Abstract:
The present invention provides the initiation of a transport layer security (TLS) session between a client device and a server using a firewall without interruption. The present invention holds a TLS hello message received from the client device until after the server has been validated. A firewall consistent with the present invention does not interrupt a transport layer control (TCP) connection that was established between the client device and the firewall before the TLS hello message was received by the firewall.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention is generally related to establishing a transport layer security (TLS) session without interruption. More specifically, the present invention relates to a client device communicating with a server through a firewall without interrupting an initial transport control protocol (TCP) session. 
         [0003]    2. Description of the Related Art 
         [0004]    Servers and client devices connecting to servers over computer networks are often protected from cyber-attacks by a firewall. Firewalls commonly intercept messages and inspect them for cyber-threats. In certain instances, a firewall also stops intercepting and inspecting messages between a client device and a server after the firewall determines that the server does not pose a threat to the client device, and that the client device does not pose a threat to the server. 
         [0005]    Firewalls in modern computer networks validate the credentials of servers and client devices before allowing them to communicate directly. An example of direct communications between a client device and a server is a transport layer security (TLS) session where a firewall stops intercepting and inspecting messages between the client device and the server. 
         [0006]    Messages encrypted over a TLS session are usually inspected by the firewall using deep packet inspection over a secure socket layer (DPI-SSL) software. When inspecting messages using DPI-SSL software, the firewall separates and parses information contained within messages received by the firewall. By inspecting data within the messages cyber-threats such as computer viruses or worms are identified and blocked. DPI-SSL is used to inspect messages received from client devices or servers that are more likely to contain a cyber-threat. 
         [0007]    Firewalls also hide from the server the internet protocol (IP) address of client devices that communicate with the server. To accomplish this firewalls communicate with servers using network address translation (NAT). NAT abstracts the real IP address of the client from the server by assigning a configured outbound IP address and a random TCP port numbers on a TCP connection between the firewall and server. Typically, a client device only sees TCP packets including a server IP address and server TCP port number. Because of this, the client has no idea that the connection is going through a NAT&#39;d firewall, and the server only sees the connection coming from the firewall without being aware of the actual client IP address of the client device or the TCP port used to communicate with the client device. Communications between the firewall and the server use network addresses and port numbers that are virtual, random, and that are dynamically assigned. For example, when establishing a transfer control protocol (TCP) connection between a firewall and a server, NAT designates a configured NAT IP address, assigns a dynamically obtained NAT TCP port number. These NAT address and NAT port numbers are unique and are associated with a specific TCP connection. The NAT TCP port number is usually picked from a large range of valid TCP port numbers. The source/client IP address and source/client TCP port number are mapped to the NAT IP address and NAT TCP port number in the TCP packets from client that are sent to the firewall. Likewise, the destination/NAT IP address and destination/NAT TCP port number in the packets from the server to the firewall are mapped to the client IP address and client TCP port number respectively. These mappings are associated with a particular client-server TCP connection and are stored in the firewall. Each time a new TCP connection is established, new NAT port numbers are assigned to the new TCP session. 
         [0008]    After a firewall has established a TLS session, the firewall passes through the messages associated with the TLS session without inspecting them for cyber-threats using a direct point to point connection through the firewall. This direct point to point connection is commonly referred to as a tunnel through the firewall. In certain instances, the messages tunneled through the firewall are forwarded according to a NAT mapping that hides real IP address of the client device from the server. 
         [0009]    Before a client device attempts to initiate a TLS session with a server, the client device must establish a TCP connection between the client device and a firewall, and the firewall must establish a TCP connection between the firewall and the server. 
         [0010]    When establishing a TCP connection between a client device and a firewall, the client device first transmits to the firewall a TCP synchronize message, next the client device receives a TCP synchronize acknowledgement message from the firewall, and then the client device transmits a TCP acknowledgment to the firewall. Establishing the TCP connection between the firewall and the server includes a series of similar steps, where the firewall transmits a TCP synchronize message to the server, the firewall receives a TCP synchronize acknowledgement message from the server, and the firewall transmits a TCP acknowledgement to the server. After a TCP connection between the client device and the firewall is established, the client device initiates a TLS handshake by sending a TLS hello message to the firewall that addresses the server. 
         [0011]    After the TLS client hello message is sent to the firewall, the firewall obtains the DNS name of the server from the TLS client hello message, when it is available. The DNS name of server is part of a TLS extension in the TLS client hello message is optional, and is therefore not always included in a TLS client hello message. If the domain name of server is available in the TLS client hello, the firewall compares information in the client hello with information stored in memory at the firewall. The information stored at the firewall is usually stored in either a user defined exclusion list, a dynamic exclusion list, or in both. The TLS handshake between the firewall and the server enforces local policies for TLS connections, where servers are identified using a domain name service (DNS) hostname provided by the server in the server certificate. The DNS hostname is compared against the user defined exclusion list or the dynamic exclusion list to determine whether subsequent TCP messages from the client device addressing the server should be bypassed from interception and inspection by deep packet inspection secure socket layer (DPI-SSL) software. Once it is determined that subsequent TCP messages should be bypassed from interception and inspection, legacy implementations terminate the TCP connection between the client device and firewall because the firewall has already initiated a TLS session with the server for this client connection. The firewall then adds the certificate information to an entry in the dynamic exclusion list. Firewalls terminate the TCP connection between the client and the firewall because their design expects that a user of a client device will either refresh the previous TCP connection or establish a new TCP connection between the client and the firewall. Once a TCP connection between the client device and firewall is refreshed or a new TCP connection is established, legacy firewall implementations use the new entry in the dynamic exclusion list to determine if the connection should be intercepted or tunneled prior to establishing a TLS session with the server. 
         [0012]    After the TCP connection between the client device and the firewall is terminated, a user of the client device experiences delay. This is because the client device must refresh the session or initiate a new TCP connection. In certain instances applications installed on the user device experience a fatal error when the initial TCP connection is interrupted. When an application program terminates or closes due to a fatal error, the user of the client device must also re-start the application before a TLS session can be initiated. By interrupting the TLS session between the client device and the firewall, a user of the client device is subjected to unnecessary delays caused by the firewall terminating the client device/firewall TCP connection. 
         [0013]    What is needed is a system and a method for initiating a TLS session that does not terminate an existing TCP connection between a client device and a firewall. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  illustrates a block diagram of a computing device that may be used to implement various embodiments of the present invention. 
           [0015]      FIG. 2  illustrates a client device, a firewall, and a server communicating with each other. 
           [0016]      FIG. 3  illustrates a flow chart of a method for initiating a transport layer security (TLS) session in a flawless manner. 
           [0017]      FIG. 4  illustrates a flow chart of communications between a firewall and a server when validating that the server is a trusted server, and when initiating a second transport control protocol (TCP) session between the firewall and the server. 
           [0018]      FIG. 5  illustrates a flow chart of the firewall interacting with a client device and a server when establishing a TLS session. 
       
    
    
     SUMMARY OF THE PRESENTLY CLAIMED INVENTION 
       [0019]    The present invention initiates a TLS session without interrupting an initial TCP connection between a client device and a firewall. Firewalls built according to the present invention initiate a TLS session flawlessly because they do not interrupt the TCP connection between the client device and a firewall. The present invention initiates a TLS session after a first TCP session has been initiated between a client device and a firewall, and after a first TCP session has been initiated between the firewall and a server by holding the TLS hello message in the firewall. 
         [0020]    The initiation of a TLS session between a client device and a server begins with a client device sending a TLS hello message to the server through a firewall. The firewall holds the TLS hello message until after the firewall validates the credentials of the server. While holding the TLS hello message from the client device, the firewall initiates a TLS session with the server by sending a new TLS client hello message to the server. The server then responds by sending a TLS server hello message, and a TLS server certificate message to the firewall. The new TLS client hello message includes information that is similar to, yet not identical to information included in the TLS client hello message held by the firewall. The server certificate message received by the firewall includes a “server certificate,” and the server certificate includes information that will be used by the firewall to validate the server. 
         [0021]    After the firewall receives the server certificate from the server, the firewall validates the server by comparing information in the retrieved server certificate with information stored at the firewall. The information stored in the firewall is used to validate the server by comparing the information in the server certificate with a trusted well-known set of certificate authority (CA) certificates stored in the firewall. When the information in the server certificate matches the information stored at the firewall, the firewall determines that the server is a trusted server. 
         [0022]    After validating that the server is a trusted server, the firewall classifies the connection as inspect (also known as intercept) or bypass (also known as tunnel). The information stored at the firewall is usually stored in either a user defined exclusion list, a dynamic exclusion list, or in both. 
         [0023]    In the case of bypass, the firewall initiates a second TCP connection with the server. After establishing a second TCP connection between the firewall and the server, the firewall forwards the original TLS hello message received from client to server. By holding the TLS hello message until after the server is validated the TCP connection between the client device and the firewall is not interrupted. Since the TCP connection between the client device and the firewall is not interrupted, a user of the client device no longer experiences delays when initiating a TLS session. The present invention, therefore, improves the efficiency of establishing a TLS session by not interrupting a TCP session between the client device and the firewall. 
         [0024]    The client device is not aware that the firewall initiated a TLS session with the server using the new TLS client hello message, and the client device is not aware that the firewall has validated the credentials of the server while holding the original TLS client hello message. 
         [0025]    After the TLS session between the client device and the server is established, all subsequent TCP messages between the client device and the server are passed through the firewall without being intercepted or inspected by the firewall. The firewall, thus, “transparently passes through” TCP messages from the client device addressing the server, and transparently passes through TCP messages from the server addressing the client device. The terms: transparently passes messages, transparently passes through messages, transparently passing messages, transparently passing through messages, or bypassed messages used in this disclosure refer to messages that are received over a first port at the firewall and are transmitted over another port at the firewall without being intercepted or inspected by the firewall. The present invention initiates a TLS session between the client device and the server without interrupting the initial TCP connection between the client device and the firewall even when the TLS client hello message received from the client device does not include the DNS name of server, a common name identifying the server, or a domain name. 
       DETAILED DESCRIPTION 
       [0026]      FIG. 1  is a block diagram of a device for implementing the present technology.  FIG. 1  illustrates an exemplary computing system  100  that may be used to implement a computing device with the present technology. Note that  FIG. 1  is exemplary and that all features shown in the figure may not be included in a firewall implementing the present invention. System  100  of  FIG. 1  may be implemented in the contexts of the likes of clients and servers. The computing system  100  of  FIG. 1  includes one or more processors  110  and memory  120 . Main memory  120  may store, in part, instructions and data for execution by processor  110 . Main memory  120  can store the executable code when in operation. The system  100  of  FIG. 1  further includes mass storage  130 , which may include resident mass storage and portable storage, antenna  140 , output devices  150 , user input devices  160 , a display system  170 , peripheral devices  180 , and I/O devices  195 . 
         [0027]    The components shown in  FIG. 1  are depicted as being connected via a single bus  190 . However, the components may be connected through one or more data transport means. For example, processor unit  110  and main memory  120  may be connected via a local microprocessor bus, and the storage  130 , peripheral device(s)  180 , and display system  170  may be connected via one or more input/output (I/O) buses. 
         [0028]    Mass storage device  130 , which may include mass storage implemented with a magnetic disk drive, an optical disk drive, FLASH memory, or be a portable USB data storage device. Mass storage device  130  can store the system software for implementing embodiments of the present invention for purposes of loading that software into main memory  120 . The system software for implementing embodiments of the present invention may be stored on such a portable medium and input to the computer system  100  via the portable storage device. 
         [0029]    Antenna  140  may include one or more antennas for communicating wirelessly with another device. Antenna  140  may be used, for example, to communicate wirelessly via Wi-Fi, Bluetooth, with a cellular network, or with other wireless protocols and systems. The one or more antennas may be controlled by a processor  110 , which may include a controller, to transmit and receive wireless signals. For example, processor  110  executes programs stored in memory  120  to control antenna  140 , transmit a wireless signal to a cellular network, and receive a wireless signal from the cellular network. 
         [0030]    The system  100  as shown in  FIG. 1  includes output devices  150  and input devices  160 . Examples of suitable output devices include speakers, printers, and monitors. Input devices  160  may include a microphone, accelerometers, a camera, and other devices. Input devices  160  may also include an alpha-numeric keypad, such as a keyboard, for inputting alpha-numeric and other information, or a pointing device, such as a mouse, a trackball, stylus, or cursor direction keys. I/O devices  195  include network interfaces, and touch screens. Network interfaces used the present invention may be any computer network (wired or wireless) known in the art, including, yet are not limited to Ethernet, or 802.11. 
         [0031]    Display system  170  may include a liquid crystal display (LCD), LED display, a plasma display, or be another suitable display device. Display system  170  receives textual and graphical information, and processes the information for output to the display device. 
         [0032]    Peripherals  180  may include any type of computer support device to add additional functionality to the computer system. For example, peripheral device(s)  180  may include a modem or a router. 
         [0033]    The components contained in the computer system  100  of  FIG. 1  are those typically found in computing system, such as but not limited to a desktop computer, laptop computer, notebook computer, netbook computer, tablet computer, smart phone, personal data assistant (PDA), or other computer that may be suitable for use with embodiments of the present invention and are intended to represent a broad category of such computer components that are well known in the art. Thus, the computer system  100  of  FIG. 1  can be a personal computer, hand held computing device, telephone, mobile computing device, workstation, server, minicomputer, mainframe computer, or any other computing device. The computer can also include different bus configurations, networked platforms, multi-processor platforms, etc. Various operating systems can be used including Unix, Linux, Windows, Macintosh OS, Palm OS, and other suitable operating systems. 
         [0034]      FIG. 2  illustrates a client device, a firewall, and a server communicating with each other. Communications  240  in the figure are communications  240  between the client device  210  and the firewall  220 . Communications  250  are communications between the firewall  220  and the server  230  using a wired or a wireless computer network. Communications  240 / 250  are transmitted over a network interface. 
         [0035]      FIG. 3  illustrates a flow chart of a method for initiating a transport layer security (TLS) session in a flawless manner. A flawless TLS session initiation  300  includes steps  310 ,  320 ,  330 ,  340 , and  350  in the figure. In step  310  the firewall receives a TLS hello message from a client device, and next in step  320  the firewall holds the TLS hello message. In step  330 , the firewall initiates a TLS session with the firewall by sending a new TLS client hello message to the server. This new TLS client hello message is similar to, yet is not identical to the TLS client hello message being held by the firewall. Furthermore the client device is not aware that the firewall performs this step. After transmitting the new TLS client hello message to the server, the server responds (this step is not depicted in  FIG. 3 ) by sending a server certificate to the firewall, and information in the server certificate is used by the firewall to validate the server. 
         [0036]    In step  340  of the flow chart, the firewall compares information retrieved from the server with information stored at the firewall when authenticating that the server is a trusted server. In step  350  the firewall transmits the held TLS hello message to the server when initiating a TLS session between the client device and the server. Finally, in step  360  of the flow chart, messages transmitted between the client device and the firewall are transparently passed through the firewall without interception or inspection. 
         [0037]      FIG. 4  illustrates a flow chart of communications between a firewall and a server when validating that the server is a trusted server, and when initiating a second transport control protocol (TCP) connection between the firewall and the server.  FIG. 3  and  FIG. 4  include the corresponding steps of holding a TLS hello message  320  &amp;  410 , sending a new TLS client hello message from the firewall to the server  330  &amp;  420 , comparing information  340  &amp;  450 , and transmitting the held TLS hello message  350  &amp;  480  to the server. 
         [0038]    The flow chart in the figure begins with step  410 , holding the TLS hello message. In step  420 , the firewall initiates a TLS session with the firewall by sending a new TLS client hello message to the server. As in  FIG. 3 , this new TLS client hello message is similar to, yet is not identical to the TLS client hello message being held by the firewall. Furthermore the client device is not aware that the firewall performs this step. 
         [0039]    In step  430  the firewall receives a TLS server hello message from the server, and in step  440  the firewall receives the server certificate from the server. In step  450  the firewall compares information in the server certificate with information stored at the firewall when validating that the server is a trusted server. In step  460  a first TCP connection between the firewall and the server is reset, in step  470  a second TCP connection is initiated between the firewall and the server, and in step  480  the held TLS hello message is transmitted to the server using the second TCP connection. 
         [0040]    Steps  340 / 450  in  FIGS. 3-4  compare information contained in the certificate requested from and retrieved by the firewall with information stored at the firewall. Information compared in step  340  &amp;  450  when validating the server include, yet are not limited to the name of a server, the DNS hostname of the server, a domain name associated with the server, and a common name. In instances where a TLS session previously interrupted is re-established, the firewall will use information stored in a peer certificate on the server. In these embodiments, the peer certificate will include less information than the information contained in the certificate that was used to originally validate the server. 
         [0041]    Comparison information stored at the firewall will be stored in memory in a user defined exclusion list, a dynamic exclusion list, or in both the user defined exclusion list and the dynamic exclusion list. When information in the user defined exclusion list corresponds to information in the certificate, that information will be copied to the dynamic exclusion list. In certain embodiments, the dynamic exclusion list is a data structure that stores or deletes information according to administration rules of the firewall. An example of such an administrative rule is when the firewall deletes an entry in the dynamic exclusion list after a user terminates a TLS session, and does not refresh that session for more than 24 hours. 
         [0042]    In certain embodiments of the present invention, the firewall hides a real internet protocol (IP) address of client devices from the server that communicate with the server. To accomplish this, the firewall translates network addresses using a technique that is commonly known as network address translation (NAT). NAT abstracts the real IP address of the client from the server by assigning a configured outbound IP address and a random TCP port numbers on TCP connection between the firewall and server. 
         [0043]    Communications between the firewall and the server use network addresses and port numbers that are virtual, random, and that are dynamically assigned. For example, when establishing a transfer control protocol (TCP) connection between a firewall and a server, NAT designates a configured NAT IP address, assigns a dynamically obtained NAT TCP port number. These NAT address and NAT port numbers are unique and are associated with a specific TCP connection. The source/client IP address and source/client TCP port number are mapped to the NAT IP address and NAT TCP port number in the TCP packets from client that are sent to the firewall. Likewise, the destination/NAT IP address and destination/NAT TCP port number in the packets from the server to the firewall are mapped to the client IP address and client TCP port number respectively. These mappings are associated with a particular client-server TCP connection and are stored in the firewall. Each time a new TCP connection is established, new NAT addresses and port numbers are assigned to the new TCP session. 
         [0044]    The first TCP connection established between the firewall and the server communicates over a first port of the firewall, and a port of the server. The second TCP connection established between the firewall and the server communicates over a second port of the firewall, and a port of the server. The firewall will store in memory an association that identifies which specific port of the firewall is associated with the specific port of the server for a particular TCP connection. When a TCP connection is reset the association will be cleared. Data structures for storing these associations in memory are not limited to a hash table, a linked list, or a tree. Associations that are stored in these data structures also associate a network address of the firewall with a port number at the firewall, and associate a network address of the server with a port number at the server when TCP connections are made dynamically. 
         [0045]      FIG. 5  illustrates a flow chart of the firewall interacting with a client device and a server when establishing a TLS connection. The figure begins with step  510 , this step establishes a first TCP connection between a client device and a firewall. Next in step  520 , a first TCP connection is established between the firewall and a server. Then in step  530  a TLS session is initiated flawlessly using the flawless TLS initiation  300  described in respect to  FIG. 3 . The term “flawlessly” means that a currently established TCP session or connection between the client device and the firewall will not be interrupted when initiating a TLS session. 
         [0046]    After initiating the TLS session flawlessly, the firewall in step  540  passes messages between the client device and the server without intercepting or inspecting those messages using. Messages passed though between the client device to the server are “tunneled” through the firewall without inspection. Network tunneling is the passing of private information over a public network without exposing the private information to inspection by the public. 
         [0047]    The firewall in the present invention acts as a proxy-less DPI-SSL inspection engine where messages between the client device and the server do not use an internet protocol address of the firewall. Messages using TCP connections that are not currently being passed through are inspected, and messages using TCP connections that are currently being bypassed (passed through) are not inspected. When inspecting messages using DPI-SSL software, the firewall separates and parses information contained within messages received by the firewall. By inspecting data within the messages, cyber-threats such as computer viruses or worms are identified and blocked. 
         [0048]    Client devices are known to re-transmit TLS hello messages when they have not received a response to a previous TLS hello message. In the present invention, when a re-transmitted TLS hello message corresponds to a TLS hello message that is currently being held by the firewall is received, the firewall will overwrite the previous TLS hello message with the later TLS hello message and hold the re-transmitted TLS hello message. 
         [0049]    In certain instances, a TLS session is re-established with a different server with the same common name, a domain name service (DNS) hostname, that serves the same content as the server that a client device originally connected to. In such an instance, the different server is addressed using an IP address of the different server. In other instances, a TLS session is re-established from a different client device where a TLS client hello message from the different client device includes a common name associated with the server, a domain name service (DNS) hostname, or a domain name. In other circumstances, a server may always re-validate the credentials of the server and the client device whenever a TLS session is re-initiated. 
         [0050]    While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. The description are not intended to limit the scope of the presently claimed invention or to limit the scope of embodiments of the present invention. The present descriptions are intended to cover alternatives, modifications, and equivalents consistent with the spirit and scope of the disclosure.