Patent Publication Number: US-8533823-B2

Title: System and method for source IP anti-spoofing security

Description:
RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 10/850,505, filed May 20, 2004, now U.S. Pat. No. 7,516,487 (the &#39;487 patent), issued Apr. 7, 2009, which is incorporated herein by reference for all purposes. The &#39;487 patent is a continuation in part of U.S. patent application Ser. No. 10/631,366 filed Jul. 31, 2003, now U.S. Pat. No. 7,523,485 (the &#39;485 patent), issued Apr. 21, 2009, which is incorporated by reference for all purposes. The &#39;485 patent claims benefit from U.S. Provisional Patent Application Serial No. 60/472,158, filed May 21, 2003, which is incorporated herein by reference for all purposes, and also claims benefit from U.S. Provisional Patent Application Ser. No. 60/472,170, filed May 21, 2003, which is incorporated herein by reference for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method of providing for enhanced security on a computer network to reduce the risk created by the spoofing of IP addresses. 
     BACKGROUND 
     As is widely known source IP Address spoofing is a common technique used in denial of service attacks (DoS). Other types of source IP address spoofing attacks are widely known, and include attacks such as distributed denial of service attacks (DDoS), Worm attacks, and Man In the Middle attacks. Spoofed Source IP Address attacks can also include Smurf attacks, NameServer attacks, and ICMP, IGMP, and UDP protocol attacks. One goal in some spoofing attacks is to spread a software virus to as many random new victims as possible, and other attacks are designed to overwhelm a computer system, and other attacks are used to steal information. 
       FIG. 1  shows a computer network  100  of the prior art. At the lowest layer (layer 1 of the OSI Networking protocols) is the physical layer which describes the actual physical elements such as cables and connectors which connect different devices of the computer network. The next layer of the system is the layer 2, the datatink layer. At this level, among other things, the MAC addresses are used to identify the devices which are interconnected on a subnet. As is widely known a MAC address is a unique address which corresponds to a device connected to a network. The MAC address is generally determined by the Ethernet board of a device which is connected to the network. 
     The computer network  100  can have a number of subnets. As shown in  FIG. 1 , the subnets are  102 ,  104 ,  106 , and  108 . Each subnet can contain a number of layer 2 devices such as switches. For example, subnet  102  is shown as having switches  110 - 126 , and subnet  104  is shown as having switches  128 - 134 . The layer 2 devices are not shown for subnets  106  and  108 , but as one of skill in the art will appreciate, most subnets will include a number of layer 2 devices such as switches or hubs. Each switch can have a number of ports to which additional switches can be coupled, or to which host devices such as end user computers, or serves, or mainframes can be connected. At the subnet level different devices are connected to the subnet and can communicate with other devices on the subnet by transmitting data packets through the switches. These data packets include the MAC address for the device to which the data packet is to be sent (the destination MAC address) and the MAC address for the host which is sending the packet (the source MAC address). In addition each host device will be assigned an IP address. The IP address is utilized by a router  136  to determine routing for data packets which are being sent by a host on one subnet to a host on a different subnet, or to a different device which may require that the packet be transmitted via the Internet  138 . The IP address is often assigned using the Dynamic Host Configuration Protocol (DHCP). Each host on a subnet will normally be assigned an IP address. As is widely known data packets generated by a host on the subnet can include information which is being sent from one host to another host, and further these data packets will include MAC addresses as described above, and a source IP address and a destination IP address. 
     Source IP spoofing occurs when an attacker host uses a source IP address, which does not correspond, or is not assigned, to its MAC address, in a transmitted data packet. For example, the attacker host may select a source IP address for a different host on a different subnet and transmit a data packet with this false, or spoofed, IP address. This data packet would then be received by the destination device, and the destination device would read the spoofed IP address and it would appear to the destination device that the data packet had come from the device which is actually assigned the source IP address which was used by the attacker host. 
     In terms of network security defenses, traditional blocks to this type for source IP spoofing were to create inbound filters on the router ports  140 - 146  that supported the subnets  102 - 108 . The router filter operates such that it knows which IP addresses should be received from a specific subnet connected to the particular port. This allows ISP&#39;s and enterprises to block randomly spoofed source IP addresses, where the spoofed IP address received on a particular port of the router, is not consistent with source IP addresses for the subnet which is coupled to the particular port of the router. Hackers have recognized the limitations inherent in this type of source IP address anti-spoofing process, and developed spoofing software tools, some of which are referred to as “zombies, and “bots” which now spoof source IP addresses from within their own subnet and subnet mask settings. For customers with large class B subnets, the router level (layer 3) type of defense is not very effective as hundreds and potentially thousands of hosts on the subnet can still be affected. 
     An Automatic Spoof detector (referred to as “Spoofwatch”) has been developed in an attempt to efficiently detect hosts performing source IP spoofing. Spoofwatch works on the premise that these hosts do not respond to ARP requests for their spoofed IP addresses. This solution has many potential shortcomings. For example, the router  136  can receive very large numbers of different source IP addresses in different data packets. Thus, a very large amount of router&#39;s processing power is consumed with generating the ARP requests and monitoring the responses. 
     A review of a number of different websites related to networking showed a number of different approaches related to preventing IP address spoofing, but each approach was very different than that discussed herein. Other techniques have been developed for providing defenses against source IP address spoofing. One of these other approaches relies on using encryption, and source IP filtering at the layer 3 level, which is after the data packets have been transmitted from the subnet to the router. 
     Additional information regarding different approaches to combating source IP spoofing can be found at http://www.cisco.com/en/US/tech/tk86/tk803/technologies_tech_note09186a00800a7828.shtml. Additional websites provide discussion regarding the risks associated with source IP address spoofing and provide some discussion for ways to combat spoofing, see for example: http://www.sans.org/rr/threats/spoofed.php; http://www.cert.org/incident_notes/IN-2000-04.html; http://www.anml.iu.edu/PDF/Automatic_Spoof_Detector.pdf; and http://www.linuxgazette.com/issue63/sharma.html. 
     In order to increase the efficiency and effectiveness of combating source IP spoofing it would be beneficial to provide source IP spoofing at lower level in the network hierarchy, in a manner which has not previously been provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an overview of a system of the prior art. 
         FIG. 2  shows an embodiment of a network device of the present invention. 
         FIG. 3  shows a method of an embodiment of the present invention. 
         FIG. 4  shows a method of an embodiment of the present invention. 
         FIG. 5  shows an embodiment of a system herein. 
         FIG. 6  shows a method of an embodiment herein, for validating learned source IP addresses. 
         FIG. 7  shows a method of an embodiment herein for determining is a source IP address should be aged out. 
         FIG. 8  shows a method of an embodiment herein for determining if a possible spoofed source IP attack is occurring. 
         FIG. 9  shows a graph for an embodiment herein for determining an expected number of failed validation attempts. 
     
    
    
     DETAILED DESCRIPTION 
     One approach to improving defenses against Source IP Spoofing is to attack the problem at the subnet or Layer 2 level. Because ISP&#39;s and Universities have been hard hit with spoofing attacks, a feature that stops Source IP Spoofing at the Layer 2 subnet level provides a number of advantages. ISPs and Universities frequently have very large subnets, and as a result, utilizing defenses against spoofing at the router lever can consume an inordinate amount of the router&#39;s processing power. In the past networking devices, such as switches, at the subnet level did not analyze source IP addresses in data packets sent by hosts on the subnet, and in particular it is believed that networking devices at the layer 2 level did not analyze source IP address information in data packets to provide anti-spoofing security procedures based on an analysis of source IP address information in data packets transmitted by hosts on the subnet. Some layer 2 switching devices did provide for some security on ports of the switch, where the source host MAC address was used on an inbound filter on the port to which the host was connected. However, this MAC address type of port security did not provide effective protection against a host attacker that was spoofing source IP addresses. 
       FIG. 2  shows a view of an embodiment of a network device  200  of the present invention. This network device provides some layer 2 switching functionality where the MAC addresses of different hosts coupled to the subnet are utilized in applying switching procedures and identifying different hosts on the subnet. The network device includes a number ports  202 - 232 . End user host devices (not shown) such as personal computers can be coupled to these ports  202 - 232 , and it is possible for other network devices such as hubs or additional switches to be connected to a port of the network device  200 . A subnet could include one of the network devices  200 , or could include a large number of network devices  200  coupled together and connected with a large number of hosts to form a large subnet. The operation of the network device  200  allows for passing data packets received on a port through the network device switching  234  and then transmitting the received data packets through a different port, such that the data packet is transmitted to an intended destination device. 
     The basic switching operation of such a network device is well known. The network device  200  contains a processing device which operates to analyze data packets received on a port to identify the MAC address of the host sending the data packet. Each data packet can include at least the MAC address of the device sending the data packet (source MAC address) and the MAC address of the device to which the data packet is to be sent to (destination MAC address). As discussed above each host on the subnet can also have an IP address. In the past where one host device on a subnet is sending data packets to other host devices on the same subnet, a switching device would refer to a MAC address look up table to determine which port the destination host was on, and the data packet would be transmitted through the port which is connected with the destination host. A typical layer 2 switch would not analyze the source IP address in connection with this switching function, and would not use the source IP address to provide source IP anti-spoofing operations. 
     In the network device  200 , however, additional functions are provided which allow for utilizing source IP address information. As described herein much of this additional functionality is described in connection with a port security processor  242 . Functions of the port security processor can be implemented in a single processor, which is programmed to provide a number of different functions, or aspects of the functions of the port security could be implemented by different processors which work cooperatively to provide the functions herein. As shown in the embodiment of  FIG. 2  the port security processor  242  includes a MAC address detector  238  which detects when a new host has been coupled to one of the ports  202 - 232  of the network device  200 ; the MAC address for the new host is stored in a table which correlates the MAC address for the new host with the port to which it is coupled. In one embodiment, this MAC address table would be stored in an ACL-CAM discussed in more detail below. In addition when a new MAC address is identified, a source IP address detector  236  operates to identify the source IP address which corresponds to the MAC address for the new host. This source IP address and the corresponding MAC address are then stored in the table such that each MAC address and source IP address is correlated with each other as a source IP address/MAC address pair. 
     The table which stores the IP address/MAC address pairs can be implemented using a number of different devices. In the embodiment shown in  FIG. 2  the table is embodied as an access control list, which are data fields, included in a content addressable memory  240 , which is referred to as an ACL-CAM. By utilizing a content addressable memory where the functionality of the memory is determined by hard wiring (as opposed to a CPU which requires the loading of software), the switching of the of data packets is done at a very high speed, and once the MAC address has been determined and the source IP address has been learned, a CPU  244  of the network device  200  is able to operate to monitor and control other aspects of the of the operation of the network device, and the ACL-CAM will control access and switching through the ports. 
     Some prior switches allowed for initially learning source IP addresses for MAC addresses, however, these prior switches were not used for protecting against source IP address spoofing. One limitation of utilizing a prior switch was that these were not designed to allow for the fact that source IP addresses are generally not static, so the source IP address for a MAC address can change over time. 
     In the network device  200 , the source IP address detector  236  automatically learns the source IP address for each MAC address entering a port of the network device  200 . The port security processor  242  of the network device  200  also provides for dynamically adjusting inbound source IP address anti-spoofing blocking criteria for each port, and a system administrator can specify how many devices or IP addresses to permit per port of the network device  200 . For example, the port security processor  242 , can be programmed to receive input from a system administrator&#39;s computer  246 , which can be coupled to the network device  200  by a secure port  248 , and to provide information to the system administrator&#39;s computer  246 . Using the ability to input commands to the network device  200  a system administrator can control aspects of the port security operation, as well as other aspects of the operation of the network device  200 . For example, the system administrator could control the maximum number of source IP addresses which are learned from a port, and the network device  200  would reject any data packets with new source IP addresses that exceed the maximum number. 
     In one embodiment, the port security processor  242  will periodically poll ports for the learned IP addresses which are stored in the table to ensure that the host devices with the learned source IP addresses are still coupled to the port. If it is determined that a host device having the learned source IP address is no longer coupled to a port then source IP address for the host that is no longer present can be removed from the table so as to allow a new source IP address to have access on the port. 
     The network device  200  extends port security features beyond the MAC address filtering procedures that were used in prior layer 2 devices. The port security processor  242  allows source IP anti-spoofing protection to be activated selectively on a port-by-port basis. The port security processor  242  uses the source IP address detector  236  to automatically learn the source IP addresses for each host device attached to the port. To determine if the data packets received at a port contain a new source IP address that has not been learned, the ACL-CAM compares the source IP address and the MAC address in a received data packet, with the table of IP address/MAC address pairs. 
     When a new MAC Source Address is detected on a port by virtue of a received data packet identifying a new MAC source address, the source IP address detector  236  learns the association of source MAC address and the corresponding source IP address, Once the pair is learned, the ACL-CAM  240  is programmed with the information and switching of the network device proceeds to switch data packets normally. 
     The source IP address detector  236 , can learn the source IP address for a host in a number of different ways. For example, the source IP address can be learned by using a reverse address resolution protocol (RARP) which provides for sending out the MAC address on the subnet, and in response an RARP server sends a data packet to identify the IP address which corresponds to the sent MAC address. Thus, listening to the RARP server communications can provide one way for determining a source IP address and MAC address pair. The source IP address detector  236  could also learn the IP address for a host by listening to the DHCP response packet being returned to the host. This response contains the source TP Address for the host. When a DHCP packet is detected, the entry in the table for the MAC address receiving the DHCP packet with a source IP address is cleared and the source IP address provided in the DHCP packet is loaded into the table. This utilization of the DHCP response works well where the port is set to allow for one IP address. It should be noted that if a port is set to allow for more than one IP address for the port, then relying on the DHCP response alone may be insufficient, as the DHCP response may not allow for an unambiguous correlation of the source IP address to the correct MAC address. However, this ambiguity could be resolved if the DHCP request and the DHCP response were both tracked. Another technique provides for watching for the IP header information in a data packet when the host first transmits a data packet through a port. If a static IP address is used and the port is set to 1 IP Address, the user can unplug the cable (causing a link down) to reset the table, or a timer can be used to clear the table. Another technique provides for trapping (listening to) ARP requests and ARP reply messages to learn the source IP address and MAC address pairs, and storing the pairs in ACL-CAM for each port. 
     In addition to the above, it should be recognized that care must be taken to learn the correct source IP address. For example, in order to support delayed technologies, such as the widely known IEEE 802.1X standard which may postpone IP Address assignment until the port is authenticated, a tiered, or delayed, approach is required to successfully detect the source IP address. This approach could allow for first identifying the MAC address and then waiting for an appropriate amount of time to learn the correct IP address. It is important to avoid learning the IP addresses assigned by Microsoft or Apple operating systems which may be provided to a host when a DHCP server cannot initially be found. It can be advantageous to delay IP address learning process until a certain amount of traffic has passed through the port. Further the risk of possibly learning the wrong IP address can be reduced by allowing an system administrator to seed the IP learning process with the IP address properties that are consistent with the subnet to which the host is coupled. For example, if the IP Subnet is 10.32.1.0/24, then this information can be used to seed the IP learning process and only match a MAC address with an IP addresses consistent with the subnet 10.32.1.0/24 addresses. This will eliminate false learning of a default 169.0.0.0 addresses assigned by Microsoft or Apple operating systems when a DHCP server is not initially located. It is also advantageous to confirm that the learned IP address is correct by performing a reverse IP check. 
     Once the learned IP address has been confirmed, and the IP address/MAC address pair has been stored in the ACL-CAM, blocking procedures are applied to the port. If there is more than one host device allowed per port, this process of determining MAC address and source IP address pairs will be repeated for each learned IP address on the port. Once a new source IP Address is confirmed, the inbound blocking procedures are applied to the port and includes the new source IP Address. Additional MAC address and IP address pairs will continue to be learned until the maximum number of MAC and IP Addresses is reached. The source IP anti-spoofing procedures should be compatible with existing MAC address port security and 802.1X Port Authentication features. In order to provide for compatibility the following order of execution can be used. First, MAC address port security is utilized, which confirms that a MAC address present on a port is a valid MAC address, and if it is not MAC port security procedures block data packets from the invalid MAC address. If 802.1X Port Authentication is enabled, the user will be prompted for the 802.1X user credentials to authenticate the port and either permit or deny data packet traffic based on the success or failure of the IEEE 802.1x authentication process. Source IP Security is then used, if enabled, to check if the maximum number of source IP addresses has been learned for a port. If the maximum number of source IP address has not been learned for the port, then the source IP address will be learned and confirmed, and the pairing of the source IP address with MAC address will be set in a table. 
     In order for the blocking to be efficient and fast, it should be implemented in hardware, such as a CAM, as opposed to using a CPU where the operation would be slower. The source IP anti-spoofing methods can provide for different types of security. For example, one aspect of the operation described herein provides for allowing an system administrator to set a maximum number of source IP addresses for a port. By limiting the number of the source IP addresses which can transmit data packets through a port of the network device  200 , the risk of certain types of spoofing attacks (such as DoS attacks) can be prevented. This aspect of the operation provides for blocking of data packets at the port of the network device based on the source IP address contained in the data packet. For example, by limiting the number of source IP addresses on a port, an attacking host could only use a limited number of spoofed IP addresses before the maximum number of source IP addresses for the port would be exceeded. 
     A second operation of the network device  200  blocks data packets at the port, where the data packet contains a source MAC address and source IP address pair which does not match one of the previously identified MAC address/IP address pairs stored in the table. In this operation, a host device which is attempting to use a source IP address which does not match the correct source IP address for the MAC address will be blocked at the port. This second part of the operation provides for a high level of security against source IP spoofing attacks. 
     It should be noted that embodiments of the methods and systems herein can be provide for a significant amount of flexibility, which can provide a system administrators and ISPs with a powerful tool to combat source IP spoofing. For example, as discussed above, in one embodiment the number of source IP Addresses which can be associated with each switch port can be selected by an administrator. If more than one source IP address is permitted per port, then source IP spoofing is possible if the attacking host is using a validated MAC address, unless the source IP Security process as been activated to provide for port access based on correlating the learned source IP Address to its MAC address, such that access on a port is blocked, or permitted, based on the matching of the MAC/IP address pair in a received data packet with a MAC/IP address pair stored in a table. It should be recognized, however, that even without providing security based on the MAC/IP address pairs, some degree of protection against Source IP spoofing is provided by allowing the number of source IP addresses on the port to be controlled. 
     The port security processor  242  can also include a source IP age out timer  250 . The port security processor  242  can allow an administrator to specify a time period for a source IP age-out timer. This timer can clear the ACL-CAM, or other possible table, of source IP addresses, every n seconds to allow the network device  200  to support downstream hubs and switches for multi-host configurations. If a timer were not provided then source IP addresses which were previously on the port might prevent new source IP addresses from gaining access to the port, where the maximum number of source IP addresses would be exceeded. In short, it would not be practical to maintain a link for a source IP address indefinitely. Where an administrator did not want to have a time out entry they could specify a zero “0” for the timer, and a default of 60 or 120 seconds or other appropriate time could be provided. Additionally, or alternatively, the source IP age out timer could also be flow based, which would provide that if the flow stops for a source IP address, for a period of n seconds, then the source IP address can be removed (aged out) from the table. 
     An embodiment of the system and method can also provide for capturing the information when a possible IP spoofing attempt has occurred. This information could then be used to generate syslog messages which could be transmitted and recorded in a log to record information regarding the operation of the system, including possible IP spoofing attempts. 
     As shown by the discussion above, it is important in many applications of the source IP security operations herein, that the table containing the learned source IP addresses be dynamic, such that the table can be updated so that timed out source IP addresses can be removed and new learned source IP addresses can be added to the table. Further, being able to change entries in the table allows for the fact that the source IP addresses assigned for a given host can change over time. This means that when the learned source TP addresses are stored in an ACL-CAM, these addresses should not be saved when a write only memory operation is performed. 
     The port security processor  242  should be programmed such that an administrator can view the source IP addresses learned and/or assigned to the port. Further, the IP security device should allow and administrator to view the setup and configuration of the timers and source IP address ACL&#39;s. The port security processor  242  should provide commands which allow an administrator to clear a single entry from the table, and to allow the administrator to clear all entries from the table. Further, debugging tools may also be provided to allow administrators to troubleshoot the security procedures for their particular environments. 
       FIG. 3  is a flowchart showing a method  300  of an embodiment of the method herein. The method provides for receiving  302  a data packet from a host on a port of the network device. The data packet is analyzed  304  to determine its source IP address. The determined source IP address is then compared  306  with the source IP address, or addresses, which have previously been learned and stored in a table as source IP address which is permitted access to the network through the port. If it is a previously learned IP address then the data packet is passed  308  through the port. If it is not a previously learned source IP address, then it is treated as a new source IP address. Once it is determined a new IP address is on the port, then it must be determined  310  if the maximum number of source IP addresses are present on the port. If the maximum number of source IP addresses are present on the port, then the data packet with the new source IP address is blocked  312  and the information relating to the blocking of the port is transmitted in a trap and syslog message. If the maximum number of the source IP addresses are not present on the port, then a learn source IP routine is performed  314  (various methods related to this are described above). Typically the learn source IP routine will include doing a reverse IP check to confirm the source IP address. If the confirmation or learning of the source IP address fails then the data packet containing the new source IP address is blocked  316  and syslog message can be generated. If the learning and confirmation of the new source IP address is successful, then the new source IP address is stored  318  a table indicating that the new source IP address is permitted access through the port. As discussed above this storing of the new source IP address in a table, can include storing the information in an ACL-CAM. Further a syslog message could also be transmitted in connection this operation. At this point the data packet with new source IP packet can be passed  320  through the port. Further, an additional operation not shown in the flow chart includes the operation of the age-out timer which would provide for removing previously learned source IP addresses which are stored in the table and determined to no longer be present on the port. 
     In general in connection with the reverse IP checking described above, this type of operation may also be referred to as source IP checking. It should also be recognized that in some embodiments some source IP addresses will be statically configured. Where a source IP address is dynamically mapped, the source IP check can be done by sending out an ARP request to the host. If the response comes back and matches with the MAC address previously learned then the source IP check is successful. If the MAC address does not match then the source IP check has failed. 
       FIG. 4  shows a method  400  of another embodiment of the invention. The method  400  provides for receiving a data packet on a port and determining its source MAC address  402 . The determined source MAC address is compared  404  with MAC addresses shown in a table  404 . A determination  406  is made as to whether the source MAC address is new, which would mean that it is not in the table. If the source MAC address was previously stored in the table, then the source IP address and MAC address pair for the received data packet is compared  408  with the source IP address/MAC address pairs in the table. If the pair for the received data packet is found in the table then the received data packet is passed  410  through the port. If the pair for the received data packet is not found in the table, then the received data packet is blocked, or dropped  412  at the port. 
     If the received data packet at the port has a MAC address which is new, then the source IP address for the received data packet is learned  414  using one of the processes described above. After the source IP address has been learned, a reverse IP check  416  is done to confirm the source IP address. If the reverse IP check is successful  418 , then the table is programmed  420  with the IP address/MAC address pair, and the packet is passed  422 . If the reverse IP check is not successful then the received data packet is blocked  424 , or dropped at the port. It should also be recognized that some computers, or host workstations, may have firewalls which do not do not respond to RARP, or ICMP, or some other reverse IP techniques. Thus, in these situations the data flow from a host can be monitored to validate that the host is utilizing the proper MAC/IP address pair. 
     An embodiment herein provides for network device which includes layer 2 switching functions, where the switch provides for protection from spoofing of the Source IP address. The valid set of IP hosts, or subnets, can be configured statically, or learned dynamically by looking at the address information from various protocols, such as ARP and RARP, as discussed above. There can be provided two phases to the learning of the host addresses: the learning from the different control or data packets, and then validation of the learned information. Once the learned addresses are validated, the data packets from the validated source IP addresses will be forwarded. If the port security is enabled on the port then all other data packets from different source IP address will be dropped. 
     The security functions of an embodiment herein can be particularly advantageous when implemented on edge switches, or a riser switch feeding off from the edge switch, where the edge switches or riser switches are network devices that provide connectivity to different hosts connected to the subnet, and the data packets received by these switches contain both source IP and MAC addresses. This is illustrated in  FIG. 5 .  FIG. 5  shows a system  500  of an embodiment herein. The router  502  operates to receive data packets originating in the subnet  520 . The router  502  can receive data packets being transmitted through switches  504 - 510  of the subnet  520 . These data packets received by the router  502  will contain both a MAC address and a source IP address, so it is possible to implement the source learning and validation at the router  502  for data packets which are outgoing from the subnet  520 . However, it should be noted that it can be very advantageous to implement the anti source IP spoofing functions at switches which are directly coupled to the hosts. The advantage of moving this process closer to the hosts is that the potential spoofed IP attack could be stopped essentially at the very port to which the attacking host is connected. Switch  506  is shown as having hosts  512 - 516  connected to ports of the switch. In fact there many implementations there could be hundreds or thousands of hosts connected to switches of the subnet. 
     In light of the above, it should be recognized that the anti-source IP spoofing function herein can advantageously be implemented on ports of the switches, as opposed to waiting to implement these functions at ports of the router. It should be noted that in implementation where a particular switch is not implementing anti source IP spoofing functions, then the router may need to do its own reverse source IP checking and validation, and further it will be recognized that a layer 2 switch will not be able to validate a host&#39;s source IP address when the host is connected to the system such that a router is positioned between the host and the layer 2 switch. This is because the layer 2 switch would typically see the MAC address of the router, rather than the MAC address of the host. 
     The Source IP Anti spoofing functions herein can co-exist with existing CAM-ACL features, such as, Flow Based functions, and Rule Based ACL functions, where flow based controls call for specific policies to be applied on host by host basis, and a rule based approach can allow for general or specific rules to be applied to groups of hosts, or ports. The implementation of the Source IP anti-spoofing functions can be done such that it is compatible with other features which can be provided in a network device, these features can include MAC address filtering; 802.1X port authentication; dynamic VLAN configuration; and dynamic user policies. Thus, the system and method herein allow for the learning and validation of source IP addresses, and for dynamically binding policies with the learned source IP addresses. Further, the filtering herein can be implemented in hardware, and can be applied to both dynamically learned source IP addresses and to statically assigned source IP addresses. 
     An embodiment herein can provide for generating system log information which records events regarding the learning and validation of source IP addresses. The learning and validation of the source IP addresses as well as the generation of system log information can consume the switches CPU processing power. Thus, in some applications it may be beneficial to provide additional features which will further reduce the amount of CPU processing required to implement the anti-source IP spoofing functions herein. One embodiment could provide a mirroring operation where information which would otherwise be processed by the CPU can be mirrored (transmitted) to another processing device which can share some of the processing demands with the CPU of the switch. 
     An additional aspect of the learning and validating the source IP addresses, is that controls and policies can be implemented not only on data packets coming into the switches, additionally outgoing policies can be applied on data packets being transmitted from a port. For example, a specific source IP address may be blocked from transmitting data packets through certain specified ports of the switch. An embodiment herein can also apply rate limiting policies, where the number of data packets transmitted to a particular host during a given time frame can be limited. 
     The source IP address for hosts connected to ports of a switch can be either statically configured, or learned dynamically. The learning of source IP addresses can provide for looking at the control packets, such as, ARP, RARP, DHCP, BOOTP, ICMP, etc. After the source IP address is initially learned the source IP address can be validated using RARP or ICMP procedures. Once a learned source IP address has been validated, the source IP address will be programmed into a CAM-ACL, and can be utilized in connection with layer 4 features that use the CAM. For example, these features can include rate limiting on certain ports, rate limiting for certain hosts, and providing for application control. Further as was the case with the above references to Layer 2 and Layer 3, Layer 4 is another functional layer of a network identified and discussed generally in the International Standards Organization, standard ISO/IEC 7498, which defines a 7-layer model for describing interconnected systems. It is referred to as the Open Systems Interconnection (OSI) model, and is incorporated herein by reference in its entirety. The Layer 4 is the transport layer which provides for transfer of data between hosts and flow control. Additional aspects of the learning of source IP address and L4 CAM programming are discussed below. 
     In many instance it will be desirable to assign certain hosts or devices of a network system with a static source IP address. A source IP address can be statically configured by an administrator of the system using a command line interface (CLI) of the network device. Examples of devices where is would be beneficial to provide a static IP address could include an application server, or database server, or source IP addresses for different network devices such as the switches or routers. 
     For dynamically assigned source IP addresses a number of different options can be used to determine the source IP address for a host. One of these options is to perform snooping of ARP requests. This snooping of ARP requests means that the switch will listen to ARP requests which are transmitted by hosts of the subnet, and then to the response to the ARP requests, and this response will include the MAC address and the source IP address for a host. After initially learning the MAC address and the source IP address for a host, the switch will probe or ping the host using ARP or ICMP to receive a confirmation that the host is actually responding to data packets sent to the learned source IP address. Once this has been confirmed, or validated the switch will identify the learned source IP address as valid, and record the Source IP address and corresponding MAC address in the CAM-ACL. During the transient state, which occurs after initially learning a new source IP address but prior to confirming the source IP as valid, the switch will continue to forward data packets with the new source IP addresses for the period of time, during which the validity of source IP addresses is being confirmed. If the source IP address is not confirmed as valid within a designated time period, then subsequent data packets having the new source IP address which has not been confirmed as valid will be dropped. 
     Another option for learning dynamically assigned source IP addresses, utilizes the fact that, in some systems, on boot-up a host on the subnet sends RARP broadcasts with its MAC address, and a RARP server responds with the source IP to be used by the host having the MAC address in the RARP broadcast. By listening to the RARP response from the RARP server, the source IP address can be learned by monitoring the RARP response on the port where the corresponding host having the corresponding MAC is coupled. 
     As discussed above the data packets from the DHCP server can also be analyzed to determine the assigned source IP address. The operation of some computer networks provide that a host and a DHCP server exchange DHCP messages by which the DHCP server assigns an IP address to the host. In such a network, the network device, switch, of the present invention with the source IP protection enabled, can listen to the DHCP messages and learn the IP address assigned to a host by the DHCP server. 
     Some systems will utilize BOOTP data packets; and the general operation in connection with BOOTP data packets is the same as for the DHCP operation. In the BOOTP systems on boot-up, a host sends a bootp broadcast message with its MAC address. The Bootp server responds with the IP address for that MAC address. By snooping into (analyzing) the BOOTP server response the IP address can be learned for that particular port on which the MAC address was learned. 
     Additionally, the source IP address can be learned by temporarily retaining a data packet transmitted by a host, and then generating an ARP request and waiting for an ARP response from the host to confirm the source IP address. 
     As discussed above the system and method herein also provide for monitoring an aging of the source IP address. A method of an embodiment herein would provide that statically configured Source IP Addresses on the subnets are never aged out. These static source IP addresses would be removed when they have been removed from the subnet configuration. For dynamically assigned and learned source IP addresses, the method can provide for aged source IP to be removed. 
     Layer4 CAM Programming 
     The network device can provide two variations for the layer 4 (L4) CAM-ACL programming. One of these variations provides for rule based control where groups of source IP addresses are provided with rule defined access through the network to other addresses. A second variation, is a flow based model where specific source IP address are provided with certain flow protocols determined on a source IP address specific basis. An embodiment herein provides for programming a Layer4 CAM-ACL entry with the source IP as one of the specified addresses on the port. In one implementation there is support for the Rule based CAM-ACL programming using hardware filtering, features such as TOS/DSCP honoring (e.g. providing a specified quality of service QOS or type of service TOS), marking (such prioritizing traffic or packet types), and rate limiting. (DSCP stands for Differential Service Code Point. It is a 6 bit value in an IP header that can serve as a quality of service information for the switch, and it can be advantageous to allow the network device to change the value of the DSCP). 
     In the absence of any specific ACL bound rules applied to the port, the switch will be programmed for a session entry to forward all the IP traffic with the validated source IP addresses, and deny all other source IP traffic. This rule would typically be applied for every new IP source learned on the switch. 
     In the Layer4 CAM programming, there is an implicit Source IP/Subnet clause for the configured or learned source address of the trusted subnet. The extended ACL will not have a source &lt;clause&gt; of its own for a port that is configured with this feature. Thus, where a source IP and MAC address pair has been learned and verified then groups of ACL rules or clauses from the CAM can be applied to the learned source IP address. 
     Rule Based ACL 
     When a rule based ACL group using extended ACL is configured, rules which are pre-programmed into the CAM can be applied to learned and verified addresses. The extended ACL can operate to provide a number of functions, including operating to restrict application services. This programming can then be repeated, or updated, with an implicit permit Source IP clause for each secured host source IP address. A secured host source IP address could be a source IP address which has been learned and validated or it could be a statically assigned source IP address. 
     In one embodiment if the Source IP addresses are configured statically, whenever an ACL clause (e.g. a policy rule) is bound to a port, the system will program the clauses with an implicit Source IP permit clause. Once the ACL entry for the statically configured address is unbound from the port, the CAM associated with the ACL entry will be removed. Thus, in this manner IP/MAC address pairs can be removed from the CAM-ACL. 
     In one embodiment, for a dynamically learned source IP Address, the system will have to re-program all the ACL clauses for each Source IP Address learned with an implicit source IP permit clause added to them. If the protocol mechanism (such as the validation process) or aging process, decides that a dynamically source IP address is not to be used any more, the specific Source IP address is removed from the ACL, which can make space available for a new source IP address. 
     Statically Configured Addresses 
     When the secured hosts are statically configured, the host addresses are programmed in the CAM and only traffic from these statically configured addresses are permitted on the configured ports. All other IP traffic is dropped. The learning control packets such as DHCP, BOOTP will still be processed and forwarded via the CPU of the switch. The L4 CAM-ACL for the statically configured host address will reside in the bank that gets matched prior to the Rule CAMs. 
     If there is a conflict between the statically configured addresses and the Rule based ACL, the statically configured addresses will take precedence. In such cases, it may be desirable to have only one or the other, but not both (i.e. to provide either the statically configured CAM or the rule based ACL but not both). 
     Rule Based ACL 
     Each rule based ACL will operate to provide a keyword of “secured-host” which will be applied to a learned and validated source IP address for hosts. All other source IP addresses will be denied. Any other ACL rule without the keyword will be used as is, which means that other ACL rules will be applied to data packets received on the ports without alteration, but notably if a source IP address is not designated as a secure host then it will dropped, so none of the ACL rules would be applied to it. 
     Command Line Interface 
     Provided herein below are some examples of the syntax of code which can be generated via an interface between user, such as system administrator, and the CPU of the switch which controls the operation of the switch, and the programming of the CAM-ACL. 
     ACL 
     The ACL will provide for use of a keyword, “secured-host” which can be used to identify source IP addresses which have been learned and validated. The user can configure the ACL with this as the source subnet. 
     The language below shows exemplary syntax from an embodiment of command line interface for switch where different key words are defined and specifically the last line shows the designation of “secured host” as applying to secure IP hosts. 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 &gt;ip access-list extended 111 
                   
               
               
                   
                 &gt;permit ip ? 
               
               
                   
                 (config-ext-nacl)#permit ip 
               
               
                   
                  A.B.C.D or A.B.C.D/L 
                 IP address/Subnet mask 
               
               
                   
                 length 
               
               
                   
                  any 
                 Any destination host 
               
               
                   
                  host 
                 A single destination host 
               
            
           
           
               
               
            
               
                   
                  secured-host     Secure IP hosts 
               
               
                   
                   
               
            
           
         
       
     
     Configuration 
     As discussed above the anti source IP spoofing functions herein can be enabled on a port by port basis. At the port, or interface, level the syntax utilized in connection with the command line interface for a switch is shown as follows for enabling the anti-spoofing feature on an interface, where the last two lines of this text corresponds to the command for enabling the anti-spoofing functions. 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 srcip-security &lt;CR&gt; 
               
               
                   
                 MON-SW-BigIron Router(config-if-e1000-1/1)#srcip? 
               
               
                   
                  srcip-security       Configure source ip security for 
               
               
                   
                 this interface 
               
               
                   
                   
               
            
           
         
       
     
     In one embodiment the dynamic learning of source IP addresses is enabled by default, but commands will be available to disable the dynamic learning. In addition other options for configuration could include: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 MON-SW-BigIron Router(config-it-e1000-1/1)#srcip 
               
            
           
           
               
               
               
            
               
                   
                  A.B.C.D 
                 IP Address 
               
               
                   
                  DECIMAL 
                 Number of Allowed source IP 
               
               
                   
                 Addresses 
               
               
                   
                  ageing 
                 Disable aging 
               
               
                   
                  trusted-net 
                 Network to be trusted to learn IP 
               
               
                   
                 address 
               
               
                   
                  &lt;cr&gt; 
               
               
                   
                   
               
            
           
         
       
     
     Much of the configuration above pertains to configurations implemented at the interface or port level. Additionally a number of configurations can be applied at global level to multiple interfaces, or ports of a switch. The language below shows command line interface text associated with globally configuring the switch for different aspects of function such as setting a max-age out time for a source IP address, setting the maximum number of IP addresses per port, the disabling of the aging function of port, and other configuration commands. Of course these configurations could also be applied on a port by port basis if a user desired to do so. 
     
       
         
           
               
             
               
                   
               
             
            
               
                 MON-SW-BigIron Router(config)#srcip ? 
               
            
           
           
               
               
            
               
                  max-age 
                 Age out time for learned sourceIP 
               
               
                 entries 
               
               
                  max-ipaddr-per-interface 
                 Maximum number of addresses to learn 
               
               
                   
                 per interface 
               
               
                  ageing 
                 Disable aging 
               
               
                  trusted-net 
                 Network to be trusted to learn IP 
               
               
                 address 
               
               
                  no-dynamic-learning 
                 Disables dynamic learning of IP 
               
               
                 Addresses 
               
               
                   
               
            
           
         
       
     
     Show Commands 
     Given the above operation of the system, it is clear that at times it can be advantageous for a system administrator user to be able to view information regarding the status and configuration of the system relative to implementation of the security functions disclosed herein. A review of some of the tables and information which are available to a user of an embodiment of the system herein is provided to further illustrate aspects of an embodiment of the system herein. 
     For example using a command in the command line interface of “sh srcip-sec-table” can display all the IP addresses learned or currently being learned on all ports which have srcip-security enabled. An example of such a table is shown below. 
                            BigIron Router#sh srcip-sec-table                                     IP Address   MAC Address   Port   State   Learn-Scheme   Age                                             192.168.20.177   000b.cd3d.808a   4/20   LEARNING   DHCP   0       25.25.25.25   0007.e90f.eaa1   4/20   LEARNT   IP_DATA   80       192.168.20.50   0007.1234.1111   4/20   LEARNT   STATIC   0       192.168.20.51   0007.1234.abcd   4/20   LEARNT   STATIC   0                    
As shown the above table indicates total entries=4 total learnt entries=3.
 
     The show srcip-sec-table command can also provide for a view of other aspects of the system such as the status of an IP address, a configuration or the status of specific port the basic elements of the commands are shown below. 
     
       
         
           
               
            
               
                   
               
               
                 MON-SW-BigIron Router#sh srcip 
               
            
           
           
               
               
               
            
               
                   
                 A.B.C.D 
                 IP address 
               
               
                   
                   
               
               
                   
                 configuration 
                 Display the sourceIp-security configuration 
               
               
                   
                 ethernet 
                 slot/port 
               
               
                   
                 | 
                 Output modifiers 
               
               
                   
                 &lt;cr&gt; 
               
               
                   
                   
               
            
           
         
       
     
     A specific command for viewing the table for a specific source IP address 25.25.25.25 is shown below. 
     
       
         
           
               
            
               
                   
               
               
                 MON-SW-BigIron Router#sh srcip 25.25.25.25 
               
            
           
           
               
               
               
               
               
               
            
               
                 IP Address 
                 MAC Address 
                 Port 
                 State 
                 Learn-Scheme 
                 Age 
               
               
                   
               
               
                 25.25.25.25 
                 0007.e90f.eaa1 
                 4/20 
                 LEARNT 
                 IP_DATA 
                 0 
               
               
                   
               
               
                 HashT: Total Number of learnt SrcIp + MAC entries = 1 
               
            
           
         
       
     
     A specific command for viewing a table for viewing security configurations is shown below, where a switch is configured such that the security procedures are enabled and the number of source IP addresses per port is set at 64. 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 MON-SW-BigIron Router#sh srcip conf 
               
               
                   
                 Global Configuration:- 
               
               
                   
                   Enabled: 1 
               
               
                   
                   Max no entries per interface (global): 64 
               
               
                   
                   Age out time: 180 
               
               
                   
                   Ageing: enabled 
               
               
                   
                   Aging frequenzy: 10 
               
               
                   
                   Trusted Net(global): 0.0.0.0 
               
               
                   
                   Trusted NetMask(global): 0.0.0.0 
               
               
                   
                 Port specific configuration:- 
               
               
                   
                   Port: 4/20 
               
               
                   
                     Max Entries: 64 
               
               
                   
                     Trusted Net: 0.0.0.0 
               
               
                   
                     Trusted NetMask: 0.0.0.0 
               
               
                   
                   
               
            
           
         
       
     
     A specific command for viewing a table for showing the status of source IP addresses for a specific port 4/20 of switch is shown below. 
     
       
         
           
               
            
               
                   
               
               
                 MON-SW-BigIron Router#sh srcip e 4/20 
               
            
           
           
               
               
               
               
               
               
            
               
                 IP Address 
                 MAC Address 
                 Port 
                 State 
                 Learn-Scheme 
                 Age 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 25.25.25.25 
                 0007.e90f.eaa1 
                 4/20 
                 LEARNT 
                 IP_DATA 
                 20 
               
               
                 192.168.20.50 
                 0007.1234.1111 
                 4/20 
                 LEARNT 
                 STATIC 
                 0 
               
               
                 192.168.20.51 
                 0007.1234.abcd 
                 4/20 
                 LEARNT 
                 STATIC 
                 0 
               
               
                   
               
               
                 Total entries = 3 total learnt entries = 3 
               
            
           
         
       
     
     The language below shows the commands and table for a specific address which includes 25 and is on interface 4/20. 
     
       
         
           
               
             
               
                   
               
               
                 MON-SW-BigIron Router#sh srcip e 4/20 | incl 25 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 25.25.25.25 
                 0007.e90f.eaa1 
                 4/20 
                 LEARNT 
                 IP_DATA 
                 50 
               
               
                   
               
            
           
         
       
     
     Clear Commands 
     During the course of operation of the network device herein there may be times when it is desired to clear learned hosts. Commands can be provided specifically for clearing the CAM-ACL of the learned and validated source IP addresses. Additionally, statically configured hosts can be removed when the user removes them from the configuration from the specified vlan. 
     The language shown below provides for clearing all learned source IP addresses from a table. 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 MON-SW-BigIron Router#clear srcip? 
               
               
                   
                  srcip-security-table  Flush the srcip security table 
               
               
                   
                   
               
            
           
         
       
     
     The language shown below provides for clearing a specific learned source IP addresses from a table. 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 MON-SW-BigIron Router#clear srcip ? 
               
               
                   
                  A.B.C.D  IP Address 
               
               
                   
                 MON-SW-BigIron Router#clear srcip 25.25.25.25 
               
               
                   
                   
               
            
           
         
       
     
     The language shown below provides for clearing selected source IP addresses from a table. 
     
       
         
           
               
            
               
                   
               
               
                 MON-SW-BigIron Router#sh srcip 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Learn- 
                   
               
               
                 IP Address 
                 MAC Address 
                 Port 
                 State 
                 Scheme 
                 Age 
               
               
                   
               
               
                 192.168.20.50 
                 0007.1234.1111 
                 4/20 
                 LEARNT 
                 STATIC 
                 0 
               
               
                 192.168.20.51 
                 0007.1234.abcd 
                 4/20 
                 LEARNT 
                 STATIC 
                 0 
               
               
                   
               
               
                 Total entries = 2 total learnt entries = 2 
               
            
           
         
       
     
     The language below can be used to provide commands to clear all the learned hosts from the specified interface. 
     Interface e 1/1 
     clear srcip-host &lt;CR&gt; 
     It should be noted that the numerous examples of command language provided above are merely illustrative. One of skill in the art will recognize that the specific command language can be varied, as can different aspects of the network devices operation, and still be such that it is with the scope of the general teaching herein. 
     The above described system and method is very flexible and can provide for a range of different implementation. For example, where desired the source IP address can be validated against the subnet. The system can also provide for a trusted-subnet that the user can configure. 
     In order to minimize the demands placed on the CPU of the switch, data packets with source IP packets which do not pass the security procedures herein can be dropped, without maintaining a record of which source IP addresses have been dropped. However, other embodiments can provide a log of denied packets. In order to keep a record of discarded packets, but still provide for some protection of the CPU resources, the operation can provide for forwarding discarded packets to a mirror port. 
       FIG. 6  shows a flow chart of a method  600  of an embodiment herein. The initial step in the method is learning  602  the source IP address provided by a host connected to a subnet. This learned source IP address would be a new source IP address in that it would not be stored CAM-ACL and it would not have previously been associated with the MAC address of the host which it is presently associated with. As discussed above the source IP address could be learned a number of different ways. After a new source IP address is learned, the network device will operate to attempt to validate  604  the source IP address. Examples of different ways of validating learned source IP address are discussed above. Following the validation attempt, a determination  606  is made as to whether the validation attempt was successful. If the validation attempt was not successful then a determination  608  as to the number of attempts is made. If three attempts have been made then data packets with the new source IP address, which has been learned but not validated, are dropped  610 . If it is determined that 3 or more attempts to validate the learned source IP address have not been made, then another attempt  604  is made to validate the source IP address. If at  606  it is determined that the source IP address was validated, then the CAM-ACL is programmed with the new source IP address, which has been learned and validated, and as discussed above the CAM-ACL can operate to match the source IP address with the MAC address for the host. Further, it should be recognized that the number of attempts made to validate a newly learned source IP address could be varied such that more or less than 3 attempts are made to validate a source IP address. 
       FIG. 7  shows a flow chart of a method  700  of an embodiment herein. The method  700  provides for determining if a source IP address stored in the CAM-ACL should be aged out, which would result in the source IP address being removed from the CAM ACL and another source IP address and MAC address pair could take its place. At step  702  a source IP address in the CAM-ACL is identified. This source IP address would a source IP address that had previously been learned and validated. A determination  704  is made as to whether the last activity detected from the host having the identified source IP address, occurred more than predetermined age out time in the past. For example when was the last data packet received from the host, and was this data packet received more than 2 minutes ago. If the last activity does not exceed an age out time period, then another source IP address can be checked to see if it should be aged out. If it is determined that the age out time period has been exceeded, then the processor of the switch can generate an ARP request to check  706  and see if the identified source IP address is still up on the subnet. Additionally, other procedures, for example ICMP could be used to the see if the host with the identified source IP address is still up on the subnet. A determination  708  is then made to regarding whether the source IP was validated—for example, did the host respond to an ARP request. If the source IP was not validated then the source IP address is deleted  710  from the CAM-ACL. If the source IP address is validated then the MAC and source IP address are maintained  712  in the CAM-ACL, and another source IP address can be identified and analyzed starting at  702 . 
       FIG. 8  shows a flow chart of a method  800  of an embodiment herein. The method  800  provides for identifying  802  the number of attempts for new source IP addresses to come up on a port of a network device, or to otherwise to transmit or receive data packets through a port of the network device. As discussed above when a new source IP addressed is learned a method herein provides for attempting to then validate the new source IP address. The method  800 , further provides for tracking  804  the number of new source IP addresses which are not successfully validated during a given time period. The number of unsuccessful validations for the given time period is then analyzed to determine  806  if the number of new source IP addresses is higher than expected. If the number of unsuccessful attempts to validate new source IP addresses does not exceed a threshold level, then the method will continue to apply  808  the normal processing operation described above. 
     However, if the number of new source IP addresses that cannot be validated exceeds a threshold number, then the system will go into a possible attack mode  810 . In the possible attack mode the system will operate to drop all data packets coming from new source IP addresses. This means that new source IP address and MAC address pairs will not be added to the CAM-ACL while the system is in a possible attack mode. Further, while in this mode the system will not attempt to validate new source IP addresses. This possible attack mode functions to protect the CPU from becoming overloaded with trying to validate an exceptionally high number of new source IP addresses, which could occur when a spoofed source IP address attack is occurring. 
     At the step of determining if the number source IP addresses which cannot be validated is high, the system can utilize a predetermined threshold number for an allowable number of unsuccessful validation attempts. This predetermined number could be input by a system administrator. Alternatively, instead of using a predetermined threshold number, the system could provide for a degree of self monitoring and learning, such that the system will observe historic operation of the system to determine, based on past operation, an expected number of unsuccessful validation attempts for a given time period on a given port. 
       FIG. 9  is a graph  900  illustrates aspects of the operation of system where the expected number of unsuccessful validation attempts is determined based on the past operation of the system. Axis  902  shows time, and axis  904  shows the number of unsuccessful validation attempts. During time t 0  the operation of the system is shows a relatively constant number of unsuccessful validation attempts per unit time, where the number of unsuccessful validation attempts is shown by line  906 . During time period t 1  a substantial increase in the number of unsuccessful validation attempts is observed. Time period t 1  is followed by time periods t 2  and t 3  which show increasing numbers of unsuccessful validation attempts. The operation of the system could be implemented in a number of different ways. For example, the system could provide that after three time periods (t 1 , t 2 , t 3 ) of increasing number of unsuccessful validation attempts the system goes into possible attack mode. For example, in a situation where ten unsuccessful validation attempts normal occur during typical two minute time interval, and then the system sees a number of consecutive two minute time intervals where the number of unsuccessful validation attempts increases to 20 per two minute time intervals, the system could be under an IP spoofing attack. While in the possible attack mode the system could continue to monitor the number of data packets received from new source. If the system determines that it is receiving a high number of new source IP address, then the system could continue to operate in possible attack mode. When the system determines that the number of new source IP addresses has decreased to an expected range, then the system could go back to normal operation. In another embodiment, the system could operate such that when it goes into possible attack mode, it remains in possible attack mode for a predetermined amount of time, and then after the predetermined amount of time it goes back into normal operating mode. 
     Another aspect of an embodiment herein is that it allows for dynamic application of forwarding policy or rules. The forwarding of packets on a network device including switching functionality can be based, for example, on the destination MAC address. The policy based forwarding extends the forwarding criteria to include the protocol type, the application ports, such as, UDP/TCP ports, MTU, rate-limiting etc. For example, certain types of data packet protocols could be restricted from being transmitted to devices on certain ports. Or newly learned source IP addresses may be restricted from transmitting or receiving data from devices on other ports of the system. These policies can be extended to hosts learned as a result of the dynamic learning and validation procedures. The forwarding criteria can augmented with the newly learned and validated source IP address, or with statically MAC address/IP address pairs. The traffic from the validated hosts automatically uses the forwarding policies. The traffic from other dynamic source IP Addresses which fail to be validated would be discarded. 
     The switching device can allow a user to configure forwarding policies so that a user can specify the subset of data traffic which passes these forwarding policies will be forwarded, while other data traffic that does not pass the forwarding policies will be discarded or dropped. 
     These forwarding policies are configured using the Extended Access Control Lists, where a user could choose the specific hosts, specific subnets for source and destination ip addresses, specific application ports for UDP/TCP, etc. 
     The Source IP Anti-spoofing will restrict the forwarding decisions to apply to only a subset of hosts that are considered to be validated. The traffic from all other hosts will get discarded. The forwarding policies are modified when the hosts are validated using the validation techniques discussed above. 
     The following example illustrates the forwarding policy and its modification to the set of validated hosts. 
     Access-list 120 permit udp any host 10.10.10.1 
     Access-list 120 deny ip any any 
     The above access-list will permit any UDP application traffic to the destination of 10.10.10.1. All other traffic is discarded. The above access-list does not specify the source ip address that should be allowed. Thus the forwarding policy will apply source IP addresses, and apply this to all the source ip addresses. 
     With the Source-IP Anti spoofing running, say only the hosts 20.20.20.1, 20.20.20.2 and 20.20.20.3 are validated. All other hosts are denied. Thus, the above access-list will now only allow the forwarding from the above hosts, with UDP application port, and to destination address of 10.10.10.1. All other source ip addresses, such as, 20.20.20.5 etc. will be discarded even if they have the UDP protocol type, and to the destination address of 10.10.10.1. 
     In one embodiment forwarding policies are defined to alter, or control, data packet forwarding based on criteria which can be defined, or modified, by a system administrator. In this embodiment ACLs are used to specify this forwarding criterion. The example below illustrates simple forwarding criteria using the ACL. 
     Ip access-list extend 111 
     permit secure-host any 
     deny ip any any 
     The above clause will allow packets from validated hosts to go through. The rest of the traffic is denied. This criteria would apply to hosts on the subnet where this ACL criteria is applied. 
     When Source-IP Anti-spoofing is applied, the anti-spoofing procedures validate a list of hosts on the subnet that are considered valid. Once the hosts are validated, the above forwarding policies applied via the ACL are modified. The ACL is extended to include only the set of validated hosts. 
     This operation allows a system administrator to continue to use the forwarding policies of the ACL, and the system administrator does not have to modify the forwarding policy for each validated host. Instead, the policies are automatically extended/modified to allow the traffic from valid hosts. On a large network with several subnets, this saves the user from having to modify their policy for each host. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. This is especially true in light of technology and terms within the relevant art(s) that may be later developed. Thus, the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.