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A server that has more than one network interface is said to be multihomed. An interface is normally a network card such as an Ethernet card. Typically these will be named eth0 and eth1. There are other types of interface, such as the loopback interface, dial-up PPP interfaces, and so on. In this section we will be dealing with two Ethernet interfaces, but netfilter will behave the same for any type of interface. Although there are many possible configurations for a multihomed host, the simplest is to have each interface on a different IP network, which means that each interface has a unique IP address. We will look at two ways of handling this configuration under CentOS, first by acting as a router and forwarding traffic between one subnet and another. We will then look at IP masquerading, which allows a private subnet to access the Internet by sharing a single IP address. It is quite straightforward to turn your CentOS server into a router. All that is necessary is to enable IP forwarding. To do this, edit /etc/sysctl.conf and set net.ipv4.ip_forward = 1. To apply this change, run sysctl p. If you have a firewall you must configure which traffic is allowed to be forwarded between interfaces. Because the forwarded packets are processed by the RH Firewall 1 INPUT chain, you must add a rule to that chain. So far, none of the rules presented have been concerned with which interface the packets are arriving on. A router has multiple interfaces, so this becomes an issue. The iptables command provides parameters to allow you to specify which interface the packet arrived on and will be output on. These are i and o, respectively. Each parameter takes the name of the interface such as eth0. To allow traffic from one network to another, the general rule format is iptables I RH Firewall 1 INPUT i first interface s first IP subnet \ o second interface d second IP subnet j ACCEPT iptables I RH Firewall 1 INPUT i second interface s second IP subnet \ o first interface d first IP subnet j ACCEPT Because you need to permit traffic to be forwarded from the first network to the second, and traffic from the second back to the first, you need to have two rules. If you have the subnet 192.168.1.0/24 on eth0 and 192.168.2.0/24 on eth1, you would add these two rules: iptables o eth1 iptables o eth0 I d I d RH Firewall 1 INPUT i eth0 s 192.168.1.0/24 \ 192.168.2.0/24 j ACCEPT RH Firewall 1 INPUT i eth1 s 192.168.2.0/24 \ 192.168.1.0/24 j ACCEPT
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CHAPTER 10 NETWORK SECURITY
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Now that your server is acting as a router, you must also refine your host access rules to include a destination address. This will ensure that rules which permit incoming connections to your server do not inadvertently permit a connection to be made to a host on the other network. Listing 10-9 shows a rule that permits SSH connections from one subnet to the server IP address. Listing 10-9. Restricting the Destination Address A RH Firewall 1 INPUT s 192.168.3.0/24 d 192.168.3.1 \ m state state NEW m tcp p tcp dport 22 j ACCEPT Unless you have been allocated a whole subnet from your ISP, routing alone is not sufficient to get the workstations on the Internet. You must also use a technique called IP masquerading.
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Using netfilter for IP Masquerading
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So far we have only looked at netfilter for filtering packets both from a single network interface and multiple interfaces. If your server has more than one interface, then there are many other things that netfilter can do. One very useful feature is IP masquerading. This is often referred to as Network Address Translation (NAT). However, masquerading is subtly different from NAT because we are hiding an entire subnet behind a single IP address. Masquerading is different from filtering and forwarding packets because it requires actually altering the packets for both the IP addressing and the protocol port numbers. The main purpose of IP masquerading is to allow many hosts to access the Internet using a single IP address. Windows users might be familiar with Internet Connection Sharing, which is similar to what can be done with masquerading. To use masquerading you need to use the MASQUERADE target, which can only be used in the POSTROUTING chain in the nat table. The table is specified with the t parameter to the iptables command. Along with the MASQUERADE target you must specify the output interface. It is the address on the output interface that will be used for the masquerading. Once masquerading is enabled, you must also add a rule that will permit traffic from the internal network to be forwarded. You must have IP forwarding enabled, as shown earlier. Finally, your routing table must have an entry so that Internet traffic is routed via the masquerading network interface. You will normally have set this up already when you performed your network configuration. Listing 10-10 shows the iptables commands that will enable masquerading. Listing 10-10. Setting Up Masquerading # Assuming eth1 is the Internet, masquerade internal traffic to the Internet iptables t nat A POSTROUTING o eth1 j MASQUERADE # Assuming our internal subnet on eth0 is 192.168.3.0/24 and # the server is 192.168.3.1, allow the Internet traffic to be forwarded iptables I RH Firewall 1 INPUT i eth0 s 192.168.3.0/24 \ d \! 192.168.3.1 j ACCEPT You can use service iptables save to save all of these rules so they apply at startup or you can add them to /etc/sysconfig/iptables as previously show. In order to access the Internet, workstations must have the address of your server (136.186.3.1) as their default router. When connections from the workstations are made over the Internet, the source address will be that of the server rather than that of the workstations.
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