Linux IP Masquerade HOWTO
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# 7.6. ( NAT vs. Proxy ) - How does IP Masquerade differ from Proxy or NAT services?

    Proxy:  Proxy servers are available for: Win95, NT, Linux, Solaris, etc.

                Pro:    + (1) IP address ; cheap
                        + Optional caching for better performance (WWW, etc.)

                Con:    - All applications behind the proxy server must both SUPPORT
                          proxy services (SOCKS) and be CONFIGURED to use the Proxy
                          server
                        - Screws up WWW counters and WWW statistics

    	 A proxy server uses only (1) public IP address, like IP MASQ, and acts
    	 as a translator to clients on the private LAN (WWW browser, etc.).
    	 This proxy server receives requests like TELNET, FTP, WWW,
    	 etc. from the private network on one interface.  It would then in turn,
    	 initiate these requests as if someone on the local box was making the
    	 requests.   Once the remote Internet server sends back the requested
    	 information, it would re-translate the TCP/IP addresses back to the
    	 internal MASQ client and send traffic to the internal requesting host.
    	 This is why it is called a PROXY server.

    		Note:  ANY applications that you might want to use on the
    			internal machines *MUST* have proxy server support
    			like Netscape and some of the better TELNET and FTP
    			clients.  Any clients that don't support proxy servers
    			won't work.

    	 Another nice thing about proxy servers is that some of them
    	 can also do caching (Squid for WWW).  So, imagine that you have 50
    	 proxied hosts all loading Netscape at once.  If they were installed
    	 with the default homepage URL, you would have 50 copies of the same
    	 Netscape WWW page coming over the WAN link for each respective computer.
    	 With a caching proxy server, only one copy would be downloaded by the
             proxy server and then the proxied machines would get the WWW page from
             the cache.  Not only does this save bandwidth on the Internet
             connection, it will be MUCH MUCH faster for the internal proxied
             machines.

    MASQ:	 IP Masq is available on Linux and a few ISDN routers such
     or	 as the Zytel Prestige128, Cisco 770, NetGear ISDN routers, etc.
    1:Many
     NAT
    		Pro: 	+ Only (1) IP address needed (cheap)
    			+ Doesn't require special application support
    			+ Uses firewall software so your network can become
    			  more secure

    		Con:	- Requires a Linux box or special ISDN router
    			  (though other products might have this..  )
    			- Incoming traffic cannot access your internal LAN
    			  unless the internal LAN initiates the traffic or
    			  specific port forwarding software is installed.
    			  Many NAT servers CANNOT provide this functionality.
    			- Special protocols need to be uniquely handled by
    			  firewall redirectors, etc.  Linux has full support
    			  for this (FTP, IRC, etc.) capabilty but many routers
    			  do NOT (NetGear DOES).

    	 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.  You WILL need to install special Linux modules for things
    	 like RealAudio, FTP, etc. to work)!

    	 Also, many users operate IP MASQ for TELNET, FTP, etc. *AND* also setup a
    	 caching proxy on the same Linux box for WWW traffic for the additional
    	 performance.

    NAT:	 NAT servers are available on Windows 95/NT, Linux, Solaris, and some
    	 of the better ISDN routers (not Ascend)

    		Pro: 	+ Very configurable
    			+ No special application software needed

    		Con:	- Requires a subnet from your ISP (expensive)

    	 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.  After that, all traffic is re-written from the NAT
    	 public IP address to the NAT private 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.

For an excellent and very comprehensive description of the various forms of NAT, please see:

- http://www.suse.de/~mha/linux-ip-nat/diplom/nat.html/

Here is another good site to learn about NAT, although many of the URLs are old but still valid:

- http://www.linas.org/linux/load.html/

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