Firewall Configuration Strategies Chapter 3

Learning Objectives Set up firewall rules that reflect an organization’s overall security approach Understand the goals that underlie a firewall’s configuration Identify and implement different firewall configuration strategies Employ methods of adding functionality to your firewall

Establishing Rules and Restrictions for Your Firewall Rules give firewalls specific criteria for making decisions about whether to allow packets through or drop them All firewalls have a rules file—the most important configuration file on the firewall

The Role of the Rules File Establishes the order the firewall should follow Tells the firewall which packets should be blocked and which should be allowed Requirements Need for scalability Importance of enabling productivity of end users while maintaining adequate security

Restrictive Firewalls Block all access by default; permit only specific types of traffic to pass through

Strategies for Implementing a Security Policy Follow the concept of least privilege Spell out services that employees cannot use Use and maintain passwords Choose an approach Open Optimistic Cautious Strict Paranoid

Connectivity-Based Firewalls Have fewer rules; primary orientation is to let all traffic pass through, then block specific types of traffic

Overview to Firewall Configuration Strategies Criteria Scalable Take communication needs of individual employees into account Deal with IP address needs of the organization

Scalability Provide for the firewall’s growth by recommending a periodic review and upgrading software and hardware as needed

Productivity The stronger and more elaborate the firewall, the slower the data transmissions Important features of firewall: processing and memory resources available to the bastion host

Productivity

Dealing with IP Address Issues If service network needs to be privately rather than publicly accessible, which DNS will its component systems use? If you mix public and private addresses, how will Web server and DNS servers communicate? Let the proxy server do the IP forwarding (it’s the security device)

Firewall Configuration Strategies

Firewall Configuration Strategies Settle on general approaches; establish rules for them Deploy firewalls, routers, VPN tunnels, and other tools in a way that will implement rules Use security components to defend against common attacks

Using Security Components to Defend Against Attacks

Screening Router Filters traffic passing between one network and another Simple, minimally secure Two interfaces—external and internal—each with its own unique IP address Performs IP forwarding, based on an access control list (ACL)

Screening Router

Stateful Packet Filtering

Dual-Homed Host A workstation with an internal interface and an external interface to the Internet Disadvantage Host serves as a single point of entry to the organization

Screened Host Similar to dual-homed host, but the host is dedicated to performing security functions Sits exposed on the perimeter of the network rather than behind the firewall Requires two network connections Also called a dual-homed gateway or bastion host

Screened Host

Two Routers, One Firewall Router positioned on the outside Performs initial, static packet filtering Router positioned just inside the network Routes traffic to appropriate computers in the LAN being protected Can do stateful packet filtering

Two Routers, One Firewall

DMZ Screened Subnet Screened subnet Three-pronged firewall Network exposed to external network, but partially protected by a firewall Three-pronged firewall Three network interfaces connect it to: External network DMZ Protected LAN Service network Screened subnet that contains an organization’s publicly accessible server

DMZ Screened Subnet

Three-Pronged Firewall with Only One Firewall Advantages Simplification Lower cost Disadvantages Complexity Vulnerability Performance

Common Service Network Systems Those that contain Web and mail servers Those that contain DNS servers Those that contain tunneling servers

Multiple-Firewall DMZs Achieve the most effective Defense in Depth Help achieve load distribution Added security offsets slowdown in performance Two or more firewalls can be used to protect Internal network One DMZ Two DMZs Branch offices that need to connect to main office’s internal network

Two Firewalls, One DMZ Two firewalls used to set up three separate networks (tri-homed firewall) Internal protected network (behind DMZ) External private network or service network (within DMZ) External network (outside DMZ) Advantage Enables control of traffic in the three networks

Two Firewalls, One DMZ

Two Firewalls, Two DMZs Setting up separate DMZs for different parts of the organization helps balance the traffic load between them

Two Firewalls, Two DMZs

Multiple Firewalls to Protect Branch Offices

Load Distribution Through Layering of Firewalls

Reverse Firewalls Inspect and monitor traffic going out of a network rather than trying to block what’s coming in Help block Distributed Denial of Service (DDoS) attacks

Specialty Firewalls Protect specific types of network communications (eg, e-mail, instant-messaging) Examples Mail Marshal and WebMarshal by Marshal Software OpenReach includes a small-scale packet-filtering firewall for its VPN VOISS Proxy Firewall (VF-1) by VocalData Speedware Corporation sells its own firewall software

Approaches That Add Functionality to a Firewall Network Address Translation (NAT) Encryption Application proxies VPNs Intrusion detection systems (IDSs)

NAT Converts publicly accessible IP addresses to private ones and vice versa; shields IP addresses of computers on the protected network from those on the outside

NAT

Encryption Takes a request, turns it into gibberish using a private key; exchanges the public key with the recipient firewall or router Recipient decrypts the message and presents it to the end user in understandable form

Encryption

Application Proxies Act on behalf of a host; receive requests, rebuild them from scratch, and forward them to the intended location as though the request originated with it (the proxy) Can be set up with either a dual-homed host or a screened host system

Application Proxies Dual-homed setup Screened subnet system Host that contains the firewall or proxy server software has two interfaces, one to the Internet and one to the internal network being protected Screened subnet system Host that holds proxy server software has a single network interface Packet filters on either side of the host filter out all traffic except that destined for proxy server software

Application Proxies on a Dual-Homed Host

VPNs Connect internal hosts with specific clients in other organizations Connections are encrypted and limited only to machines with specific IP addresses VPN gateway can: Go on a DMZ Bypass the firewall and connect directly to the internal LAN

VPN Gateway Bypassing the Firewall

Intrusion Detection Systems Can be installed in external and/or internal routers at the perimeter of the network Built into many popular firewall packages

IDS Integrated into Perimeter Routers

IDS Positioned Between Firewall and Internet

Chapter Summary How to design perimeter security for a network that integrates firewalls with a variety of other software and hardware components Rules and restrictions that influence configuration of a security perimeter Security configurations that either perform firewall functions or that use firewalls to create protected areas