|The Unix and Internet Fundamentals HOWTO, by Eric Raymond|
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Linux IP Masquerade HOWTO
by David A. Ranch, November 13, 2005. This document describes how to enable the Linux IP Masquerade feature on a given Linux host. IP Masquerade is a form of Network Address Translation or NAT which NAT allows internally connected computers that do not have one or more registered Internet IP addresses to communicate to the Internet via the Linux server's Internet IP address. For example, if a Linux host ( the MASQ gateway ) is connected to the Internet via PPP, Ethernet, etc., the IP Masquerade feature allows other "internal" computers connected to this Linux box (via PPP, Ethernet, etc.) to also reach the Internet as well. Linux IP Masquerading allows for this functionality even though these internal machines don't have an officially assigned IP address. To other machines on the Internet, the outgoing traffic will appear to be from the IP MASQ Linux server itself. MASQ allows a set of machines to invisibly access the Internet via the MASQ gateway. Masq or 1:Many NAT is similar to a proxy server in the sense that the server will perform IP address translation and fake out the remote server ( WWW for example ) as if the MASQ server made the request instead of an internal machine. The major difference between a MASQ and PROXY server is that MASQ servers don't need any configuration changes to all the client machines. Just configure them to use the linux box as their default gateway and everything will work fine. Network Address Translation is the name for a box that would have a pool of valid IP addresses on the Internet interface which it can use. Whenever the Internal network wanted to go to the Internet, it associates an available VALID IP address from the Internet interface to the original requesting PRIVATE IP address. Once the associated PUBLIC NAT address becomes idle for some pre-determined amount of time, the PUBLIC IP address is returned back into the public NAT pool. The major problem with NAT is, once all of the free public IP addresses are used, any additional private users requesting Internet service are out of luck until a public NAT address becomes free.
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