Lesson 18-Internet Architecture

Overview

Internet services.

Develop a communications architecture.

Design a demilitarized zone.

Understand network address translation.

Design partner networks.

Internet Services

Services to offer.

Services not to offer.

Services to Offer

The organization may provide the following services

through Internet:

Mail.

Encrypted e-mail.

Web.

Internal access to Internet.

External access to internal systems.

Control services.

Mail

Mail service is generally offered to internal employees to send

and receive messages.

It requires that at least one server be established to receive

inbound mail.

Outbound mail can move through the same server or directly

through desktop systems.

Organization may choose to establish relays for public mail to

be sent to discussion groups.

Encrypted E-mail

It is better to encrypt the contents of the e-mail to protect

any sensitive information.

Systems like desktop software and network appliances

placed in mail stream provide encrypted e-mail.

Web

To publish information via Web, the organization needs to

establish a Web server.

Web servers can provide static content or dynamic content.

HTTPS is used for Web pages that contain sensitive

information or require authentication.

File Transfer Protocol (FTP) server allows external

individuals to get or send files.

Internal Access to Internet

Most common services that employees are allowed to

access are:

HTTP (port 80) and HTTPS (port 443)

FTP (ports 21 and 20)

Telnet (port 23) and SSH (port 22)

POP-3 (port 110) and IMAP (port 143)

NNTP (port 119)

External Access to Internal Systems

External access to sensitive internal systems is a delicate

matter.

The two forms of external access are employee access or

non-employee access.

External access may be accomplished through VPNs, dial-

up lines, leased lines, or unencrypted access over the

Internet.

Control Services

These services are required for smooth function of network

and Internet connection.

DNS - Domain Name Service is used to resolve system

names into IP addresses.

Control Services

ICMP - Internet Control Message Protocol provides services

such as ping and messages that help the network function

efficiently.

NTP - Network Time Protocol is used to synchronize time

between various systems.

Services Not to Offer

Internet architecture should be designed to accommodate

the services that are required by the organization.

Services not to be offered due to security risk are NetBIOS,

Unix RPC, NFS, “r” services, TFTP, Remote Control

Protocols, and SNMP.

Develop a Communications Architecture

Primary issues for establishing an organization’s Internet

connection are throughput requirements and availability.

Availability requirements of the connection should be set by

the organization.

Develop a Communications Architecture

Single-line access

Multiple-line access to a single ISP

Multiple-line access to multiple ISPs

Single-Line Access

Standard single-line access architecture

Single-Line Access

The following potential failures make single-line access

suitable for non-business-critical Internet connections:

Router failure.

CSU failure.

Cut local loop.

Damage to the telephone company’s CO (central office).

POP failure at the ISP.

Multiple-Line Access to a Single ISP

They are used to overcome the single point of failure issues

with the single ISP architecture.

Shadow link or redundant circuit services offered by

different ISPs provide a second communication link in case

of failure.

Multiple-line access to a single ISP has Single-POP access or

Multiple-POP access.

Multiple-Line Access to a Single ISP

Single-POP access:

An ISP can provide fail-over access by setting up a redundant

circuit to the same POP.

It addresses failures in router, CSU, phone company circuit to

CO, and ISP equipment.

Benefit to this architecture is the low cost of the redundant

circuit.

Multiple-Line Access to a Single ISP

Multiple-POP access:

Running second connection to a second POP additional

availability and reliability can be obtained.

Border Gateway Protocol (BGP) protocol, run by ISP, specifies

routes between entities with such dual connections.

Single point failures of local loop and CO can be overcome if

the organization’s facility has two local loop connections.

Multiple-line Access to Multiple ISPs

If architected correctly, use of multiple ISPs can reduce the

risk of loss of service dramatically.

Issues that occur in choosing ISPs are complexity of using

different ISPs, thorough knowledge in ISPs, and physical

routing of connections.

Working with multiple ISPs also involve routing and IP

address space issues that must be resolved.

Design a Demilitarized Zone

Defining the DMZ.

Systems to place in DMZ.

Appropriate DMZ architectures.

Defining the DMZ

A DMZ is created by providing a semi-protected network zone.

The DMZ is delineated with network access controls, such as

firewalls or heavily filtered routers.

Any system that can be directly contacted by an external user

should be placed in a DMZ since they can be attacked.

External system’s access to sensitive systems must be

avoided.

Systems to Place in DMZ

Layout of systems between the DMZ and the internal network

Systems to Place in DMZ

DMZ can have either both internal and external mail servers

or a single firewall mail server.

Using Web server for receiving user’s input and application

server for processing it provides protection to the database

server.

All externally accessible systems should be placed in the DMZ.

The organization’s ISP can provide alternate DNS services.

Appropriate DMZ Architectures

The three common architectures are router and firewall,

single firewall, and dual firewall.

These architectures have their own advantages and

disadvantages; hence organizations must choose the

appropriate one.

Appropriate DMZ Architectures

Router and firewall architecture:

Router and firewall architecture involves risk to systems on the

Internet. The risk can be reduced using filters on the router.

Risk to systems can also be reduced by locking them so that

only services offered by DMZ run on them.

Appropriate DMZ Architectures

Single firewall architecture:

A single firewall can be used to create a DMZ using a third

interface.

The single firewall becomes a single point of failure and a

potential bottleneck for traffic, unless in fail-over configuration.

Single firewall architecture is simple compared to the router

and firewall architecture.

Appropriate DMZ Architectures

Dual firewall architecture:

Dual firewall architecture uses two firewalls to separate DMZ

from external and internal networks.

Dual firewalls increase cost of architecture and require

additional management and configuration.

Understand Network Address Translation

Any organization that plans to install a firewall will have to

deal with addressing issues.

In most networks, the firewall performs the NAT function of

translating one or more addresses into other addresses.

NAT can also provide a security function as hidden

addresses of internal systems are not visible to the

Internet.

Understand Network Address Translation

Private class addresses are used on internal networks behind a

firewall that performs NAT.

These addresses provide an organization with flexibility in

designing its internal addressing scheme.

Static NAT is a one- to-one configuration that allows accessing

internal network addresses from the Internet.

Static NAT maps a single real address from the organization’s

external network to a system on the DMZ.

Understand Network Address Translation

Dynamic NAT maps many internal addresses to a single

real address.

Dynamic NAT creates a practical limit of about 64,000

simultaneous connections.

Dynamic NAT is useful for desktop clients who use the

Dynamic Host Configuration Protocol (DHCP).

Design Partner Networks

Partner networks are generally established to exchange certain files

or pieces of data between organizations.

Architectures and methodologies of Internet connection can be used

for partner networks as their requirements do not differ much.

Rules must be added to firewall to allow systems at the partner

organization and internal systems to access partner DMZ systems.

NAT should be used when connecting to partner networks.

Summary

Organizations can offer services like mail, encrypted e-mail, Web,

internal access to Internet, external access to internal systems, and

control services.

Control services include DNS, ICMP, and NTP.

To reduce security risks, services that are not required should not

be offered.

Types of Internet architectures are single-line access, multiple-line

access to a single ISP, and multiple-line access to multiple ISPs.

Summary

Establishing a not truly trusted, semi-secure zone outside of the

trusted network creates a DMZ.

Router and firewall, single firewall, and dual firewall are the

three DMZ architectures.

Firewall performs the NAT function of translating one or more

addresses into other addresses.

Partner networks are generally established to exchange data

between organizations.