USOO5915087A
`Patent Number:
`11
`(45) Date of Patent:
`
`5,915,087
`Jun. 22, 1999
`
`0 743 777 A2 11/1996 European Pat. Off. ........ HO4L 29/06
`2287 619 9/1995 United Kingdom ............ HO4L 12/22
`96/13113 5/1996 WIPO ............................ HO4L 29/06
`96/31035 10/1996 WIPO ............................ HO4L 12/24
`97/13340 4/1997 WIPO .............................. HO4L 9/00
`97/16911 5/1997 WIPO .....
`HO4L 29/06
`97/26731
`7/1997 WIPO .............................. HO4L 9/00
`97/29413 8/1997 WIPO.
`OTHER PUBLICATIONS
`
`International Search Report, PCT Application No. PCT/US
`95/12681, 8 p. (mailed Apr. 9, 1996).
`News Release: “100% of Hackers Failed to Break Into One
`Internet Site Protected by SidewinderTM", Secure Comput
`ing Corporation (Feb. 16, 1995).
`News Release: "Internet Security System Given Product of
`the Year' Award", Secure Computing Corporation (Mar. 28,
`1995).
`
`(List continued on next page.)
`Primary Examiner Joseph E. Palys
`ASSistant Examiner Pierre E. Elisca
`Attorney, Agent, Or Firm-Schwegman, Lundberg,
`Woessner & Kluth, PA.
`
`United States Patent (19)
`Hammond et al.
`
`54) TRANSPARENT SECURITY PROXY FOR
`UNRELIABLE MESSAGE EXCHANGE
`PROTOCOLS
`
`75 Inventors: Scott Hammond; Jeffery Young;
`Edward B. Stockwell, all of St. Paul
`s
`s
`
`Minn.
`73 Assignee: Secure Computing Corporation,
`Roseville, Minn.
`
`21 Appl. No.: 08/763,933
`
`Dec. 12, 1996
`22 Filed:
`(51) Int. Cl. ................................................ G06F 11/00
`52 U.S. Cl. ......................................................... 395/18701
`58 Field of Search ......................... 395/18701, 2006,
`395/2005.9,186,188.01,380/25, 4,340,825.31,
`825.34
`
`56)
`
`References Cited
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`
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`57
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`-
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`tracted from th
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`
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`
`time. Reply messages are then Sent using the address of the
`Server entity So that the proxy is transparent to the requester.
`
`25 Claims, 6 Drawing Sheets
`
`:
`I
`I
`
`228
`TIMER
`
`224
`PORT
`MANAGER
`
`226
`SECURITY
`MONITOR
`
`an an a
`
`
`
`RELAY
`
`
`
`-222
`
`--21 O
`
`214
`
`CLIENT
`
`MESSAGES
`MESSAGE 2
`MESSAGE 1. STACK 1 STACK 2
`
`MESSAGES
`
`MESSAGE 2
`MESSAGE 1
`
`216
`
`SERVER
`
`220
`
`230
`
`GUEST TEK EXHIBIT 1010
`Guest Tek v. Nomadix, IPR2018-01668
`
`

`

`5,915,087
`Page 2
`
`U.S. PATENT DOCUMENTS
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`5,623,601
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`... 395/187.01
`5,636,371
`6/1997 Yu ......
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`

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`Inc.).
`
`

`

`U.S. Patent
`U.S. Patent
`
`Jun. 22, 1999
`Jun. 22, 1999
`
`Sheet 1 of 6
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`

`1
`TRANSPARENT SECURITY PROXY FOR
`UNRELIABLE MESSAGE EXCHANGE
`PROTOCOLS
`
`5,915,087
`
`FIELD OF THE INVENTION
`The present invention relates to network Security Systems
`and in particular to a Security System for Securely exhchang
`ing messages using unreliable protocols.
`BACKGROUND OF THE INVENTION
`Networks connect many computers together allowing
`them to exchange data via communications lines. Several
`Standards defining how Such data eXchanges should occur
`have been developed and implemented to ensure that com
`15
`puters and computer programs using the same protocols can
`Successfully exchange data. One of the problems associated
`with the ability to exchange data is ensuring that a requestor
`entity, Such as a user on a network, Sometimes referred to as
`a client, is authorized to Send messages to and to receive data
`from a Server entity, Such as another computer.
`Firewalls are devices, Such as programs or Separate com
`puter Systems which were introduced in order to address the
`Security problems associated with connecting a once private
`network Such as a local area network connecting computers
`in an office, to an “Internet', where the data transmissions
`are open to eaves dropping, and the potential exists for
`“hostile' outsiders to disrupt network service or tamper with
`or attack Systems residing on the private network.
`There are a number of different classes of firewalls, each
`designed to address different types of Security concerns. In
`Spite of the different approaches, all firewalls perform a
`function know as “relaying", where Protocol Data Units
`(PDUs) are received by the firewall from a sending appli
`cation entity and forwarded to a receiving application entity,
`possibly with some modifications to the original PDU. Since
`firewalls are designed to enforce a Security policy, Some
`information, or context, must be extracted from the PDUs
`and Subjected to a set of rules. Based on the outcome of the
`rules check, the firewall performs an action; the PDU is
`either relayed, modified and relayed, or rejected in Some
`fashion. The precise action is chosen by the designer of the
`firewall in order to affect the behavior of the system such
`that the Security policy is Satisfied. The action is of course
`subject to the constraints of the protocol the firewall is
`designed to Support.
`The Internet uses a simple transport protocol to provide a
`process to process communication Service called User Data
`gram Protocol (UDP) UDP is a protocol for processes to
`eXchange datagrams Such as messages between processes
`coupled via a network, Internet Protocol (IP) in this case.
`One important feature of the UDP protocol is that there is no
`assurance that a message will get through. It is said to be an
`unreliable communications protocol for this reason. No
`continuous connection is established, and Since there is no
`maintenance of the States of messages to ensure delivery,
`there is very little overhead in implementing the UDP
`communication protocol. It is Suitable for transfer of data
`Such as network Video, where there is no desire to Spend time
`reconstructing lost frames of live Video, and for audio
`communications, where the same considerations apply.
`Processes communicating using UDP indirectly indentify
`each other using an abstract locator, often called a port or
`mailbox of a known host device along with the address of
`the host. Many common processes receive messages at fixed
`ports on each device on which they run. One process, known
`as a Domain Name Server (DNS) receives messages at port
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`53 for example. Following a first communication at Such a
`port, processes may then agree on a different port number,
`which frees up the original port for other processes. A
`configuration file contains a list of hosts and ports which
`packets should be relayed between.
`This points to a difficulty in implementing firewalls which
`protect Servers from illegal messages. The firewall must find
`a way to accept messages that are not addressed to it. There
`is a need for this to be done with further multilevel checking
`of the messages without confusing the processes attempting
`to communicate. There is a further need to do this without
`modifying the client that is Sending and receiving messages.
`
`SUMMARY OF THE INVENTION
`A proxy for unreliable message based protocol commu
`nication Systems accepts messages from a requesting pro
`ceSS on a client that are intended for a Server with a different
`address than a host on which the proxy is running. It also
`Sends messages back to the client, using the address of the
`Server. The proxy interrogates messages Sent by the
`requestor in accordance with defined authentication proce
`dures. In one embodiment, the proxy transparently receives
`and forwards messages in accordance with a defined Security
`policy. Messages are interrogated for conformance to
`desired protocols, and optionally further decoded to add
`additional application specific filtering.
`In one embodiment, the proxy comprises a monolithic
`computer program which interfaces with a dual Stack net
`working firewall which monitors the requestor's address
`which is extracted directly from the message and checks the
`address against an access control list. If the address is
`invalid, the proxy causes the message to be deleted. Since
`the requestor does not see the proxy, it does not need to be
`modified to work with the proxy. The dual stack refers to the
`division of the firewall into an external Stack and an internal
`Stack. The external Stack deals with communications coming
`in from external computer Systems, and the internal Stack
`handles communications with internal Systems coupled to
`the internal Stack that the firewall is designed to protect. The
`monolithic nature of the proxy arrises from the fact that it
`sits on top of both Stacks and is able to communicate with
`both StackS via a single, or monolithic process. This means
`that only one process is needed for the proxy. Multiple
`instances of the process may be operating, one for each
`Session of messages to be transferred between a requester
`and Server. This avoids the consumption of Significant
`resources exhibited by non-monolithic prior proxies when
`many Sessions are operating.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a block diagram of a computer System imple
`menting the present invention.
`FIG. 2 is a combined block diagram and logical message
`flow diagram of one embodiment of the present invention.
`FIG. 3 is a logical block diagram of a table used to track
`message flow and ports for the embodiment of FIG. 2.
`FIGS. 4a-C are pseudo code flow diagrams of a proxy
`operating on the computer System of FIG. 1 which controls
`message flow.
`DESCRIPTION OF THE EMBODIMENTS
`In the following detailed description of the preferred
`embodiments, reference is made to the accompanying draw
`ings which form a part hereof, and in which are shown by
`way of illustration specific embodiments in which the inven
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`tion may be practiced. It is to be understood that other
`embodiments may be utilized and structural changes may be
`made without departing from the Scope of the present
`invention.
`There has been an explosion in the growth of computer
`networks as organizations realize the benefits of networking
`their personal computers and WorkStations. Increasingly,
`these networks are falling prey to malicious outsiders who
`hack into the network, reading and Sometimes destroying
`Sensitive information. Exposure to Such attacks has
`increased as companies connect to outside Systems Such as
`the Internet.
`To protect themselves from attacks by malicious
`outsiders, organizations are turning to mechanisms for
`increasing network Security. One Such mechanism is
`described in “SYSTEM AND METHOD FOR PROVID
`ING SECURE INTERNETWORK SERVICES”, U.S.
`patent application Ser. No. 08/322,078 filed Oct. 12, 1994 by
`Boebert et al., the discussion of which is hereby incorpo
`rated by reference. Boebert teaches that modifications can be
`made to the kernel of an operating System in order to add
`type enforcement protections. Type enforcement adds an
`additional level of protection to the process of accessing
`files. This is quite helpful for a BSD 4.4 UNIX type
`operating System wherein once a proceSS receives privileges,
`it uses those privileges to access other network files. This
`can lead to a dangerous breach of network Security. Boebert
`further teaches a Secure computer that is used to connect a
`private network having a plurality of WorkStations to a
`public network. A protocol package Such as TCP/IP running
`on the Secure computer implements a communications pro
`tocol used to communicate between each workStation and
`the Secure computer.
`Program code running on the Secure computer is used to
`communicate through the private network to the WorkSta
`tion's protocol package. In one embodiment, the Secure
`computer is an Intel Pentium-based machine running a
`hardened form of BSD/OS Unix. A system based on a 90
`Mhz. Pentium microprocessor with 32 megabytes of
`memory, 2 gigabytes of hard disk Space, a DAT tape for
`backup and a CD-ROM for software loads has been found
`to be adequate. Likewise, program code running on the
`Secure computer is used to communicate through a public
`network interface to a public network Such as the Internet.
`In an Internet embodiment, the program code used to
`communicate with the Internet is part of a set of Internet
`protocols which communicate with computers on the Inter
`net through an Internet connection. In one embodiment,
`different protocols may be used when communicating with
`different entities on the Internet In one embodiment, a top
`wrapper package operating in the Internet protocols is used
`to sit on the external, public network So that information
`about external problems can be logged. Such a System is
`currently being Sold under the brand name, Sidewinder, by
`the assignee hereof. Certain aspects of the Sidewinder
`product related to dual Stack network Separation are further
`55
`described in “SYSTEM AND METHOD FOR ACHIEV
`ING NETWORKSEPARATION”, U.S. patent application
`Ser. No. 08/599,232 filed Feb. 9, 1996, the discussion of
`which is hereby incorporated by reference. Internal and
`external networks are separated by have using Separate
`Stacks in programming Separated into internal and external
`burbs. A Security manager sits at the top of the Stacks and
`ensures that communications between the burbs are checked
`for predetermined Secure characteristics before being trans
`ferred to the other burb.
`The use of acceSS control lists to check whether an entity
`is authorized to communicate with another entity is
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`described in “GENERALIZED SECURITY POLICY
`MANAGEMENT SYSTEM AND METHOD, U.S. patent
`application Ser. No. 08/715,668, filed Sep. 18, 1996 and
`assigned to the same assignee hereof, the discussion of
`which is hereby incorporated by reference. In essence, a
`Security monitor maintains an address based acceSS control
`list used to identify authorized clients from which message
`receipt is acceptable.
`The current invention is an extension to the Sidewinder
`product. As shown in FIG. 1 generally at 110, a network
`communication controller Such as a computer System com
`prises a processor 112 coupled to a random acceSS memory,
`RAM 114. While only a single bus 116 is shown connecting
`the RAM 114 and processor 112 to a communications port
`118 and disk drive or other storage medium 120, it will be
`recognized by those skilled in the art that it represents
`Several different buSSes in a Standard personal computer
`architecture. The communications port represents various
`communications options in computer Systems, Such as eth
`ernet cards, modems and other communication devices.
`In FIG. 2, operation of an improved firewall product
`comprising an improved computer program extension to the
`Sidewinder product is indicated generally in block repre
`Sentation at 210. The computer program is usually Stored on
`the disk drive 120 and run or executed on top of an operating
`system running on the processor 112 out of RAM 114. It
`should be noted that disk drive 120 is used herein to
`represent various Storage medium by which the computer
`program 210 may be stored and distributed. It also repre
`Sents a communication medium in which the proxy may be
`temporarily Stored while being transferred or transmitted to
`computer system 110. Computer program 210 further com
`prises a proxy 212 which is used to process communications
`between processes. Also shown in FIG. 2 are a client 214
`and Server 216 which each have processes thereon trying to
`communicate using a UDP protocol for Sending datagrams
`comprising messages. They also may communicate using
`other protocols, but the embodiments herein will be
`described with respect to message exchange using unreliable
`communication protocols such as UDP on top of IP
`Processes communicating using UDP indirectly indentify
`each other using an abstract locator, often called a port or
`mailbox of a known host or server device along with an IP
`address of the host. Many common processes receive mes
`Sages at fixed ports on each device they run on. The Domain
`Name Server (DNS) receives messages at port 53 for
`example. Following a first communication at Such a port,
`processes may then agree on a different port number, which
`frees up the original port for other processes. Note that there
`is no connection that is maintained. The message is Sent by
`a client or Server, and the underlying network does its best
`to route it to the correct host using a destination address and
`port contained in the message.
`The Sidewinder security system provides the ability to
`accept messages intended for Selected Servers. Each UDP
`datagram or message comprises a packet with a header and
`data. Among the fields of the header are the length of the
`packet, the protocol identifier-17 for UDP, a checksum, and
`importantly for the embodiments of the invention described,
`a Source address and port identifier and a destination address
`and port identifier. The Sidewinder security system has
`networking modifications which allow it to accept a message
`intended for any destination address.
`Messages, Such as message 1, from client 214 intended
`for a server attached to the firewall are received at a first
`stack 220 even though the firewall has a different address
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`than the server. The firewall simply keeps a list of addresses
`of Server attached to it, and accepts all messages intended for
`Such servers. They are then relayed by a relay 222 to a port
`manager 224 which receives the message, and requests the
`operating System to bind it to a port which may be based on
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`an address in the message if it is not already So bound. A
`Security monitor 226 is operatively coupled to the port
`manager 224 to monitor messages from clients for conform
`ance to predefined conditions and to prevent the further
`routing of non conforming messages. A timer represented at
`228 provides a time Stamp to time how long a port is being
`held open waiting for further messages from either the client
`or the server. A link table 310, shown in FIG.3 keeps track
`of which ports are currently bound and hence monitored for
`further messages, and also provides mapping for routing of
`messages with desired addresses.
`Referring to FIG. 2 again, if a message is approved for
`Sending on to the Server 216, a new port is bound if not
`already bound, and noted in the link table. The message is
`then sent via relay 222 to a second stack 230 with an address
`and port identifying it as originating from firewall 210. The
`message is then Sent on to the intended Server 216. The
`Server may respond to the message, but will use the address
`and port used by the firewall. The original client is transpar
`ent to the server, which only knows of the firewall.
`When and if the server 216 responds to the message via
`a message 2, it uses the address and port Specified by the
`firewall. The port manager uses the link table to monitor for
`messages on Specific ports identified in the table.
`Alternatively, the operating System may maintain a Separate
`table and provide Such monitoring. In any event, the link
`table is then used to identify the original client address and
`port, as well as the address and port of the firewall on which
`the first message was recieved from the client. The proxy
`then causes those addresses to be used in Sending the reply
`message from the Server on through the Stacks and relay to
`the client. In essence, it spoofs the addresses Such that the
`message appears to the client to have originated from the
`Server. In essence, the network addressing Structure of the
`Server remains unknown to the client, providing Security to
`the message exchange.
`In one practical embodiment, the client