Security information for DNS

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Security information for DNS

Domain Name System (DNS) was originally designed as an open protocol and is therefore vulnerable to attackers. Windows Server 2003 DNS has improved the ability to prevent an attack on your DNS infrastructure through the addition of security features. Before considering which of the security features to use, you should be aware of the common threats to DNS security and the level of DNS security in your organization.

DNS security threats

The following are the typical ways in which your DNS infrastructure can be threatened by attackers:

• Footprinting is the process by which DNS zone data is obtained by an attacker to provide the attacker with the DNS domain names, computer names, and IP addresses for sensitive network resources. An attacker commonly begins an attack by using this DNS data to diagram, or footprint, a network. DNS domain and computer names usually indicate the function or location of a domain or computer in order to help users remember and identify domains and computers more easily. An attacker takes advantage of the same DNS principle to learn the function or location of domains and computers in the network.

• Denial-of-service attack is when an attacker attempts to deny the availability of network services by flooding one or more DNS servers in the network with recursive queries. As a DNS server is flooded with queries, its CPU usage will eventually reach its maximum and the DNS Server service will become unavailable. Without a fully operating DNS server on the network, network services that use DNS will become unavailable to network users.

• Data modification is an attempt by an attacker (that has footprinted a network using DNS) to use valid IP addresses in IP packets the attacker has created, thereby giving these packets the appearance of coming from a valid IP address in the network. This is commonly called IP spoofing. With a valid IP address (an IP address within the IP address range of a subnet), the attacker can gain access to the network and destroy data or conduct other attacks.

• Redirection is when an attacker is able to redirect queries for DNS names to servers under the control of the attacker. One method of redirection involves the attempt to pollute the DNS cache of a DNS server with erroneous DNS data that may direct future queries to servers under the control of the attacker. For example, if a query were originally made for example.microsoft.com and a referral answer provided a record for a name outside of the microsoft.com domain, such as malicious-user.com, then the DNS server would use the cached data for malicious-user.com to resolve a query for that name. Redirection can be accomplished whenever an attacker has writable access to DNS data, such as with insecure dynamic updates.

Mitigating DNS security threats

DNS can be configured to mitigate the common DNS security issues discussed above. The following table lists five main areas on which to concentrate when determining your DNS security.

Three levels of DNS security

The following three levels of DNS security will help you understand your current DNS configuration and enable you to increase the DNS security of your organization.

Low-level security

Low-level security is a standard DNS deployment without any security precautions configured. Only deploy this level of DNS security in network environments where there is no concern for the integrity of your DNS data or in a private network where there is no threat of external connectivity.

• The DNS infrastructure of your organization is fully exposed to the Internet.

• Standard DNS resolution is performed by all DNS servers in your network.

• All DNS servers are configured with root hints pointing to the root servers for the Internet.

• All DNS servers permit zone transfers to any server.

• All DNS servers are configured to listen on all of their IP addresses.

• Cache pollution prevention is disabled on all DNS servers.

• Dynamic update is allowed for all DNS zones.

• User Datagram Protocol (UDP) and Transmission Control Protocol/Internet Protocol (TCP/IP) port 53 is open on the firewall for your network for both source and destination addresses.

Medium-level security

Medium-level security uses the DNS security features available without running DNS servers on domain controllers and storing DNS zones in Active Directory.

• The DNS infrastructure of your organization has limited exposure to the Internet.

• All DNS servers are configured to use forwarders to point to a specific list of internal DNS servers when they cannot resolve names locally.

• All DNS servers limit zone transfers to servers listed in the name server (NS) resource records in their zones.

• DNS servers are configured to listen on specified IP addresses.

• Cache pollution prevention is enabled on all DNS servers.

• Nonsecure dynamic update is not allowed for any DNS zones.

• Internal DNS servers communicate with external DNS servers through the firewall with a limited list of source and destination addresses allowed.

• External DNS servers in front of your firewall are configured with root hints pointing to the root servers for the Internet.

• All Internet name resolution is performed using proxy servers and gateways.

High-level security

High-level security uses the same configuration as medium-level security and also uses the security features available when the DNS Server service is running on a domain controller and DNS zones are stored in Active Directory. In addition, high-level security completely eliminates DNS communication with the Internet. This is not a typical configuration, but it is recommended whenever Internet connectivity is not required.

• The DNS infrastructure of your organization has no Internet communication by internal DNS servers.

• Your network uses an internal DNS root and namespace, where all authority for DNS zones is internal.

• DNS servers that are configured with forwarders use internal DNS server IP addresses only.

• All DNS servers limit zone transfers to specified IP addresses.

• DNS servers are configured to listen on specified IP addresses.

• Cache pollution prevention is enabled on all DNS servers.

• Internal DNS servers are configured with root hints pointing to the internal DNS servers hosting the root zone for your internal namespace.

• All DNS servers are running on domain controllers. A discretionary access control list (DACL) is configured on the DNS Server service to only allow specific individuals to perform administrative tasks on the DNS server.

• All DNS zones are stored in Active Directory. A DACL is configured to only allow specific individuals to create, delete, or modify DNS zones.

• DACLs are configured on DNS resource records to only allow specific individuals to create, delete, or modify DNS data.

• Secure dynamic update is configured for DNS zones, except the top-level and root zones, which do not allow dynamic updates at all.