EP1766934A1

EP1766934A1 - Method, computer programme, device and system for protecting a server against denial-of-service attacks

Info

Publication number: EP1766934A1
Application number: EP05788506A
Authority: EP
Inventors: Yannick Carlinet; Yvon Gourhant; Patrick Trabe
Original assignee: France Telecom SA
Current assignee: Orange SA
Priority date: 2004-07-08
Filing date: 2005-07-08
Publication date: 2007-03-28
Also published as: WO2006013291A1; US20080052402A1; FR2872980A1

Abstract

The invention concerns in particular a method for protecting a server (10) against denial-of-service attacks using a protocol whereby a communication session setup between a client (26) and the server (10) is requested by the client for providing a service. An intermediate equipment (30) intercepts the service request emitted by the client (10) and verifies whether the client (26) is present in the table of clients considered as reliable. If the client (10) is not present in the table, the intermediate equipment (30) replies to the request and includes in the reply an identifier. If the client (10) sends an acknowledgement within a predetermined delay including the identifier, the client is included in the table of clients considered as reliable.

Description

A method, computer program, device and system for protecting a server against denial of service attacks.

The present invention relates to a method, a computer program, a device and a system for protecting a server against denial of service attacks.

More specifically, the invention relates to such a method in which, during recovery of a communication session between a client and the server, recovery of this session being required by the customer for the provision of a service, at least a part following data is exchanged:

the server receives a service provision request sent by the client; the server returns a service provision agreement to the client;

- In a predetermined time by the server, it waits for an acknowledgment of receipt of the agreement, from the customer. In general, the server is able to handle multiple service provision requests. For this purpose, it includes a buffer memory in which it stores the requests it receives, pending the corresponding acknowledgments that must reach it before the expiration of the predetermined time. This time runs from the sending of the service provisioning agreement by the server. The buffer has a predetermined size and can therefore store a predetermined maximum number of service provision requests.

A denial of service attack is to use the protocol for establishing a communication session with the server, as previously described, for:

- Send a service provision request to the server that we want to attack;

- receive the service provisioning agreement from the server; and

- do not return the acknowledgment of receipt expected by the server.

Thus, a malicious user can synchronously transmit a large number of denial of service attacks to the server, from one or more client terminals called "zombies", so as to quickly fill the server buffer . The server is then no longer able to receive new requests for service provision, for example from other well-intentioned users, and can no longer fulfill its function of providing service.

A first preventative type solution to protect a server against such attacks is to increase the size of its buffer or reduce the delay predetermined by the server during which it waits for the acknowledgment to be issued by the client.

Increasing the size of the buffer is not a satisfactory solution since the size of this buffer is itself limited by the memory generally available on the server. Similarly, reducing the predetermined waiting time for the acknowledgment of receipt is not satisfactory since it is likely to harm well-intentioned users who wish to actually request a service from the server, but do not not having a connection at sufficient speed to be able to send back to the server an acknowledgment of receipt in a too short time.

Another reactive solution to protect a server against such attacks is to divert all data addressed to the attacked server to another server generally called "black hole", as soon as attacks to this server are detected, so that it is the black hole that receives all attacks rather than the server itself. The function of the black hole is to receive the data and destroy it without processing it.

However, this solution does not allow to treat differently the malicious attacks and the real requests of service provision emitted by legitimate customers. In addition, when this solution is applied, it can be considered that the attack has worked since the attacked server is no longer able to provide the service.

Another solution is described in document US 2004/0015721 and consists in using an intermediate equipment disposed between the client and the server. This intermediate device has the function of behaving as the server vis-à-vis the client and as the client vis-à-vis the server. In this way, the client actually establishes a first communication session with the intermediate equipment, then the intermediate equipment establishes a second communication session with the server if the first session is correctly established.

The effect of this solution is that it is no longer the server, but the intermediate equipment, which receives the attacks from a malicious client, but that in return must manage two communication sessions, one between the client and the intermediate equipment, the other between the intermediate equipment and the server, instead of a single communication session between the client and the server.

The invention aims to improve existing methods of protecting a server against denial of service attacks by providing a method capable of protecting a server against such attacks at least as efficiently as in the US document 2004/0015721, without requiring the management of two communication sessions.

The invention therefore relates to a method of protecting a server against denial of service attacks using a protocol according to which an establishment of a communication session between a client and the server is required by the client for the provision of services. a service, the method comprising the following steps: a) interception of a service provision request issued by a client and intended for the server, the request not being transmitted to the server; b) verification if the client is present in a table of customers deemed reliable; c) if the client is present in the table, transmission of the request to the server; d) if the customer is absent from the table, implement the following steps: e) sending a service provision agreement to the customer; f) upon receipt of an acknowledgment of the agreement from the customer under predetermined conditions, entry of the customer in the table and sending to the customer a signal informing him that the establishment of the communication session failed.

Steps b) to f) of this method are for example implemented by an intermediate equipment.

In this case, the intermediate equipment maintains a table with a list of customers deemed reliable. When a customer is registered in the table, the intermediate equipment does not interrupt the establishment of a session required by this customer. On the other hand, if the customer is not registered in the table, that is to say if it is not considered reliable by the intermediate equipment, the establishment of the session is automatically interrupted.

Thus, it manages only one communication session to be established between the client and the server, the intermediate equipment intervening only to possibly interrupt recovery of this session required by the client. It will be noted that when the conditions predetermined by the intermediate equipment are verified, the establishment of the session is interrupted by the intermediate equipment, and not diverted to another terminal. Denial of service attacks have no effect on the server or on any other terminal.

According to a particular embodiment of the invention, the predetermined conditions are that the acknowledgment is received within a predetermined time after sending the service provision agreement. -AT-

In this embodiment, the client is entered in the table by the intermediate equipment for example if, during a previous session establishment, the customer has returned a service delivery agreement acknowledgment of receipt issued by the customer. intermediate equipment within the predetermined time by the server. In this case, each first attempt to establish a communication session by a client with the server fails due to the intermediate equipment has not yet registered this client in the table. In fact, this first session setup attempt is a test run by the staging equipment to verify that the client is sending an acknowledgment within the time required by the server. If the customer returns the acknowledgment in time, he is then considered to be a reliable customer and is entered in the table by the intermediate equipment. The fact that the client returns the acknowledgment in time proves that the client does not use a spoofed IP address (a technique commonly used in an attack). Thus, during a second attempt to set up a communication session by this client with the server, the intermediate equipment will not interrupt this session establishment, according to the invention.

Preferably, the criterion predetermined by the intermediate equipment is a waiting time of the acknowledgment of the agreement weaker than that predetermined by the server. This embodiment is particularly interesting when service provisioning requests are issued by clients whose access to the server is made via a high-speed network, that is to say in time of delay weaker than the Internet. Indeed, in this case, the delay to return an acknowledgment of receipt of the agreement may be lower. The fact that this lower delay is imposed by the intermediate equipment and not by the server allows the latter to receive any other requests for service provision from other customers with access to lower data rates. .

According to another embodiment of the invention, the predetermined conditions are that the acknowledgment contains a value equal to a unique key previously introduced in the service provisioning agreement.

Preferably, the unique key is customer-specific and is calculated a first time at the time of sending the service provisioning agreement and a second time at the time of receipt of the acknowledgment.

This embodiment is particularly advantageous since it is not necessary for the intermediate equipment to keep in its buffer memory, for a predetermined duration, service supply requests, while waiting. corresponding acknowledgments. Indeed, in this embodiment, the intermediate equipment sends to the customers who have issued a request for service provision, a service provision agreement without storing the original request. It is only when he receives an acknowledgment of a service provision agreement that he compares the value contained in this acknowledgment with a key that he calculates. Thus, this intermediate server is much less vulnerable to denial of service attacks since its processing capacity is not limited by its buffer memory.

Thanks to these intermediary equipments, the remote server is less in demand since the computing load necessary for the verification of the reliability of the customers is distributed on various intermediate equipments. In addition, these intermediate devices are preferably located near the customers so that the network connecting the remote server to the intermediate equipment is not cluttered by the different messages issued during a denial of service attack.

The invention also relates to a computer program for protecting a server against denial of service attacks using a protocol according to which an establishment of a communication session between a client and the server is required by the client for providing a service, the program containing instructions for implementing steps b) to f) previously defined.

The invention also relates to a device for protecting a server against denial of service attacks using a protocol in which an establishment of a communication session between a client and the server is required by the client for the provision of services. a service, the device comprising means for implementing steps b) to f) previously defined.

Optionally, the means for implementing steps b) to f) comprise a computer program according to the invention.

Finally, the invention also relates to a system for protecting a server against denial of service attacks using a protocol in which an establishment of a communication session between a client and the server is required by the client for the providing a service, the system comprising a server adapted for providing a service that may be required by a customer, characterized in that the system comprises an intermediate device formed by a protection device as defined above.

A protection system of a server according to the invention may further include the feature that the intermediate equipment is a firewall disposed between the server and a client access network to the server. The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the appended drawings in which:

- Figure 1 shows schematically the general structure of an installation comprising a system according to a possible embodiment of the invention; FIG. 2 represents the successive steps of a method for protecting a server according to a first embodiment of the invention;

FIG. 3 represents the successive steps of a method according to a second embodiment of the invention;

FIG. 4 represents the successive steps of a method according to a third embodiment of the invention.

The installation represented in FIG. 1 comprises a first server 10 adapted for the provision of a predetermined service to different clients.

The server 10 is connected to a high-speed network 12, for example an ADSL link which is itself connected to an operator network 14. Intermediate equipment 16 can be arranged at the interface of the operator network 14 and the network This intermediate equipment 16 is for example a firewall.

The installation comprises a second server 18 also adapted for providing a predetermined service to different customers.

This server 18 is connected to a private local area network 20, itself connected to the operator network 14. An intermediate equipment 22, as well as a router 24, can be arranged at the interface of the operator network 14 and the high-speed network 12. The intermediate equipment 22 is for example a firewall, as the intermediate equipment 16.

The installation represented in FIG. 1 further comprises a first client terminal 26 capable of requiring the provision of a service on the part of the server 10 or the server 18. This client terminal 26 is connected to a high-speed network 28, for example identical to the high-speed network 12, that is to say an ADSL link. This high-speed network 28 is itself connected to the operator network 14 via intermediate equipment 30, such as a firewall. Finally, the installation includes a second client terminal 32, also likely to require the provision of a service from the server 10 or the server 18. It is connected to a packet data transmission network 34, such as than the Internet. The Internet network 34 is itself connected to the operator network 14 via a router 36 directly connected to a control platform 38 and an intermediate equipment 40. The intermediate equipment 40 is for example a firewall, as the intermediate equipment 16, 22 and 30. -T-

The set of intermediate equipment 16, 22, 30 and 40 is managed by a conventional system 42 under the control of the operator of the network 14.

To enable the provision of a service to requesting client terminals, such as terminals 26 and 32, the server 10 comprises means for establishing a communication session with remote terminals.

More specifically, the server 10 comprises means 43 for receiving a service provision request issued by any client. It further comprises means 44 for issuing a service provision agreement to the customer who transmitted the request. Finally, it comprises means 45 for triggering a predetermined delay waiting for an acknowledgment of the agreement it has issued, this acknowledgment must be from the client who transmitted the request. The server 18 also includes the same means 43, 44 and 45 as the server 10.

To allow the protection of the servers 10 and 18 against denial of service attacks that may come from the client terminals 26 and 32, the intermediate equipment 16, 22, 30 and 40 include means 46 for interrupting the establishment of a session. required by a client, if a predetermined criterion by these intermediate equipment is verified during the data exchange necessary for the establishment of the session.

For example, the criterion predetermined by an intermediate device is a waiting time for an acknowledgment lower than that predetermined by the server 10 or 18. To do this, the intermediate equipment concerned comprises means 47 for triggering this weak delay.

In general, the waiting time implemented on a server such as the server 10 or 18 is of the order of a few tens of seconds, while the low delay of an intermediate device can be set to only 3 seconds.

This criterion of low delay is advantageously implemented on intermediate equipment located at the interface of networks with low delay or lightly loaded because it imposes a shorter response time from a client. On the other hand, intermediate equipment located at the interface of two networks, at least one of which is at a rate comparable to the Internet, should not apply this criterion to interrupt sessional establishments.

It is for this reason that in the embodiment shown in FIG. 1, each of the intermediate devices 16, 22, and 30 includes means 47 for triggering a small delay, but not the intermediate equipment 40. predetermined by an intermediate device may also be the absence of a customer of a table maintained by this intermediate equipment when the latter intercepts a request for service provision from that client. In this case, such a table is stored in storage means 48 which are regularly updated by the intermediate equipment concerned.

This criterion can be implemented on each intermediate equipment 16, 22, 30 and 40.

In a preferred embodiment which will be described more specifically with reference to FIG. 3, the client is entered in the table by the intermediate equipment if, during a previous session establishment, the client has sent back an acknowledgment of receipt. service delivery agreement issued by the intermediate equipment within the predetermined time by the server whose customer has requested the provision of a service.

A first embodiment of a method for protecting the server 10 or 18 according to the invention will now be described with reference to FIG. 2, as part of a data exchange between the client 26 and the server 10. This The method is advantageously implemented by the intermediate equipment 30 located at the interface of the operator network 14 and the high-speed network 28.

According to this method, during a first step 50, the client terminal 26 transmits, via the broadband network 28, a service provision request for the server 10. This request is intercepted by the intermediate equipment 30 and then passes through the operator network 14 and the high-speed network 12, to reach the server 10 during a step 52.

In the next step 54, the server 10 returns a service provisioning agreement to the client terminal 26. The transmission of this agreement activates the means 45 for triggering the predetermined delay by the server 10, during a step 56. The service provisioning agreement issued by the server 10 is intercepted by the intermediate equipment 30, which triggers the activation of the means 47 for triggering the short delay predetermined by the intermediate equipment, during a step 58 Once this lower waiting period triggered by the intermediate equipment 30, the service provisioning agreement reaches the client terminal 26, via the high-speed network 28, during a step 60.

If at the expiration of the short delay triggered by the intermediate equipment 30, it has still not received an acknowledgment to be issued by the client terminal 26, the intermediate equipment 30 interrupts the establishment of the session required by the client terminal 26, during a step 62 during which it transmits to the server 10 a signal informing it of this interruption. Thus, the server 10, which had stored the service provision request of the client terminal 26 in its buffer memory, can release it before the expiry of its own waiting period. Denial of service attacks possibly transmitted from the terminal 26 are thus neutralized by the intermediate equipment 30, without affecting the server 10, which can receive other requests for service provision from other client terminals.

Of course, if following step 60, in which the client terminal 26 receives the service provision agreement, the latter sends an acknowledgment to the server 10 before the expiry of the small delay imposed by the equipment. intermediate 30, the establishment of the communication session required by the client terminal 26 is not interrupted.

A second embodiment of a method for protecting the server 10 or 18 according to the invention will now be described with reference to FIG. 3, as part of a data exchange between the client 32 and the server 10. This method is advantageously implemented by the intermediate equipment 40 located at the interface of the operator network 14 and the Internet network 34.

In accordance with this method, the client terminal 32 issues a first service provision request to the server 10. This first service provision request is issued in a step 70. It is transmitted over the Internet 34 and reaches the router 36 which, under the control of the control platform 38 redirects to the intermediate equipment 40 so that it can intercept it. The intermediate equipment 40 receives this service supply request and checks whether the identification number corresponding to the client terminal 32 is missing from a table that it keeps up to date.

This is the case since this request is the first that the client terminal addresses the server 10. The intermediate equipment 40 thus intercepts the establishment of the session required by the client terminal 32 and responds to the request, instead of the server 10, during a step 72 of returning to the client terminal 32 of a service provision agreement. The intermediate equipment intercepts the request prevents its transmission to the server 10. The return of the service provision agreement by the intermediate equipment 40 triggers a waiting period, predetermined by the intermediate equipment, an accused of receiving the agreement, corresponding to the wait time of the server 10.

In the next step 76, the client terminal 32 returns an acknowledgment of the agreement it has received. As before, this acknowledgment is redirected to the intermediate equipment 40 by the router 36 under the control of the control platform 38. If this acknowledgment reaches the intermediate equipment 40 before the expiry of the timeout triggered in step 74, this triggers the registration of the client terminal 32 in the table maintained by the intermediate equipment 40. This registration of the client terminal 32 in the table of the intermediate equipment 40 attests that this client terminal 32 issued a service provision request which was not a denial of service attack. This client terminal is therefore considered a trusted terminal by the intermediate equipment 40. The registration in the table of the intermediate equipment 40 may be temporary, that is to say itself subject to a delay.

After having received the acknowledgment sent by the client terminal 32 during the step 76, the intermediate equipment 40 interrupts the establishment of the session that it has established with the client terminal in place of the server 10 and returns a signal informing the client terminal 32 that the connection failed, during a step 78. Indeed, this session can not be taken over by the server 10, since for the establishment of a communication session between the client terminal 32 and the server 10, the server 10 must have itself generated the sequence number of the acknowledgment that it receives, when issuing the agreement.

Later, the client terminal 32 issues a second service provision request to the server 10. This service provision request is sent by the client terminal 32 during a step 82. This service provision request is intercepted by the service terminal 32. intermediate equipment 40 which verifies, as before, whether the client terminal 32 is absent from the table it keeps up to date. This is not the case, so the service provision request sent by the client terminal 32 during the step 82 is transmitted and received by the server 10 during a step 84. Then, during a step 86 , the server 10 returns a service provisioning agreement to the client terminal 32, and triggers during a step 88 a waiting time for an acknowledgment from the client terminal 32.

If, as illustrated in FIG. 3, the client terminal 32 returns, before the expiration of the time imposed by the server 10, an acknowledgment during a step 90, restoration of the communication session between the client terminal 32 and the server 10 may continue without being interrupted by the intermediate equipment 40. In this second embodiment of a method according to the invention, it will be noted that the server protected by the intermediate equipment is not at all requested when is the victim of a denial of service attack.

A third embodiment of a method for protecting the server 10 or 18 according to the invention will now be described with reference to FIG. 4, as part of a data exchange between the client 32 and the server 10. This method is advantageously implemented by the intermediate equipment 40 located at the interface of the operator network 14 and the Internet network 34.

According to this method, the client terminal 32 sends a first service provision request to the server 10. This first service provision request is issued during a step 100. It is transmitted over the Internet 34 and reaches the router 36 which, under the control of the control platform 38 redirects it to the intermediate equipment 40 so that it can intercept it. The intermediate equipment 40 receives this service supply request and checks whether the identification number corresponding to the client terminal 32 is missing from a table that it keeps up to date. This is the case since this request is the first that the client terminal sends to the server 10. The intermediate equipment 40 thus intercepts the establishment of the session required by the client terminal 32.

The service provision request issued by the client 32 includes an identifier of this client, for example its IP address. Then, upon receipt of this service provision request, the intermediate equipment 40 calculates, by means of a predefined algorithm, a key that is a function of the IP address of the client 32. A secret algorithm is used if although only the intermediate equipment 40 is able to calculate this key. In a next step 102, the intermediate equipment 40 responds to the request instead of the server 10 by returning to the client terminal 32 a service provisioning agreement. This service provision agreement contains a value equal to the key that the intermediate equipment has calculated. For example, the intermediate equipment 40 may include the value in the service provisioning agreement in the form of a sequence number. This sequence number is a field conventionally used in packet data transmission protocols such as TCP. Unlike the embodiment described above, the intermediate equipment 40 does not store the service provision request and does not trigger any delay. Thus, it does not fill its buffer.

In a next step 104, the terminal 32 returns an acknowledgment of the agreement it has received. In order to specify the number of the packet that the client terminal has received, it includes in its acknowledgment the sequence number of the service provision agreement. However, this sequence number corresponds to the value equal to the unique key.

As previously, this acknowledgment is redirected to the intermediate equipment 40 by the router 36 under the control of the control platform 38.

On receipt of this acknowledgment, the intermediate equipment 40 extracts the IP address of the client terminal 32 and the value it contains.

In a next step, the intermediate equipment 40 calculates a key from the IP address it has extracted from the acknowledgment, and then compares the extracted value with the newly calculated key. If these two keys are identical, the intermediate equipment considers that the client terminal 32 is reliable and can then trigger the registration of the client terminal 32 in the updated table. This registration of the client terminal 32 in the table of the intermediate equipment 40 attests that this client terminal 32 has issued a service provision request that was not a denial of service attack.

Thus, in this embodiment, the intermediate equipment can test the reliability of a client terminal 32 that has issued a request for service provision without having to temporarily fill its buffer.

Then, during a step 106, the intermediate equipment 40 sends to the client terminal 32 a signal informing the client terminal 32 that the connection has failed.

Later, the client terminal 32 issues a second service provision request to the server 10. As the client terminal 32 has been added to the table maintained by the intermediate equipment 40, this request will be transmitted to the server 10 which will accept the establishment of the session. The following steps are identical to those described in the second embodiment.

In this third embodiment, it will be noted that the server 10 is protected by the intermediate equipment since it is not requested at all for a denial of service attack. In addition, it should be noted that this intermediate equipment can not also be the victim of a denial of service attack since it does not keep the requests for service provision in memory.

The method of not storing service provisioning requests may further be implemented directly on the server. Indeed, the server is no longer likely to see its buffer quickly filled and is thus protected against denial of service attacks. In this case, by exception to the general definition of the invention, the request is actually transmitted to the server but the latter only takes it into account from the step of transmitting the request to the server.

It is clear that a system and method according to the invention can effectively protect a server against denial of service attacks, without requiring the management of several communication sessions

Claims

A method of protecting a server (10, 18) against denial of service attacks using a protocol whereby an establishment of a communication session between a client (26, 32) and the server is required by the client for providing a service, the method comprising the steps of: a. interception of a service provision request issued by a client for the server (10), the request not being transmitted to the server; b. verification if the client is present in an array of customers deemed reliable; vs. if the client is present in the table, transmission of the request to the server; d. if the client is not present in the table, implement the following steps: e. sending (72) a service provision agreement to the customer; f. in the event of receiving an acknowledgment of receipt of the agreement from the customer under predetermined conditions, recording the customer in the table and sending (78) to the customer a signal informing him that the establishment of the communication failed.

The method of claim 1, wherein the predetermined conditions are that the acknowledgment is received within a predetermined time after sending the service provisioning agreement.

The method of claim 1, wherein the predetermined conditions are that the acknowledgment contains a value equal to a unique key previously entered in the service provisioning agreement.

4. The method of claim 3, wherein the unique key is customer-based and is calculated a first time at the time of sending the service provision agreement and a second time at the time of receipt of the accused. reception.

A computer program for protecting a server against denial of service attacks using a protocol whereby an establishment of a communication session between a client and the server is required by the client for the provision of a service , the program containing instructions for the implementation of steps b) to f) of claim 1.

6. Device for protecting a server against denial of service attacks using a protocol whereby an establishment of a communication session between a client and the server is required by the client for the provision of a service, the W 2

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device comprising means for carrying out the steps b) to f) of claim 1.

7. Device according to claim 6, wherein the means for the implementation of steps b) to f) comprise a computer program according to claim 5.

8. System for protecting a server against denial of service attacks using a protocol whereby an establishment of a communication session between a client (26, 32) and the server is required by the client for the provision of a service, the system comprising a server (10, 18) adapted for providing a service that may be required by a customer (26, 32), characterized in that the system comprises an intermediate equipment (16, 22, 30, 40) formed by a protective device according to claim 6 or 7.

The protection system of a server according to claim 8, wherein the intermediate equipment (16, 22, 30, 40) is a firewall disposed between the server (10, 18) and a network (28, 34). ) access of the client (26, 32) to the server.