Dynamic DNS and NAT Router/Firewalls
As discussed in Chapter 2, "Introduction to Networking," to conserve the limited number of IP addresses available for Internet purposes, most home router/firewalls use network address translation (NAT) to map a single, public, DHCP-obtained IP address to the many private IP addresses within your network.
NAT can fool the operation of some DDNS client software. In these cases, the software can report only the true IP address of the Linux box's NIC interface. If the Linux box is being protected behind a NAT router/firewall, then the NIC reports in its data stream to the DDNS provider a private IP address that no one can reach directly via the Internet. The reported value is therefore invalid.
Some DDNS providers use more intelligent clients, such as DDclient, that can be configured to let the DDNS provider record the public IP address from which the data stream is originating. Once this is done, you'll have to also configure your router/firewall to do port forwarding to make all HTTP traffic destined for the IP address of the router/firewall to be exclusively address translated using NAT and forwarded to a single server on your home network. If your firewall is Linux based, then the examples in Chapter 14, "Linux Firewalls Using iptables," will be helpful. Many Web-based small office/home office (SOHO) firewalls have easy interfaces to configure port forwarding; please refer to your manufacturer's manual on how to do this.
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