Pm Firewalls

7/30/2019 Pm Firewalls

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Firewalling Techniques

Prabhaker Mateti


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ACK

Not linux specific

Some figures are from 3com


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Components of the Firewall System

Bastion Host

Packet-filtering router

Application-level gateway (or proxy server) Circuit-level gateway


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Dual Homed Gateway

A system that has

two or more network interfaces, each of which

is connected to a different network.

Acts to block or filter some or all of the

traffic trying to pass between the networks.


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Bastion Host

Runs general purpose

operating system

hardened to resist

attack


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Proxy services

Proxy servers on a bastion host can prohibit

direct connections from the outside and reducedata-driven attacks.


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Circuit Relay

Determines if the connection isvalid according to rules

opens a session and permitstraffic only from the allowed source and

possibly only for a limited period

of time. Whether a connection is valid is

based upon: destination IP address and/or port

source IP address and/or port

time of day

protocol

user password


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Demilitarized Zone (DMZ)

a neutral zone between the private LAN

and the public Internet.

FTP servers, Web servers and the like are

located in DMZ.


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Location of a Firewall

Untrusted

Network

Firewall DMZ

Internal LAN

External LANwww

SMTP


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An Application Gateway: Problem

Allow select internal users to telnet

outside.

Users authenticate themselves to

create telnet connection

A gateway used in this sense is

different from a standard gateway.


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An Application Gateway: Solution

Router filter blocks all telnet connections not originatingfrom gateway.

For authorized users, gateway sets up telnet connection todest host. Gateway relays data between 2 connections

host-to-gatewaytelnet session

gateway-to-remotehost telnet session

applicationgateway

router and filter


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Packet Filtering Router


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Packet Filtering Router

Decide not only how, but shoulda packet beforwarded

Not best when detail protocol knowledgerequired for decision Proxy may be a better choice

Lots of leverage as all hosts behind areprotected

Can provide unique capabilities Rejecting forged internal or external packets (address

spoofing)

Recognition of malformed packets


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Packet-Filtering Router

Service-Dependent Filtering

Some typical filtering rules include:

Permit incoming Telnet sessions only to a

specific list of internal hosts

Permit incoming FTP sessions only to

specific internal hosts

Permit all outbound Telnet sessions

Permit all outbound FTP sessions

Deny all incoming traffic from specific

external networks

Service-Independent Filtering

Source IP Address Spoofing Attacks. Source Routing Attacks. In Tiny

Fragment Attacks. Tiny fragment attacks are designed to circumvent

userdefined filtering rules; the hacker hopes that a filtering router

will examine only the first fragment and allows all other fragments to

pass. A tiny fragment attack can be defeated by discarding all packets

where the protocol type is TCP and the IP FragmentOffset is equal to

1.

Defining packet filters can be a complex task

Generally, the packet throughput of a router decreases as the number

of filters increases.


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Filtering by Service

Characteristics of internal to external telnetconnection Source is inside,

destination is outside,

is TCP, destination port 23, source port > 1023,

first packet an outbound SYN

Characteristics of ext to int opposite

Risk: trusting the port implies trusting the serveron that port Any service can be run from any port by root

Can telnet fromport 23, for example


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Security Policy

It is important to note that an Internet firewall is not just a router, abastion host, or a combination of devices that provides security for anetwork.

The firewall is part of an overall security policy that creates aperimeter defense designed to protect the information resources ofthe organization.

This security policy must include published security guidelines toinform users of their responsibilities; corporate policies definingnetwork access, service access, local and remote userauthentication, dial-in and dialout, disk and data encryption, andvirus protection measures; and employee training. All potentialpoints of network attack must be protected with the same level of

network security. Setting up an Internet firewall without acomprehensive security policy is like placing a steel door on a tent.

[From a web based article]


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Security Policy Describes a

Perimeter Defense


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A Connection Circumventing an

Internet Firewall


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Benefits of an Internet Firewall

Without a firewall, each host system on the privatenetwork is exposed to attacks from other hosts on theInternet.

Firewalls offer a convenient point where Internet securitycan be monitored and alarms generated.

An Internet firewall is a logical place to deploy a NetworkAddress Translator (NAT) that can help alleviate theaddress space shortage and eliminate the need torenumber when an organization changes Internet serviceproviders (ISPs).

An Internet firewall is the perfect point to audit or logInternet usage.

An Internet firewall can also offer a central point ofcontact for information delivery service to customers.


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Limitations of an Internet Firewall

Creates a single point of failure.

Cannot protect against attacks that do not go through thefirewall.

Cannot protect against the types of threats posed bytraitors or unwitting users.

Cannot protect against the transfer of virus-infectedsoftware or files.

Cannot protect against data-driven attacks. A data-driven attack occurs when seemingly harmless data ismailed or copied to an internal host and is executed tolaunch an attack.


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Limitations of firewalls and

gateways IP spoofing

router cant know if data really comes from claimedsource

If multiple apps. need special treatment, each

has own app. gateway. client software must know how to contact

gateway. e.g., must set IP address of proxy in Web browser

Tradeoff degree of communication with outside world, level of

security

Performance problem


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Three Myths of Firewalls

Firewalls make the assumption that the only way in or out of a corporatenetwork is through the firewalls; that there are no "back doors" to yournetwork. In practice, this is rarely the case, especially for a network whichspans a large enterprise. Users may setup their own backdoors, usingmodems, terminal servers, or use such programs as "PC Anywhere" so thatthey can work from home. The more inconvenient a firewall is to your usercommunity, the more likely someone will set up their own "back door"

channel to their machine, thus bypassing your firewall. Firewalls make the assumption that all of the bad guys are on the outside of

the firewall, and everyone on the inside of the can be consideredtrustworthy. This neglects the large number of computer crimes which arecommitted by insiders.

Newly evolving systems are blurring the lines between data andexecutables more and more. With macros, JavaScript, Java, and otherforms executable fragments which can be embedded inside data, a securitymodel which neglects this will leave you wide open to a wide range ofattacks.

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