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- Microsoft Proxy Server 2.0

Un proxy server è un programma (e, per estensione, il computer su cui esso viene eseguito) che svolge la funzione di "agente" per gli utenti di altri computer.

Difatti, configurando i propri programmi di comunicazione perchè sfruttino un proxy - tipicamente situato su un computer diverso dal proprio - tutte le richieste di comunicazione, come ad esempio la richiesta di caricamento di un ipertesto, non saranno fatte dal proprio computer direttamente al sito remoto, ma verranno fatte al proxy server; il programma di proxy si occuperà poi di contattare il sito remoto, farsi spedire l'oggetto desiderato, e "girarlo" al computer dell'utente. Apparentemente, questa può sembrare una perdita di tempo (e talvolta lo è, se il sistema non è ben studiato); tuttavia, l'operazione può presentare molti vantaggi: ad esempio il proxy dispone solitamente di una propria "cache", nella quale memorizza tutti gli oggetti recuperati negli ultimi tempi; se un utente richiede di caricare un oggetto, ad esempio un ipertesto, che è già stato richiesto qualche tempo prima da un altro utente, e si trova quindi memorizzato sul proxy, esso provvede a spedirglielo direttamente, senza dover ricontattare il sito remoto, e quindi molto più velocemente di quanto succederebbe se il computer dovesse attendere la lunga e lenta connessione con il sito remoto."

Il servizio Proxy viene abitualmente utilizzato principalmente per permettere l’interfacciamento tra la rete Internet pubblica e quella Intranet privata.

Per meglio capire questo concetto, bisogna innanzitutto parlare di indirizzi IP pubblici e privati. Ogni computer collegato ad Internet deve essere univocamente identificato da un indirizzo a 32bit, che di solito viene rappresentato nella notazione a.b.c.d.
Pertanto, il numero di PC collegabili ad Internet risulta essere di circa 2³² (in realtà un po’ meno, dovuto agli indirizzi NET e Broadcast inutilizzabili).
Tuttavia questo numero è in realtà abbastanza piccolo, se confrontato in scala mondiale. Pertanto attualmente si tende a ridurre al minimo il numero di indirizzi IP effettivi, utilizzando le classi di indirizzi private.
Queste sono range di indirizzi che possiedono –per convenzione- due caratteristiche:

non sono necessariamente univoci a livello mondiale; l’univocità è richiesta soltanto nell’ambito della LAN.
Non possono essere direttamente collegati alla rete Internet pubblica.

Vengono solitamente utilizzati per assegnare l’indirizzo IP (in modo statico o dinamico) ai PC di una rete locale. Una di queste macchine, generalmente il server, possiede oltre alla scheda rete collegata alla LAN anche un’interfaccia dotata di indirizzo IP pubblico (ovvero reale) che utilizza per effettuare l’interscambio dei pacchetti IP tra le due reti.
Questa interfaccia può essere:

Una linea asincrona collegata ad un modem,
Una scheda di rete collegata ad una linea dedicata,
Una scheda di rete collegata ad un router: è questa la configurazione adottata.

In pratica le funzioni che il proxy espleta sono fondamentalmente:

mappatura e redirezione dei pacchetti tra IP pubblici e privati;
filtro sui pacchetti in uscita;
filtro sui pacchetti in entrata (firewall)
cache tra i siti interni ed esterni.

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Vengono di seguito riportate le caratteristiche salienti del Proxy Microsoft, tratte direttamente dal sito Web ufficiale. Il Proxy Server 2.0 è un ottimo proxy (ma non il migliore), molto funzionale e ricco di opzioni interessanti, peccato il prezzo!!:

Microsoft Proxy Server 2.0 Features
*Feature*	*Description*
*EXTENSIBLE FIREWALL SECURITY*
Packet Layer Security with Dynamic Packet Filtering	New! Microsoft Proxy Server 2.0 supports inbound and outbound packet filtering. Unlike other packet filtering firewalls, Proxy Server intelligently & dynamically determines which packets to allow to pass through to the secured network’s circuit & application layer proxy services. Rather than force a network manager to manually pre-define and permanently open a set of ports for different applications, this feature opens ports automatically only as needed, then closes the ports when the communication ends. This approach minimizes the number of exposed ports in either direction and provides a unique measure of hassle-free security.
Circuit Layer Security	Protect your Intranet via the Winsock proxy and the new SOCKS proxy. These services provide application-transparent circuit gateways. Microsoft Proxy Server 2.0 provides multi-platform access to Telnet, RealAudio, NetShow, IRC, and several other Internet services. Unlike other circuit layer proxies, Microsoft Proxy Server 2.0 circuit layer security works with dynamic packet filtering for enhanced security and ease of use.
Application Layer Security	Microsoft Proxy Server 2.0 understands and interprets commands within the application protocols (such as HTTP, FTP, and Gopher) from client PCs. Proxy Server acts on behalf of the client PC to interact with the Internet resource. The network topology and IP or IPX addresses are not revealed to the outside network.
Real-time Security Alerts	New! Now you can be notified immediately if your network is under attack so you can take action. Microsoft Proxy Server 2.0 supports several alerting thresholds and variables for great flexibility.
Reverse Proxy	New! Now you can place your web server behind Proxy Server to publish to the World Wide Web without compromising the security of the web server or its data. Proxy Server "impersonates" a Web server to the outside world, while your Web server maintains access to internal network services.
Reverse Hosting	New! This extension of reverse proxy allows several web servers sitting behind Microsoft Proxy Server to publish to the Internet, providing great flexibility and security in Web publishing. These additional web servers can publish independently or appear as directories in a single large virtual web server
Server Proxying	New! Microsoft Proxy Server 2.0 has the ability to listen for inbound packets destined to a server computer that is connected behind the Proxy Server computer. Proxy Server then forwards the incoming requests. For example, incoming mail can be directed to your Microsoft Exchange Server computer.
Extensive Logging Support	Microsoft Proxy Server 2.0 logs via log files or to ODBC databases so network managers have a complete profile of inbound and outbound traffic moving through the Proxy Server computer. Logging has been expanded in Microsoft Proxy Server 2.0 to include alert information and other new firewall-related activity.
Virtual Private Networking with Routing & Remote Access Service Update	You can use Microsoft Proxy Server 2.0 on the same server with Routing and Remote Access Service for Windows NT Server to connect branch offices to a corporate network via the Internet. Using the Internet as a Virtual Private Network provides big cost savings compared to traditional Wide Area Network (WAN) options. This provides all-in-one access and control for use with Internet and connectivity to a multi-site Intranet.
Secure Sockets Layer Tunneling	Microsoft Proxy Server permits SSL tunneling, which provides an encrypted path between the client and remote server. This feature is useful for secure Internet transactions and other applications.
Full authentication / logging	The built-in WinSock Proxy performs full access control, encrypted authentication, and logs all transactions.
Complementary Third Party Applications – the Virtual Bundle	A variety of third party products "plug in" to Microsoft Proxy Server 2.0 for value-added or specialized security. For example, you can use filters to prevent viruses, Java scripts or ActiveX controls from being downloaded into your secured network. Third party applications work with Proxy Server via the Internet Server Application Programming Interface (ISAPI). This extensibility gives customers great choice and flexibility.
*PERFORMANCE AND COST-SAVINGS*
Array-Based Content Caching	New! Proxy Server now allows you to set up distributed caching among multiple Proxy Server computers. Arrays allow a group of Proxy Server computers to be treated and administered as a single, logical entity. Arrays provide load balancing, fault tolerance, scalability, and ease of administration.
Hierarchical Content Caching	New! Proxy Server now enables caching across a hierarchical connection of individual Proxy Server computers or arrays, enabling distributed deployment to branch offices and departments. Requests from clients are sent upstream through the hierarchy until the requested object is found.
Cache Array Routing Protocol	New! This is a new approach for performing scalable array-based and hierarchical-based caching that has been submitted to the IETF. The new protocol, developed by Microsoft, provides substantial advantages over alternative approaches in performance to enable linear scalability with cache arrays or hierarchies. Microsoft Proxy Server 2.0 is the first product to make use of this protocol.
Active Intelligent Caching	Microsoft Proxy Server 2.0 automatically determines which web sites are most used and how frequently their content is refreshed. Proxy Server uses this information to proactively pre-load that web content into its cache during periods of low network use. This provides a consistent, accelerated Internet experience for all users accessing these web sites, without requiring network manager intervention.
FTP and HTTP Cache Support	New! Now you can cache not only HTTP 1.0 objects - you can also cache HTTP 1.1 and FTP objects. There is greater control over the Time-to-Live (TTL) setting, as well, with Microsoft Proxy Server version 2.0.
Hypertext Transfer Protocol (HTTP) version 1.1	New! Implementation of HTTP 1.1 allows Proxy Server to use persistent client-to-proxy server connections, persistent proxy server-to-Internet server connections, read-range, and virtual hosts. Full support of HTTP 1.1 helps Microsoft Proxy Server deliver significant performance gains. (NOTE: Full HTTP 1.1 support requires use of the Internet Information Server 4.0 HTTP engine which is not supported in the initial beta)
Improved Cache and Proxy Performance	New! Microsoft Proxy Server 2.0 offers unbeaten performance to meet real-world requirements of fulfilling requests not only from the cache but also from across a live Internet connection – all while performing the full range of security and managed access. Microsoft Proxy Server 2.0 is up to 40% faster than Proxy Server 1.0
*EASY, COMPREHENSIVE MANAGEMENT SUPPORT*
Windows NT Server Integration	Microsoft Proxy Server capitalizes on features that make Windows NT Server a secure, scalable network operating system. This includes the best integration with the Windows NT Server directory service for easier manageability and reduced total cost of ownership. Microsoft Proxy Server supports a single user logon for network services and applications so user accounts do not have to be re-created for Proxy Server.
User Access Control	Network managers can use Microsoft Proxy Server to set detailed user & group permission lists by Internet protocol in the Web Proxy, WinSock Proxy, and SOCKS Proxy components
Site Filtering	Network managers can specify a list of Internet addresses (IP addresses, IP address ranges, or URLs) to be exclusively permitted or denied for access by users behind the Proxy Server.
GUI-Based Administration	Microsoft Proxy Server’s Internet Service Manager provides an easy to understand way to administer a local or remote proxy server computer, proxy array, or proxy hierarchy.
HTML-Based Administration	New! You can administer Microsoft Proxy Server locally or remotely via a web browser for added management flexibility and ease-of-use. You can even create customized HTML error pages. (NOTE: HTML-based admin is available as a Web download.)
Command Line Administration	New! This tool lets you manage Microsoft Proxy Server through MS-DOS prompts if that is your preference. You can configure and manage one or more local or remote servers with this tool.
Array Administration	New! Multiple proxy servers can be administered simultaneously via transparent Array based administration. This allows change to be propagated to other proxy servers with a single mouse click.
Configuration Backup & Restore	New! You can now back up your server configuration to a file or roll back to a previous configuration.
Client Auto-Configuration	New! You can automatically configure Web Proxy clients by using predefined JavaScripts or by creating your own scripts for great ease-of-use and fast enterprise-wide deployment. You can also use the Internet Explorer Administration Kit or Microsoft Systems Management Server to automate the proxy client installation.
SNMP Support	A network manager can monitor and examine the current status of any Microsoft Proxy Server on the network using an SNMP console such as HP OpenView for added flexibility and reduced cost of ownership.
*FLEXIBLE NETWORKING AND APPLICATIONS SUPPORT*
IPX-to-IP Gateway	Unlike other proxy servers, Microsoft Proxy Server 2.0 does not require that network managers "rip and replace" existing legacy IPX network with IP networks. This built-in IPX-to-IP gateway can be much less expensive than other solutions. Windows 95 and Windows NT Workstation 4.0 clients are supported.
Auto-Dial Connection	This unique feature dynamically connects your network to your ISP, as needed, providing even more cost savings and user access control.This also includes a backup route to the Net if primary path is busy.
SOCKS Support	New! You can now configure Microsoft Proxy Server as a SOCKS server or as a SOCKS client to an upstream SOCKS server for easy access to rich Internet services for Macintosh, Unix or other client PCs.
Unbeaten LAN & WAN Connectivity Options	Microsoft Proxy Server 2.0 can be used with over 2,000 LAN and WAN cards that have earned the Windows NT Compatible logo. This provides unbeaten customer choice and flexibility.
Great Protocol Support	Great variety of protocols are built in to Proxy Server plus more protocols can be added, The Web Proxy supports: HTTP, HTTP-S, FTP, and Gopher. The WinSock Proxy includes: AlphaWorld, AOL, Archie, Echo, Enliven, IMAP4, IRC, Microsoft NetShow, MSN, NNTP, POP3, RealAudio, SMTP, Telnet, and VDOLive. Other protocols can be added with the WinSock Proxy service and with the SOCKS Proxy service.

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