Patent Publication Number: US-7584352-B2

Title: Protection against denial of service attacks

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention generally relates to information processing systems and more specifically to a mechanism for protecting a server computer system or a server in a server farm from denial of service attacks. 
   2. Description of the Background Art 
   Applicant&#39;s commonly-owned co-pending U.S. patent application Ser. No. 09/650,524 entitled “Method for Protecting Web Servers Against Various Forms of Denial-of-Service Attacks” is incorporated by reference herein in its entirety. 
   A denial of service attack (DOSA) is an assault on a network site that overwhelms the network with so many requests for service that regular traffic is either slowed or completely interrupted. DOSAs against well-known web sites, such as the FBI&#39;s site, have become a problem in recent years. Moreover, as the threat of terrorism becomes more acute, countermeasures against denial of service attacks present a daunting challenge to operators of web sites. These attacks can take several forms such as attacks based on ICMP (Internet Control Message Protocol) packets, UDP (user datagram protocol) packets, and TCP (transmission control protocol) packets. Many types of DOSAs and related terms are described below. 
   Also described below are some known countermeasures to shield servers and/or server farms against a DOSA. Unfortunately, the methods used today are DOSA-type specific and some of them can actually contribute to slowing down the server by expending too much of the server&#39;s time and resources in verifying connections. A method is needed which can handle various forms of DOSAs without tying up memory or processor time. 
   Definition of Terms 
   ACK 
   Acknowledgment of receipt of transmission, used in communications networks. 
   AES 
   Advanced Encryption Standard. A state-of-the-art encryption standard that is being developed by the NIST. It is expected to replace DES. See “Computer Desktop Encyclopedia,” 9 th  Edition, McGraw-Hill 2001 by Alan Freedman (hereafter “Freedman”) 
   CAM 
   Content Addressable Memory (CAM). Also known as “associative storage,” content addressable memory differs from RAM in the way values in memory are returned. A standard RAM returns the value stored at a particular address. A CAM returns a value based upon the contents of memory. CAMs can provide a performance advantage over RAMs since they can compare a desired value against an entire set of stored values simultaneously and return the address where the value is stored in a single clock cycle. 
   Denial of Service Attacks 
   A set of one or more packets that is sent to a web site or other site on the Internet for the purpose of “bogging down” or disabling the site. Denial of service attacks against the FBI web site and Yahoo and eBay have been in the news in recent years. 
   DES 
   Data Encryption Standard. A NIST-standard secret key cryptography method that uses a 56-bit key. DES is based on an IBM algorithm which was further developed by the U.S. National Security Agency. It uses the block cipher method which breaks the text into 64-bit blocks before encrypting them. There are several DES encryption modes. The most popular mode exclusive ORs each plaintext block with the previous encrypted block. DES decryption is very fast and widely used. The secret key may be used in more than one session. Or, a key can be randomly generated for each session, in which case the new key is transmitted to the recipient using a public key cryptography method such as RSA. See Freedman. 
   Firewall 
   A firewall is an information processing system that sits between the Internet and a server or between the Internet and an enterprise network to protect the server or the enterprise network from various forms of Internet-borne attacks. 
   Leaky Bucket 
   A mechanism used in certain networks to limit the influx of bursty traffic (data that arrives in spurts). The input traffic is temporarily stored in a buffer (the “bucket”) and then is output (“leaks”) at a steady rate. If the input rate exceeds the programmed rate for a certain amount of time, the buffer overflows and traffic is discarded. 
   NIST 
   National Institute of Standards &amp; Technology, Washington, D.C. The standards-defining agency of the U.S. government, formerly the National Bureau of Standards. See Freedman. 
   Ping Attacks 
   This is an attack in which the attacker attempts to “bog down” a site by sending it a stream of “ping” packets. A ping is an ICMP echo request packet. A network node is supposed to respond to an ICMP echo request with an ICMP echo response. A ping is normally used to determine if a host is up and if the path to and back from the host is working. But if an attacker sends large numbers of pings very rapidly, the host can get bogged down responding to pings, making it difficult for the host to provide service to legitimate users. 
   Random Early Drop 
   Random Early Drop is a defense against a TCP SYN (or SYN Flood) attack. When a new TCP SYN packet comes along, the TCP implementation in a server determines whether any free TCP control blocks are available. If so, the server allocates one to the new “partially established” connection. If not, it uses a TCP control block from an existing “partially established” TCP connection. Since “real” TCP connections will not normally stay in the “partially established” state for very long—because the third packet in the 3-way handshake will normally arrive “shortly” after the sending of the second packet in the 3-way handshake, the re-use of the “partially established” TCP connection&#39;s control block will usually not impact legitimate traffic but it will reclaim control blocks so that they can be used for legitimate TCP connections. 
   Rate Limiting 
   Rate limiting is a useful defense against some DOSAs. A mechanism in front of the server (or server farm) can protect a server/server farm from attacks that are not TCP-based by limiting the rate at which non-TCP packets are allowed into the server/server farm. Rate limiting can minimize the impact of attacks that are not TCP-based and since there is no reason that a typical web server should be receiving large volumes of non-TCP traffic, the rate limiting of non-TCP traffic does not affect any “real” traffic. (Note that it may be useful to allow (some) ICMP packets into a server farm to diagnose network problems, for example; but there is no reason why a web server farm should be subjected to ICMP packets at a rate of 10 gigabits/second, for example. This rate limiting can be done using what is known in the networking industry as a “leaky bucket.” But rate-limiting of TCP packets cannot be used as a defense against TCP-based attacks because the “real” traffic to a server farm (e.g. web traffic or mail traffic) is TCP-based, and thus such rate limiting would limit the flow of the “real” traffic as well as the “attack” traffic. 
   Rijndael Algorithm 
   Rijndael is a block cipher, designed by Joan Daemen and Vincent Rijmen as a candidate algorithm for the AES. The cipher has a variable block length and key length. Keys with a length of 128, 192, or 256 bits to encrypt blocks with a length of 128, 192 or 256 bits (all nine combinations of key length and block length are possible) can be generated. Both block length and key length can be extended very easily to multiples of 32 bits. Rijndael can be implemented very efficiently on a wide range of processors or in special-purpose hardware. The Rijndael algorithm was selected by NIST as the base for AES. 
   Smurf Attacks 
   This is an attack in which the attacker sends “broadcast” ping packets to a large group of machines with a forged source address such that the source address in the packet is that of the “target” node, i.e., the machine that the attacker wants to disable. When the broadcast ping is received by the presumably large number of machines, they each respond to the ping by sending an echo reply to the address that was in the source address field in the broadcast ping packet. Since the source address is that of the target, the target gets swamped with large numbers of echo replies. The target throws these away but the load on the target and the load on the link connecting the target to the network is large and this load can bog down both the node and the link. 
   Spoofing 
   Faking the source address of a transmission. 
   SYN 
   A synchronization packet used in establishing a TCP connection. 
   SYN cookies (also known as TCP SYN cookies or TCP cookies) 
   A TCP SYN cookie is a mechanism for dealing with a SYN flood. 
   A TCP SYN flood (see SYN flood below) was generally considered to be an insoluble problem. See, for example, “Practical UNIX and Internet Security,” by Garfinkel and Spafford, page 778:
         The recipient will be left with multiple half-open connections that are occupying limited resources. Usually, these connection requests have forged source addresses that specify nonexistent or unreachable hosts that cannot be contacted. Thus, there is also no way to trace the connections back. . . . There is little you can do in these situations . . . any finite limit can be exceeded.       

   SYN cookies solve this problem using cryptographic techniques. A SYN cookie is a particular choice of an initial TCP sequence number by a TCP server that can be sent in a SYN-ACK. The particular choice of sequence number allows the server to
         participate in the 3-way handshake without tying up any memory on the server; and   do this in such a way that an attacker cannot fake a legitimate third packet in the 3-way handshake.
 
The sequence number can be a combination of the target server&#39;s IP address and port number and the source client&#39;s IP address and port number. The Rijndael algorithm can be employed to generate a hard-to-guess value so that an attacker cannot fake a third packet in the 3-way handshake.
 
SYN Flood (Also Known as a TCP SYN Flood)
       

   A TCP SYN flood attack works as follows. The attacker sends a large number of TCP SYNs to the target. The attacker typically uses fake source addresses in these packets so that these packets cannot be easily traced back to the attacker. The attacker also typically uses a different source address in each packet to make it hard for the target to know that it is the subject of a TCP SYN attack. Each time a SYN is received, the target responds with the second packet in the 3-way handshake. It also allocates some “resources,” such as a TCP control block that will be used to keep track of what&#39;s going on in the TCP connection. If enough TCP SYNs come in, lots of resources get tied up. Those resources are not freed for some time because the target is expecting the third packet in the 3-way handshake but the third packet never arrives. The targets may eventually time-out these partial connections and free these resources but if enough resources get tied up, the target runs out of resources and is not able to respond to legitimate traffic. 
   TCP 
   TCP is the Transmission Control Protocol. TCP is used to provide a “reliable” connection between 2 nodes in an internet. TCP uses sequence numbers, checksums, acknowledgements (or ACKS) and retransmissions of lost or garbled data to provide reliability. TCP is used by applications in which it is important to deliver bytes correctly such as mail, web browsing, etc. 
   TCP ACK Flood 
   This is an attack in which the attacker sends a flood of TCP packets in which the ACK flag is set. The ACK flag is normally set to acknowledge the reception of ungarbled data but an attacker may set the ACK flag in a flood of packets in an attempt to make his packets appear to be part of an established and legitimate TCP connection so that the attack packets won&#39;t get filtered out by a firewall. 
   TCP Connection/Watching SYNs and FINs 
   A TCP connection has two “ends” where each is defined by an IP address and a TCP port number. A TCP connection is established in a 3-way handshake via an exchange of SYN and ACK packets and ended via a 3-way exchange of FIN (French for “end”) packets. If one keeps track of the SYN and FIN exchanges one can determine if a TCP packet is part of a legitimate TCP connection or if it is an attack packet from some attacker that ought to be discarded. 
   UDP 
   UDP is the User Datagram Protocol. The UDP doesn&#39;t provide sequence numbers, acknowledgements (ACKs), retransmissions or reliability. It is used by applications in which the “timeliness” of packet delivery is important and the retransmission of lost or garbled data is not important. IP telephony is an example in which the timeliness of packet delivery is important and the retransmission of lost or garbled packets is not useful. 
   UDP Flood 
   An attack in which the attacker sends large numbers of UDP packets. A UDP flood is similar to a ping attack. 
   Wire-speed 
   Wire speed is the ability to process packets as fast as they are received from the network. 
   3-way Handshake 
   A TCP connection is set up with a 3-way “handshake” that works as follows. The initiator sends a SYN packet that includes some “synchronizing” information. The server responds with a SYN packet that includes some synchronizing information of its own. The server&#39;s SYN packet also acknowledges (ACKs) the synchronizing information that was received from the initiator. The initiator ACKs the information in the server&#39;s SYN packet. 
   5-tuple 
   The 5 items of information that are found in a TCP packet that uniquely identify a particular TCP connection: the destination IP address, the source IP address, the protocol field that indicates that the IP packet includes a TCP segment, the TCP destination port and the TCP source port. Since the protocol field (which indicates TCP) does not necessarily need to be stored in a table of TCP connections, a table of TCP connections might store only the other 4 items (4-tuples). 
   There are many forms of denial of services attacks and there is a need for a countermeasure against DOSAs that overcomes the shortcomings of the prior art. 
   SUMMARY OF THE INVENTION 
   A mechanism for protecting at least one target server computer from denial of services attacks launched by at least one source client computer comprises a crypto engine and a fast lookup engine. The crypto engine is used to generate and validate TCP SYN cookies to defend against TCP SYN flood attacks. The fast lookup engine is used to quickly determine whether a packet that purports to be part of a previously established TCP connection actually is part of a legitimately established connection to defend against TCP ACK attacks. The mechanism also optionally includes a “leaky bucket” mechanism for rate-limiting non-TCP traffic to defend against non-TCP-based attacks. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a network using a system for protecting against DOSAs. 
       FIG. 2  is a flow chart illustrating a method for performing a 3-way handshake to establish a connection, according to a known method. 
       FIG. 3  is a block diagram of a system for protecting a network node against DOSAs. 
       FIG. 4  is a flow chart illustrating a method using a crypto engine and a fast lookup engine for protecting a server against DOSAs, according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   FIG.  1 —Block Diagram of a Communications Network 
   Referring to  FIG. 1 , there is shown a simplified version of a communications network  100  wherein an embodiment of the invention can be advantageously deployed to defend against DOSAs. In this embodiment the network  100  operates according to the TCP/IP suite of protocols but the principles of the invention also apply to any packet switched network that uses packet integrity confirmation techniques analogous to that of the TCP/IP. The network  100  comprises (includes but is not limited to) a plurality of servers  102 ,  104 ,  106 - 106   n , and  108 , all connected to the Internet  110 . 
   The network also comprises firewalls  112  and  114 , a router  116 , and a plurality of personal computers (PCs)  118 - 122 . Assume that the user of unit  118  (the source client computer) launches a TCP-based attack against a target server  102  which is protected by a firewall unit  112 . The firewall unit  112  is an information processing system which can be adapted to prevent DOSAs according to an embodiment of the invention. The firewall unit  112  may be so adapted by using specialized hardware such as one or more application-specific integrated circuits (ASICs) or by programming general purpose programming hardware. The firewall unit  112  DOSA mechanism protects the server  102  by discarding TCP traffic that fraudulently purports to be part of an existing TCP connection (i.e., a TCP packet that has the ACK bit set in the TCP header when the TCP connection defined by the packet&#39;s TCP 5-tuple has not been established in a TCP 3-way handshake). It also shields the server  102  from TCP SYN packets that attempt to establish new TCP connections until those attempts are “validated” in a 3-way handshake by the DOSA protection mechanism embodied in the firewall unit  112 . It passes TCP packets that are part of a legitimate TCP connection on to the server  102  and passes packets from the server  102  back to the client  118  (after modifying those packets as described below). 
   FIG.  2 —Flowchart Illustrating the 3-way Handshake 
     FIG. 2  illustrates a method for protecting against denial of service attacks between a source client and a target server according to the prior art. The flowchart  200  shows two different paths that can be followed, depending upon whether the packet is a TCP packet or not. The first step is the receipt of a packet at a target server or server farm in step  202 . In step  204  a test is made to determine if the arriving packet is a TCP packet. If the packet is determined to be a TCP packet, then in step  205  a determination is made as to whether the TCP packet is the first packet in a 3-way handshake. If it is, then in step  206 , it is established that this packet is trying to initiate a TCP connection, with its SYN bit set to ON and its ACK bit set to OFF (SYN=1, ACK=0) in the TCP header. The SYN bit indicates that the packet contains an initial sequence number (i.e. synchronizing information) that the client will use for data that is sent from the client to the server. This is the first step in the 3-way handshake that is used to initiate a connection. If, in step  205 , the arriving TCP packet is not the first packet in a 3-way handshake, that is, if it appears to be part of an existing TCP connection, the packet will be admitted in step  214 . 
   In step  208 , the server  102 , after receiving the TCP SYN packet from the client  118 , will acknowledge the first SYN packet by returning the second packet in the 3-way handshake. This packet header will have its ACK bit set to ON, to acknowledge the client&#39;s initial sequence number and the SYN bit will be ON as well (SYN=1, ACK=1). This SYN bit indicates that the packet contains an initial sequence number that the server will use for data that is sent from the server  102  to the client  118 . At this point the server  102  will allocate memory resources for this partially established connection in the expectation that it will receive the third packet in the 3-way handshake acknowledging the server&#39;s initial sequence number. When this packet is received by the client  118 , the client ACK&#39;s the server&#39;s initial sequence number and the connection is established. Note that since there can be many simultaneous (established or partially established) connections at the server  102  and at the client  118 , both the server  102  and the client  118  need to look at the 5-tuples in a TCP packet to match a given packet up with an appropriate TCP connection. 
   If the third packet in the 3-way handshake arrives as expected in step  210 , it will have its SYN bit OFF and its ACK bit ON (SYN=0, ACK=1). In the meantime, while waiting for the third packet to arrive to complete the handshake process, the server  102  has tied up its memory resources. When the packet does arrive, the server  102  accepts this packet and the new TCP connection will be established. However, the source client node  118  could also be part of a denial of service attack known as a TCP SYN attack, or TCP SYN flood. In a TCP SYN flood, the attacker sends a large number of TCP SYN packets that appear to be requests for new connections from a large number of distinct clients. The idea is to get the server  102  to respond to each of these packets with the second packet in the 3-way handshake and allocate resources (e.g. memory) to each of these partially completed connections. The memory will remain tied up until the third packet arrives to complete the 3-way handshake. But in a TCP SYN attack the third packet in the 3-way handshake is never sent and a great deal of memory can be tied up in partially established connections limiting the server&#39;s ability to respond to legitimate requests. Eventually, the server  102  may free resources, but there is the danger that the server  102  will not be able to respond to legitimate TCP traffic. 
   If in step  204  it is determined that the packet is not a TCP packet, then the server  102  may use “rate limiting” as a defense in step  216 . This is where a mechanism, such as a firewall unit  112 , will accept non-TCP packets, but at a limited rate. This defense is sometimes used in the networking industry and is more commonly known as a “leaky bucket.” Unfortunately, rate limiting is not an adequate defense against TCP-based attacks. 
   The 3-way handshake process ends in step  212  with the server  102 , or the firewall unit  112 , establishing the TCP connection which will subsequently be used to send data from the client  118  to the server  102  and then to send data back from the server  102  to the client  118 . 
   FIG.  3 —Block Diagram of a System for Protecting Against DOSAs 
   Referring to  FIG. 3 , there is shown a highly simplified block diagram of the components of the firewall unit  112 . According to a preferred embodiment, the mechanism for protection against denial of services attacks is deployed in the firewall unit  112 . It should be understood that the firewall unit  112  comprises a CPU  300  that is adapted to operate according to the principles of the invention, a memory (e.g., DRAM and/or disk storage)  302 , itself comprising a fast lookup table  304 , a lookup table engine  314  and a high-speed crypto engine  306 . In another embodiment, the fast lookup table  304  could be part of the fast lookup engine  314 . These are all connected by a bus (or a set of buses)  308 . The crypto engine  306  is used to generate and validate TCP SYN cookies at high speeds. The fast lookup engine is used to validate, at high speed, whether or not a TCP packet that purports to be part of an already established connection really is part of an established connection. 
   The firewall unit  112  also comprises a network interface  310  that receives and transmits packets to and from the network  100  and a server interface  312  that communicates with the server  102 . In general, the DOSA protection mechanism distinguishes between packets that are part of an existing TCP connection and those that fraudulently purport to be legitimate by using the fast lookup engine  314  to determine if a packet&#39;s 5-tuple is in the lookup table  304 . The crypto engine  306  and the fast lookup engine  314  are used to defend against TCP-based attacks (i.e. SYN floods and ACK floods). DOSA attacks based on traffic other than TCP packets can be handled with a “rate limiting” mechanism such as a “leaky bucket”. 
   This mechanism is readily adaptable for use in different environments. Our example illustrates the mechanism as part of a firewall  112 , but other embodiments can and should be contemplated within the scope and spirit of the invention. The DOSA mechanism can be used from within a network adapter in a server or in a network box, such as a router, firewall, web cache, load-balancer, content switch, etc. which will sit “in front” of a server or server farm. The mechanism provides distinct advantages when used in different embodiments. For example, placing the DOSA protection mechanism in the router provides the advantage of allowing a variety of different server types to be protected, whereas placing the DOSA mechanism in a network adapter in a server provides additional scalability. 
   The CPU  300  protects against a TCP SYN flood by implementing an encrypted sequence number scheme (cookie scheme), and protects against TCP ACK attacks using a fast lookup engine  314 . An advantage of the cookie scheme is that memory resources are not tied up because information regarding partially completed TCP connections does not need to be stored. The firewall unit  112 , using the CPU  300 , can confirm that a packet completes a legitimate 3-way handshake (described further in  FIG. 4 ) without having had to store anything in memory. This is accomplished through the use of the crypto engine  306  and the cookie scheme. The crypto engine  306  generates and validates a unique sequence number for an incoming packet. Since the sequence number generated by the crypto engine  306  is a function of a secret key as well as information from the initial SYN packet, attackers cannot forge packets that appear to complete the 3-way handshake. It is critical to employ an effective encryption method to choose sequence numbers for the cookie scheme. If an attacker can guess the sequence number that a server will send in response to a client&#39;s initial SYN packet, the attacker would be able to establish fraudulent TCP connections and tie up resources on the server. 
   There are different ways to generate these unique sequence numbers. One way is to use the Rijndael algorithm which can be implemented efficiently on a wide range of processors ranging from specially-programmed general purpose processors to specialized hardware. In a preferred embodiment of this invention, the Rijndael algorithm can be used to create a sequence number combining identifying information from both the source client  118  and the target server  102  (such as the IP address and port number), along with the client&#39;s initial sequence number and some measure of the current time. Additionally, other encryption algorithms and methods could be contemplated within the scope of this invention. 
   Once a legitimate TCP connection has been established the 5-tuple that defines that connection is stored in the lookup table  304 . These stored 5-tuples are used to confirm the validity of a TCP packet that purports to be part of an existing connection. The table lookups must be performed quickly and efficiently due to the high volume of TCP packets that are received at web servers. There are several known table lookup technologies available today which could be used for the fast lookup engine  314 . 
   One way to implement a fast lookup engine  314  is through the use of Content Addressable Memory (CAM). This is an ideal method to use because you can compare a search field (in this case, the packet&#39;s 5-tuple) with all of the entries in the lookup table  304  simultaneously. 
   The CDROM drive  316  is an example of an input/output device for receiving computer readable media comprising program instructions according to an embodiment of the invention. It will be understood by those skilled in the art that other computer readable media can also be advantageously used within the spirit and scope of the invention. 
   FIG.  4 —Flowchart Illustrating a Preferred Embodiment 
   Referring to  FIG. 4 , a method  400  illustrates the use of the crypto engine  306  and the fast lookup engine  314 , in combination with a cookie encryption scheme, to defend against TCP-based denial of service attacks. The process begins at step  402  when a packet arrives at target server  102 . In this example we will assume that the packet is a TCP packet. Non-TCP packets would be dealt with by employing known protection methods, such as rate limiting. We will focus our example on this method&#39;s handling of TCP packets. 
   In step  404  the firewall unit  112  determines if this packet is the first packet in a 3-way handshake that initiates a new connection. If the arriving packet purports to be part of an already existing connection (perhaps by having the ACK bit set), then in step  405  the fast lookup engine  314  does a table lookup in the lookup table  304 , searching for the packet&#39;s 5-tuple of identifying information. According to this method, any packet which is part of an existing connection will have its information stored in the lookup table  304 . If the information is not found in the lookup table  304  (step  407 ), the packet is not considered legitimate and is dropped or discarded in step  422 . As stated earlier, the packet is considered to be part of an already established TCP connection if its 5-tuple of information can be found in the lookup table  304 . This is a much more reliable method for determining if a packet is part of a connection than the known method which merely checks the SYN and ACK bits. Since the ACK bit is always set and the SYN bit is always clear once a TCP connection is established some simple implementations have used these bits to determine if a packet is part of an established connection. But an attacker can easily defeat this check by sending a stream of packets that have the SYN bit clear and the ACK bit set in a TCP ACK flood. The fast-lookup mechanism  314  allows the firewall unit  112  to defend against ACK attacks or ACK floods. And the crypto mechanism  306  allows the firewall unit  112  to defend against TCP SYN attacks or SYN floods. 
   If the arriving packet is attempting to initiate a connection, as confirmed in step  404 , then in step  406  the firewall unit  112 , using the crypto engine  306 , generates a unique sequence number for this connection, as in the known TCP SYN cookie scheme. The sequence number generated by the crypto engine  306  may be a function of a secret key and the client&#39;s IP address and port number, the target server&#39;s IP address and port number, the client&#39;s initial sequence number and the current time. The Rijndael algorithm could be used to generate this sequence number. Note that the time could be one of various measurements of date and time. For example, on a Unix system this is typically the number of milliseconds that have elapsed since midnight, Jan. 1, 1970. This is just one example of how the time portion of the sequence number may be defined. 
   After the sequence number is generated in step  406 , the firewall unit  112  sends back the second packet of the 3-way handshake which includes this sequence number, in step  408 . The packet will be sent back to the source client  118  with its SYN bit ON and its ACK bit ON to identify that this packet is the second packet of the 3-way handshake. The next step occurs when the firewall unit  112  receives the third packet of the 3-way handshake in step  410 . If this packet has its SYN bit OFF and its ACK bit ON, this will confirm that the packet purports to be the third packet of a 3-way handshake. The firewall unit  112 , after verifying that the SYN bit is OFF and the ACK bit is ON, uses the crypto engine  306  to validate the sequence number (step  412 ) that the packet is ACKing (which should be the sequence number originally sent to the source client  118  in the second packet of the 3-way handshake in step  408 ). If the sequence number does not match the sequence number previously generated by the crypto engine  306 , the packet is discarded in step  422 . If the packet includes the correct sequence number the legitimacy is confirmed and the packet is accepted. The fast lookup engine  314  will add the packet&#39;s 5-tuple identifying information to the lookup table  304  in step  414 . It is important to note that this example refers to the “5-tuple” of identifying information, but other configurations of tuples could also be employed in keeping with the spirit and scope of the invention. 
   The firewall unit  112  needs to perform additional functions in step  416 . After validating the 3-way handshake, the firewall unit  112  opens a connection to the server/server farm and passes data through the connection. It plays the part of the server  102  in communicating with the client  118  and it plays the part of the client  118  in communicating with the server  102 . But since the sequence number chosen by the server  102  will in general be different from the sequence number that the firewall unit  112  chose in communicating with the client  118 , the firewall unit  112  needs to adjust sequence numbers of packets from the server  102  to the client  118  and the corresponding acknowledgment numbers of packets from the client  118  to the server  102 . The firewall unit  112  will also need to adjust TCP checksums appropriately. The connection ends with the TCP shutdown sequence. The connection is shut down in two stages, one for each “direction” (client→server and server→client). First, one end sends a packet with the header&#39;s FIN bit set, which the other end ACKnowledges; then, when the other end is also finished sending its data, it sends out a FIN packet, which the other end ACKs, thereby closing the connection. The firewall unit  112  will also handle this shutdown procedure. 
   Since the firewall unit  112 , as one possible embodiment of the invention, includes a fast lookup engine  314  and a high-speed crypto engine  306  to shield the target server  102  from denial of service attacks, the target server&#39;s  102  processor will not be slowed or impaired. Using the encrypted sequence number scheme (SYN cookie scheme with the crypto engine  306 ) by itself would prevent some attackers from launching successful attacks, but the addition of the fast lookup engine  314  provides additional security. Consequently, the system will not get bogged down or expend its resources on partially completed TCP connections or TCP ACK attacks. 
   The embodiment described herein is illustrative of the invention; however other implementations are contemplated within the scope of the following claims.