Network Security

DMZ (De-Militarized Zone)

General Framework

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

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. • • • • 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 • 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.

•   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

172.16.2.0/24

Specify Traffic exiting corporate network

• 10.1.1.1/24 • • • • • 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?

Limit Traffic allowed into corporate

172.16.2.0/24

network

• 10.1.1.1/24 • • • • • 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

Limit Traffic to Corporate Network

172.16.2.0/24 10.1.1.1/24 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 • • • • • • 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

172.16.2.0/24

Protect the DMZ Network

10.1.1.1/24 • • • • • • • • 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

172.16.2.0/24

Deter Spoofing

10.1.1.1/24 • • • • • • • • • 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