Part IV Implementing Network Services in SUSE Linux in .NET

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Part IV Implementing Network Services in SUSE Linux
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For any incoming connections you wish to have on a firewall, you can append a rule in the same way you did with the SSH connection.
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The order of rules
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You must be very conscious of the order you set rules in a chain because netfilter passes the TCP/IP packet through the rules in the order they are inserted into the kernel. If you wish to insert a rule at the top of the list (that is, making it the first rule that is executed), you can use the -I (insert) parameter to iptables. For example, if you are allowing SSH into your firewall from the Internet, but you know that you do not want a certain IP address to have access to SSH, you have to insert the REJECT/DROP rule before the general SSH rule:
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iptables A INPUT p tcp dport ssh i eth0 j ACCEPT iptables I INPUT p tcp dport ssh i eth0 s 10.32.1.4 j DROP
In this example, using the -s option to specify a source IP address, we have inserted the DROP rule before the general SSH acceptance rule. When a TCP/IP packet has been inserted into a chain, it is checked in order with each rule. If one of the rules matches the TCP/IP packet, it is then sent to the target specified (ACCEPT, DROP, REJECT) immediately. In the case of our inserted SSH DROP rule, it fires off packets destined for the SSH port to the DROP target before it gets to the ACCEPT SSH rule. In essence, all the TCP/IP packets sequentially go through every rule in the chain until they are directed to a target. If none of the rules fires off a packet to a target, that packet is dealt with by the default policy, which is to kill the packet in this case.
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Network Address Translation
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While one of the main uses of netfilter is its packet filtering functions, another very important aspect of netfilter is its NAT functions. Network Address Translation (NAT) is the process whereby the source or destination IP address of a packet is seamlessly changed when it passes through the firewall.
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CrossReference 6 contains some more information about NAT.
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Source NAT
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Source NAT (SNAT) works on packets forwarded through the firewall before a packet leaves for the outbound network. For this to work, you must deal with the packets before any routing decisions have been made, and the POSTROUTING chain must be used to implement Source NAT. The main purpose of SNAT is to hide private networks behind a firewall with a public IP address. This drastically reduces the cost of acquiring public IP addresses and allows you to use non-routable addresses in your internal network.
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Note The POSTROUTING chain deals with any packets that are about to be sent out to the network card. This includes any packets that are routed onto other destinations. In the case of SNAT, this is the only chain that you want to use because, for example, it makes no sense to source NAT traffic coming into the firewall INPUT chain.
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23 Implementing Firewalls in SUSE Linux
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Figure 23-2 details a home network that uses netfilter to SNAT our internal network.
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SUSE 9.1 192.168.1.3 192.168.1.1 eth0 217.41.132.74 eth1 Linux Firewall Internet
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OSX 192.168.1.0/24 DHCP
Figure 23-2: Network using a netfilter firewall In this scenario, all of the machines are behind a netfilter firewall that not only protects the machines, but also provides SNAT for outgoing connections. For SNAT to work, IP forwarding must be enabled. To do this, enter a 1 into /proc/sys/net/ipv4/ip_forward.
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bible:~ # echo 1 > /proc/sys/net/ipv4/ip_forward
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This will immediately enable IP forwarding on your Linux machine. This is a volatile operation, and once your machine has been rebooted, IP forwarding will be turned off by default. To set IP forwarding on by default, edit the file /etc/sysconfig/sysctl and change IP_FORWARD from no to yes and re-run SuSEconfig. While editing the sysctl file, make sure that DISABLE_ECN is set to yes.
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Tip ECN is Enhanced Congestion Notification. This is a new feature of TCP/IP that allows machines to notify you that a network route is congested. It is a great feature, but unfortunately is not in widespread circulation and can stop your network traffic from traversing the Internet correctly if it goes through a router that does not support ECN. We have been on customer sites where their network just stopped working for certain sites for no reason. Turning off ECN fixed this.
When IP forwarding has been enabled, you can insert the SNAT rule into the POSTROUTING chain. In the home network, you need to source NAT all the internal traffic (192.168.1.0/24) to the firewall public address of 217.41.132.74. To do this, you need to insert a SNAT rule into the NAT table.