Chapter 33

How Remote Access Works


The days of working at an office every day from 9:00 a.m. to 5:00 p.m. and only occasionally working into the night are long gone. Today, people may be telecommuting from home, they may be on the road, and they may work evenings or weekends from their home office. The days of the virtual office are here, and intranets are an important part of making that a reality.

Since intranets hold so much of a corporation's resources, and since so much work these days is collaborative work done via the network, people need access to the intranet in order to do any work. That means they need some remote way of gaining access to the intranet.

Typically, remote access is gained via a modem. The most common method is to dial into a remote access server and its associated modem bank. They dial in using one of the Internet's standard dial-in protocols, either the Point-to-Point Protocol (PPP) or the Serial Line Interface Protocol (SLIP). SLIP is an older protocol and has fast been falling out of favor because the PPP protocol is more robust, especially when it comes to handling errors. Part of the process of dialing in involves identification of the user. Some remote access servers hang up and call the individual back at a pre-determined phone number.

After someone logs into the remote access server, he or she can log into machines on the intranet just like in the office. The intranet's firewall allows packets sent via the remote access server to enter the intranet. Once they've logged in, they have full access to the intranet, although at dial-in speeds instead of at higher speeds available when actually at the office.

Providing dial-in access in this manner is expensive, because corporations have to maintain large banks of modems that can be dialed into, and because they have to pay for the costs of long-distance and 800 telephone numbers.

A solution developed by Microsoft, 3Com, US Robotics, and others is called the Point-to-Point Tunneling Protocol (PPTP). This protocol allows 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 a long-distance one, and the banks of modem pools aren't needed.

PPTP also allows for people to use other network protocols, such as IPX or NetBIOS, so they can access parts of the corporate network that aren't TCP/IP-based. And it also allows for secure transmission of data. It does this by encrypting the data being sent, and encapsulating it and the other network protocols inside an IP packet. That IP packet is then sent out over the Internet through a technique called tunneling. On the receiving end, the outer IP envelope is stripped off, and the protocols and data inside the packet used. The person now has full access to the intranet and other corporate network resources, and has done it by making a local phone call.

Providing Remote Access to an Intranet

In today's increasingly mobile world, it's important that people be able to access a corporate intranet from their homes or from the road. This illustration shows how that access can be gained via a new protocol called the Point-to-Point Tunneling Protocol (PPTP).

  1. Before the PPTP protocol, when people wanted to gain access to an intranet they usually dialed into a remote access server through its modem bank. After logging into the server, they were then able to get access to the intranet's resources. One drawback of this approach is that it required the corporation to pay for long-distance or 800 telephone access and maintain the modem banks, which can easily cost millions of dollars a year.
  2. The PPTP protocol allows people to gain access to an intranet by dialing into an Internet Service Provider (ISP) and requesting to be sent to the intranet. The connection to the ISP is made using the normal PPP Internet dial-in protocol. Since ISP calls can be local calls, this cuts down tremendously on telecommunications costs. It also means that the intranet need not have sizable modem pools available to answer every incoming call, another significant cost-savings.
  3. The ISP has special software and hardware installed that uses the PPTP protocol. An important component of gaining access to an intranet is to ensure that any data sent to and from it is secure. The PPTP protocol can encrypt the data in the IP packet it receives. It then takes that encrypted packet and encapsulates it inside another IP packet, sometimes called an envelope. PPTP also allows remote users to get at corporate network information that uses other protocols than TCP/IP, such as IPX and NetBIOS. It does this by encapsulating it inside the IP packet as well.
  4. The ISP sends the envelope with the encrypted data inside it through the public Internet to the intranet. No one can read what is inside the envelope since the data is encrypted. When data is sent in this manner, it is called tunneling.
  5. The data is sent through a firewall to a server on an intranet. This server has the hardware and software necessary to handle the incoming PPTP packets.
  6. The person trying to get at intranet data will have to log into this server with a user name and password, just as he or she would have to if directly connected to the intranet, as a way to keep out intruders. PPTP uses two protocols for allowing people to log in, the Password Authentication Protocol (PAP) and the Challenge Handshake Authentication Protocol (CHAP). See Chapter 17 for more on how these protocols work.
  7. The intranet server strips off the outside envelope. It then decrypts the data inside the envelope. The person can now make full use of the intranet-or other network resources. All packets that pass between the intranet and the user will go through this tunneling technique.