Network Security

DMZ (De-Militarized Zone)

J. Wang. Computer Network Security Theory and Practice. Springer 2008

General Framework

What is a DMZ?

• A DMZ is a computer network that sits between a trusted internal network, such as a corporate private LAN, and an untrusted external network, such as the public Internet

• Also known as a – Data Management Zone or – Demarcation Zone – Perimeter Network

Typical components of DMZ network• Web servers that need to be made

available to the general public, such as company's primary Web presence advertising its products or services.

• Public DNS servers that resolve the names in your domain for users outside your organization to the appropriate IP addresses.

• Public FTP servers on which you provide files to the public– Downloads of your product

manuals or – Software drivers

• Anonymous SMTP relays that forward e-mail from the Internet to internal mail server(s)

• Servers running h complex e-commerce Internet and extranet applications

• Proxy Servers

Split Configurations• Mail services can be split between

servers on the DMZ and the internal network. – Internal mail server handles e-

mail from one computer to another on the internal network.

– Mail that comes in or is sent to computers outside the internal network over the Internet is handled by an SMTP gateway located in the DMZ.

• For e-commerce systems– Front-end server, directly

accessible by Internet users is in the DMZ,

– Back-end servers that store sensitive information are on the internal network.

DMZ with two firewalls• DMZ that uses two firewalls,

called a back to back DMZ. • An advantage of this configuration

is that you can put a fast packet filtering firewall/router at the front end (the Internet edge) to increase performance of your public servers,

• Place a slower application layer filtering (ALF) firewall at the back end (next to the corporate LAN) to provide more protection to the internal network without negatively impacting performance for your public servers

Tri-homed DMZ• When a single firewall is used to

create a DMZ, it's called a trihomed DMZ.

• The firewall computer or appliance has interfaces to three separate networks:– The internal interface to the

trusted network (the internal LAN)

– The external interface to the untrusted network (the public Internet)

– The interface to the semi-trusted network (the DMZ)

Creating a DMZ Infrastructure

• Two important characteristics of the DMZ are:

• A different network ID from the internal network – A DMZ can use either public or

private IP addresses, depending on its architecture • subnet the IP address

block that is assigned by your ISP

• If using private IP addresses for the DMZ, a Network Address Translation (NAT) device will be required

• It is separated from both the Internet and the internal network by a firewall

Security of DMZ

• The level of security within the DMZ also depends on the nature of the servers that are placed there. We can divide DMZs into two security categories:

• DMZs designed for unauthenticated or anonymous access

• DMZs designed for authenticated access

Host Security on the DMZ

• Be sure to set strong passwords and use RADIUS or other certificate based authentication for accessing the management console remotely.

• To allow you to manage the router through a Web page, it runs an HTTP server. It is a good security practice to disable the HTTP server, as it can serve as a point of attack.

• username richard privilege 15 secret bigXdogYlover

• Router(config)# username natalie privilege 15 secret BIGxDOGyLOVER

• Router(config)# ip http server • Router(config)# ip http authentication local

• Set up your VTY access for SSH (optional, but recommended): – Router(config)# username name secret

password – Router(config)# line vty 0 4– Router(config-line)# transport input ssh– Router(config-line)# transport output ssh – Router(config-line) login local

Different privilege levels to users Router(config)#privilege exec all level 5 show ip

Example Network

Specify Traffic exiting corporate network

• The corporate network zone houses private servers and internal clients. No other network should be able to access it.

• Configure an extended access list to specify which traffic can exit out the network

• GAD(config)#access-list 101 permit ip 10.10.10.0 0.0.0.255 any

• GAD(config)#access-list 101 deny ip any any

• GAD(config)#interface fa1• GAD(config-if)#ip access-group 101 in

– Can Host A ping the Web Server?– Can Host A ping Host B?– Can Host B ping the Web Server?– Can Host B ping Host A?

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Limit Traffic allowed into corporate network

• traffic can be allowed into the corporate network must be limited.

• Traffic entering the corporate network will be coming from either the Internet or the DMZ.

• Allow all traffic that originated from the corporate network can be allowed back into that network. Enter the following:– GAD(config)#access-list 102 permit tcp any any

established• Permit ICMP into the network. This will allow the

internal hosts to receive ICMP messages– GAD(config)#access-list 102 permit icmp any any

echo-reply– GAD(config)#access-list 102 permit icmp any any

unreachable• No other traffic is desired into the corporate

network– GAD(config)#access-list 102 deny ip any any

• Finally, apply the access-list to the corporate network Fast Ethernet port.– GAD(config)#interface ethernet1– GAD(config-if)#ip access-group 102 out

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Limit Traffic to Corporate Network• Traffic that can be allowed into the corporate

network must be limited.• Traffic entering the corporate network will be

coming from either the Internet or the DMZ.• Allow all traffic that originated from the corporate

network can be allowed back into that network. Enter the following:– GAD(config)#access-list 102 permit tcp any

any established• Permit ICMP into the network. This will allow the

internal hosts to receive ICMP messages– GAD(config)#access-list 102 permit icmp any

any echo-reply– GAD(config)#access-list 102 permit icmp any

any unreachable• No other traffic is desired into the corporate

network– GAD(config)#access-list 102 deny ip any any

• Finally, apply the access-list to the corporate network Fast Ethernet port.– GAD(config)#interface ethernet1– GAD(config-if)#ip access-group 102 out

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Can Host A ping the Web Server?Can Host A ping Host B?Can Host B ping the Web Server?Can Host B ping Host A

Protect the DMZ Network• The DMZ network will house only one external server that will

provide World Wide Web services• Configure an extended access list to protect the DMZ network

– GAD(config)#access-list 111 permit ip 10.1.1.0 0.0.0.255 any

– GAD(config)#access-list 111 deny ip any any– GAD(config)#interface ethernetfa0– GAD(config-if)#ip access-group 111 in

• Specify which traffic can enter the DMZ network. Traffic entering the DMZ network will be coming from either the Internet or the corporate network requesting World Wide Web services.

• Configure an outbound extended access-list specifying that World Wide Web requests be allowed into the network. – GAD(config)#access-list 112 permit tcp any host 10.1.1.10

eq www• What command would be entered to allow

– DNS, Email and FTP requests into the DMZ?• For management purposes, it would be useful to let corporate

users ping the Web Server but not for Internet users. – GAD(config)#access-list 112 permit icmp 10.10.10.0

0.0.0.255 host 10.1.1.10– GAD(config)#access-list 112 deny ip any any

• GAD(config)#interface fa ethernet 0• GAD(config-if)#ip access-group 112 out

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Deter Spoofing

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• Spoofing - A common method to attempt to forge a valid internal source IP addresses.

• To deter spoofing, it is decided to configure an access list so that Internet hosts cannot easily spoof an internal network addresses.

• Three common source IP addresses that hackers attempt to forge are valid internal addresses (e.g., 10.10.10.0), loopback addresses (i.e.,127.x.x.x), and multicast addresses (i.e., 224.x.x.x – 239.x.x.x).

• GAD(config)#access-list 121 deny ip 10.10.10.0 0.0.0.255 any

• GAD(config)#access-list 121 deny ip 127.0.0.0 0.255.255.255 any

• GAD(config)#access-list 121 deny ip 224.0.0.0 31.255.255.255 any

• GAD(config)#access-list 121 permit ip any any• GAD(config)#interface serial 0• GAD(config-if)#ip access-group 121 in