel would not know what to look for when it tries to track specific connections. The NAT helpers on the other hand, are extensions of the connection tracking helpers that tell the kernel what to look for in specific packets and how to translate these so the connections will actually work. For example, FTP is a complex protocol by definition, and it sends connection information within the actual payload of the packet. So, if one of your NATed boxes connect to a FTP server on the Internet, it will send its own local network IP address within the payload of the packet, and tell the FTP server to connect to that IP address. Since this local network address is not valid outside your own network, the FTP server will not know what to do with it and hence the connection will break down. The FTP NAT helpers do all of the translations within these connections so the FTP server will actually know where to connect. The same thing applies for DCC file transfers (sends) and chats. Creating these kind of connections requires the IP address and ports to be sent within the IRC protocol, which in turn requires some translation to be done. Without these helpers, some FTP and IRC stuff will work no doubt, however, some other things will not work. For example, you may be able to receive files over DCC, but not be able to send files. This is due to how the DCC starts a connection. First off, you tell the receiver that you want to send a file and where he should connect to. Without the helpers, the DCC connection will look as if it wants the receiver to connect to some host on the receiver's own local network. In other words, the whole connection will be broken. However, the other way around, it will work flawlessly since the sender will (most probably) give you the correct address to connect to.

Note If you are experiencing problems with mIRC DCCs over your firewall and everything works properly with other IRC clients, read the mIRC DCC problems section in the Common problems and questions appendix.

As of this writing, there is only the option to load modules which add support for the FTP and IRC protocols. For a long explanation of these conntrack and nat modules, read the Common problems and questions appendix. There are also H.323 conntrack helpers within the patch-o-matic, as well as some other conntrack as well as NAT helpers. To be able to use these helpers, you need to use the patch-o-matic and compile your own kernel. For a better explanation on how this is done, read the Preparations chapter.

Note that you need to load the ip_nat_irc and ip_nat_ftp if you want Network Address Translation to work properly on any of the FTP and IRC protocols. You will also need to load the ip_conntrack_irc and ip_conntrack_ftp modules before actually loading the NAT modules. They are used the same way as the conntrack modules, but it will make it possible for the computer to do NAT on these two protocols.

proc set up

At this point we start the IP forwarding by echoing a 1 to /proc/sys/net/ipv4/ip_forward in this fashion:

echo "1" > /proc/sys/net/ipv4/ip_forward

Warning! It may be worth a thought where and when we turn on the IP forwarding. In this script and all others within the tutorial, we turn it on before actually creating any kind of IP filters (i.e., iptables rule-sets). This will lead to a brief period of time where the firewall will accept forwarding of any kind of traffic for everything between a millisecond to a minute depending on what script we are running and on what box. This may give malicious people a small time-frame to actually get through our firewall. In other words, this option should really be turned on after creating all firewall rules, however, I have chosen to turn it on before loading any rules to maintain consistency with the script breakdown currently used in all scripts.

In case you need dynamic IP support, for example if you use SLIP, PPP or DHCP you may enable the next option, ip_dynaddr by doing the following :

echo "1" > /proc/sys/net/ipv4/ip_dynaddr

If there is any other options you might need to turn on you should follow that style. There's other documentation on how to do these things and this is out of the scope of this documentation. There is a good but rather brief document about the proc system available within the kernel, which is also available within the Other resources and links appendix. The Other resources and links appendix is generally a good place to start looking when you have specific areas that you are looking for information on, that you do not find here.

Note! The rc.firewall.txt script, and all other scripts contained within this tutorial, do contain a small section of non-required proc settings. These may be a good primer to look at when something is not working exactly as you want it to, however, do not change these values before actually knowing what they mean.

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.

Setting up default policies

Quite early on in the process of creating our rule-set, we set up the default policies. We set up the default policies on the different chains with a fairly simple command, as described below.

iptables [-P {chain} {policy}]

The default policy is used every time the packets do not match a rule in the chain. For example, let's say we get a packet that matches no single rule in our whole rule-set. If this happens, we must decide what should happen to the packet in question, and this is where the default policy comes into the picture. The default policy is used on all packets that does not match with any other rule in our rule-set.

Caution! Do be cautious with what default policy you set on chains in other tables since they are simply not made for filtering, and it may lead to very strange behaviors.

Setting up user specified chains in the filter table

Now you have a good picture of what we want to accomplish with this firewall, so let us get on to the actual implementation of the rule-set. It is now high time that we take care of setting up all the rules and chains that we wish to create and to use, as well as all of the rule-sets within the chains.

After this, we create the different special chains that we want to use with the -N command. The new chains are created and set up with no rules inside of them. The chains we will use are, as previously described, icmp_packets, tcp_packets, udp_packets and the allowed chain, which is used by the tcp_packets chain. Incoming packets on $INET_IFACE, of ICMP type, will be redirected to the chain icmp_packets. Packets of TCP type, will be redirected to the tcp_packets chain and incoming packets of UDP type from $INET_IFACE go to udp_packets chain. All of this will be explained more in detail in the INPUT chain section below. To create a chain is quite simple and only consists of a short declaration of the chain as this: