Abstract:
An improved network content filtering system and method utilize the network address translation functionality of a shared network connection to redirect outgoing packets from a client intended for a destination web server to an access controlling web server instead. Before a session to the destination web server is established, the access controlling web server either approves or refuses the connection, providing a content filtering mechanism. If the connection is refused, the access controlling web server may substitute other content for a filtered URL. In order to identify the client, the shared connection may additionally embed an identifier token in the redirected traffic, so as to customize the filtering action or to facilitate billing functions.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates generally to network access and, more particularly, relates to filtering of content retrievable from a wide area network such as the Internet.  
         [0002]     BACKGROUND OF THE INVENTION  
         [0003]     With the explosion of the Internet in recent years, an increasing amount of valuable information has become available online. The Internet has become a global community, rich with resources and communications facilities. However, the Internet is also a frontier that remains largely unregulated, and hence contains many instances of harmful or objectionable material. For example, web sites containing violent or pornographic materials are common, as are sites advocating extremist viewpoints. Additionally, perusers of the Internet are often bombarded with unsolicited advertising that they may find annoying or offensive.  
         [0004]     Accordingly, it is often desirable to filter the content that may be retrieved from the Internet. For example, a parent or teacher may wish to prevent a child from viewing materials on violent, pornographic, or bigoted sites. Additionally, users may wish to avoid the receipt of unsolicited advertisements contained within a page being viewed.  
         [0005]     Certain schemes to effect content filtering are known. For example, Net Nanny® resides on a personal computer (PC) client and works by checking intended URL&#39;s with a local list of URL&#39;s corresponding to disallowed sites. If the intended URL is on the list, the user is denied access to the site.  
         [0006]     Most PC&#39;s and other client computers are not connected directly to the Internet. Such computers may instead be linked to the Internet through a router, or “gateway.” For example, an Internet service provider may provide Internet access for a home computer through a shared connection. Additionally, some computers, especially those in a commercial environment, reside on a local area network (LAN), which is connected to the Internet through a gateway, which may be a firewall as well.  
         [0007]     The placement of the gateway between the LAN, or the home computer, and the Internet has allowed for content filtering by way of what has come to be known as a “proxy server.” Also called an application level gateway, a proxy server is essentially an application that intervenes between a sender and a receiver. Proxy servers generally employ network address translation (NAT), a technique which presents a single IP address to the Internet regardless of which particular computer behind the server sent the message. Thus, the proxy server directs all user requests to the Internet as if they were coming from a single IP address, and distributes responses back to the appropriate users.  
         [0008]      FIG. 1  illustrates the functionality of a typical proxy server when used for content filtering. As shown, a client  200  transmits a packet for a connection to a URL on the Internet to a gateway  204 . In addition to other functions such as address translation and protocol compliance, the gateway  204  instantiates an application level proxy  206  connected to the client via a connection  212 . The proxy  206  may contact a local or remote database  208  of disallowed sites to determine whether the requested URL corresponds to a disallowed site. If so, the connection is refused; if the requested URL does not correspond to a disallowed site, the proxy  206  establishes a connection  214  to the remote server  210  corresponding to the requested URL. During the same session, subsequent transmissions are passed by the proxy  206  between the connections  212  and  214 .  
         [0009]     The proxy server suffers many shortcomings as a means of filtering Internet content. Most importantly, use of a proxy server is slow, given that time must be spent to instantiate the proper proxy. Furthermore, all subsequent packets, even to a previously approved site, are still handled and passed off via the proxy, incurring additional transmission time. Additionally, the use of a proxy sever in this way often requires a reconfiguration of the client application, increasing administrative overhead for the local network.  
         [0010]     Another system for filtering Internet content uses the PICS rating system. According to this method, a client browser is configured to first query a PICS/RSACi server regarding a requested URL. If the server indicates that the URL is not disallowed, then the browser proceeds to access the requested URL without any further intervention from the PICS/RSACi server. This system is inadequate in that it allows a clever user to bypass the filtering mechanism at the browser level without facing additional hurdles thereafter. Also, this system increases administrative and overhead costs in that it requires each client machine to be configured to provide the desired filtering communications.  
       SUMMARY OF THE INVENTION  
       [0011]     In view of the foregoing, the present invention provides a method and system for network access control that extends the Network Address Translation (NAT) capabilities of a gateway, firewall, or other shared connection node to redirect communication packets, from a client on a first network destined for a target server on a second network, to an access control server, which then indicates whether access to a resource on the target server should be allowed. In particular, when the client sends handshake packets intended for the target server to the gateway or other shared connection, the gateway redirects the handshake packets to the access control server by rewriting the packet destination address. The access control server sends a response to the gateway which the gateway interprets to either allow or disallow access of the client to the resource on the target server. If access is allowed, all subsequent packets in that session are simply inspected on the fly by the gateway to determine when a connection to a different destination is attempted.  
         [0012]     This method operates much more efficiently than existing filtering mechanisms due to its limited intervention in an approved session, as well as its ability to function without instantiating proxies or reconfiguring clients. The filtering function provided by the invention is also difficult to circumvent by local client users because it does not reside on the client machine. Additionally, the invention provides a mechanism whereby content filtering takes place with reference to distributed rather than centralized listings or standards, increasing the variety of lists that may be used.  
         [0013]     Additional features and advantages of the invention will be made apparent from the following detailed description of illustrative embodiments which proceeds with reference to the accompanying figures.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     While the appended claims set forth the features of the present invention with particularity, the invention, together with its objects and advantages, may be best understood from the following detailed description taken in conjunction with the accompanying drawings of which:  
         [0015]      FIG. 1  is a schematic diagram generally illustrating a prior art filtering mechanism;  
         [0016]      FIG. 2  is a block diagram generally illustrating an exemplary computer system with which the present invention may be used;  
         [0017]      FIG. 3  is a simplified diagram of a network environment having a client, a gateway, an intended server, and a control server for access control, for implementing an embodiment of the invention;  
         [0018]      FIG. 4  is a diagram of network communications in the environment of  FIG. 3  in a case where access to a desired URL is allowed; and  
         [0019]      FIG. 5  is a diagram of network communications in the environment of  FIG. 3  in a case where access to a desired URL is not allowed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     Turning to the drawings, wherein like reference numerals refer to like elements, the invention is illustrated as being implemented in a suitable computing environment. Although not required, portions of the invention will be described in the general context of computer-executable instructions, such as program modules, being executed by a computer. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the invention may be practiced with other computer system configurations, including hand-held devices, multi-processor systems, microprocessor based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. The invention may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices.  
         [0021]     With reference to  FIG. 2 , an exemplary system for implementing a network client machine includes a general purpose computing device in the form of a conventional personal computer  20 , including a processing unit  21 , a system memory  22 , and a system bus  23  that couples various system components including the system memory to the processing unit  21 . The system bus  23  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory includes read only memory (ROM)  24  and random access memory (RAM)  25 . A basic input/output system (BIOS)  26 , containing the basic routines that help to transfer information between elements within the personal computer  20 , such as during start-up, is stored in ROM  24 . The personal computer  20  further includes a hard disk drive  27  for reading from and writing to a hard disk  60 , a magnetic disk drive  28  for reading from or writing to a removable magnetic disk  29 , and an optical disk drive  30  for reading from or writing to a removable optical disk  31  such as a CD ROM or other optical media.  
         [0022]     The hard disk drive  27 , magnetic disk drive  28 , and optical disk drive  30  are connected to the system bus  23  by a hard disk drive interface  32 , a magnetic disk drive interface  33 , and an optical disk drive interface  34 , respectively. The drives and their associated computer-readable media provide nonvolatile storage of computer readable instructions, data structures, program modules and other data for the personal computer  20 . Although the exemplary environment described herein employs a hard disk  60 , a removable magnetic disk  29 , and a removable optical disk  31 , it will be appreciated by those skilled in the art that other types of computer readable media which can store data that is accessible by a computer, such as magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, random access memories, read only memories, and the like may also be used in the exemplary operating environment.  
         [0023]     A number of program modules may be stored on the hard disk  60 , magnetic disk  29 , optical disk  31 , ROM  24  or RAM  25 , including an operating system  35 , one or more applications programs  36 , other program modules  37 , and program data  38 . A user may enter commands and information into the personal computer  20  through input devices such as a keyboard  40  and a pointing device  42 . Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to the processing unit  21  through a serial port interface  46  that is coupled to the system bus, but may be connected by other interfaces, such as a parallel port, game port or a universal serial bus (USB). A monitor  47  or other type of display device is also connected to the system bus  23  via an interface, such as a video adapter  48 . In addition to the monitor, personal computers typically include other peripheral output devices, not shown, such as speakers and printers.  
         [0024]     The personal computer  20  preferably operates in a networked environment using logical connections to one or more remote computers, such as a remote computer  49 . The remote computer  49  may be another personal computer, a server, a router, a network PC, a peer device and/or other common network node, and typically includes many or all of the elements described above relative to the personal computer  20 , although only a memory storage device  50  has been illustrated in  FIG. 2 . The logical connections depicted in  FIG. 2  include a local area network (LAN)  51  and a wide area network (WAN)  52 . Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets and the Internet.  
         [0025]     In a LAN networking environment, the personal computer  20  is connected to the local network  51  through a network interface or adapter  53 . In a WAN networking environment, the personal computer  20  typically includes a modem  54  or other means for establishing communications over the WAN  52 . The modem  54 , which may be internal or external, is connected to the system bus  23  via the serial port interface  46 . In a networked environment, program modules depicted relative to the personal computer  20 , or portions thereof, may be stored in the remote memory storage device. It will be appreciated that the network connections shown are exemplary and other means of establishing a communications link between the computers may be used.  
         [0026]     In the description that follows, the invention will be described with reference to acts and symbolic representations of operations that are performed by one or more computers, unless indicated otherwise. As such, it will be understood that such acts and operations, which are at times referred to as being computer-executed, include the manipulation by the processing unit of the computer of electrical signals representing data in a structured form. This manipulation transforms the data or maintains it at locations in the memory system of the computer, which reconfigures or otherwise alters the operation of the computer in a manner well understood by those skilled in the art. The data structures where data is maintained are physical locations of the memory that have particular properties defined by the format of the data. However, while the invention is being described in the foregoing context, it is not meant to be limiting as those of skill in the art will appreciate that various of the acts and operations described hereinafter may also be implemented in hardware.  
         [0027]     In overview, a system is provided for controlling the information available to a network client residing on a first network, the network client being connectable to an intended information server and a controlling information server residing on a second network via a gateway which resides on both networks. In operation the controlling information server may maintain a list referring to information which is not to be made available to the network client. At the time that the network client requests information from the intended information server, the gateway redirects the request to the controlling information server, which references the list and returns to the gateway an indication of whether the requested information is to be made available to the network client. If the information is to be made available, the gateway establishes a connection between the network client and the intended information server. If the information is not to be made available, the gateway establishes a connection between the network client and the controlling information server.  
         [0028]     Now referring to  FIG. 3  wherein certain aspects of the invention are illustrated in greater detail, a client  300  residing on a local network  310  is communicably connected via a local network connection or otherwise, to a gateway  302 . The client  300  may be a PC, workstation or other network capable machine, while the gateway  302  is preferably a firewall, router, or other connection node disposed between the client and a wide area or local area network. 304 . The gateway  302  preferably resides on both networks. The network  304  is preferably the Internet, but may alternatively be any other similar distributed linked resource system.  
         [0029]     In order to retrieve information from the Internet, for instance from intended server  306 , the client  300  sends a packet to the gateway  302  to be forwarded to the intended web site. The Internet content within the packet may be embedded in a LAN protocol at this stage, requiring formatting into an Internet protocol, typically TCP/IP, prior to transmission by the gateway.  
         [0030]     Each node in a TCP/IP network is assigned an “IP address,” which is typically composed of four numbers separated by periods, but which may be composed of more numbers depending upon the protocol used. (For example, a new generation of IP, referred to as IPv 6 , increases the address space from 32 to 128 bits). Nodes may be clients, servers, routers, and so on. Typically, the address is split between a Net ID, which allows the packet to be routed to other networks, and a Host ID. The exact way in which the address is split between these components is determined by the class system being used, which is indicated via the first three bits of the first byte of the address.  
         [0031]     Typically, all of the client machines attached to the local network served by the gateway  302  may be mapped to a single IP address with respect to the other network. To accomplish this, the gateway usually also performs what is known as Network Address Translation (NAT) on any outgoing packets. This entails rewriting the source address in the outgoing packet to correspond to the IP address of the gateway on the other network. On incoming packets, the procedure is reversed, and the packets are routed to the appropriate client. This technique serves both to conserve Internet address space and to hide internal network addresses from possible intruders.  
         [0032]     By way of example, referring again to  FIG. 3 , the client  300  typically sends a packet to the intended destination server  306  via the following process: the client  300  prepares a packet containing, among other things, a source IP address corresponding to the client (for example, 10.1.1.2), and a destination address corresponding to the server  306  (for example, 18.62.0.6). Following standard TCP/IP protocol routing procedure, the client  300  has been configured to send all packets destined off the local network to router  302  on its internal interface (for example, 10.1.1.4). Prior to forwarding the packet to the Internet, the NAT component of the router  302  modifies the packet&#39;s source address to correspond to the router&#39;s own Internet IP address (for example, 192.101.186.3). At the same time, the router  302  records other session-identifying information, so that the procedure can be accurately reversed for incoming packets. This is necessary because, although not shown, several other computers may also routinely access the Internet via the same router  302 . Typically, if the requested URL is not found on the destination server  306 , the destination server  306  returns an error code, such as “Error  404 : Object not found.” For more detailed information regarding TCP/IP networking, the reader is referred to  Internetworking With TCP/IP, Volume I: Principles, Protocols, and Architecture , by Douglas E. Comer, published by Prentice Hall (1995).  
         [0033]     In accordance with an aspect of the present invention, the network address translation capability described above is modified to provide a content filtering mechanism. Referring to  FIG. 3 , a server  308 , which may be an ordinary web server, will be labeled herein as an Access Controlling Web Server (ACWS). The ACWS  308  preferably hosts a list of disallowed URL&#39;s, which it recognizes itself to correspond to. As will be described in fuller detail hereinafter, the gateway  302  uses its packet access during network address translation to initially alter the destination as well as the source address of a packet, such that the packet is redirected to the ACWS instead of the intended server  306 . Based on a response from the ACWS  308 , the gateway  302  decides either to allow all subsequent session transmissions between the client  300  and the server  306 , or alternatively to refuse a connection to the server  306 , preferably establishing instead a connection to the ACWS  308 .  
         [0034]     The communications of the invention will be described hereinafter with reference to standard HTTP packets. It will be understood by those skilled in the art that the contents of each packet will be tailored to accomplish the particular transmission in the desired fashion. For example, the GET URL packet will likely reference a particular URL. Generally, within the HTTP protocol, a session is established by way of a handshaking process. This handshaking process consists of a SYN packet from the client, a SYN-ACK packet from the destination, and an ACK packet from the client. This exchange is typically followed by a GET URL packet sent from the client, and a data exchange comprised of DATA and ACK packets between the client and destination. According to an embodiment of the invention, the gateway first alters this ordinary course of events by redirecting the initial handshaking such that it takes place not between the client  300  and server  306 , but between the gateway  302  and the ACWS  308 .  
         [0035]     Certain of the communications involved in the redirection process of a preferred embodiment are illustrated in  FIG. 4 . The illustrated exchange corresponds to a situation wherein the requested URL is not a disallowed URL. To initiate a session, the client  302  in step  1  sends a typical SYN packet destined for the original server  306  to the gateway  302 . Typically, agreed upon ports correspond to well-known applications. For example, HTTP applications are usually on port “ 80 ”, so that a web server is located by specifying its address and port ( 80 ). Thus, the SYN packet will typically be addressed to port “ 80 ” of the original server  306 . This combination, or some other event, may be used by the gateway  302  to detect the start of a new session and hence to begin redirection. Thus, upon receiving this SYN packet, the gateway  302  may change the packet source IP address pursuant to ordinary NAT, and further changes the packet destination IP address to be that of the ACWS  308 . Thus, in step  2 , the ACWS receives the packet originally destined for server  306 .  
         [0036]     The ACWS responds in an ordinary manner by transmitting a SYN-ACK packet to the gateway  302  at the indicated IP address in step  3 , which is forwarded to the client  300  in step  4 , again via-ordinary NAT. In steps  5  and  6 , an ACK packet is passed from the client to the ACWS via the gateway similarly to the transmissions of steps  1  and  2 . At this point, still unaware of the redirection, the client sends a GET URL packet destined for the server  306  in step  7 . As with the previous outgoing packets, the gateway  302  redirects this GET URL packet to the ACWS in step  8 . As with many typical servers, the ACWS maintains or accesses a list of URL&#39;s to which it corresponds. In an embodiment of the invention, this list is preferably a list of disallowed URL&#39;s. Upon checking the list, if the ACWS does not locate an entry corresponding to the URL requested in the GET URL packet, the ACWS returns a standard error message, such as “Error  404 : Object not found,” to the gateway  302  in step  9 .  
         [0037]     In response to receipt of this error message, the gateway  302  determines that the requested URL is not a disallowed URL. Thus, in step  10  through  13 , the gateway replays, and responds to, the initial handshaking packets to the original server  306 . To facilitate this exchange, the gateway has preferably maintained a record of the packets involved in the handshaking process. The result of this sequence is to establish a connection between the client and the intended server  306  without apprising the client of the initial redirection. Alternatively, the client may be apprised of the redirection, but it is preferable in the interest of speed and convenience that the client not be required to take additional steps thereafter to effect a connection to the desired server once a URL has been approved. Once steps  2 , 3 , 6 , and  8  have been repeated between the gateway.  302  and the server  306  in steps  10 - 13 , a connection is established between the client  300  and the server  306 . A data exchange thereafter takes place in steps  15  et seq., with the gateway  302  intervening essentially only to accomplish ordinary NAT and to monitor packets for attempts to start a new session. Although only two data exchanges are shown, there may be an arbitrary number of data exchanges at this point.  
         [0038]     If the requested URL is a disallowed URL rather than an allowed URL, the network steps and communications may be as illustrated in  FIG. 5 . In particular, the handshaking sequence of steps  1 - 8  are preferably the same as the like-numbered steps in  FIG. 4 . However, it may be that upon receipt of the GET URL packet in step  8 , the ACWS  308  finds a corresponding entry in its listing of disallowed URL&#39;s. In this event, the ACWS  308  preferably returns data, rather than an error message, to the gateway  302 . Upon receiving this data, the gateway  302  preferably performs the standard reverse mapping of the Network Address Translation, forwarding the data to the client  300 . Thus a connection is established between the client  300  and ACWS  308 , and the client continues in communication with the ACWS  308  rather than the intended destination server  306 .  
         [0039]     In this situation, the data provided by the ACWS  308  to the client  300  is any desired content. For example, if the desired URL corresponded to advertising material, the ACWS  308  may substitute alternative advertising materials, or some other informative or entertaining material to fill the user interface space allocated for the filtered advertisement. Likewise, if the desired URL corresponded to offensive or inappropriate content, the ACWS  308  may supply an advertisement, or other inoffensive or appropriate material to fill the user interface space allocated for the filtered material. Alternatively, the ACWS  308  could simply provide a notation that content had been filtered or that a connection was not made, a warning or other message, or other filler material such as a design or solid color.  
         [0040]     It may be desirable, in keeping with the invention, to allow different filtering with respect to different clients. This is easily accomplished by the gateway  302 , by redirecting to different ACWS&#39;s depending upon the identity of the client. One benefit of the invention in allowing distributed content filtering, is the elimination of reliance on any single list service. This allows for greater customization and control of the filtering process and parameters.  
         [0041]     Along similar lines, it may be desirable to apprise the ACWS of the identity of the client. This may aid in performing authentication, billing functions, customization of response, and so on. One way to accomplish this notification is to embed an identifying token in the initial HTTP GET packet application header, subsequently adjusting sequence and acknowledgment numbers to reflect the change in packet size. Such a token identifies the client and could additionally identify a particular user. Using this method, the added identifying functionality is accomplished transparently to the client, and accordingly to the user.  
         [0042]     In an alternative embodiment, the response of the ACWS is inverted from that described above. That is, certain ACWS&#39;s could respond to a request for a disallowed URL by transmitting an error message, or a “not OK” message, while responding to an allowed URL request with an “OK” message, instead of an error message. In such an embodiment, the gateway  302  would modify its behavior in accordance with this alternate response scheme, so as to enable connections only to allowed URL&#39;s. Accordingly, on receipt of an “OK” response, the gateway would make the desired connection and step out of the process. Examples of potential ACWS&#39;s which behave in this manner are existing RSACi Web servers.  
         [0043]     It will be appreciated that an improved system and method of network content filtering has been described, which overcomes many shortcomings inherent in prior content filtering methods. The described system and method additionally enable distributed filtering relying on a wide variety of independent content listings, allowing for greater customization and ease of maintenance. All of the references cited herein are hereby incorporated in their entireties by reference.  
         [0044]     In view of the many possible embodiments to which the principles of this invention may be applied, it should be recognized that the embodiments described herein with respect to the drawing figures are meant to be illustrative only and should not be taken as limiting the scope of invention. For example, those of skill in the art will recognize that certain elements of the illustrated embodiments shown in software may be implemented in hardware and vice versa or that the illustrated embodiments can be modified in arrangement and detail without departing from the spirit of the invention. Therefore, the invention as described herein contemplates all such embodiments as may come within the scope of the following claims and equivalents thereof.