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.