Abstract:
An improved firewall for providing network security is described. The improved firewall provides for dynamic rule generation, as well using conventional fixed rules. This improvement is provided without significant increase in the processing time required for most packets. Additionally, the improved firewall provides for translation of IP addresses between the firewall and the internal network.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/461,798 filed Aug. 2, 2006, which is a continuation of U.S. patent application Ser. No. 10/893,283 filed Jul. 19, 2004 (now U.S. Pat. No. 7,107,612), which is a continuation of U.S. patent application Ser. No. 09/525,369 filed Mar. 15, 2000 (now U.S. Pat. No. 6,772,347, issued on Aug. 3, 2004), which is a continuation-in-part of U.S. patent application Ser. No. 09/283,730, filed on Apr. 1, 1999 (now U.S. Pat. No. 6,701,432, issued on Mar. 2, 2004), the disclosures of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of computer networks. In particular, the present invention relates to a method, apparatus and computer program product for providing network security. 
     BACKGROUND OF THE INVENTION 
     A packet switch communication system includes a network of one or more routers connecting a plurality of users. A packet is the fundamental unit of transfer in the packet switch communication system. A user can be an individual user terminal or another network. A router is a switching device that receives packets containing data or control information on one port and, based on destination information contained within the packets, routes the packets out another port to their final destination, or to some intermediary destination(s). Conventional routers perform this switching function by evaluating header information contained within the packet in order to determine the proper output port for a particular packet. 
     As known, a communications network can be a public network, such as the Internet, in which data packets are passed between users over untrusted, i.e., non-secure communication links. Alternatively, various organizations, typically corporations, use what is known as an intranet communications network, accessible only by the organization&#39;s members, employees, or others having access authorization. Intranets typically connect one or more private servers, such as a local area network (LAN). The network configuration in a preferred embodiment of this invention can include a combination of public and private networks. For example, two or more LANs can be coupled together with individual terminals using a public network, such as the Internet. A network point that acts as an entrance to another network is known in the art as a gateway. 
     Conventional packet switched communication systems that include links between public and private networks typically include means to safeguard the private networks against intrusions through the gateway provided at the interface of the private and public networks. The means designed to prevent unauthorized access to or from a private are commonly known as firewalls, which can be implemented in both hardware and software, or a combination of both. Thus, a firewall is a device that can be coupled in-line between a public network and a private network for screening packets received from the public network. 
     Referring to  FIG. 1 , a conventional packet switch communication system  100  can include two (or more) private networks  102   a  and  102   b  coupled by a public network  104  for facilitating the communication between a plurality of user terminals  106 . Each private network  102  can include one or more servers and a plurality of individual terminals. Each private network  102  can be an intranet, such as a LAN. Public network  104  can be the Internet, or other public network having untrusted links for linking packets between private networks  102   a  and  102   b . In a preferred embodiment, at each gateway between a private network  102  and public network  104  there is a firewall  110 . 
     The architecture of an illustrative prior art firewall is shown in  FIG. 2   a . The firewall  110  generally includes one or more public network links  120 , one or more private network links  122 , and memory controller  124  coupled to the network links by a PCI bus  125 . Memory controller  124  is also coupled by a memory bus  129  to a memory (RAM)  126  and a firewall engine, implemented in a preferred embodiment as an ASIC  128 . The firewall engine ASIC  128  performs packet screening prior to routing packets through to private network  102 . The firewall engine ASIC  128  processes the packets to enforce an access control policy, screening the packets in accordance with one or more sets of rules. The rules are described in more detail below. A central processor (CPU)  134  is coupled to memory controller  124  by a CPU bus  132 . CPU  134  oversees the memory transfer operations on all buses shown. Memory controller  124  is a bridge connecting CPU bus  132 , memory bus  129 , and PCI bus  125 . 
     In operation, packets are received at public network link  120 . Each packet is transferred on bus  125  to, and routed through, memory controller  124  and on to RAM  126  via memory bus  129 . When firewall engine  128  is available, packets are fetched using memory bus  129  and processed by the firewall engine  128 . After processing, the packet is returned to RAM  126  using memory bus  129 . Finally the packet is retrieved by the memory controller  124  using memory bus  129 , and routed to private network link  122 . The screening rules implemented by the firewall engine  128  are typically searched in linear order, beginning with the internal rule memory. Certain aspects of the rule structure are described below. 
     As known in the art, a rule is a control policy for filtering incoming and outgoing packets. Rules specify actions to be applied as against certain packets. When a packet is received for processing through a rule search, the packet&#39;s IP header, TCP header, or UDP header may require inspecting. A rule will generally include, at a minimum, source/destination IP addresses, UDP/TCP source/destination ports and transport layer protocol. Additional criteria may be used by the rules as well. 
     Generally, the address information is used as matching criterion—in other words to match a rule, a packet must have come from a defined source IP address and its destination must be the defined destination IP address. The UDP/TCP source/destination port specifies what client or server process the packet originates from on the source machine. The firewall engine can be configured to permit or deny a packet based upon these port numbers. The rule may include a range of values or a specific value for a TCP/UDP port. The transport layer protocol specifies which protocol above the IP layer, such as TCP or UDP, the policy rule is to be enforced against. 
     The firewall engine described above essentially screens packets using an access control list (ACL), and may be referred to as an ACL engine. That is, it performs a simple comparison of various matching criteria of an incoming IP packet—typically source, destination, port and protocol—to each rule in a rule set in sequence. Based upon this comparison, an incoming IP packet is either allowed or denied. A data-flow chart for this firewall engine is shown in  FIG. 5 . 
     It will be appreciated that using a fixed set of rules can be restrictive in many practical applications. Therefore, it is desirable to provide a system and method capable of adding rules to the rule set of the firewall engine dynamically—that is, to extract from a sequence of packets information, such as the port number and IP address, and generate new rules using this information. However, generating these new rules dynamically would increase the complexity of the comparison and decrease the speed of the firewall engine. There is therefore a need in the art for a firewall engine which can generate rules dynamically, based upon information extracted from incoming packets, with a limited impact on the speed of the firewall engine. 
     SUMMARY OF THE INVENTION 
     In accordance with a preferred embodiment, an apparatus, method and computer program product for providing network security is described. The apparatus includes an engine for sorting incoming IP packets into initially allowed and initially denied packets using a fixed set of rules. The packets are then further sorted by a second engine. In one embodiment, the engine further sorts the initially denied packets into allowed packets and denied packets, using dynamically generated rules. The denied packets are dropped and the allowed packets are permitted to enter the network. 
     Likewise, the method includes the step of sorting incoming IP packets into initially allowed and initially denied packets using a fixed set of rules. The packets are then further sorted. In one embodiment, additional steps include sorting the initially denied packets into allowed packets and denied packets, using dynamically generated rules. The denied packets are dropped and the allowed packets are permitted to enter the network. 
     Finally, the computer program product sorts incoming IP packets into initially allowed packets and initially denied packets. In one embodiment, the computer program product further sorts the initially denied packets into allowed packets and denied packets, using dynamically generated rules. The denied packets are dropped and the allowed packets are permitted to enter the network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary packet switch communications system. 
         FIG. 2   a  illustrates a firewall with an application-specific integrated circuit (ASIC). 
         FIG. 2   b  illustrates a firewall with a local bus and an application-specific integrated circuit (ASIC). 
         FIG. 3  illustrates an exemplary rule structure for use by a firewall. 
         FIG. 4  is a flow diagram for a firewall screening process. 
         FIG. 5  is a data-flow chart for a prior art firewall. 
         FIG. 6  is a data-flow chart for a firewall in accordance with one embodiment of the invention. 
         FIG. 7   a  is a logic diagram for processing incoming packets in accordance with the invention. 
         FIG. 7   b  is a logic diagram for processing outgoing packets in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A conventional firewall may be implemented in software, or in hardware as shown in  FIG. 2   a . Alternatively, a hybrid of software and hardware may also be used to implement a firewall. The firewall of  FIG. 2   a  uses a memory bus  129  to communicate between the ASIC  128 , the RAM  126 , and the memory  130 , which stores the rules used by the firewall.  FIG. 2   b  shows a high-speed firewall that employs a local bus  202  for improved processing speed. A high-speed firewall is described in pending parent application Ser. No. 09/283,730, the contents of which is hereby incorporated by reference. Exemplary high-speed firewalls include NetScreen Technology, Inc.&#39;s integrated firewall products, described at www.netscreen.com and related web pages. Selected web pages describing NetScreen&#39;s high-speed firewalls are provided as Appendix A to this application. 
     As shown in  FIG. 2   b , the high-speed firewall includes a hardware ASIC  204  to implement the firewall engine. The firewall engine retrieves packets stored in memory and processes each packet to enforce an access control policy. The processing by the firewall engine includes retrieving rules from a rule set, and screening the packets in accordance with the retrieved rules. In a specific embodiment, the rules may be stored in an internal memory in the ASIC  204 , or may be retrieved from a separate rule memory  206  via the local bus  202 . In a preferred embodiment, frequently accessed rule sets may be stored in the internal memory, with less-frequently accessed rule sets being stored in the separate rule memory  206 . 
     The structure  500  of a rule used by a firewall engine in accordance with one embodiment of the present invention is shown in  FIG. 3 . A rule will generally include, at a minimum, source/destination IP addresses  502   503 , UDP/TCP source/destination ports  504   505  and transport layer protocol  510 . Additional information used by the rules may include: a range of values for the UDP/TCP source/destination port  504   505 ; a counter  506  to keep track of the number of times the rule has been matched; a general mask (GMASK)  511  to indicate whether to ignore or check certain information in the packet header; source/destination IP address mask  508  to indicate whether to ignore part of an IP address, typically a specified number of the least significant bits; a searching control field  512  to tell the firewall engine to search in the separate rule memory  206  and to give a starting address; and a response action field  514  to specify the action to be taken if the rule is matched. 
     The address information is used as matching criterion—to match a rule, a packet must have come from the defined source IP address  502  and its destination must be the defined destination IP address  503 . Part of the address may be masked using the source/destination IP address mask  508 . The UDP/TCP source/destination port  504   505  specifies what client or server process the packet originates from on the source machine. The firewall engine can be configured to permit or deny a packet based upon these port numbers. The rule may include a range of values or a specific value for a TCP/UDP port. The counter  506  is used to track the number of times a rule has been matched, and at some threshold value will trigger a certain action, such as deny, log or alarm. The transport layer protocol  510  specifies which protocol above the IP layer, such as TCP or UDP, the policy rule is to be enforced against. 
     Referring to  FIGS. 2   b  and  4 , a process  600  executed by the firewall engine in the ASIC  204  is shown for screening packets using both the on-chip and off-chip rule memories. The firewall engine process begins at step  602 . A packet is received at an interface (public network interface  122 ) and transferred to dual-ported memory  203  using a DMA process executed by memory controller  124  ( 604 ). 
     CPU  134  reads the packet header information from packet memory and writes the packet information into special registers on ASIC  204  ( 606 ). These registers are mapped onto the system memory space, so CPU  134  has direct access to them. In an exemplary hardware firewall, the registers include: a source IP register; a destination IP register; a port register; a protocol register; and an acknowledge register, for storing the acknowledge bit from the packet. 
     CPU  134  also specifies which rule set to search by writing to a rule set specifier register ( 608 ). CPU  134  issues a command to the firewall engine located in the ASIC  204  by writing to a control register to initiate the ASIC rule search ( 610 ). Alternatively, the firewall engine may first check a stored look-up table with criteria relating to ongoing current applications or services, before searching the rules. In that case, the firewall engine first compares the contents of the special registers to the contents of a look-up table, where the look-up table includes the IP address, port and protocol corresponding to each current application or service. For example, if the packet is an FTP packet for an FTP that is ongoing, this information will be in the look-up table. If, on the other hand, the packet is an FTP packet for a newly-initiated FTP, the information will not be in the look-up table. 
     If the information is not in the look-up table, or if a look-up table is not used, the firewall engine then compares the contents of the special registers to each rule in sequence ( 611 ) until a match is found ( 612 ). The search stops when a match is found ( 613 ). Alternatively, for certain rules, known as counter rules, the firewall engine will increment the count register and continue the search. If the count threshold is exceeded, or if the search locates a match for a non-counter rule, the search results are written to a status register  616 . Likewise, if no match is found, and the entire set of rules has been examined, the search results are written to the status register. In addition, when a match is found, if a look-up table is used the information identifying the current application, such as the IP address, port and protocol, are written to the look-up table so that later packets in the current application may be processed using the look-up table instead of a rule search. 
     During the search, CPU  134  polls the status register to check whether the firewall engine is busy or has completed the search. When the CPU  134  determines that the search is complete, the CPU  134  executes certain actions against the current packet based on the information in the status register, such as permit or deny the packet, signal an alarm, and log the packet. 
     The process described above is a prior art one-pass search process, as illustrated in  FIG. 5 ; the ACL engine  621  conducts a search through an optional look-up table, and then through rules, as illustrated in  FIG. 4 , to determine whether a given packet matches a rule in the set and take action on that basis. The rules use a set of matching criteria—for example, source and destination IP address, and port number, indicating the application. These rules are fixed and use known matching criteria. The ACL engine  621  then allows some packets  622 , and denies or drops, others  623 . 
     As shown in  FIG. 6 , in a preferred embodiment, the IP packets  620  enter the ACL engine  621 . As in the prior art, the ACL engine  621  conducts a search, using fixed rules. The ACL engine then outputs allowed packets  632 , and initially denied packets  633 . 
     Unlike the prior art, the firewall engine that embodies one aspect of the present invention includes additional dynamic filtering, which further processes the packets. In particular, the initially denied packets  633  are processed by a dynamic filter  637 , which allows some of the initially denied packets to pass through the firewall and enter the private network. The dynamic filter  637  conducts a search through an additional set of rules, which are dynamically generated. The dynamic filter  637  generates rules using criteria such as port number and IP address, which are extracted from incoming packets for applications, such as RealAudio, Netmeeting (which uses the H3232 protocol) and network file system (NFS). 
     For example, when an FTP is initiated, the first sequence of FTP packets, which includes information on the port number and the IP address, will be passed by the rules in the ACL engine  621 . The dynamic filter  637  then extracts port number and IP address from this first sequence of packets, and generates new rules, similar to the fixed rules used by the ACL, including these criteria. Later sequences of FTP packets will be denied by the ACL engine  621 , but the dynamic filter  637  will pass the packets based on the new, dynamically-generated rules. The way in which the search through the dynamically-generated rules is conducted is similar to the approach used in the ACL engine  621 . The dynamic filter then drops packets which are finally denied  636 , and allows other initially denied packets, which meet the additional access control requirements, to pass  635  through the firewall and enter the private network. 
     This approach to processing the incoming IP packets has advantages over the prior art. Using dynamically-generated rules allows for more flexible access policy. However, if dynamic rule generation was included in the ACL engine  621 , the processing speed would be decreased. The dynamic filter  637  used in accordance with the present invention, following the ACL engine  621 , advantageously allows the use of dynamically-generated rules, without increasing the processing time for those IP packets, which are initially allowed  632  by the ACL engine  621  based on the fixed rule set. 
     Another preferred embodiment, as shown in  FIG. 6 , additionally allows for network address translation (NAT), to enable IP addresses, port numbers and message authentication codes (MACs) in the private network to be concealed from the public network. The public network can only access this information for the firewall. Thus, the destination information in the headers in the incoming packets must be changed, to reflect the private network IP addresses, port numbers and MAC. Furthermore, source information in the headers of outgoing packets must also be changed, to reflect the firewall network IP address, port number and MAC. 
     However, depending on the particular application used, information relating to the IP address or port number may be embedded in the packet content or payload, as well as in the header. In that case, the packet payload for an incoming packet must be translated to reflect the internal IP address and port number, as shown in  FIG. 7   a . Likewise, the packet payload for an outgoing packet must be translated to reflect the firewall address and port number, as shown in  FIG. 7   b.    
     As shown in  FIG. 6 , the dynamic analyzer  638  examines those packets which are initially allowed  632  by the ACL engine  621 . The dynamic analyzer  638  determines whether a given packet may require modification, due to embedded address or port number information. The dynamic analyzer  638  then separates packets which may require modification  640  from packets which do not require modification  639 . Packets which include IP address or port number information are identified by reading a protocol-specific field in the header. The dynamic analyzer  638  allows those initially allowed packets  632  and  635  which do not require modification  639  to pass through the firewall  642  into the private network. 
     The packets  640  which may require modification are then passed to an application-specific handler  641 . The application-specific handler  641 , as its name suggests, processes packets  640  for a particular application, such as FTP or NFS. The application-specific handler examines the protocol, session, port number and IP address, as well as the payload. In one embodiment, the application-specific handler may modify certain packets, which have been allowed  632  and  635 . If the IP address or port number in the packet header have been changed, for an incoming packet, or must be changed, for an outgoing packet, the application-specific handler translates the payload to reflect the change. In another embodiment, multiple application-specific handlers  641  may be provided, to process packets for different applications. For example, the firewall may include both an FTP-specific handler and an NFS-specific handler. 
     In another embodiment, the application-specific handler  641  may include the capability to send a “reset” packet to inform the TCP processor sending the denied packets that the connection has been denied. The connection is thereby rejected, rather than merely dropped. The rejection will prevent the TCP processor sending the denied packets  636  from continuing to try to connect with the network, thereby avoiding wasted bandwidth. 
     In conjunction with the software functionality description provided in the present disclosure, an apparatus in accordance with the preferred embodiments may be programmed using methods known in the art as described, for example, in Francise et. al.,  Professional Active Server Pages  2.0, Wrox Press (1998), and Zaration,  Microsoft C++ 6.0  Programmer&#39;s Guide , Microsoft Press (1998), the contents of each of which is hereby incorporated by reference into the present application. 
     While preferred embodiments of the invention have been described, these descriptions are merely illustrative and are not intended to limit the present invention. 
     For example, while the preferred embodiment discusses primarily a hardware implementation of a firewall, the scope of the preferred embodiments is not so limited. Those skilled in the art will recognize that the disclosed software and methods are readily adaptable for broader network analysis applications.