Firewalls, etc.

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Outline

• Intro• Various firewall technologies:

– Static Packet Filtering (or nonstateful packet filter)

– Dynamic Packet Filtering (or stateful packet filter)

– Circuit-level gateways– Application-level gateways (aka. Proxy servers)

• Firewall configurations

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Firewalls• A firewall, in the real world, is built between buildings to

prevent a fire started in one building from spreading to another

• A digital firewall serves similar purpose, by preventing security breaches that occur in one zone from spreading to another zone

• In a way, firewalls can be considered as delimiters that together define the perimeter of a network

• A firewall prevents unwanted and/or unauthorized traffic from entering into or getting out of a given network (the ‘protected’ network)

• Also called ‘secure Internet gateways’ or ‘security gateways’

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RFC2828 Internet Security Glossary (by R. Shirey, May 2000)

$ filtering router • An internetwork router that selectively prevents the

passage of data packets according to a security policy.

• A filtering router may be used as a firewall or part of a firewall. • A router usually receives a packet from a network and decides

where to forward it on a second network. A filtering router does the same, but first decides whether the packet should be forwarded at all, according to some security policy.

• The policy is implemented by rules (packet filters) loaded into the router. The rules mostly involve values of data packet control fields (especially IP source and destination addresses and TCP port

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RFC2828 Internet Security Glossary (by R. Shirey, May 2000)

$ bastion host• A strongly protected computer that is in a network

protected by a firewall (or is part of a firewall) and is the only host (or one of only a few hosts) in the network that can be directly accessed from networks on the other side of the firewall.

• Filtering routers in a firewall typically restrict traffic from the outside network to reaching just one host, the bastion host, which usually is part of the firewall.

• Since only this one host can be directly attacked, only this one host needs to be very strongly protected, so security can be maintained more easily and less expensively.

• However, to allow legitimate internal and external users to access application resources through the firewall, higher layer protocols and services need to be relayed and forwarded by the bastion host. Some services (e.g., DNS and SMTP) have forwarding built in; other services (e.g., TELNET and FTP) require a proxy server on the bastion host.

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Firewalls• According to RFC2828 Internet Security Glossary (by R. Shirey, May

2000):

$ firewall - An internetwork gateway that restricts data

communication traffic to and from one of the connected networks (the one said to be "inside" the firewall) and thus protects that network's system resources against threats from the other network (the one that is said to be "outside" the firewall).

- A firewall typically protects a smaller, secure network (such as a corporate LAN, or even just one host) from a larger network (such as the Internet). The firewall is installed at the point where the networks connect, and the firewall applies security policy rules to control traffic that flows in and out of the protected network.

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Firewalls- A firewall is not always a single computer. For example, a

firewall may consist of a pair of filtering routers and one or more proxy servers running on one or more bastion hosts, all connected to a small, dedicated LAN between the two routers. The external router blocks attacks that use IP to break security (IP address spoofing, source routing, packet fragments), while proxy servers block attacks that would exploit a vulnerability in a higher layer protocol or service. The internal router blocks traffic from leaving the protected network except through the proxy servers.

- The difficult part is defining criteria by which packets are denied passage through the firewall, because a firewall not only needs to keep intruders out, but usually also needs to let authorized users in and out.

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Firewalls- a more precise definition

• According to Cheswick & Bellovin (1994)• A firewall system is a collection of components

placed btwn two networks that collectively have the following 3 properties:

1. All traffic (inside out, or outside in) must pass through the firewall.

2. Only authorized traffic (as defined by the local security policy) are allowed to pass. firewall policy

3. The firewall itself is immune to penetration.

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Firewalls- additional, stronger features

• A firewall is able to:1. enforce strong authentication for users who wish to

establish inbound or outbound connections

2. associate data streams that are allowed to pass through the firewall with previously authenticated and authorized users

• Use of application gateways is needed to support these higher-level features.

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Compare Various Firewall Technologies

Types Packet filters Application gateways

Sub-types StaticDynamic

Circuit-levelApplication-level

OSI layers

Network layer (or Internet layer in TCP/IP)

Transport layer or higher

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Static Packet Filtering• Aka ‘screening routers’• Stateless, meaning that each IP packet must be

examined in isolation from what has happened in the past (and what may happen in the future), forcing the filter to make a decision to permit or deny each packet individually based on the packet-filtering rules no concept of session lead to problems when more than one connection is created in a

protocol such as FTP

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Dynamic Packet Filtering• Aka stateful packet filter• A dynamic packet filter maintains state information about

past IP packets to make more intelligent decisions about the legitimacy of present and future IP packets

• State information are stored in an internal database• Subsequent packets belonging to the same association

can pass quickly through the stateful inspection device

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Circuit-level Gateways/Firewalls

• A proxy server for TCP or UDP (at the transport layer)• Goal: To allow a TCP/IP application to traverse (i.e., securely use) a

firewall• Is Located and running on a firewall• Relays TCP connections:

- They intercept TCP connection being made to a host behind them and complete the handshake on behalf of that host.- As soon as the connection is made, only data packets belonging to the connection are allowed to go through.

• It does not interfere with the data stream. Making it different from an application-level gateway

• Example: SOCKS (RFC1928SOCKS Protocol Version 5. By M. Leech, M. Ganis, Y. Lee, R. Kuris, D. Koblas, L. Jones. March 1996)

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SOCKS• The implementation of the SOCKS protocol typically involves

the recompilation or relinking of TCP-based client applications to use the appropriate encapsulation routines in the SOCKS library. ‘socksified’ clients

• Procedure for TCP-based clients – When a TCP-based client wishes to establish a connection to an object

that is reachable only via a firewall, it must open a TCP connection to the appropriate SOCKS port on the SOCKS server system. The SOCKS service is conventionally located on TCP port 1080.

– If the connection request succeeds, the client enters a negotiation for the authentication method to be used, authenticates with the chosen method, then sends a relay request.

– The SOCKS server evaluates the request, and either establishes the appropriate connection or denies it.

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Application-level Gateways

• A proxy server that allows a specific application protocol to traverse a firewall.

• A sample scenario: The packet filter of a firewall blocks all inbound Telent and FTP sessions, unless the sessions are terminated by a bastion host.– Multiple application gateways may be running on the bastion

host a proxy server for FTP, a proxy server for Telent, …– A user who wishes to connect inbound to an intranet server must

have his Telnet or FTP client connect to the application gateway.

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Application-level Gateways

• To properly authenticate the user, an application gateway must have access to authentication and authorization information, either locally or remotely:– User-level authentication info may be stored locally on

the firewall– User-level authentication info may be stored in a

centralized authentication server (e.g., RADIUS, TACACS+)

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Trade-offs of Firewalls

• Advantages:1. Provides basic access control services for an intranet

2. Provides a centralized filtering/gateway function

3. (To some degree) Relieves individual hosts the responsibility of having a filter or firewall itself

4. Centralized management of filtering rules

• Limitations: next

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Trade-offs of Firewalls

• Limitations:1. Cannot protect sites and corporate intranets against insider attacks

internal / intranet firewalls2. Can be circumvented by tunneling unauthorized application

protocols in authorized ones3. Little protection against attacks embedded in the data field of a

packet (e.g., virus-infected programs or data files, malicious Java applets, malicious ActiveX controls, …)

4. May foster a false sense of security lax security within the firewall perimeter

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Positioning of Firewalls• Positioning a firewall is as important as using the right type of

firewall and configuring it correctly.• Goals:

– The positioning of the firewall should effectively screen the types of traffic.

– The positioning of the firewall should ensure there are no back doors into the protected network. (The firewall cannot be bypassed.)

• Some guidelines:– Position the firewall to clearly delimit the public network and the private

network.– Use an additional firewall to protect a critical segment.– Build a DMZ to hold servers that need to be accessed from the public

network. Do not put them inside the private network!– Most firewalls are stateful, so avoid ‘asymmetric routing’!– Use layering (stacked) firewalls.

Why? 2 reasons (a. layered protection; b. allowing different filtering rules)

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Dual-Homed Firewalls• A dual-homed host is a host with two network interfaces,

each of which is connected to a different network.• A dual-homed firewall is a dual-homed host on which IP

routing and forwarding are disabled.– IP packets can no longer be routed or forwarded between the

two networks.– Data can only be transferred from one network to the other if

there is an application gateway running on the firewall to do that.– See diagram next.

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outer screening routerinner screening

router

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Dual-Homed Firewalls• The outer screening router makes sure that

– All incoming packets have the bastion host as the destination address.

– All outgoing packets have the bastion host as the source address.

• The inner screening router makes sure that– All incoming packets have the bastion host as the

source address.– All outgoing packets have the bastion host as the

destination address.• Packet filtering rules need to be configured.

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Screened Subnet Firewalls• Two separate screening routers: outer, inner

(see diagram next)• DMZ: the subnet in between the two screening

routers• The bastion host is part of the DMZ.• A packet must pass both screening routers

before it reach the intranet.• When configured properly (e.g., using NAT),

only the bastion host and other servers in the DMZ can be seen from the Internet.

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outer screening router

inner screening router

the DMZ

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Layering Firewalls

• Examples:The DCSL Network