23 Implementing Firewalls in SUSE Linux
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Port scanning is when a machine automatically tries to connect to a range of TCP/IP ports on a machine to see if there are any services listening. It is used not only by crackers, but also by legitimate users who wish to see what services are available on a server. You should port scan only hosts that you have been allowed to interrogate. Port scanning a machine usually triggers alarms on a system, and you may get into trouble depending what the administrator is feeling like that day.
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This example fully illustrates that network security must be considered as a whole, not just as a threat from the Internet.
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Configuring a Firewall with iptables
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To configure a firewall on Linux, you need to get used to the iptables command, which is used to manipulate the kernel packet filtering settings from user space. (Refer to 6 for more information on TCP/IP, because an understanding of TCP/IP is needed.)
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Note The terms user space and kernel space are used a lot in the Unix community. When something runs in kernel space, it is under the control and the constraints of the kernel. Something running in kernel space could be a kernel module or the packet filtering code. When something is in user space, it uses the system libraries and is not under the strict control of the kernel. We use iptables (user space) to tell the kernel space filtering code (netfilter) what it needs to do with the TCP/IP packets it receives. When a TCP/IP packet is received by the kernel, it is passed and acted upon in kernel space by the netfilter code.
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The kernel filtering code uses chains to signify where a packet is in the kernel. Figure 23-1 gives an overview of how the kernel sees a TCP/IP packet. This also helps us to see how iptables interacts with these packets later in the chapter.
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Figure 23-1: Overview of the kernel chains
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Part IV Implementing Network Services in SUSE Linux
The filtering chains are extremely important to the operation of the filtering code because they determine whether or not a packet should be interpreted by the kernel. The chains themselves represent the final destination of the packet: INPUT The packet is destined for the firewall itself. OUTPUT The packet originated from the firewall. FORWARD The packet is passing through the firewall (neither originates from nor is destined for the firewall). Consider these examples to show how the chains work in a normal firewall: My firewall at home is Linux based, and it does a few things that most small firewalls do: It provides my non-routable addresses with a public Internet address via Network Address Translation (NAT), and runs an SSH server for me to log in remotely to my network.
Note When setting up a firewall appliance, you need to enable IP forwarding. IP forwarding allows packets to be routed from one network interface to another in the Linux machine. This is integral to the whole process of routing packets and the Linux machine s acting as a router. Most iptables firewalls that protect a network run on low-cost, low CPU powered hardware.
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When a TCP/IP packet leaves my laptop, it is sent to the default route, which is my iptables firewall on my router. When the firewall receives the packet, it analyzes it to find its destination. As it sees that the packet is not destined for the machine itself, it is sent to the FORWARD chain. When in the FORWARD chain, the packet will traverse all firewall rules until it is either dropped or is sent to the outbound network interface (my ADSL router) for further processing. The important part of the scenario is that any non-local packets (destined or originating from the machine) are passed to the forward chain (for forwarding!). When I SSH into my firewall from the Internet, a TCP/IP packet attempts to open an SSH connection for me. In the same way that the packet will reach the firewall as in the forwarding example, the kernel analyzes the packet to see where it is destined. As my machine is the final destination for the packet, it is inserted into the INPUT chain for further processing. If the packet is allowed through, it is passed over to the kernel to be handed over to user space (which is normal when no firewalling is used). The OUTPUT chain is slightly different because it does not deal with traffic from the network. An OUTPUT chain is triggered only when a packet originates from the machine itself. For example, if you are logged into the machine and initiate an FTP connection to the outside world, this is considered a packet that traverses the OUTPUT chain.
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