Attack defending system and attack defending method

• US 7,958,549 B2
• Filed: 07/25/2007
• Issued: 06/07/2011
• Est. Priority Date: 08/20/2002
• Status: Active Grant

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Yamagata, Masaya, Nakae, Masayuki
• Primary Examiner(s): Brown; Christopher J
• Application Number:
  US11/782,801
• Publication Number:
  US 20080282339A1
• Time in Patent Office: 1,413 Days
• Field of Search: 726/11, 726/13, 726/21
• US Class Current: 726/11
• CPC Class Codes: H04L 63/0227 Filtering policies mail message filtering H04L51/212
• CPC Class Codes: H04L 63/0263 Rule management
• CPC Class Codes: H04L 63/101 Access control lists [ACL]
• CPC Class Codes: H04L 63/1408 by monitoring network traffic monitoring network traffic per se H04L43/00
• CPC Class Codes: H04L 63/1491 using deception as countermeasure, e.g. honeypots, honeynets, decoys or entrapment
• CPC Class Codes: H04L 63/20 for managing network security; network security policies in general filtering policies H04L63/0227

Abstract

An attack defending system allows effective defense against attacks from external networks even when a communication system uses a communication path encryption technique such as SSL. A firewall device and a decoy device are provided. The firewall device refers to the header of an input IP packet and, when it is determined that the input IP packet is suspicious, it is guided into the decoy device. The decoy device monitors a process providing a service to detect the presence or absence of attacks. When an attack has been detected, an alert including the attack-source IP address is sent to the firewall device so as to reject subsequent packets from attack source.

Claims

1. An attack defending system provided at an interface between an internal network and an external network, comprising a decoy device and a firewall device, wherein the firewall device inputs an input IP packet from the external network and forwards it to one of the decoy device and the internal network, wherein

the firewall device comprises:

a microprocessor programmed to execute;

a destination selector for selecting one of the internal network and the decoy device as a destination of the input IP packet based on header information of the input IP packet and a distribution condition; and

a confidence manager for managing confidence levels for source IP addresses of a plurality of input IP packets,

wherein the destination selector obtains a confidence level for a source IP address of the input IP packet from the confidence manager and selects a destination of the input IP packet depending on whether the confidence level satisfies the distribution condition,

the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.

2. The attack defending system according to claim 1, wherein, when a confidence level for a source IP address is retrieved, the confidence manager updates the confidence level for the source IP address.

3. The attack defending system according to claim 2, wherein the confidence manager updates the confidence level by adding a predetermined value to the confidence level for the source IP address.

4. The attack defending system according to claim 2, wherein the confidence manager updates the confidence level by adding a variable to the confidence level for the source IP address, wherein the variable becomes smaller as a packet size of the input IP packet corresponding to the confidence level becomes larger.

5. The attack defending system according to claim 2, wherein the confidence manager updates the confidence level when the input IP packet is a packet conforming to a predetermined protocol.

6. The attack defending system according to claim 1, wherein the confidence manager comprises:

a first confidence memory for storing the confidence levels for the source IP addresses of the plurality of input IP packets and latest updated times at which the confidence levels were updated, respectively;

a second confidence memory for storing a copy of the confidence levels stored in the first confidence memory;

a first updating processor for updating a confidence level stored in the first confidence memory when the destination selector has obtained the confidence level;

a copying processor for copying the confidence levels stored in the first confidence memory to the second confidence memory at regular intervals; and

a second updating processor for searching the second confidence memory for a confidence level corresponding to a latest updated time from which a predetermined time period has elapsed, to update a found confidence level,

wherein the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.

7. The attack defending system according to claim 6, wherein the copying processor searches the first confidence memory for a confidence level corresponding to a latest updated time from which a predetermined time period has elapsed and deletes an entry corresponding to a found confidence level from the first confidence memory.

8. The attack defending system according to claim 6, wherein the second updating processor decrease the found confidence level by a predetermined value.

9. The attack defending system according to claim 6, wherein the second updating processor deletes the found confidence level from the second confidence memory.

10. The attack defending system according to claim 1, wherein the decoy device comprises an attack detector for detecting presence or absence of an attack by executing a service process for the input IP packet transferred from the firewall device.

11. The attack defending system according to claim 10, wherein the confidence manager updates a confidence level of a source IP address of the input IP packet transferred to the decoy device depending on whether the attack detector detects an attack based on the input IP packet transferred from the firewall device.

12. The attack defending system according to claim 1, wherein the decoy device comprises:

an event memory for temporarily storing events related to at least network input/output, file input/output, and process creation/termination; and

an event manager for analyzing cause-effect relations of the events stored in the event memory to form links among the events.

13. The attack defending system according to claim 1, wherein the decoy device comprises:

an attack detector for detecting an attack from an execution status of the service process according to a rule having at least one of domain constraint and type constraint added thereto.

14. The attack defending system according to claim 12, wherein the decoy device further comprises:

an attack detector for detecting an attack from an execution status of the service process according to a rule having at least one of domain constraint and type constraint added thereto.

15. The attack defending system according to claim 14, wherein the attack detector searches the links to extract at least, a generation event of a process generating an event to be inspected and a network reception event by which the event to be inspected is generated, when determination is made based on the domain constraint and the type constraint.

16. The attack defending system according to claim 1, further comprising a mirroring device for copying at least a file system from a server on the internal network to the decoy device, wherein when an attack is detected by the decoy device, the mirroring device copies at least the file system from the server on the internal network to the decoy device.

17. An attack defending method using a decoy device in a firewall device provided at an interface between an internal network and an external network, comprising:

a microprocessor programmed to execute;

preparing a filtering condition and a distribution condition for input IP packets;

holding confidence levels for source IP addresses of a plurality of input IP packets;

determining whether an input IP packet inputted from the external network is to be accepted, based on header information of the input IP packet and a filtering condition corresponding to the input IP packet;

selecting one of the internal network and the decoy device as a destination of the input IP packet accepted, based on whether the confidence level satisfies the distribution condition;

detecting presence or absence of an attack by executing a service process for the input IP packet forwarded to the decoy device; and

managing the filtering condition corresponding to the input IP packet forwarded to the decoy device depending on whether an attack is detected based on the input IP packet,

wherein the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.

18. The attack defending method according to claim 17, wherein the header information of an input IP packet includes at least one of a source IP address and a destination IP address thereof, wherein a destination of the input IP packet is determined depending on whether the header information of the input IP packet satisfies the distribution condition.

19. The attack defending method according to claim 17, wherein the distribution condition is a guiding list containing a set of IP addresses unused in the internal network, wherein the input IP packet is transferred to the decoy device when a destination IP address of the input IP packet matches an unused IP address contained in the guiding list.

20. The attack defending method according to claim 17, wherein the distribution condition is updated depending on whether an attack is detected based on the input IP packet transferred to the decoy device.

21. The attack defending method according to claim 17, wherein the filtering condition managing step comprises:

setting the filtering condition with a limited validity period, corresponding to the header information of the input IP packet forwarded to the decoy device; and

when the limited validity period has elapsed, setting the filtering condition to a default filtering condition.

22. The attack defending method according to claim 17, wherein the filtering condition managing step comprises:

generating a filtering condition corresponding to a combination of an attack category of an attack detected and address information of the input IP packet; and

dynamically updating the filtering condition according to the filtering condition generated.

23. The attack defending method according to claim 17, wherein the attack detecting step comprises:

temporarily storing events related to at least network input/output, file input/output, and process creation/termination; and

analyzing cause-effect relations of the events to form links among the events.

24. The attack defending method according to claim 17, wherein the attack detecting step comprises:

extracting events related to at least network input/output, file input/output, and process creation/termination; and

comparing the extracted events with a rule having domain constraint and type constraint added thereto.

25. The attack defending method according to claim 23, wherein the attack detecting step further comprises:

comparing the events with a rule having domain constraint and type constraint added thereto.

26. The attack defending method according to claim 25, wherein the links are searched to extract at least, a generation event of a process generating an event to be inspected and a network reception event by which the event to be inspected is generated, when determination is made based on the domain constraint and the type constraint.

27. The attack defending method according to claim 24, wherein the rule describes a constraint related to an access source including a network domain and a constrain related to processes processing accesses and its sequence.

28. The attack defending method according to claim 25, wherein the rule describes a constraint related to an access source including a network domain and a constrain related to processes processing accesses and its sequence.

29. An attack defending method using a decoy device in a firewall device provided at an interface between an internal network and an external network, comprising:

preparing a distribution condition of IP packets;

holding confidence levels for source IP addresses of a plurality of input IP packets;

selecting one of the internal network and the decoy device as a destination of the input IP packet depending on whether the confidence level satisfies the distribution condition,

the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.

30. The attack defending method according to claim 29, wherein the confidence level holding step comprises:

storing the confidence levels for the source IP addresses of the plurality of input IP packets and latest updated times at which the confidence levels were respectively updated, into a real-time confidence database;

updating a confidence level stored in the real-time confidence database each time the confidence level is accessed;

copying the confidence levels stored in the real-time confidence database to a long-term confidence database at regular intervals;

searching the long-term confidence database to find a confidence level corresponding to a latest updated time from which a predetermined time period has elapsed; and

updating a confidence level found.

31. The attack defending method according to claim 29, further comprising:

at the decoy device, detecting presence or absence of an attack by executing a service process for the input IP packet transferred from the firewall device.

32. The attack defending method according to claim 31, further comprising:

updating a confidence level of a source IP address of the input IP packet transferred to the decoy device depending on whether an attack is detected by the decoy device based on the input IP packet transferred from the firewall device.

33. The attack defending method according to claim 31, wherein the attack detecting step comprises:

temporarily storing events related to at least network input/output, file input/output, and process creation/termination; and

analyzing cause-effect relations of the events to form links among the events.

34. The attack defending method according to claim 31, wherein the attack detecting step comprises:

extracting events related to at least network input/output, file input/output, and process creation/termination; and

comparing the extracted events with a rule having domain constraint and type constraint added thereto.

35. The attack defending method according to claim 33, wherein the attack detecting step further comprises:

comparing the events with a rule having domain constraint and type constraint added thereto.

36. The attack defending method according to claim 35, wherein the links are searched to extract at least, a generation event of a process generating an event to be inspected and a network reception event by which the event to be inspected is generated, when determination is made based on the domain constraint and the type constraint.

37. The attack defending method according to claim 34, wherein the rule describes a constraint related to an access source including a network domain and a constrain related to processes processing accesses and its sequence.

38. The attack defending method according to claim 35, wherein the rule describes a constraint related to an access source including a network domain and a constrain related to processes processing accesses and its sequence.

39. A firewall device connected to a decoy device, provided at an interface between an internal network and an external network, wherein the firewall device inputs an input IP packet from the external network and forwards it to one of the decoy device and the internal network, comprising:

a microprocessor programmed to execute;

a destination selector for selecting one of the internal network and the decoy device as a destination of the input IP packet based on header information of the input IP packet and a distribution condition; and

a confidence manager for managing confidence levels for source IP addresses of a plurality of input IP packets,

wherein the destination selector obtains a confidence level for a source IP address of the input IP packet from the confidence manager and selects a destination of the input IP packet depending on whether the confidence level satisfies the distribution condition,

the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.

40. A firewall device connected to a decoy device, provided at an interface between an internal network and an external network, wherein the firewall device inputs an input IP packet from the external network and forwards it to one of the decoy device and the internal network, comprising:

a microprocessor programmed to execute;

a first destination selector;

a second destination selector; and

a confidence manager for managing confidence levels for source IP addresses of a plurality of input IP packets,

wherein the first destination selector selects one of the second destination selector and the decoy device as a destination of the input IP packet based on header information of the input IP packet and a first predetermined condition; and

the second destination selector obtains a confidence level for a source IP address of the input IP packet from the confidence manager and selects a destination of the input IP packet depending on whether the confidence level satisfies a second predetermined condition,

the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.

41. A firewall device connected to a decoy device, provided at an interface between an internal network and an external network, wherein the firewall device inputs an input IP packet from the external network and forwards it to one of the decoy device and the internal network, comprising:

a microprocessor programmed to execute;

a packet filter for determining whether the input IP packet inputted from the external network is to be accepted, based on header information of the input IP packet and a filtering condition corresponding to the input IP packet;

a destination selector for selecting one of the internal network and the decoy device as a destination of the input IP packet accepted and forwarded by the packet filter, based on the header information of the input IP packet and a distribution condition;

a confidence manager for managing confidence levels for source IP addresses of a plurality of input IP packets; and

a filtering condition manager for managing the filtering condition corresponding to the input IP packet forwarded to the decoy device depending on whether the attack detector detects an attack based on the input IP packet,

wherein the destination selector obtains a confidence level for a source IP address of the input IP packet from the confidence manager and selects a destination of the input IP packet depending on whether the confidence level satisfies the distribution condition,

the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.

42. A program for implementing an attack defending system on a computer, the attack defending system including a decoy device and a firewall device, which are provided at an interface between an internal network and an external network, the program comprising:

a microprocessor programmed to execute;

preparing a set of filtering conditions and a distribution condition of IP packets;

holding confidence levels for source IP addresses of a plurality of input IP packets;

determining whether an input IP packet inputted from the external network is to be accepted, based on header information of the input IP packet and a filtering condition corresponding to the input IP packet;

selecting one of the internal network and the decoy device as a destination of the input IP packet accepted, based on whether the confidence level satisfies the distribution condition;

detecting presence or absence of an attack by executing a service process for the input IP packet forwarded to the decoy device; and

managing the filtering condition corresponding to the input IP packet forwarded to the decoy device depending on whether an attack is detected based on the input IP packet,

wherein the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.

43. A program for implementing an attack defending system on a computer, the attack defending system including a decoy device and a firewall device, which are provided at an interface between an internal network and an external network, the program comprising:

a microprocessor programmed to execute;

preparing a distribution condition of IP packets;

holding confidence levels for source IP addresses of a plurality of input IP packets;

selecting one of the internal network and the decoy device as a destination of the input IP packet depending on whether the confidence level satisfies the distribution condition,

wherein the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.

44. A program for implementing a firewall device on a computer, wherein the firewall is connected to a decoy device and is provided at an interface between an internal network and an external network, the program comprising:

a microprocessor programmed to execute;

preparing a set of filtering conditions and a distribution condition of IP packets;

holding confidence levels for source IP addresses of a plurality of input IP packets;

determining whether an input IP packet inputted from the external network is to be accepted, based on header information of the input IP packet and a filtering condition corresponding to the input IP packet;

selecting one of the internal network and the decoy device as a destination of the input IP packet accepted, based on whether the confidence level satisfies the distribution condition;

instructing the decoy device to detect presence or absence of an attack by executing a service process for the input IP packet forwarded to the decoy device; and

managing the filtering condition corresponding to the input IP packet forwarded to the decoy device depending on whether an attack is detected based on the input IP packet,

wherein the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.

45. An attack defending system provided at an interface between an internal network and an external network, comprising a decoy device and a firewall device, wherein the firewall device inputs an input IP packet from the external network and forwards it to one of the decoy device and the internal network, wherein the firewall device comprises:

a microprocessor programmed to execute;

a destination selector for selecting one of the internal network and the decoy device as a destination of the input IP packet, based on request data included in the input IP packet and a distribution condition; and

a confidence manager for managing a confidence level of request data,

wherein the destination selector obtains a confidence level of the request data included in the input IP packet from the confidence manager and determines a destination of the input IP packet depending on whether the obtained confidence level of the request data included in the input IP packet satisfies the distribution condition,

the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.

46. The attack defending system according to claim 45, wherein the destination selector selects both of the internal network and the decoy device as a destination of the input IP packet when the obtained confidence level of the request data included in the input IP packet is not smaller than a predetermined threshold.

47. The attack defending system according to claim 45, wherein the destination selector comprises an input buffer for temporarily storing the input IP packet when the obtained confidence level of the request data included in the input IP packet is smaller than a predetermined threshold,

wherein, when the decoy device verifies that the request data included in the input IP packet is safe, the destination selector automatically transfers the input IP packet from the input buffer to the internal network.

48. The attack defending system according to claim 46, wherein the destination selector comprises an input buffer for temporarily storing the input IP packet when the obtained confidence level of the request data included in the input IP packet is smaller than the predetermined threshold,

wherein, when the decoy device verifies that the request data included in the input IP packet is safe, the destination selector automatically retransfers the input IP packet from the input buffer to the internal network.

49. The attack defending system according to claim 45, wherein the decoy device comprises:

an event memory for temporarily storing events related to at least network input/output, file input/output, and process creation/termination while executing a server process; and

an event manager for analyzing cause-effect relations of the events stored in the event memory to form links among the events.

50. The attack defending system according to claim 45, wherein the decoy device comprises:

an attack detector for detecting an attack from an execution status of a service process according to a rule having at least one of domain constraint and type constraint added thereto.

51. The attack defending system according to claim 49, wherein the decoy device further comprises:

an attack detector for detecting an attack from an execution status of a service process according to a rule having at least one of domain constraint and type constraint added thereto.

52. The attack defending system according to claim 51, wherein the attack detector searches the links to extract at least, a generation event of a process generating an event to be inspected and a network reception event by which the event to be inspected is generated, when determination is made based on the domain constraint and the type constraint.

53. The attack defending system according to claim 45, wherein the firewall device further comprises:

an encryption processor for decrypting an encrypted input IP packet and encrypting an output IP packet.

54. The attack defending system according to claim 45, further comprising a mirroring device for copying at least a file system from a server on the internal network to the decoy device, wherein when an attack is detected by the decoy device, the mirroring device copies at least the file system from the server on the internal network to the decoy device.

55. An attack defending method using a decoy device in a firewall device provided at an interface between an internal network and an external network, comprising:

preparing a set of filtering conditions and a distribution condition of IP packets;

holding confidence levels for source IP addresses of a plurality of input IP packets;

determining whether an input IP packet inputted from the external network is to be accepted, based on header information of the input IP packet and a filtering condition corresponding to the input IP packet;

selecting one of the internal network and the decoy device as a destination of the input IP packet accepted, based on whether the confidence level satisfies the distribution condition;

detecting presence or absence of an attack by executing a service process for the input IP packet forwarded to the decoy device; and

managing the filtering condition corresponding to the input IP packet forwarded to the decoy device depending on whether an attack is detected based on the input IP packet,

wherein the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.

56. A non-transitory computer readable recording medium with a computer program recorded thereon for implementing an attack detecting system on a computer, wherein the attack detecting system uses a decoy device and a firewall device provided at an interface between an internal network and an external network, the computer program executed by the computer comprising:

preparing a set of filtering conditions and a distribution condition of IP packets;

holding confidence levels for source IP addresses of a plurality of input IP packets;

determining whether an input IP packet inputted from the external network is to be accepted, based on header information of the input IP packet and a filtering condition corresponding to the input IP packet;

selecting one of the internal network and the decoy device as a destination of the input IP packet accepted, based on whether the confidence level satisfies the distribution condition;

detecting presence or absence of an attack by executing a service process for the input IP packet forwarded to the decoy device; and

managing the filtering condition corresponding to the input IP packet forwarded to the decoy device depending on whether an attack is detected based on the input IP packet,

wherein the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.

57. The non-transitory computer readable recording medium according to claim 56, wherein the distribution condition is a confidence management table containing a plurality of entries, each of which comprises a combination of request data and its confidence level,

wherein the attack defending method further comprises:

when an entry matching the request data included in the input IP packet is found in the confidence management table, extracting a confidence level from the entry; and

when no entry matching the request data included in the input IP packet is found in the confidence management table, creating a new entry comprised of a combination of the request data and an initial confidence level in the confidence management table,

the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.

58. The non-transitory computer readable recording medium according to claim 57, wherein, in the destination selecting step, when the confidence level extracted from the confidence management table is not smaller than a predetermined threshold, both of the internal network and the decoy device are selected as a destination of the input IP packet.

59. The non-transitory computer readable recording medium according to claim 57, wherein, in the destination selecting step, when the confidence level extracted from the confidence management table is not smaller than a predetermined threshold, the input IP packet is temporarily stored and, if no attack is detected, then the input IP packet is automatically transferred to the internal network.

60. An attack defending system provided at an interface between an internal network and an external network, comprising:

a firewall device comprising a programmed microprocessor;

a decoy device comprising a programmed microprocessor; and

at least one confidence management server comprising a programmed microprocessor,

wherein the firewall device transmits a request message including at least a part of data of an input IP packet, to the at least one confidence management server, and

the at least one confidence management server generates a confidence level for the input IP packet from data included in the request message in response to the request message, and transmits a response message including at least the confidence level back to the firewall device,

the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets,

the firewall device uses a destination selector for selecting one of the internal network and the decoy device as a destination of the input IP packet based on header information of the input IP packet and a distribution condition.

61. The attack defending system according to claim 60, wherein the firewall device comprises:

a destination selector for selecting one of the internal network and the decoy device as a destination of the input IP packet, based on header information of the input IP packet and a distribution condition; and

a management server connection section for transmitting the request message to the at least one confidence management server and obtaining the confidence level for the input IP packet as a response to the request message,

wherein the destination selector selects a destination of the input IP packet depending on whether the confidence level for the input IP packet satisfies the distribution condition.

62. A non-transitory computer readable recording medium with a computer program recorded thereon for implementing an attack detecting system on a computer, wherein the attack detecting system is provided at an interface between an internal network and an external network, the computer program executed by the computer comprising:

assigning at least one requisite confidence level to each of a plurality of decoy devices, which correspond to a server on the internal network;

holding a distribution condition used to distribute an IP packet based on the at least one requisite confidence level and confidence levels for a plurality of IP packets;

when an IP packet is inputted, obtaining a confidence level of the input IP packet; and

determining a decoy device having a requisite confidence level, which is not greater than the obtained confidence level, as a destination of the input IP packet,

the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.

63. An attack defending method in an attack defending system provided at an interface between an internal network and an external network, wherein the attack defending system comprises a firewall device, a decoy device, and at least one confidence management server, wherein the firewall device transmits a request message including at least a part of data of an input IP packet, to the at least one confidence management server, and

the at least one confidence management server generates a confidence level for the input IP packet from data included in the request message in response to the request message, and transmits a response message including at least the confidence level back to the firewall device,

the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets,

the firewall device uses a destination selector for selecting one of the internal network and the decoy device as a destination of the input IP packet based on header information of the input IP packet and a distribution condition.

64. The attack defending method according to claim 63, wherein the destination selector selects a destination of the input IP packet depending on whether the confidence level for the input IP packet satisfies the distribution condition.

65. A non-transitory computer readable recording medium with a computer program recorded thereon for implementing an attack detecting system on a computer, wherein the attack detecting system is provided at an interface between an internal network and an external network, wherein the attack defending system comprises a firewall device, a decoy device, and at least one confidence management server, the computer program executed by the computer, comprising:

receiving a request message from the firewall device, wherein the request message includes at least a part of data of an input IP packet;

generates a confidence level for the input IP packet from data included in the request message in response to the request message; and

transmitting a response message including at least the confidence level back to the firewall device,

the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets,

the firewall device using selecting one of the internal network and the decoy device as a destination of the input IP packet based on header information of the input IP packet and a distribution condition.

66. A non-transitory computer readable recording medium with a computer program recorded thereon for implementing an attack detecting system on a computer, wherein the attack detecting system is provided at an interface between an internal network and an external network, wherein the attack defending system comprises at least a decoy device, a firewall device and a confidence management server, the computer program executed by the computer comprising:

transmitting a request message from the firewall device to the confidence management server, wherein the request message includes at least a part of data of an input IP packet;

receiving a response message from the confidence management server, the response message including at least a confidence level of the input IP packet calculated from data included in the request message; and

selecting one of the internal network and the decoy device as a destination of the input IP packet depending on whether the confidence level of the input IP packet satisfies a predetermined distribution condition,

the confidence level is calculated based on the header information of the input IP packet and an input history of previous input IP packets.