network security topologies (1)

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  • 7/31/2019 Network Security Topologies (1)


    Jason Kennedy

    March 23, 2004

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    Perimeter Security Topologies

    Demilitarized zone (DMZ)

    Network Address Translation


    Virtual Local Area Networks

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    Perimeter networks permit communicationbetween the organization and third


    Technology closely related to perimeter

    networks is network address translation(NAT). .

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    It is critical to create a strong networkperimeter that protects internal resources

    from threats outside the org.Problems can occur from:

    The internet (no power to enforce security)

    External networks (business partners,customers, suppliers)

    Need to block undesirable network traffic

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    Goal is to selectively admit or deny traffic(or data flows) from other networks

    based on a number of criteria, such as: Type of protocol

    Source of request



    Admitted or denied based on companiessecurity policy.

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    Enforced primarily by firewalls

    Firewalls used to create choke points onthe network perimeter.

    Firewall inspects each packet forcompliance with the security policy.

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    To have a successful network security perimeter, thefirewall must be the gateway for all communicationsbetween trusted networks and untrusted and unknown

    networks. Each network can contain multiple perimeter networks.

    Three types:

    The outermost perimeter

    Internal perimeter

    The innermost perimeter

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    Identifies the separation point between the assets you control and theassets you dont control.

    This is the router you use to separate your network from your ISPs



    Represent additional boundaries where you have other securitymechanisms in place.

    Ex. When a manager creates a new policy, each network that makes

    up that topology must be classified as one of three types of networks:


    Semi trusted


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    The Outermost perimeter is the most insecure area ofyour network infrastructure.

    Normally reserved for routers, firewalls, and public

    Internet servers, such as HTTP, FTP, and Gopherservices

    The easiest area to gain access to and therefore themost frequently attacked.

    Sensitive company info should not be put in this area.

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    1. Know your enemy

    Consider who might attack

    Identify motivations for an attack

    What could they do?

    2. Counting the Cost

    Weigh costs against benefits

    3. Identifying Any Assumptions Dont assume hackers know less than you

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    4. Controlling Your Secrets

    Use passwords and encryption keys

    Have a limited number of secrets

    5. Knowing Your Weaknesses Understand system weak points

    Areas of potential danger

    6. Limiting the Scope of Access

    Create barriers in your system, so if intruders attack onepoint of the system, they do not automatically have accessto other points.

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    7. Understanding Your Environment

    Know what is expected and unexpected from yoursystem.

    Any traffic or patterns that stray from the normshould be investigated.

    8. Limiting Your Trust

    Know which software you rely on S/W has bugs too!

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    DMZ are areas that are within the autonomous system, but are notas tightly controlled as the networks interior.

    Used by companies that want to host its own Internet services,without sacrificing unauthorized access to its private network.

    Sits between the Internet and an internal networks line of

    defense, and is usually some combination of firewalls and bastionhosts.

    Basically involves adding multiple firewall layers of security

    between the Internet and a companies critical data and businesslogic.

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    A typical DMZ configuration includes:

    Outer firewall b/t the Internet and the Web Server processingthe requests originating on the company Web site.

    Inner firewall b/t the Web Server and the appl. Server to whichit is forwarding requests. Date resides behind this.

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    How it works in a small business:

    A separate computer receives requests from users within theprivate network for access to Web sites or other companyresources on the public network.

    The bastion host then initiates sessions for these requests on

    the public network. The bastion is not able to initiate a sessionback into the private network. It can only forward packets thathave been requested.

    Users on the public network outside the company can accessonly the hosts on the DMZ. They can only view your website.

    Use filtering to impair an attackers ability to have a

    vulnerable host communicate to the attackers host.

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    Other security tips:

    Filter the source of the IP address to determine if itsis one on the DMZ network.

    Solid understanding of network traffic.

    FTP and DNS initiate outbound connections.Special considerations should be given to theseprotocols.

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    Intranet A network topology or the application (Web portal) that

    enterprises use as a single point of access to deliver servicesto employees and business units.

    Also called a campus network.

    Main purpose is to share company info and company

    resources among employees. Extranet

    Private network that uses the Internet protocol and the publictelecommunication system to securely share part of abusinesss info or operations with suppliers, vendors, partners,

    customers, or other businesses. For users outside of the company.

    Requires firewall mgt., the use of digital certificates,encryption, and the use of VPNs.

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    An Internet standard that enables a local area networkto use one set of IP addresses for internal traffic and asecond set of addresses for external traffic.

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    Serves two main purposes:

    1. Provides a type of firewall by hiding internal IP addresses

    2. Enables a company to use more internal IP addresses.

    When communication between a privately addressedhost and a public network (the Internet) is needed,address translation is required. This is where NATcomes in.

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    NAT is like the receptionist in a large officeLets say you left instructions with the receptionist not to forward any

    calls to you unless you request it.

    Later on, you call a potential client and leave a message for thatclient to call you back.

    You tell the receptionist that you are expecting a call from this clientand to put the client through when he/she calls back.

    The client calls the main number to your office, which is the onlynumber the client knows.

    When the client tells the receptionist that he/she is looking for you,the receptionist checks the lookup table that matches your namewith your extension.

    The receptionist knows that you requested this call, and forwards youthe call (message).

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    NAT routers sit on the border between public andprivate networks.

    NAT works by creating bindings between addresses.

    Static NAT a one to one mapping between public andprivate addresses.

    Dynamic NAT maps an unregistered IP address to aregistered IP address from a group of registered IP


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    In static NAT, the computer with the IP address of192.168.32.10 will always translate to

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    Edge devices that run dynamic NAT create binding on

    the fly by building a NAT table.

    Connections initiated by private hosts are assigned a

    public address from a pool.

    As long as the private hosts has an outgoingconnection, it can be reached by incoming packetssent to this public address.

    When the connection expires, the binding expires, andthe address is returned to the pool for REUSE.

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    In dynamic NAT, the computer with the IP address192.168.32.10 will translate to the first availableaddress in the range from to213.18.123.150.

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    Port Address Translation (PAT)

    Used to allow many hosts to share a single IP address bymultiplexing streams differentiated by TCP/UDP port numbers

    Ex. Suppose private hosts and bothsend packets from source port 1108. A PAT router mighttranslate these to a single public IP address two different source ports, lets say 61001 and 61002.Response traffic received for port 61001 is routed back to, while port 61002 traffic is routed back to192.168.0.3:1108.

    Commonly implemented on Small Office / Home Officerouters (SOHO)

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    Technology that enables a network to securely send itsdata through an untrusted or shared networkinfrastructure.

    Works by encrypting and encapsulating the securedtraffic within packets carried by the second network.

    VPN is the best known example of tunneling

    Tunnel is actually an agreement between routers on

    how the data is encrypted.

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    Virtual local area networks

    A way of dividing a single physical network switch amongmultiple network segments or broadcast domains.

    Ability to configure multiple LANs on a single switch

    Trunk allows switches to share many VLANs over asingle physical link

    Routers needed to make different VLANs talk

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