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/

This is a great URL for learning about other NAT solutions for Linux as well as other platforms:

• "http://www.uq.net.au/~zzdmacka/the-nat-page/