Patent Publication Number: US-6904459-B1

Title: Methods and systems for preventing socket flooding during denial of service attacks

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims the benefit of U.S. provisional application Ser. No. 60/189,096, filed Mar. 14, 2000, which provisional application is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. The Field of the Invention 
   The present invention relates to computer networks. Specifically, the present invention relates to methods and system for preventing socket flooding during denial of service attacks. 
   2. The Prior State of the Art 
   Computer networks, and in particular the Internet, have transformed the way people communicate and do business. In these computer networks, computer systems may often communicate using a request/response protocol. For example, a requesting client computer system (“client”) will transmit a request for a service to a responding server computer system (“server”). The responding server then uses data from within the request in order to fulfill the request. 
   For example, a client may compose a request for a Web page. In such a request, there would typically be request data such as the Uniform Resource Locator (“URL”) identifying the Web page, the address of the client, and any other data that would be needed or helpful for the server to retrieve the Web page and transmit that Web page to the client. For each request, a typical server would allocate resources such as memory space, processing time or pooled function calls for receiving the request data. Upon processing of the request data, the server would then free up these allocated resources. 
   While the vast majority of individuals use computer networks in a responsible manner, there are a few individuals who maliciously desire to harm others using computer networks. One particular harmful scheme is to impair the operation of another&#39;s server. This may be accomplished by, for example, repeatedly transmitting requests to the server without sending any request data. 
   Unaware of the malicious nature of the attack, the server will unknowingly attempt to accommodate each request by allocating memory, processing time and/or pooled function calls for each request. However, in the described harmful scheme, since no request data is sent, the server cannot finish processing the request until it has received data from the client. Until it has finished processing the request, the allocated resources are tied up and unavailable for subsequent requests. The server will eventually time out the connection and reclaim the resources after a certain time, but the timeout period is relatively long compared to the time it takes an attacker to flood the computer with requests. Eventually, during this timeout period, the server will deplete its ability to allocate resources resulting in denials of service for subsequent legitimate requests during the timeout period. This effectively shuts down operation of the server during the timeout period resulting in a loss of service for legitimate requests. 
   Therefore, what are desired are methods and systems for reducing the incidence of service denials due to an attack in which requests are repeatedly made to the server without transmitting any request data. 
   SUMMARY OF THE INVENTION 
   The present invention relates to methods and systems for preventing or at least reducing the impact of denial of service attacks. Denial of service attacks occur when a client repeatedly sends connection requests to a server without sending corresponding request data. Without adequate protection, the server will allocate resources for each connection request. However, since no request data is sent, the server cannot finish processing the request and sits idle waiting for data from the client The resources are hence not freed up for subsequent requests. Eventually, the resources are expended to a point where the server cannot respond to any other requests, legitimate or not. Thus, the server is effectively shut down by the denial of service attack. 
   In accordance with the present invention, an effective method of reducing the impact of denial of service attacks is presented. In one embodiment, the method is implemented in large part using Winsock modules. For each connection request received by the server from one or more clients, the server attempts to establish a connection to accommodate the corresponding request. In the Winsock implementation, the Winsock extension Winsock( )AcceptEx( ) is used to try to establish a connection. 
   Next, the connection request is mapped to a corresponding listen socket. For each connection request that the server cannot currently handle, the connection request is placed in the backlog queue corresponding to the listen socket to which the connection request mapped. The backlog queues are monitored, for example, by calling a Winsock( )select( ) module and passing in those listen sockets that correspond to the monitored backlog queues. The backlog queues are determined to be used, for example, if the Winsock( )select( ) module returns. 
   If one or more of the backlog queues have entries, then the method determines which connection sockets have connections but no corresponding request data. This identification may be accomplished using, for example, the Winsock( )getsockopt( ) module. These connection sockets are suspected to be serving a malicious connection request since there is a connection but no request data received which is indicative of a denial of service attack. Thus, these connection sockets are disconnected. 
   The present invention allows for the early detection of denial of service attacks by immediately taking action once the backlog queue has entries, rather than waiting until the server becomes dysfunctional. If a denial of service attack were to occur, highly suspect connection sockets corresponding to the denial of service attack would be disconnected thereby freeing up resources for legitimate requests. Even if the denial of service attack were to continue, the method would continue to disconnect the maliciously established connections thereby allowing more legitimate connection requests to be satisfied even during the denial of service attack. This improves the security of the server against denial of service attacks and diminishes the malicious motive for generating denial of service attacks in the first place. 
   There is some risk associated with closing a connection socket simply when it has a connection but no received data. For example, the connection socket may not have been created as a result of a malicious connection request. Instead, it may be that the connection request was legitimate in that the associated connection socket just happened to be in a stage where the connection was just made but the soon to arrive request data simply has not arrived yet. In this case, a legitimate connection request would be denied. 
   However, this case would typically be relatively rare. For example, the legitimate connection request would not be denied unless the backlog queue had entries in it which should in itself be relatively rare. Secondly, even though the backlog queue is full, the period of time between the time a connection is made and the time the data is received is relatively brief for a legitimate connection request. Thus, the chance that the legitimate connection request would be executing in that brief period is also relatively small. 
   Notwithstanding this small risk, the method may be further optimized to reduce the chances for denying legitimate connection requests even further by allowing the systems administrator to specifying a grace period between the time the backlog queue is determined to be used and the time the identified connection sockets are disconnected. If, during this grace period, the server is able to handle the connection requests in the backlog queue, no connection sockets will be disconnected. 
   Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the manner in which the above-recited and other advantages and features of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
       FIG. 1  illustrates an exemplary system that provides a suitable operating environment for the present invention; 
       FIG. 2  is schematically illustrates a client and server communicating using a standard request/response protocol; 
       FIG. 3  illustrates a server-implemented process for responding to requests; 
       FIG. 4  illustrates a series of listen sockets implements using a Winsock module as existing on a server; and 
       FIG. 5  illustrates a server-implemented method of protecting against or at least reducing the impact of denial of service attacks. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention extends to both methods and systems for preventing denial of services due to socket flooding caused by a denial of service attack. The embodiments of the present invention may comprise a special purpose or general purpose computer including various computer hardware, as discussed in greater detail below. 
   Embodiments within the scope of the present invention also include computer-readable media for carrying or having computer-executable instructions or data structures stored thereon. Such computer-readable media can be any available media which can be accessed by a general purpose or special purpose computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code means in the form of computer-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer. When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or a combination of hardwired or wireless) to a computer, the computer properly views the connection as a computer-readable medium. Thus, any such a connection is properly termed a computer-readable medium. Combinations of the above should also be included within the scope of computer-readable media Computer-executable instructions comprise, for example, instructions and data which cause a general purpose computer, special purpose computer, or special purpose processing device to perform a certain function or group of functions. 
   FIG.  1  and the following discussion are intended to provide a brief, general description of a suitable computing environment in which the invention may be implemented. Although not required, the invention will be described in the general context of computer-executable instructions, such as program modules, being executed by computers in network environments. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Computer-executable instructions, associated data structures, and program modules represent examples of the program code means for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represent examples of corresponding acts for implementing the functions described in such steps. 
   Those skilled in the art will appreciate that the invention may be practiced in network computing environments with many types of computer system configurations, including personal computers, 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 local and remote processing devices that are linked (either by hardwired links, wireless links, or by a combination of hardwired or wireless links) through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices. 
   With reference to  FIG. 1 , an exemplary system for implementing the invention includes a general purpose computing device in the form of a conventional computer  120 , including a processing unit  121 , a system memory  122 , and a system bus  123  that couples various system components including the system memory  122  to the processing unit  121 . The system bus  123  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)  124  and random access memory (RAM)  125 . A basic input/output system (BIOS)  126 , containing the basic routines that help transfer information between elements within the computer  120 , such as during start-up, may be stored in ROM  124 . 
   The computer  120  may also include a magnetic hard disk drive  127  for reading from and writing to a magnetic hard disk  139 , a magnetic disk drive  128  for reading from or writing to a removable magnetic disk  129 , and an optical disk drive  130  for reading from or writing to removable optical disk  131  such as a CD-ROM or other optical media. The magnetic hard disk drive  127 , magnetic disk drive  128 , and optical disk drive  130  are connected to the system bus  123  by a hard disk drive interface  132 , a magnetic disk drive-interface  133 , and an optical drive interface  134 , respectively. The drives and their associated computer-readable media provide nonvolatile storage of computer-executable instructions, data structures, program modules and other data for the computer  120 . Although the exemplary environment described herein employs a magnetic hard disk  139 , a removable magnetic disk  129  and a removable optical disk  131 , other types of computer readable media for storing data can be used, including magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, RAMs, ROMs, and the like. 
   Program code means comprising one or more program modules may be stored on the hard disk  139 , magnetic disk  129 , optical disk  131 , ROM  124  or RAM  125 , including an operating system  135 , one or more application programs  136 , other program modules  137 , and program data  138 . A user may enter commands and information into the computer  120  through keyboard  140 , pointing device  142 , or other input devices (not shown), such as a microphone, joy stick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to the processing unit  121  through a serial port interface  146  coupled to system bus  123 . Alternatively, the input devices may be connected by other interfaces, such as a parallel port, a game port or a universal serial bus (USB). A monitor  147  or another display device is also connected to system bus  123  via an interface, such as video adapter  148 . In addition to the monitor, personal computers typically include other peripheral output devices (not shown), such as speakers and printers. 
   The computer  120  may operate in a networked environment using logical connections to one or more remote computers, such as remote computers  149   a  and  149   b.  Remote computers  149   a  and  149   b  may each be another personal computer, a server, a router, a network PC, a peer device or other common network node, and typically include many or all of the elements described above relative to the computer  120 , although only memory storage devices  150   a  and  150   b  and their associated application programs  136   a  and  136   b  have been illustrated in FIG.  1 . The logical connections depicted in  FIG. 1  include a local area network (LAN)  151  and a wide area network (WAN)  152  that are presented here by way of example and not limitation. Such networking environments are commonplace in office-wide or enterprise-wide computer networks, intranets and the Internet. 
   When used in a LAN networking environment, the computer  120  is connected to the local network  151  through a network interface or adapter  153 . When used in a WAN networking environment, the computer  120  may include a modem  154 , a wireless link, or other means for establishing communications over the wide area network  152 , such as the Internet. The modem  154 , which may be internal or external, is connected to the system bus  123  via the serial port interface  146 . In a networked environment, program modules depicted relative to the computer  120 , 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 communications over wide area network  152  may be used. 
     FIG. 2  illustrates a requesting client computer system  210  (hereinafter, “a client”) and a responding server computer system  220  (hereinafter, “a server”) which communicate over a network  230 . In a typical request/response communication protocol such as HyperText Transport Protocol (“HTTP”), the client  210  transmits a connection request  240  to the server  220  over the network  230 . The server  240  then provides a connection in response to the connection request and transmits a connection confirmation message  250  back to the client  210 . The client  210  then transmits request data  260  to the server  220 . The request data  260  includes information helpful in identifying what the request is as well as information helpful in fulfilling the request. If appropriate for the request, the server  220  then transmits a response  270  back to the client  210  over the network  230 . 
   The server computer system  220  is a “server” computer system in that it provides a service in the form of a connection and a response to the client computer system  210 . The server may also obtain the services of other computer systems over the network. In this context, the server  220  may also be a client computer system. The client computer system  210  is a “client” computer system in that it is served by the server providing the connection and generating the response. The client computer system  210  may provide services to yet other computer systems. In this context, the client computer system may also be a server computer system. The client  210  and the server  220  may each be structure similar to the computer  120  or may contain a subset or superset of the elements described above for the computer  120 . 
     FIG. 3  illustrates a flowchart of a method  300  performed by the server  220  when responding to requests from the client  210 . The method is initiated by the server  220  monitoring the network  230  for connection requests destined for the server  220  (step  310 ). The method continues as the server  220  detects such connection requests (step  320 ). The remainder of the method  300  is performed for each detected connection request. 
   For each connection request, a connection is established using a means or step for establishing a connection request. Specifically, for each connection request, the connection request is mapped to a specific listen socket (step  330 ). If the server is implementing the WINDOWS® operating system, the server may call a Winsock module to map the request to the listen socket.  FIG. 4  schematically illustrates a Winsock module  410  and associated listen sockets  420  and will be used in describing the remaining steps of FIG.  3 . As apparent to those of ordinary skill in the art, a listen socket allows the server to listen for the expected request data The Winsock module may create one or more listen sockets  420 A through  420 H. Step  330  maps the request to one of these listen sockets  420 . 
   If the server is able to accommodate the connection request (“Yes” in decision block  340 ), the server allocates resources (step  360 ) such as memory space, processing time or pooled function calls for receiving and processing the expected request data. The server computer system then receives the request data (step  370 ) and processes the request data (step  380 ). Once the server has completed processing the request, the server frees up the previously allocated resources and disconnects (step  390 ). 
   On the other hand, if the server  220  is unable to handle the connection request (“No” in decision block  340 ), then the connection request is placed in a backlog queue for future handling (step  350 ). As shown in  FIG. 4 , each listen socket  420 A through  420 H has a corresponding backlog queue  430 A through  430 H. If the server cannot handle the connection request, the connection request is passed into the queue corresponding to the listen socket that the connection request mapped to in step  330 . Although each listen socket has a request queue in  FIG. 4 , in an alternative embodiment, a more general backlog queue may be shared between one or more or all of the listen sockets. In this alternative, the server computer system may map the request to the listen socket after the connection request is drawn from the backlog queue during future processing. 
   The method  300  will now be explained in the context of a WINDOWS® operating system using a Winsock module to establish connections. For each detected connection request, the Winsock module maps the connection request to a listen socket (step  330 ). To establish a connection to the listen socket, a module may be called that accepts connections and waits for request data before completing. For example, an extension of the Winsock module called Winsock( )AcceptEx( ) is called and the corresponding listen socket is passed in along with the new connection socket that represents the connection to the listen socket. The Winsock( )AcceptEx( ) is completed when request data is beginning to be received from the network in step  370 . 
   Winsock may allocate a pool having a fixed number of Winsock( )Accept( ) calls available for creating new connections. If the entire pool of Winsock( )Accept( ) calls are already processing new connections, then the server is not currently able to satisfy subsequent connection requests (“No” in decision block  340 ). In this case, the connection request is placed in the backlog queue corresponding to listen socket (step  350 ). 
   In normal operation, it should preferably be very rare that the server  220  cannot currently handle a connection request However, a denial of service attack may often result in the server being unable to currently handle connection requests. In this description and in the claims, a “denial of server attack” is defined as the repetitious transmission of connection requests without a subsequent transmission of request data needed to process the requests. In such a denial of service attack, the method  300  of  FIG. 3  will proceed through step  360  in which resources are allocated. However, the server does not receive subsequent request data as in step  370 . Therefore, the allocated resources are never freed up in step  390 . Since connection requests are repeatedly made, the amount of allocated resources rises until the server can no longer allocate resources and thus must deny legitimate requests for service. 
   In the context of the Winsock module, the repeated connection requests will result in repeated calls of the Winsock( )AcceptEx( ) module. However, none of the Winsock( )AcceptEx( ) modules will complete since no request data is sent during a denial of service attack. Thus, the pool of Winsock( )AcceptEx( ) modules will gradually deplete. Eventually, the server  220  will not be able to handle new connection requests, legitimate or not, and the connection requests will be placed in the backlog queue. Eventually, the backlog queue will also be filled up and thus new connection requests will not be saved and thus will never be handled. 
     FIG. 5  illustrates a flowchart of a method  500  that prevents or at least reduces the impact of these denial of service attacks. As mentioned above, when the server  220  cannot currently handle a connection request, the connection request is place in a backlog queue. The method  500  monitors this backlog queue (step  510 ). Accordingly, embodiments within the scope of the present invention include a means and/or step for monitoring the backlog queue. Any method of monitoring the backlog queue will suffice so long as the method is capable of determining whether of not there are entries in the backlog queue. In the example shown in  FIG. 4 , each listen socket has a corresponding backlog queue. The method  500  may monitor these backlog queues by, for example, calling modules that scan the backlog queues to determine usage. On such module is a Winsock extension called Winsock( )select( ). A list of listen sockets is passed into the Winsock( )select( ) function. The Winsock( )select( ) module monitors the backlog queue of each of the listens sockets in the list of listen sockets passed into the Winsock( )select( ) module. 
   Next, the method  500  determines if the backlog queue is being used (step  520 ). Any method for determining that the backlog queue is being used will suffice. In the above example where the Winsock( )select( ) extension of Winsock is used to monitor the backlog queue, the determination is made by the very fact that the Winsock( )select( )extension module returns. The Winsock( )select( ) extension module returns when one or more of the listen sockets have entries in their corresponding backlog queues. 
   Next, the method  500  resets one or more connection sockets upon notification that the backlog queue is being used (step  530 ). Accordingly, embodiments within the scope of the present invention include a means and/or step for resetting one or more connection sockets upon notification that the backlog queue is being used. 
   As part of the step for resetting one or more listen sockets, the method  500  includes a step of determining which connection sockets have established connections, but have not received any data (step  540 ). In the context of using Winsock, the server computer system  220  enumerates all the connection sockets that have been created using a currently called Winsock( )AcceptEx( ) function. For each of these currently called Winsock( )AcceptEx( ) connection sockets, the extension Winsock( )getsockopt( ) is used to determine whether or not a connection has been established. If a connection has been established, then the connection socket is suspected of being caused by a malicious connection request since a connection has been made, yet no request data has been sent (otherwise, the Winsock( )AcceptEx( ) module would not be currently called but would have been completed). Thus, this connection socket may be disconnected (step  550 ) since it is assumed that a connection socket having a connection but no request data is most likely the result of a denial of service attack. 
   There is some risk associated with closing a connection socket simply because it has a connection but no received request data. For example, the connection socket may not have been created as a result of a malicious connection request. Instead, it may be the connection request was legitimate in that the associated connection socket just happened to be in a stage where the connection was just made but the soon to arrive request data simply has not arrived yet. In this case, a legitimate connection request would be denied. 
   However, this case would typically be relatively rare. For example, the legitimate connection request would not be denied unless the backlog queue had entries in it which should in itself be relatively rare. Secondly, even though the backlog queue is full, the period of time between the time a connection is made and the time the data is received is relatively brief for a legitimate connection request. Thus, the chance that the legitimate connection request would be executing in that brief period is also relatively small. On the other hand, using this method would substantially reduce the impact of denial of service attacks. Thus, the advantages of the method in reducing the impact of denial of service attacks would typically outweigh the relatively small risk of denying legitimate connection requests. 
   Notwithstanding this small risk, the method may be further optimized to reduce the chances for denying legitimate connection requests even further. For example, the server computer system  220  may be configured to allow for a specified grace period after entries are detected in the backlog queue before connections are disconnected. If, during this grace period, the server handles the connection requests in the backlog queue, no connection sockets are disconnected. 
   The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.