Part IV Implementing Network Services in SUSE Linux in .NET

Display Code 128B in .NET Part IV Implementing Network Services in SUSE Linux
Part IV Implementing Network Services in SUSE Linux
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The NAT table is used specifically for address translation rules. This includes source and destination address translation. bible:~ # iptables t nat A POSTROUTING s o eth1 j SNAT to
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Here, we have told iptables to edit the nat table (-t nat) by appending a rule to the POSTROUTING chain. We have stated that any traffic from the network (-s) and destined to leave the firewall through eth1 (-o) should be source address NAT d to In the example, note that we have tried to be as descriptive as possible concerning what traffic should be subject to the SNAT, detailing the source IP address (specifying the network address with netmask) and the network adaptor that the traffic will leave on. You know that the traffic you need to be SNAT d will leave the eth1 interface because you want to SNAT only traffic that is heading out to the Internet. This can be through the eth1 interface only. Any traffic that is sent back to the machines behind the firewall (for example, during the three-way handshake) will be translated back by the firewall (it remembers connection states) and the destination address will automatically be set to the address of the machine on the private network that initiated the connection.
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Allowing the packets to be forwarded
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It is all well and good setting up SNAT, but the astute of you will probably realize that you have already told netfilter not to allow any forwarded traffic through the firewall (the default FORWARD policy is DROP). To correct this, you need to allow the firewall to forward these packets before they can be manipulated by the SNAT rule. To do this, you need to enable forwarding for traffic from the private network to the Internet:
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bible:~ # iptables A FORWARD s i eth0 -o eth1 j ACCEPT
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Here, iptables is being used to append (-A) to the FORWARD chain (any traffic that enters and then leaves the firewall on separate interfaces). Any traffic from the network entering the firewall on interface eth0 and leaving on interface eth1 will be allowed through. So, in this example, we have told netfilter that any traffic from the network coming in on eth0 and leaving the firewall on eth1 should be allowed through. Again, we are relying on the fact that any traffic coming in on eth0 and leaving on eth1 that is from will be traffic we want to be allowed out to the Internet.
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Tip In this example, we have been quite liberal in what we are allowing our users to access on the Internet. It is usually the policy of most companies that IM clients, P2P, and IRC should not be allowed from the corporate network. As it stands, users can access anything on the Internet as if they were directly connected. For the home network example, this is fine because the users are trusted. However, if you are implementing a corporate firewall, you will probably need to have quite a few DROP rules in the FORWARD chain, or do the right thing and deny everything and allow only essential traffic (maybe only HTTP).
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23 Implementing Firewalls in SUSE Linux
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Destination NAT (DNAT) is a nice feature when building netfilter firewalls. It does the exact opposite of the SNAT function by translating the destination address of a network packet into another address. Imagine in the example in Figure 23-2 that you had a mail server on your desktop machine. If you want to give access to that machine to Internet users, you can t just tell the firewall that you want everyone to access the IP over port 25; because this is a non-routable address, Internet users would never be able to reach it. To combat this, you can tell netfilter that any traffic destined for port 25 on the public firewall address should be redirected to the machine Any return traffic to the initiating machine will have the source address of the firewall, making the connection routable. And as far as the initiating machine is concerned, it has no idea that the machine it is actually talking to is hidden behind a firewall and is on a non-routable address. To create the illusion, you need to add a DNAT rule to the NAT table for the Simple Mail Transport Protocol (SMTP) service.
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bible:~ # iptables t nat A PREROUTING p tcp -dport smtp i eth1 j DNAT todestination=
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Here, iptables has been told to work on the NAT table (-t nat) by appending to the PREROUTING chain. You have stated that any traffic that is TCP (-p tcp) based, with a destination port of SMTP (25), and entering the firewall on eth1 should be destination NAT d to In this case, all traffic for port 25 (SMTP) on the public network interface of the firewall will have its destination address changed to The port destination of 25 will be untouched (we will come to this later).
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Note When enabling DNAT, you have to insert the rules into the PREROUTING chain because a routing decision has to be made on the final destination of the packet. At this point in the netfilter processing in the PREROUTING chain, the final destination address has not been inserted into the packet, so the routing decision is still yet to be made after this for successful delivery.
In the same regard as SNAT, you still need to allow traffic destined on port 25 to to be forwarded through the firewall.
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