JP2007074087A - SYSTEM AND PROGRAM FOR DETECTING UNAUTHORIZED ACCESS AIMING AT DDoS ATTACK - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a system and a program for detecting unauthorized access aiming at DDoS attack by executing analysis based on pseudo reassemble check without assembling an IP fragment packet. <P>SOLUTION: An attacker terminal 21 adjusts a fragment offset value against a target server 22, and is transmitting an IP fragment packet for executing an attack for fragment overlap in which data contents overlap in assembling packets, or is continuously transmitting a large number of copies of IP fragment datagram having the same contents. In this case, in an unauthorized access detecting system, a reassemble check 39 confirms that relation of FO<SB>n</SB>=DL<SB>1</SB>×(n-1) is not established, because a correct value is not set in a fragment offset value of the IP fragment packet. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a system and a program that can cope with a DDoS (Distributed Denial of Service) attack.

  In general, a DDoS attack on the Internet is an attack that prevents a service provided by a server on a network and does not cause the service to function. In this DDoS attack, a large amount of IP packets are transmitted to a target server, thereby consuming network and server resources, interrupting service, or denying service.

  In recent years, the use of the Internet, which is already becoming a social infrastructure, has increased the need to provide a stable service, but DDoS attacks that devour such network lines are increasing. Therefore, it is necessary to take positive measures against such DDoS attacks even in the carrier network.

  Focusing on the IP fragment flood attack packet used in one of the above-mentioned DDoS attacks, the application of the user terminal receives a packet having a size larger than the Ethernet (registered trademark) maximum transfer unit (Ethernet (registered trademark) MTU). Also in the case of transmission, since IP fragmentation may occur, there is a problem that even regular IP fragment packets are limited.

  In order to solve this problem, when analyzing unauthorized access of an IP packet that needs to be assembled, it is possible to detect TCP / IP unauthorized access that needs to be assembled while efficiently holding the packet. An assembly check is performed (see, for example, Non-Patent Document 1).

Norimitsu Nagashima, 2 others, “Performance Evaluation of Intrusion Detection System”, Multimedia, Distributed, Collaboration and Mobile (DICOMO 2004) Symposium, Information Processing Society of Japan, July 7, 2004, P.A. 89-92

  However, in the reassembling check, a process for assembling a packet occurs at that time, and it is difficult to apply it to a high-speed network.

  Accordingly, the present invention has been made to solve the above-described problems, and an object of the present invention is not to perform an IP fragment packet assembly process and to associate a copy of a data segment included in the packet with session information. By analyzing the network by monitoring the network traffic, it is possible to distinguish between legitimate IP fragment packets and attack IP fragment packets and detect unauthorized access to DDoS attacks. It is to provide a possible system and program.

  An unauthorized access detection system according to the present invention receives an IP packet from a terminal and detects unauthorized access due to a DDoS attack from the IP packet. When the received IP packet is an IP fragment packet, A means for comparing the fragment offset value set in the header information with the offset value calculated based on the data length of the IP fragment packet is set in advance when the fragment offset value does not match the calculated offset value. The number of times the non-matching IP fragment packets are continuously received within a predetermined time is compared with a preset threshold value. If the received number is equal to or greater than the threshold value, it indicates that it is an unauthorized access due to a DDoS attack. And means for judging To.

  Further, the unauthorized access detection system according to the present invention further determines that the terminal of the source of the received IP fragment packet is a legitimate user when the fragment offset value matches the calculated offset value. The IP packet is received from the terminal, and the user information of the terminal that is the transmission source of the IP packet is collated with the user information registered in the regular user list. And a means for determining that the access is normal.

  The unauthorized access detection system according to the present invention further includes, when the IP fragment packet determined to be unauthorized access due to the DDoS attack uses TCP, the header information of the IP fragment packet in the retransmission list. Means for registering and discarding the IP fragment packet; receiving an IP packet from the terminal; collating the header information of the IP packet with the header information registered in the retransmission list; And a means for registering the user information in a regular user list, assuming that the terminal of the source of the received IP packet is a regular user.

  An unauthorized access detection program according to the present invention is provided under a system including a terminal that transmits an IP packet, a device that receives and transfers an IP packet from the terminal, and a server that receives the IP packet from the device. A program for detecting unauthorized access to the server from the terminal by a DDoS attack, and is set in the header information of the IP fragment packet when the received IP packet is an IP fragment packet received by the computer constituting the device. When the processing for comparing the fragment offset value and the offset value calculated based on the data length of the IP fragment packet does not match the fragment offset value and the calculated offset value, within a preset time, Unmatched IP fragment packet The number of times of continuous reception is compared with a preset threshold value, and when the received number of times is equal to or greater than the threshold value, a process for determining unauthorized access due to a DDoS attack is executed. .

  In addition, the unauthorized access detection program according to the present invention further determines that the terminal of the source of the received IP fragment packet is a legitimate user when the fragment offset value matches the calculated offset value. The user information is processed into a regular user list, an IP packet is received from the terminal, the user information of the terminal that has transmitted the IP packet is collated with the user information registered in the regular user list, and the collation is performed. And the process of determining that the access is normal when the two match.

  In addition, the unauthorized access detection program according to the present invention may further include a header information of the IP fragment packet when the IP fragment packet determined to be unauthorized access due to the DDoS attack uses TCP. Is registered in the retransmission list, the IP fragment packet is discarded, the IP packet is received from the terminal, and the header information of the IP packet is compared with the header information registered in the retransmission list. If they match, the terminal of the received IP packet is regarded as a legitimate user, and the process of registering the user information in the legitimate user list is executed.

  According to the present invention, an analysis is performed by a pseudo reassembly check without performing an IP fragment packet assembly process and holding a copy process of a data segment included in the packet in association with session information. I did it. As a result, it is possible to distinguish between regular IP fragment packets and attack IP fragment packets and detect unauthorized access to a DDoS attack.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a network diagram including an unauthorized access detection system according to an embodiment of the present invention. This network includes the Internet 1, unauthorized access detection systems 10 to 12, a user terminal 20, an attacker terminal 21, and a target server 22. The user terminal 20 and the attacker terminal 21 are connected to the unauthorized access detection system 11 via an access network, and the target server 22 is connected to the unauthorized access detection system 10 via an access network. In addition, the user terminal 20, the attacker terminal 21, and the target server 22 are connected via the unauthorized access detection systems 10 to 12 by the respective access networks and the Internet 1.

  The unauthorized access detection systems 10 to 12 monitor unauthorized intrusions and attacks on communication devices and corporate networks on the Internet 1 and deal with the intrusions and attacks. The user terminal 20 normally communicates with the target server 22, and the attacker terminal 21 performs unauthorized intrusion and attack on the target server 22. Here, it is assumed that the target server 22 is a server that is a target of unauthorized intrusion and attack.

  FIG. 2 is a diagram showing a configuration of the unauthorized access detection system 10 shown in FIG. The unauthorized access detection system 10 includes network interfaces 31 and 34, an unauthorized access analysis unit 32, a filter unit 33, a network interface 34, a regular user list 36, and a retransmission list 37. In FIG. 2, the components that are originally provided in the unauthorized access detection system 10 are omitted, and only the components necessary for applying the present invention are shown.

  The network interface 31 is connected to the Internet 1 shown in FIG. 1, and the network interface 34 is connected to an access network 35 having a target server 22 including, for example, an in-house network. The regular user list 36 and the retransmission list 37 are stored in a storage unit (not shown), and regular user information such as an address (source address) of a user terminal accessible to the target server 22 is registered in the regular user list 36. In the retransmission list 37, header information of discarded packets is registered. The unauthorized access analysis unit 32 includes a search unit 38 that searches the regular user list 36 and the retransmission list 37, a reassembly check unit 39 that performs pseudo-reassembly check, and a threshold determination unit 40 that performs threshold determination. The filter unit 33 performs IP packet filtering.

  FIG. 3 is a flowchart showing the operation of the unauthorized access detection system 10 shown in FIGS. 1 and 2. Hereinafter, a description will be given of a defense method against an IP fragment flood attack that can deal with a DoS attack or a DDoS attack targeting the target server 22 on the Internet 1 in the network shown in FIG.

  In FIG. 1, a case where the attacker terminal 21 is performing an IP fragment flood attack on the target server 22 under the situation where communication is performed between the user terminal 20 and the target server 22. Suppose. In this case, since the IP fragment flood attack is intended to deplete the CPU resources and the reassembly buffer of the target server 22, the attacker terminal 21 sends the IP packet of the TCP segment without the sequence to the target server 22. Alternatively, a large amount of IP packets (IP packets having the same contents in both the IP header and data) copied from the IP fragment datagram having the same contents are continuously transmitted. Further, in many cases in DDoS attacks, the attacker terminal 21 spoofs a transmission source address in order to hide the transmission source. For this reason, in the IP fragment flood attack by TCP, the attacker terminal 21 cannot detect that the above-mentioned IP packet is discarded in the middle of the route to the target server 22 because it cannot be detected. Transmission control cannot be performed.

  Referring to FIG. 3, the unauthorized access analysis unit 32 of the unauthorized access detection system 10 receives an IP packet from the network interface 31 via the Internet 1 (step S110). The search unit 38 of the unauthorized access analysis unit 32 searches the regular user list 36 for the received IP packet, and whether or not the user information that is the transmission source of the IP packet is registered, that is, whether or not the user is a regular user. These are collated (step S120).

  If it is determined that the user information that is the transmission source of the IP packet is registered in the regular user list 36, the unauthorized access analysis unit 32 determines that the IP packet is a legitimate user and sets the filter unit 33 and the network interface 34. Then, the IP packet is transmitted to the access network 35 (steps 130 and 140).

  If it is determined that the user information that is the transmission source of the IP packet is not registered in the regular user list 36, the search unit 38 searches the retransmission list 37 for the received IP packet, and the header information of the IP packet. Whether or not the same header information is registered, that is, whether or not it is recognized that the IP packet is retransmitted (step S150). Here, when the filter unit 33 discards the IP packet, header information of the discarded packet is registered in the retransmission list 37.

  In step 150, if it is determined that the same header information as the header information of the IP packet is registered in the retransmission list 37, the search unit 38 determines that the IP packet has been retransmitted. Is registered in the authorized user list 36 (step S220), and the unauthorized access analyzing unit 32 transmits the IP packet to the access network 35 through the filter unit 33 and the network interface 34 as an IP packet by the authorized user (step S220). Steps 130 and 140). Here, as described above, since the attacker terminal 21 cannot retransmit the discarded IP packet in the IP fragment flood attack, the retransmitted IP packet is assumed to be an IP packet retransmitted by an authorized user. The user information is registered in the regular user list 36.

  If it is determined in step 150 that the same header information as the header information of the IP packet is not registered in the retransmission list 37, the search unit 38 transfers the IP packet to the reassembly check unit 39, The process proceeds to step 160.

  In step 150, the search unit 38 monitors the passage of time after the filter unit 33 registers the header information of the discarded packet in the retransmission list 37. When an IP packet having the same header information as the registered header information is not received within a preset time, that is, the received IP packet is retransmitted within a preset time. If it is not recognized, the search unit 38 transfers the IP packet (subsequent IP packet) having the same header information received thereafter to the filter unit 33. Then, the filter unit 33 determines that the transferred IP packet is an attack IP packet, and stops the function of transmitting to the access network 35 via the network interface 34 for a certain period of time (reject transfer / IP packet). (Step S230, 240).

When the IP packet is transferred from the search unit 38, the reassembly check unit 39 performs the process of step 160. That is, when the IP packet is an IP fragment packet, the reassembly check unit 39 determines that the IP packet is the first IP fragment packet when the MF (More Fragments) bit included in the header information is 1 and the fragment offset value is 0. Recognize and then identify the second and subsequent IP fragment packets belonging to the same group using pair information consisting of a source IP address and destination IP address and a 16-bit identifier (Identification) that identifies the datagram. The fragment packet number is held. Further, when the transferred IP packet is the nth IP fragment packet, the reassembling check unit 39 extracts the fragment offset value FO _n included in the header information. Also, the data length of the _first IP fragment packet that has already been received is DL ₁ (DL ₁ = Total Length−Internet Header Length (total length of packet−length of header information)), and DL ₁ × (n−1) ) Is calculated, and it is confirmed whether or not the relationship of FO _n = DL ₁ × (n−1) holds. The reassembling check unit 39, when the MF bit included in the header information of the transferred IP fragment packet is 0 (indicating that it is the last IP fragment packet of the same group) or after a certain time When a timeout occurs because an IP fragment packet is not transferred, the fragment packet number held in the group is released.

If the relationship of FO _n = DL ₁ × (n−1) holds in step 160, the reassembling check unit 39 assumes that the transferred IP fragment packet is not an attack IP fragment packet but a regular IP fragment packet. The user information is registered in the regular user list 36 (step S220), and the unauthorized access analysis unit 32 transmits the IP packet to the access network 35 via the filter unit 33 and the network interface 34 (steps 130 and 140).

If the relationship of FO _n = DL ₁ × (n−1) does not hold in step 160, the reassembling check unit 39 transfers the IP fragment packet to the threshold determination unit 40, and proceeds to step 170. .

Here, the attacker terminal 21 adjusts the fragment offset value to the target server 22 and transmits an IP fragment packet for an attack that causes fragment overlap so that data contents overlap at the time of packet assembly. In this case, or when a large number of copies of IP fragment datagrams having the same contents are continuously transmitted, the reassembly check unit 39 responds to the number n of subsequent IP fragment packets received after the second. While calculating the _{DL 1 × (n-1)} , because the correct value to the fragment offset value of the transferred IP fragment packet is not _set, the relationship _{FO n = DL 1 × (n} -1) does not hold Make sure. In this case, the reassembly check unit 39 can detect an IP fragment overlap attack.

When the attacker terminal 21 transmits an IP fragment packet of a TCP segment with a missing sequence to the target server 22, the reassembly check unit 39 determines that the correct nth IP fragment packet is a search unit. 38 is not transferred. That is, to make sure that from the correct value to the fragment offset value of the transferred IP fragment packet is not set, the relationship _{FO n = DL 1 × (n} -1) does not hold. In this case, the reassembling check unit 39 can detect that the attack is a TCP segment missing an IP packet.

In order to detect the attack of the TCP segment with the missing IP packet, the reassembling check unit 39 transfers the IP fragment packet that does not satisfy the relationship of FO _n = DL ₁ × (n−1) in the same group. The time may be monitored after that. In this case, the attack can be detected by a timeout after a preset time has elapsed. For example, when the attacker terminal 21 is attacking the target server 22 with the third IP fragment packet drop, the reassembly check unit 39 determines that the fragment packet of the IP fragment packet correctly transferred before that time. Number = 2 is held. The reassembling check unit 39 eliminates the third packet dropout due to a timeout after a preset time has elapsed after an IP fragment packet that does not satisfy the relationship of FO _n = DL ₁ × (n−1) in the same group is transferred. recognize. In this case, the reassembling check unit 39 keeps the group until the MF bit included in the header information of the transferred IP fragment packet becomes 0 or until a time-out occurs because the subsequent IP fragment packet is not transferred for a certain period of time. Holds the fragment packet number.

In the above example, the reassembling check unit 39 checks whether or not the relationship of FO _n = DL ₁ × (n−1) is established. However, in general, the order of IP fragment packets may be changed on the Internet 1. Therefore, the relational expression considering reverses the functions of the order _{(FO n = DL 1 × (} n-1 ± m), the parameter m is considering reverses the functions of the order, for example, m = -2, -1,0,1,2) Thus, a pseudo reassembly check may be performed. For example, if the held fragment packet number is 10, the number of the next IP fragment packet to be transferred is 11, but the pseudo reassembly check is performed with a width of ± 2 in consideration of the change of order. Do. Specifically, whether FO _n = DL ₁ × 8, FO _n = DL ₁ × 9, FO _n = DL ₁ × 10, FO _n = DL ₁ × 11, and FO _n = DL ₁ × 12 holds. To check.

  When the IP fragment packet is transferred from the reassembling check unit 39, the threshold determination unit 40 performs the process of step 170. That is, the threshold determination unit 40 determines a specific bit pattern (IP fragment packet destination IP address, source IP address, destination port number, source port number, protocol, flag (flags: last fragment packet) from the IP fragment packet. A pattern consisting of information / MF bit indicating whether or not), an identifier (identifier: information / fragment identifier indicating that it is an IP fragment packet of the same group), etc., and a pattern in which the specific bit pattern has already been extracted It is determined whether or not the same (step S170). Specifically, the threshold determination unit 40 determines whether or not IP fragment packets having the same specific bit pattern are continuously transferred for a predetermined threshold or more within a preset time for the target server 22. .

  In step 170, when the IP fragment packet is not continuously transferred for a predetermined threshold value or more (when the IP fragment packet is not more than the threshold value), the threshold determination unit 40 transmits the IP packet to the access network via the filter unit 33 and the network interface 34. 35 (steps 130 and 140).

  In step 170, if IP fragment packets are continuously transferred for a predetermined threshold value or more, the threshold determination unit 40 determines that an IP fragment flood attack has been detected, and whether or not the protocol of the IP packet is TCP. Judgment is made (step S180).

  If the protocol is TCP in step 180, the filter unit 33 discards the IP packet determined by the threshold determination unit 40 (step S190), and registers the header information of the discarded packet in the retransmission list 37 (step S190). S200).

  In step 180, if the protocol is not TCP, the filter unit 33 accesses the subsequent attack IP fragment packet received thereafter by the unauthorized access analysis unit 32 via the network interface 34 for a certain period of time. The function of transmitting to the network 35 is stopped (reject transfer / discard IP packet) (steps S250 and S260).

As described above, according to the unauthorized access detection system according to the embodiment of the present invention, the reassembly check unit 39 extracts the fragment offset value included in the header information of the received IP fragment packet and has already received it. An offset value is calculated from the data length DL _{1 of the first} IP fragment packet, and the extracted fragment offset value is compared with the calculated offset value. Thereby, regular IP fragment packets and attack IP fragment packets can be distinguished and filtered.

  Further, according to the unauthorized access detection system according to the embodiment of the present invention, the filter unit 33 discards an IP packet when an IP fragment flood attack is detected by the threshold determination unit 40 and the protocol is TCP, The header information of the discarded packet is registered in the retransmission list 37. Thereby, filtering based on retransmission control can be realized. That is, the unauthorized access detection system 10 does not discard IP packets due to normal user traffic between the user terminal 20 and the target server 22 that has been subjected to the DDoS attack from the attacker terminal 21, and does not discard the DDoS attack traffic. On the other hand, it can be protected by bandwidth limitation of traffic or by filtering.

  The unauthorized access detection system 10 includes a volatile storage medium such as a CPU and a RAM, a nonvolatile storage medium such as a ROM, an input device such as a keyboard and a pointing device, a monitor device that displays images and data, and an external device. You may make it comprise with the computer provided with the interface for communicating with an apparatus. In this case, the functions of the network interfaces 31 and 34, the unauthorized access analysis unit 32 (the search unit 38, the reassembling check unit 39, and the threshold determination unit 40) provided in the unauthorized access detection system 10 and the filter unit 33 are as follows. Each is realized by causing the CPU to execute a program describing the function. These programs can also be stored and distributed in a storage medium such as a magnetic disk (floppy disk, hard disk, etc.), optical disk (CD-ROM, DVD, etc.), semiconductor memory, or the like.

It is a network diagram provided with the unauthorized access detection system by embodiment of this invention. It is a figure which shows the structure of an unauthorized access detection system. It is a flowchart figure which shows operation | movement of an unauthorized access detection system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Internet 10-12 Unauthorized access detection system 20 User terminal 21 Attacker terminal 22 Target server 31, 34 Network interface 32 Unauthorized access analysis part 33 Filter part 35 Access network 36 Regular user list 37 Retransmission list 38 Search part 39 Reassembling check part 40 Threshold judgment unit

Claims (6)

In a system that receives an IP packet from a terminal and detects unauthorized access due to a DDoS attack from the IP packet,
Means for comparing the fragment offset value set in the header information of the IP fragment packet with the offset value calculated based on the data length of the IP fragment packet when the received IP packet is an IP fragment packet;
If the fragment offset value and the calculated offset value do not match, the number of times the non-matching IP fragment packets are continuously received within a preset time is compared with a preset threshold, An unauthorized access detection system comprising: means for determining that an unauthorized access is caused by a DDoS attack when the number of times of reception is equal to or greater than a threshold value. The unauthorized access detection system according to claim 1,
And means for registering the user information in a regular user list, assuming that the terminal of the source of the received IP fragment packet is a regular user when the fragment offset value matches the calculated offset value;
When the IP packet is received from the terminal, the user information of the terminal that is the transmission source of the IP packet is collated with the user information registered in the regular user list, and when the collation matches, the access is normal. And an unauthorized access detection system. The unauthorized access detection system according to claim 2,
Furthermore, when the IP fragment packet determined to be unauthorized access due to the DDoS attack uses TCP, means for registering header information of the IP fragment packet in a retransmission list and discarding the IP fragment packet When,
When an IP packet is received from a terminal, the header information of the IP packet and the header information registered in the retransmission list are collated, and if the collation matches, the terminal of the transmission source of the received IP packet is An unauthorized access detection system comprising: means for registering the user information in a regular user list as a user. Unauthorized access from a terminal to a server by a DDoS attack under a system comprising a terminal that transmits an IP packet, a device that receives and transfers an IP packet from the terminal, and a server that receives the IP packet from the device , A computer that constitutes the device,
When the received IP packet is an IP fragment packet, a process of comparing the fragment offset value set in the header information of the IP fragment packet with the offset value calculated based on the data length of the IP fragment packet;
If the fragment offset value and the calculated offset value do not match, the number of times the non-matching IP fragment packets are continuously received within a preset time is compared with a preset threshold, An unauthorized access detection program for executing a process of determining unauthorized access by a DDoS attack when the number of times of reception is equal to or greater than a threshold. The unauthorized access detection program according to claim 4, wherein the computer includes:
Further, when the fragment offset value matches the calculated offset value, the terminal of the received IP fragment packet is regarded as a regular user, and the user information is processed into a regular user list;
When the IP packet is received from the terminal, the user information of the terminal that is the transmission source of the IP packet is collated with the user information registered in the regular user list, and when the collation matches, the access is normal. An unauthorized access detection program that executes a process of determining whether or not 6. The unauthorized access detection program according to claim 5, wherein the computer includes:
Further, when the IP fragment packet determined to be unauthorized access due to the DDoS attack is a packet using TCP, the header information of the IP fragment packet is registered in the retransmission list, and the IP fragment packet is discarded When,
When an IP packet is received from a terminal, the header information of the IP packet and the header information registered in the retransmission list are collated, and if the collation matches, the terminal of the transmission source of the received IP packet is An unauthorized access detection program that executes a process of registering the user information in the authorized user list as a user.

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