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<HTML><HEAD><TITLE>Chapter 33 -- How Remote Access Works</TITLE><META></HEAD><BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#0000EE" VLINK="#551A8B" ALINK="#CE2910"><H1><FONT SIZE=6 COLOR=#FF0000>Chapter 33</FONT></H1><H1><FONT SIZE=6 COLOR=#FF0000>How Remote Access Works</FONT></H1><HR><P><CENTER><B><FONT SIZE=5><A NAME="CONTENTS">CONTENTS</A></FONT></B></CENTER><UL><LI><A HREF="#ProvidingRemoteAccesstoanIntranet">Providing Remote Access to an Intranet</A></UL><HR><P>The days of working at an office every day from 9:00 a.m. to 5:00p.m. and only occasionally working into the night are long gone.Today, people may be telecommuting from home, they may be on theroad, and they may work evenings or weekends from their home office.The days of the virtual office are here, and intranets are animportant part of making that a reality.<P>Since intranets hold so much of a corporation's resources, andsince so much work these days is collaborative work done via thenetwork, people need access to the intranet in order to do anywork. That means they need some remote way of gaining access tothe intranet.<P>Typically, remote access is gained via a modem. The most commonmethod is to dial into a remote access server and its associatedmodem bank. They dial in using one of the Internet's standarddial-in protocols, either the Point-to-Point Protocol (PPP) orthe Serial Line Interface Protocol (SLIP). SLIP is an older protocoland has fast been falling out of favor because the PPP protocolis more robust, especially when it comes to handling errors. Partof the process of dialing in involves identification of the user.Some remote access servers hang up and call the individual backat a pre-determined phone number.<P>After someone logs into the remote access server, he or she canlog into machines on the intranet just like in the office. Theintranet's firewall allows packets sent via the remote accessserver to enter the intranet. Once they've logged in, they havefull access to the intranet, although at dial-in speeds insteadof at higher speeds available when actually at the office.<P>Providing dial-in access in this manner is expensive, becausecorporations have to maintain large banks of modems that can bedialed into, and because they have to pay for the costs of long-distanceand 800 telephone numbers.<P>A solution developed by Microsoft, 3Com, US Robotics, and othersis called the Point-to-Point Tunneling Protocol (PPTP). This protocolallows someone to dial into a local Internet Service Provider(ISP), and from there access their intranet. Costs come down significantly,because the call is made to a local phone number instead of along-distance one, and the banks of modem pools aren't needed.<P>PPTP also allows for people to use other network protocols, suchas IPX or NetBIOS, so they can access parts of the corporate networkthat aren't TCP/IP-based. And it also allows for secure transmissionof data. It does this by encrypting the data being sent, and encapsulatingit and the other network protocols inside an IP packet. That IPpacket is then sent out over the Internet through a techniquecalled tunneling. On the receiving end, the outer IP envelopeis stripped off, and the protocols and data inside the packetused. The person now has full access to the intranet and othercorporate network resources, and has done it by making a localphone call.<H2><A NAME="ProvidingRemoteAccesstoanIntranet"><FONT SIZE=5 COLOR=#FF0000>Providing Remote Access to an Intranet</FONT></A></H2><P>In today's increasingly mobile world, it's important that peoplebe able to access a corporate intranet from their homes or fromthe road. This illustration shows how that access can be gainedvia a new protocol called the Point-to-Point Tunneling Protocol(PPTP).<OL><LI>Before the PPTP protocol, when people wanted to gain accessto an intranet they usually dialed into a remote access serverthrough its modem bank. After logging into the server, they werethen able to get access to the intranet's resources. One drawbackof this approach is that it required the corporation to pay forlong-distance or 800 telephone access and maintain the modem banks,which can easily cost millions of dollars a year.<LI>The PPTP protocol allows people to gain access to an intranetby dialing into an Internet Service Provider (ISP) and requestingto be sent to the intranet. The connection to the ISP is madeusing the normal PPP Internet dial-in protocol. Since ISP callscan be local calls, this cuts down tremendously on telecommunicationscosts. It also means that the intranet need not have sizable modempools available to answer every incoming call, another significantcost-savings.<LI>The ISP has special software and hardware installed that usesthe PPTP protocol. An important component of gaining access toan intranet is to ensure that any data sent to and from it issecure. The PPTP protocol can encrypt the data in the IP packetit receives. It then takes that encrypted packet and encapsulatesit inside another IP packet, sometimes called an envelope. PPTPalso allows remote users to get at corporate network informationthat uses other protocols than TCP/IP, such as IPX and NetBIOS.It does this by encapsulating it inside the IP packet as well.<LI>The ISP sends the envelope with the encrypted data insideit through the public Internet to the intranet. No one can readwhat is inside the envelope since the data is encrypted. Whendata is sent in this manner, it is called tunneling.<LI>The data is sent through a firewall to a server on an intranet.This server has the hardware and software necessary to handlethe incoming PPTP packets.<LI>The person trying to get at intranet data will have to loginto this server with a user name and password, just as he orshe would have to if directly connected to the intranet, as away to keep out intruders. PPTP uses two protocols for allowingpeople to log in, the Password Authentication Protocol (PAP) andthe Challenge Handshake Authentication Protocol (CHAP). See <A HREF="ch17.htm">Chapter17</A> for more on how these protocols work.<LI>The intranet server strips off the outside envelope. It thendecrypts the data inside the envelope. The person can now makefull use of the intranet-or other network resources. All packetsthat pass between the intranet and the user will go through thistunneling technique.</OL><HR><CENTER><P><A HREF="ch32.htm"><IMG SRC="PC.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><A HREF="#CONTENTS"><IMG SRC="CC.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><A HREF="contents.htm"><IMG SRC="HB.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><A HREF="ch34.htm"><IMG SRC="NC.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><HR WIDTH="100%"></P></CENTER></BODY></HTML>⌨️ 快捷键说明
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