Securing DNS
Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2
Domain Name System (DNS) was originally designed as an open protocol. Therefore, it is vulnerable to attackers. Security features in Windows Server 2003 DNS can help you prevent an attack on your DNS infrastructure. Before considering which of the Windows Server 2003 security features to use, you should be aware of the following:
Common threats to DNS security
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. The process by which DNS zone data, including DNS domain names, computer names, and Internet Protocol (IP) addresses for sensitive network resources, is obtained by an attacker. An attacker commonly begins an attack by using this DNS data to diagram, or "footprint," a network. DNS domain names and computer names usually indicate the function or location of a domain or computer to help users remember and identify domains and computers more easily. An attacker takes advantage of this same DNS naming principle to learn the function or location of domains and computers in the network.
Denial-of-service attack. A scenario in which 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 eventually reaches its maximum, and the DNS Server service becomes unavailable. Without a fully operating DNS server on the network, network services that use DNS are unavailable to network users.
Data modification. An attempt by an attacker that has footprinted a network by using DNS to use valid IP addresses in IP packets that the attacker has created. This gives these packets the appearance of coming from a valid IP address in the network. This process is commonly called IP "spoofing." With a valid IP address — that is, 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. A scenario in which an attacker is able to redirect queries for DNS names to servers that are under the control of the attacker. One method of redirection involves an attempt to pollute the DNS cache of a DNS server with erroneous DNS data that may direct future queries to servers that are under the control of the attacker. For example, if a query is made originally for sales.wingtiptoys.com and a referral answer provides a record for a domain name that the attacker has outside the wingtiptoys.com domain, the DNS server uses the cached data for the attacker's domain to resolve a query for that name. Redirection can occur whenever an attacker has writable access to DNS data, for example, in a scenario that includes dynamic updates that are not secure.
Mitigating DNS Security Threats
The following sections explain three levels of DNS security that you can apply to your current DNS configuration. You can use these three levels of security to increase the DNS security of your organization.
Low-Level Security
Low-level security is a standard DNS deployment without any security precautions configured. You should deploy this level of DNS security only 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 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 that are 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 that are 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.
Dynamic update that is not secure is not allowed for any DNS zones.
Internal DNS servers communicate with external DNS servers through a firewall with a limited list of allowed source addresses and destination addresses.
External DNS servers in front of the firewall are configured with root hints that point to the root servers for the Internet.
All Internet name resolution is performed by using proxy servers and gateways.
High-Level Security
High-level security uses the same configuration as medium-level security. It also uses the security features that are 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 means of 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 that point to the internal DNS servers that host 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 allow only specific individuals to perform administrative tasks on the DNS server.
All DNS zones are stored in Active Directory. A DACL is configured to allow only specific individuals to create, delete, or modify DNS zones.
DACLs are configured on DNS resource records to allow only specific individuals to create, delete, or modify DNS data.
Secure dynamic update is configured for DNS zones except the top-level zones and root zones, which do not allow dynamic updates at all.
Securing DNS
The following tasks for securing DNS are described in this objective: