Microsoft Windows Server 2022 DNS Security Technical Implementation Guide

Limiting the number of concurrent sessions reduces the risk of denial of service (DoS) on any system. A DNS server's function requires it to be able to handle multiple sessions at a time, so limiting concurrent sessions could impact availability. Primary name servers must be configured to limit the actual hosts from which they will accept dynamic updates and zone transfer requests, and all name servers should be configured to limit the hosts from/to which they receive/send zone transfers. Restricting sessions to known hosts will mitigate the DoS vulnerability.

Without a means for identifying the individual that produced the information, the information cannot be relied on. Identifying the validity of information may be delayed or deterred. This requirement ensures organizational personnel have a means to identify who produced or changed specific information in transfers, zone information, or DNS configuration changes.

Failing to act on validation errors may result in the use of invalid, corrupted, or compromised information. The validation of bindings can be achieved, for example, using cryptographic checksums. Validations must be performed automatically. At a minimum, the application must log the validation error. However, more stringent actions can be taken based on the security posture and value of the information. The organization should consider the system's environment and impact of the errors when defining the actions. Additional examples of actions include automated notification to administrators, halting system process, or halting the specific operation. The DNS server should audit all failed attempts at server authentication through DNSSEC and TSIG/SIG(0). The actual auditing is performed by the operating system/network device manager, but the configuration to trigger the auditing is controlled by the DNS server.

Without the capability to generate audit records, it would be difficult to establish, correlate, and investigate the events relating to an incident or identify those responsible for one. The actual auditing is performed by the operating system/network device manager, but the configuration to trigger the auditing is controlled by the DNS server.

Without the capability to generate audit records, it would be difficult to establish, correlate, and investigate the events relating to an incident or identify those responsible for one. The actual auditing is performed by the operating system/network device manager, but the configuration to trigger the auditing is controlled by the DNS server. Because the configuration of the audit logs on the DNS server dictates which events are logged to correlate events, the permissions for configuring the audit logs must be restricted to only those with the role of information system security manager (ISSM) or those appointed by the ISSM.

The best way for a zone administrator to minimize the impact of a key compromise is by limiting the validity period of RRSIGs in the zone and the parent zone. This strategy limits the time during which an attacker can take advantage of a compromised key to forge responses. An attacker that has compromised a zone signing key (ZSK) can use that key only during the key signing key's (KSK's) signature validity interval. An attacker that has compromised a KSK can use that key for only as long as the signature interval of the RRSIG covering the DS RR in the delegating parent. These validity periods should be short, which will require frequent re-signing. To prevent the impact of a compromised KSK, a delegating parent should set the signature validity period for RRSIGs covering DS RRs in the range of a few days to one week. This re-signing does not require frequent rollover of the parent's ZSK, but scheduled ZSK rollover should still be performed at regular intervals.

In addition to network-based separation, authoritative name servers should be dispersed geographically. In other words, in addition to being located on different network segments, the authoritative name servers should not all be located in the same building. One approach is to locate some authoritative name servers in their own premises and others in their internet service provider's data centers or in partnering organizations. A network administrator may choose to use a "hidden" primary authoritative server and have only secondary servers visible on the network. A hidden primary authoritative server is an authoritative DNS server whose IP address does not appear in the name server set for a zone. If the primary authoritative name server is hidden, a secondary authoritative name server may reside in the same building as the hidden primary.

A potential vulnerability of DNS is that an attacker can poison a name server's cache by sending queries that will cause the server to obtain host-to-IP address mappings from bogus name servers that respond with incorrect information. Once a name server has been poisoned, legitimate clients may be directed to nonexistent hosts (which constitutes a denial of service) or hosts that masquerade as legitimate ones to obtain sensitive data or passwords. To guard against poisoning, name servers authoritative for .mil domains should be separated functionally from name servers that resolve queries on behalf of internal clients. Organizations may achieve this separation by dedicating machines to each function or, if possible, by running two instances of the name server software on the same machine: one for the authoritative function and the other for the resolving function. In this design, each name server process may be bound to a different IP address or network interface to implement the required segregation.

A potential vulnerability of DNS is that an attacker can poison a name server's cache by sending queries that will cause the server to obtain host-to-IP address mappings from bogus name servers that respond with incorrect information. Once a name server has been poisoned, legitimate clients may be directed to nonexistent hosts (which constitutes a denial of service) or hosts that masquerade as legitimate ones to obtain sensitive data or passwords. To guard against poisoning, name servers authoritative for .mil domains should be separated functionally from name servers that resolve queries on behalf of internal clients. Organizations may achieve this separation by dedicating machines to each function or, if possible, by running two instances of the name server software on the same machine: one for the authoritative function and the other for the resolving function. In this design, each name server process may be bound to a different IP address or network interface to implement the required segregation.

A potential vulnerability of DNS is that an attacker can poison a name server's cache by sending queries that will cause the server to obtain host-to-IP address mappings from bogus name servers that respond with incorrect information. Once a name server has been poisoned, legitimate clients may be directed to nonexistent hosts (which constitutes a denial of service) or hosts that masquerade as legitimate ones to obtain sensitive data or passwords. To guard against poisoning, name servers authoritative for .mil domains should be separated functionally from name servers that resolve queries on behalf of internal clients. Organizations may achieve this separation by dedicating machines to each function or, if possible, by running two instances of the name server software on the same machine: one for the authoritative function and the other for the resolving function. In this design, each name server process may be bound to a different IP address or network interface to implement the required segregation. Windows 2022 DNS Servers with a caching name server role must be secured against pollution by ensuring the authenticity and integrity of queried records are verified before any data is cached.

Encrypting information for transmission protects it from unauthorized disclosure and modification. Cryptographic mechanisms implemented to protect information integrity include, for example, cryptographic hash functions that have common application in digital signatures, checksums, and message authentication codes. Confidentiality is not an objective of DNS, but integrity is. DNSSEC and TSIG/SIG(0) both digitally sign DNS information to authenticate its source and ensure its integrity.

The best way for a zone administrator to minimize the impact of a key compromise is by limiting the validity period of RRSIGs in the zone and the parent zone. This strategy limits the time during which an attacker can take advantage of a compromised key to forge responses. An attacker that has compromised a zone signing key (ZSK) can use that key only during the key signing key's (KSK's) signature validity interval. An attacker that has compromised a KSK can use that key for only as long as the signature interval of the RRSIG covering the Delegation Signer (DS) Resource Record (RR) in the delegating parent. These validity periods should be short, which will require frequent re-signing. To minimize the impact of a compromised ZSK, a zone administrator should set a signature validity period of one week for RRSIGs covering the DNSKEY RRSet in the zone (the RRSet that contains the ZSK and KSK for the zone). The DNSKEY RRSet can be re-signed without performing a ZSK rollover, but scheduled ZSK rollovers should still be performed at regular intervals.

NSEC records list the resource record types for the name, as well as the name of the next resource record. This information reveals that the resource record type for the name queried, or the resource record name requested, does not exist. NSEC uses the actual resource record names, whereas NSEC3 uses a one-way hash of the name. In this way, walking zone data from one record to the next is prevented at the expense of some CPU cycles on the authoritative server and the resolver. To prevent giving access to an entire zone file, NSEC3 should be configured. To use NSEC3, RSA/SHA-1 should be used as the algorithm, as some resolvers that understand RSA/SHA-1 might not understand NSEC3. Using RSA/SHA-256 is a safe alternative.

Most enterprises have an authoritative primary server and a host of authoritative secondary name servers. It is essential that these authoritative name servers for an enterprise be located on different network segments. This dispersion ensures the availability of an authoritative name server not only in situations in which a particular router or switch fails but also during events involving an attack on an entire network segment. A network administrator may choose to use a "hidden" primary authoritative server and have only secondary servers visible on the network. A hidden primary authoritative server is an authoritative DNS server whose IP address does not appear in the name server set for a zone. If the primary authoritative name server is hidden, a secondary authoritative name server may reside on the same network as the hidden primary.

The only protection approach for content control of a DNS zone file is the use of a zone file integrity checker. The effectiveness of integrity checking using a zone file integrity checker depends on the database of constraints built into the checker. The deployment process consists of developing these constraints with the right logic, and the only determinant of the truth value of these logical predicates is the parameter values for certain key fields in the format of various RRTypes. The serial number in the SOA RDATA is used to indicate to secondary name servers that a change to the zone has occurred and a zone transfer should be performed. It should always be increased whenever a change is made to the zone data. DNS NOTIFY must be enabled on the primary authoritative name server.

The choice of digital signature algorithm will be based on recommended algorithms in well-known standards. NIST's Digital Signature Standard (DSS) (FIPS186) provides three algorithm choices: - Digital Signature Algorithm (DSA). - RSA. - Elliptic Curve DSA (ECDSA). Of these three algorithms, RSA and DSA are more widely available and hence are considered candidates of choice for DNSSEC. Both RSA and DSA have comparable signature generation speeds, but DSA is much slower for signature verification. RSA is the recommended algorithm for this guideline. RSA with SHA-1 is currently the only cryptographic algorithm mandated to be implemented with DNSSEC, although other algorithm suites (i.e., RSA/SHA-256, ECDSA) are also specified. It can be expected that name servers and clients will be able to use the RSA algorithm at a minimum. It is suggested that at least one zone signing key (ZSK) for a zone use the RSA algorithm. NIST's Secure Hash Standard (SHS) (FIPS 180-3) specifies SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512 as approved hash algorithms to be used as part of the algorithm suite for generating digital signatures using the digital signature algorithms in NIST's DSS (FIPS186). It is expected that there will be support for Elliptic Curve Cryptography in the DNSSEC. The migration path for USG DNSSEC operation will be to ECDSA (or similar) from RSA/SHA-1 and RSA/SHA-256 before 30 September 2015.

Authoritative name servers for an enterprise may be configured to receive requests from both external and internal clients. External clients need to receive RRs that pertain only to public services (public web server, mail server, etc.). Internal clients need to receive RRs pertaining to public services as well as internal hosts. The zone information that serves the RRs on both the inside and the outside of a firewall should be split into different physical files for these two types of clients (one file for external clients and one file for internal clients).

Instead of having the same set of authoritative name servers serve different types of clients, an enterprise could have two different sets of authoritative name servers. One set, called external name servers, can be located within a DMZ; these would be the only name servers that are accessible to external clients and would serve resource records (RRs) pertaining to hosts with public services (web servers that serve external web pages or provide business-to-consumer services, mail servers, etc.). The other set, called internal name servers, is to be located within the firewall and should be configured so the servers are not reachable from outside and hence provide naming services exclusively to internal clients.

Authoritative name servers (especially primary name servers) should be configured with an allow-transfer access control substatement designating the list of hosts from which zone transfer requests can be accepted. These restrictions address the denial-of-service threat and potential exploits from unrestricted dissemination of information about internal resources. Based on the need to know, the only name servers that need to refresh their zone files periodically are the secondary name servers. Zone transfer from primary name servers should be restricted to secondary name servers. The zone transfer should be completely disabled in the secondary name servers. The address match list argument for the allow-transfer substatement should consist of IP addresses of secondary name servers and stealth secondary name servers.

Discretionary Access Control (DAC) is based on the premise that individual users are "owners" of objects and therefore have discretion over who should be authorized to access the object and in which mode (e.g., read or write). Ownership is usually acquired as a consequence of creating the object or via specified ownership assignment. In a DNS implementation, DAC should be granted to a minimal number of individuals and objects because DNS does not interact directly with users and users do not store and share data with the DNS application directly. The primary objective of DNS authentication and access control is the integrity of DNS records; only authorized personnel must be able to create and modify resource records, and name servers should only accept updates from authoritative primary servers for the relevant zones. Integrity is best ensured through authentication and access control features within the name server software and the file system the name server resides on. To protect the zone files and configuration data, which should only be accessed by the name service or an administrator, access controls must be implemented on files, and rights should not be easily propagated to other users. Lack of a stringent access control policy places the DNS infrastructure at risk to malicious persons and attackers and creates the potential for a denial of service to network resources. DAC allows the owner to determine who will have access to objects they control. An example of DAC includes user-controlled file permissions. DAC models have the potential for the access controls to propagate without limit, resulting in unauthorized access to objects. When applications provide a DAC mechanism, the DNS implementation must be able to limit the propagation of those access rights.

DNS servers with an internal role only process name/address resolution requests from within the organization (i.e., internal clients). DNS servers with an external role only process name/address resolution information requests from clients external to the organization (i.e., on the external networks, including the internet). The set of clients that can access an authoritative DNS server in a particular role is specified by the organization using address ranges, explicit access control lists, etc. To protect internal DNS resource information, it is important to isolate the requests to internal DNS servers. Separating internal and external roles in DNS prevents address space that is private (e.g., 10.0.0.0/24) or otherwise concealed by some form of Network Address Translation from leaking into the public DNS system.

All caching name servers must be authoritative for the root zone because, without this starting point, they would have no knowledge of the DNS infrastructure and thus would be unable to respond to any queries. The security risk is that an adversary could change the root hints and direct the caching name server to a bogus root server. At that point, every query response from that name server is suspect, which would give the adversary substantial control over the network communication of the name servers' clients. When authoritative servers are sent queries for zones that they are not authoritative for, and they are configured as a noncaching server (as recommended), they can be configured to either return a referral to the root servers or refuse to answer the query. The recommendation is to configure authoritative servers to refuse to answer queries for any zones for which they are not authoritative. This is more efficient for the server and allows it to spend more of its resources fulfilling its intended purpose of answering authoritatively for its zone.

If a name server could claim authority for a resource record in a domain for which it was not authoritative, this would pose a security risk. In this environment, an adversary could use illicit control of a name server to impact IP address resolution beyond the scope of that name server (i.e., by claiming authority for records outside of that server's zones). Fortunately, all but the oldest versions of BIND and most other DNS implementations do not allow for this behavior. The best way to eliminate this risk is to eliminate from the zone files any records for hosts in another zone. The exceptions are glue records supporting zone delegations, CNAME records supporting a system migration, or CNAME records that point to third-party content delivery networks (CDNs) or cloud computing platforms. In the case of third-party CDNs or cloud offerings, an approved mission need must be demonstrated.

The use of CNAME records for exercises, tests, or zone-spanning (pointing to zones with lesser security) aliases should be temporary (e.g., to facilitate a migration) and not be in place for more than six months. When a host name is an alias for a record in another zone, an adversary has two points of attack: the zone in which the alias is defined and the zone authoritative for the alias's canonical name. This configuration also reduces the speed of client resolution because it requires a second lookup after obtaining the canonical name. In the case of an authoritative name server, this information is promulgated throughout the enterprise to caching servers, which compounds the vulnerability.

IPv6 link-local scope addresses are not globally routable and must not be configured in any DNS zone. Like RFC1918 addresses, if a link-local scope address is inserted into a zone provided to clients, most routers will not forward this traffic beyond the local subnet.

DNS is only responsible for resolving a domain name to an IP address. Applications and operating systems are responsible for processing the IPv6 or IPv4 record that may be returned. A denial of service could easily be implemented for an application that is not IPv6 aware. When the application receives an IP address in hexadecimal, it is up to the application/operating system to decide how to handle the response. Combining both IPv6 and IPv4 records into the same domain can lead to application problems that are beyond the scope of the DNS administrator.

Without reauthenticating devices, unidentified or unknown devices may be introduced, thereby facilitating malicious activity. In addition to the reauthentication requirements associated with session locks, organizations may require reauthentication of devices, including but not limited to the following other situations: (i) When authenticators change; (ii) When roles change; (iii) When security categories of information systems change; (iv) After a fixed period of time; or (v) Periodically. DNS does perform server authentication when DNSSEC or TSIG/SIG(0) are used, but this authentication is transactional in nature (each transaction has its own authentication performed). Therefore, this requirement is applicable for every server-to-server transaction request.

Without identifying devices, unidentified or unknown devices may be introduced, thereby facilitating malicious activity. This applies to server-to-server (zone transfer) transactions only and is provided by TSIG/SIG(0), which enforces mutual server authentication using a key that is unique to each server pair, TSIG, or using PKI-based authentication, SIG(0), thus uniquely identifying the other server. TSIG and SIG(0) are not configurable in Windows 2022 DNS Server. To meet the requirement for authentication between Windows DNS Servers, IPsec will be implemented between the Windows DNS Servers that host any non-Active Directory (AD)-integrated zones.

Authenticity of zone transfers within Windows Active Directory (AD)-integrated zones is accomplished by AD replication. Without authenticating devices, unidentified or unknown devices may be introduced, thereby facilitating malicious activity. Device authentication is a solution enabling an organization to manage devices. It is an additional layer of authentication ensuring only specific preauthorized devices can access the system. This requirement applies to server-to-server (zone transfer) transactions only and is provided by TSIG/SIG(0), which enforces mutual server authentication using a key that is unique to each server pair, TSIG, or using PKI-based authentication, SIG(0).

Primary name servers also make outbound connection to secondary name servers to provide zone transfers and accept inbound connection requests from clients wishing to provide a dynamic update. Primary name servers should explicitly limit zone transfers to be made only to designated secondary name servers. Because zone transfers involve the transfer of entire zones and use TCP connections, they place substantial demands on network resources relative to normal DNS queries. Errant or malicious frequent zone transfer requests on the name servers of the enterprise can overload the master zone server and result in a denial of service to legitimate users. Active Directory (AD)-integrated DNS servers replicate zone information via AD replication. Non-AD-integrated DNS servers replicate zone information via zone transfers.

Weakly bound credentials can be modified without invalidating the credential; therefore, nonrepudiation can be violated. This requirement supports audit requirements that provide organizational personnel with the means to identify who produced specific information in the event of an information transfer. Organizations and/or data owners determine and approve the strength of the binding between the information producer and the information based on the security category of the information and relevant risk factors. DNSSEC and TSIG/SIG(0) both use digital signatures to establish the identity of the producer of pieces of information.

The cornerstone of the PKI is the private key used to encrypt or digitally sign information. If the private key is stolen, this will lead to the compromise of the authentication and nonrepudiation gained through PKI because the attacker can use the private key to digitally sign documents and pretend to be the authorized user. Both the holders of a digital certificate and the issuing authority must protect the computers, storage devices, or whatever they use to keep the private keys. SIG(0) is used for server-to-server authentication for DNS transactions, and it uses PKI-based authentication. In cases where SIG(0) is being used instead of TSIG (which uses a shared key, not PKI-based authentication), this requirement is applicable.

To enable zone transfer (requests and responses) through authenticated messages, it is necessary to generate a key for every pair of name servers. The key can also be used for securing other transactions, such as dynamic updates, DNS queries, and responses. The binary key string that is generated by most key generation utilities used with DNSSEC is Base64 encoded. Transaction Signature (TSIG) is a string used to generate the message authentication hash stored in a TSIG Resource Record (RR) and used to authenticate an entire DNS message.

To enable zone transfer (requests and responses) through authenticated messages, it is necessary to generate a key for every pair of name servers. The key can also be used for securing other transactions, such as dynamic updates, DNS queries, and responses. The binary key string that is generated by most key generation utilities used with DNSSEC is Base64 encoded. Transaction Signature (TSIG) is a string used to generate the message authentication hash stored in a TSIG Resource Record (RR) and used to authenticate an entire DNS message.

The private keys in the key signing key (KSK) and ZSK key pairs must be protected from unauthorized access. If possible, the private keys should be stored offline (with respect to the internet-facing, DNSSEC-aware name server) in a physically secure, non-network-accessible machine along with the zone file primary copy. This strategy is not feasible in situations in which the DNSSEC-aware name server must support dynamic updates. To support dynamic update transactions, the DNSSEC-aware name server (which usually is a primary authoritative name server) must have both the zone file master copy and the private key corresponding to the zone signing key (ZSK-private) online to immediately update the signatures for the updated resource record (RR) sets. The private key corresponding to the key signing key (KSK-private) can still be kept offline.

Not configuring a local cache of revocation data could allow access to users who are no longer authorized (users with revoked certificates). SIG(0) is used for server-to-server authentication for DNS transactions, and it uses PKI-based authentication. In cases where SIG(0) is being used instead of TSIG (which uses a shared key, not PKI-based authentication), this requirement is applicable.

NSEC records list the resource record types for the name, as well as the name of the next resource record. With this information it is revealed that the resource record type for the name queried, or the resource record name requested, does not exist. NSEC uses the actual resource record names, whereas NSEC3 uses a one-way hash of the name. In this way, walking zone data from one record to the next is prevented, at the expense of some CPU cycles on the authoritative server and the resolver. To prevent giving access to an entire zone file, NSEC3 should be configured. To use NSEC3, RSA/SHA-1 should be used as the algorithm, as some resolvers that understand RSA/SHA-1 might not understand NSEC3. Using RSA/SHA-256 is a safe alternative.

The underlying feature in the major threat associated with DNS query/response (i.e., forged response or response failure) is the integrity of DNS data returned in the response. The security objective is to verify the integrity of each response received. An integral part of integrity verification is to ensure valid data has originated from the right source. Establishing trust in the source is called data origin authentication. The security objectives, and consequently the security services, that are required for securing the DNS query/response transaction are data origin authentication and data integrity verification. The specification for a digital signature mechanism in the context of the DNS infrastructure is in IETF's DNSSEC standard. In DNSSEC, trust in the public key (for signature verification) of the source is established not by going to a third party or a chain of third parties (as in public key infrastructure [PKI] chaining), but by starting from a trusted zone (such as the root zone) and establishing the chain of trust down to the current source of response through successive verifications of signature of the public key of a child by its parent. The public key of the trusted zone is called the trust anchor.

The major threat associated with DNS forged responses or failures is the integrity of the DNS data returned in the response. The principle of DNSSEC is to mitigate this threat by providing data origin authentication, establishing trust in the source. By requiring remote clients to obtain origin authentication and integrity verification assurances for the host/service name to network address resolution information obtained through the service, data origin is validated. Ensuring all name servers have static IP addresses makes it possible to configure restricted DNS communication, such as with DNSSEC, between the name servers.

The major threat associated with DNS forged responses or failures is the integrity of the DNS data returned in the response. The principle of DNSSEC is to mitigate this threat by providing data origin authentication, establishing trust in the source. By requiring remote clients to obtain origin authentication and integrity verification assurances for the host/service name to network address resolution information obtained through the service, data origin is validated.

The major threat associated with DNS forged responses or failures is the integrity of the DNS data returned in the response. The principle of DNSSEC is to mitigate this threat by providing data origin authentication, establishing trust in the source. By requiring remote clients to obtain origin authentication and integrity verification assurances for the host/service name to network address resolution information obtained through the service, data origin is validated. A DNS server is an example of an information system providing name/address resolution service. Digital signatures and cryptographic keys are examples of additional artifacts. DNS resource records are examples of authoritative data. Applications other than the DNS, to map between host/service names and network addresses, must provide other means to ensure the authenticity and integrity of response data. In the case of DNS, employ DNSSEC to provide an additional data origin and integrity artifacts along with the authoritative data the system returns in response to DNS name/address resolution queries.

The major threat associated with DNS forged responses or failures is the integrity of the DNS data returned in the response. The principle of DNSSEC is to mitigate this threat by providing data origin authentication, establishing trust in the source. By requiring remote clients to obtain origin authentication and integrity verification assurances for the host/service name to network address resolution information obtained through the service, data origin is validated. A DNS server is an example of an information system providing name/address resolution service. Digital signatures and cryptographic keys are examples of additional artifacts. DNS resource records are examples of authoritative data. Applications other than the DNS, to map between host/service names and network addresses, must provide other means to assure the authenticity and integrity of response data. In the case of DNS, employ DNSSEC to provide an additional data origin and integrity artifacts along with the authoritative data the system returns in response to DNS name/address resolution queries. If/when WINS lookups are enabled, the validity of the data becomes questionable because the WINS data is provided to the requestor unsigned and invalidated. To ensure only the DNSSEC-signed data is being returned, WINS lookups must be disabled.

The major threat associated with DNS forged responses or failures is the integrity of the DNS data returned in the response. The principle of DNSSEC is to mitigate this threat by providing data origin authentication, establishing trust in the source. By requiring remote clients to obtain origin authentication and integrity verification assurances for the host/service name to network address resolution information obtained through the service, data origin is validated. A DNS server is an example of an information system providing name/address resolution service. Digital signatures and cryptographic keys are examples of additional artifacts. DNS resource records are examples of authoritative data. Applications other than the DNS, to map between host/service names and network addresses, must provide other means to assure the authenticity and integrity of response data. In the case of DNS, employ DNSSEC to provide an additional data origin and integrity artifacts along with the authoritative data the system returns in response to DNS name/address resolution queries.

If name server replies are invalid or cannot be validated, many networking functions and communication would be adversely affected. With DNS, the presence of DS records associated with child zones informs clients of the security status of child zones. These records are crucial to the DNSSEC chain of trust model. Each parent domain's DS record is used to verify the DNSKEY record in its subdomain from the top of the DNS hierarchy down. A DNS server is an example of an information system providing name/address resolution service. Digital signatures and cryptographic keys are examples of additional artifacts. DNS resource records are examples of authoritative data. Applications other than the DNS, to map between host/service names and network addresses, must provide other means to ensure the authenticity and integrity of response data. In DNS, trust in the public key of the source is established by starting from a trusted name server and establishing the chain of trust down to the current source of response through successive verifications of signature of the public key of a child by its parent. A trust anchor is an authoritative entity represented via a public key and associated data. It is used in the context of public key infrastructures, X.509 digital certificates, and DNSSEC. When there is a chain of trust, usually the top entity to be trusted becomes the trust anchor. A certification path starts with the subject certificate and proceeds through several intermediate certificates up to a trusted root certificate. In DNS, a trust anchor is a DNSKEY that is placed into a validating resolver so the validator can cryptographically validate the results for a given request back to a known public key (the trust anchor). One way to indicate the security status of child subspaces is through the use of DS RRs in the DNS. Path validation is necessary for a relying party to make an informed trust decision when presented with any certificate not already explicitly trusted. Without path validation and a chain of trust, there can be no trust that the data integrity authenticity has been maintained during a transaction.

A mechanism to detect and prevent unauthorized communication flow must be configured or provided as part of the system design. If information flow is not enforced based on approved authorizations, the system may become compromised. Information flow control regulates where information is allowed to travel within a system and between interconnected systems. The flow of all application information must be monitored and controlled so it does not introduce any unacceptable risk to the systems or data. Application-specific examples of enforcement occur in systems that employ rule sets or establish configuration settings that restrict information system services or provide a message filtering capability based on message content (e.g., implementing key word searches or using document characteristics). Applications providing information flow control must be able to enforce approved authorizations for controlling the flow of information between interconnected systems in accordance with applicable policy. Within the context of DNS, this is applicable in terms of controlling the flow of DNS information between systems, such as DNS zone transfers.

The NRPT is used to require DNSSEC validation. The NRPT can be configured in local Group Policy for a single computer or domain Group Policy for some or all computers in the domain.

If name server replies are invalid or cannot be validated, many networking functions and communication would be adversely affected. With DNS, the presence of Delegation Signer (DS) records associated with child zones informs clients of the security status of child zones. These records are crucial to the DNSSEC chain of trust model. Each parent domain's DS record is used to verify the DNSKEY record in its subdomain, from the top of the DNS hierarchy down. Like the DNSKEY resource record, the DS Resource Record (RR) can be used to create a trust anchor for a signed zone. The DS record is smaller in size than a DNSKEY record because it contains only a hash of the public key. The DS record is not added to a zone during the signing process like some DNSSEC-related RRs, even if a delegation already exists in the zone. To add a DS record, it must be manually added or imported. Fortunately, the DS resource record set (DSSET) is automatically added as a file to the Key Primary when a zone is signed. The DSSET file can be used with the "Import-DnsServerResourceRecordDS" cmdlet to import DS records to the parent zone. A DNS server is an example of an information system providing name/address resolution service. Digital signatures and cryptographic keys are examples of additional artifacts. DNS resource records are examples of authoritative data. Applications other than the DNS, to map between host/service names and network addresses, must provide other means to ensure the authenticity and integrity of response data. DNSSEC provides the means to verify integrity assurances for the host/service name to network address resolution information obtained through the service. By using the DS RRs in the DNS, the security status of a child domain can be validated. The DS RR is used to identify the DNSSEC signing key of a delegated zone. Starting from a trusted name server (such as the root name server) and down to the current source of response through successive verifications of signature of the public key of a child by its parent, the chain of trust is established. The public key of the trusted name servers is called the trust anchor. After authenticating the source, the next process DNSSEC calls for is to authenticate the response. This requires that responses consist of not only the requested RRs but also an authenticator associated with them. In DNSSEC, this authenticator is the digital signature of an RRSet. The digital signature of an RRSet is encapsulated through a special RRType called RRSIG. The DNS client using the trusted public key of the source (whose trust has just been established) then verifies the digital signature to detect if the response is valid or bogus. This control enables the DNS to obtain origin authentication and integrity verification assurances for the host/service name to network address resolution information obtained through the service. Without indication of the security status of a child domain and enabling verification of a chain of trust, integrity and availability of the DNS infrastructure cannot be ensured.

If name server replies are invalid or cannot be validated, many networking functions and communication would be adversely affected. With DNS, the presence of Delegation Signer (DS) records associated with child zones informs clients of the security status of child zones. These records are crucial to the DNSSEC chain of trust model. Each parent domain's DS record is used to verify the DNSKEY record in its sub domain, from the top of the DNS hierarchy down. A DNS server is an example of an information system providing name/address resolution service. Digital signatures and cryptographic keys are examples of additional artifacts. DNS resource records are examples of authoritative data. Applications other than the DNS, to map between host/service names and network addresses, must provide other means to assure the authenticity and integrity of response data. DNSSEC provides the means to verify integrity assurances for the host/service name to network address resolution information obtained through the service. By using the DS Resource Records (RRs) in the DNS, the security status of a child domain can be validated. The DS RR is used to identify the DNSSEC signing key of a delegated zone. Starting from a trusted name server (such as the root name server) and down to the current source of response through successive verifications of signature of the public key of a child by its parent, the chain of trust is established. The public key of the trusted name servers is called the trust anchor. After authenticating the source, the next process DNSSEC calls for is to authenticate the response. This requires that responses consist of not only the requested RRs but also an authenticator associated with them. In DNSSEC, this authenticator is the digital signature of an RRSet. The digital signature of an RRSet is encapsulated through a special RRType called RRSIG. The DNS client using the trusted public key of the source (whose trust has just been established) then verifies the digital signature to detect if the response is valid or bogus. This control enables the DNS to obtain origin authentication and integrity verification assurances for the host/service name to network address resolution information obtained through the service. Without indication of the security status of a child domain and enabling verification of a chain of trust, integrity and availability of the DNS infrastructure cannot be assured. A trust anchor is a preconfigured public key associated with a specific zone. A validating DNS server must be configured with one or more trust anchors to perform validation. If the DNS server is running on a domain controller, trust anchors are stored in the forest directory partition in Active Directory Domain Services (AD DS) and can be replicated to all domain controllers in the forest. On standalone DNS servers, trust anchors are stored in a file named "TrustAnchors.dns". A DNS server running Windows Server 2022 also displays configured trust anchors in the DNS Manager console tree in the Trust Points container. Trust anchors can also be viewed by executing Windows PowerShell commands or "Dnscmd.exe" at a Windows command prompt.

A trust anchor is a preconfigured public key associated with a specific zone. A validating DNS server must be configured with one or more trust anchors to perform validation. If the DNS server is running on a domain controller, trust anchors are stored in the forest directory partition in Active Directory Domain Services (AD DS) and can be replicated to all domain controllers in the forest. On standalone DNS servers, trust anchors are stored in a file named "TrustAnchors.dns". A DNS server running Windows Server 2022 also displays configured trust anchors in the DNS Manager console tree in the "Trust Points" container. Trust anchors can also be viewed by executing Windows PowerShell commands or "Dnscmd.exe" at a Windows command prompt.

If data origin authentication and data integrity verification are not performed, the resultant response could be forged, it may have come from a poisoned cache, the packets could have been intercepted without the resolver's knowledge, or resource records could have been removed that would result in query failure or denial of service. Data origin authentication must be performed to thwart these types of attacks. Each client of name resolution services either performs this validation on its own or has authenticated channels to trusted validation providers. Information systems that provide name and address resolution services for local clients include, for example, recursive resolving or caching DNS servers. DNS client resolvers either perform validation of DNSSEC signatures, or clients use authenticated channels to recursive resolvers that perform such validations.

If data origin authentication and data integrity verification are not performed, the resultant response could be forged, it may have come from a poisoned cache, the packets could have been intercepted without the resolver's knowledge, or resource records could have been removed that would result in query failure or denial of service. Data integrity verification must be performed to thwart these types of attacks. Each client of name resolution services either performs this validation on its own or has authenticated channels to trusted validation providers. Information systems that provide name and address resolution services for local clients include, for example, recursive resolving or caching DNS servers. DNS client resolvers either perform validation of DNSSEC signatures, or clients use authenticated channels to recursive resolvers that perform such validations.

If data origin authentication and data integrity verification are not performed, the resultant response could be forged, it may have come from a poisoned cache, the packets could have been intercepted without the resolver's knowledge, or resource records could have been removed that would result in query failure or denial of service. Data integrity verification must be performed to thwart these types of attacks. Each client of name resolution services either performs this validation on its own or has authenticated channels to trusted validation providers. Information systems that provide name and address resolution services for local clients include, for example, recursive resolving or caching DNS servers. DNS client resolvers either perform validation of DNSSEC signatures, or clients use authenticated channels to recursive resolvers that perform such validations.

If data origin authentication and data integrity verification are not performed, the resultant response could be forged, it may have come from a poisoned cache, the packets could have been intercepted without the resolver's knowledge, or resource records could have been removed that would result in query failure or denial of service. Data origin authentication verification must be performed to thwart these types of attacks. Each client of name resolution services either performs this validation on its own or has authenticated channels to trusted validation providers. Information systems that provide name and address resolution services for local clients include, for example, recursive resolving or caching DNS servers. DNS client resolvers either perform validation of DNSSEC signatures, or clients use authenticated channels to recursive resolvers that perform such validations.

Without identifying devices, unidentified or unknown devices may be introduced, thereby facilitating malicious activity. This applies to server-to-server (zone transfer) transactions and is provided by TSIG/SIG(0), which enforces mutual server authentication using a key that is unique to each server pair, TSIG, or using PKI-based authentication, SIG(0), thus uniquely identifying the other server. TSIG and SIG(0) are not configurable in Windows 2022 DNS Server. To meet the requirement for authentication between Windows DNS Servers, IPsec will be implemented between the Windows DNS Servers that hosts any non-Active Directory (AD)-integrated zones.

DNS is a fundamental network service that is prone to various attacks, such as cache poisoning and man-in-the middle attacks. If communication sessions are not provided appropriate validity protections, such as the employment of DNSSEC, the authenticity of the data cannot be guaranteed. The combination of signing DNS zones by DNSSEC and requiring clients to send their dynamic updates securely ensures the authenticity of those DNS records when providing query responses for them.

The underlying feature in the major threat associated with DNS query/response (i.e., forged response or response failure) is the integrity of DNS data returned in the response. An integral part of integrity verification is to ensure that valid data has originated from the right source. DNSSEC is required for securing the DNS query/response transaction by providing data origin authentication and data integrity verification through signature verification and the chain of trust.

Untrusted certificate authorities (CA) can issue certificates, but the certificates may be issued by organizations or individuals that seek to compromise DOD systems or by organizations with insufficient security controls. If the CA used for verifying the certificate is not a DOD-approved CA, trust of this CA has not been established. The DOD will only accept PKI certificates obtained from a DOD-approved internal or external certificate authority. Reliance on CAs for the establishment of secure sessions includes, for example, the use of SSL/TLS certificates. TSIG and SIG(0) are not configurable in Windows 2022 DNS Server. To meet the requirement for authentication between Windows DNS Servers, IPsec must be implemented between the Windows DNS Servers. Note: If multiple certificates from the same CA are present on the DNS server, IPsec authentication might fail due to an incorrect certificate being chosen. For this purpose, an Active Directory Certificate Services (AD CS) role must be installed and configured as an Enterprise certificate authority (CA). Refer to the Microsoft Windows Server 2022 DNS Overview.pdf for references on deploying certificates for this procedure.

Information at rest refers to the state of information when it is located on a secondary storage device within an organizational information system. Mobile devices, laptops, desktops, and storage devices can be either lost or stolen, and the contents of their data storage (e.g., hard drives and nonvolatile memory) can be read, copied, or altered. Applications and application users generate information throughout the course of their application use. The DNS server must protect the confidentiality and integrity of shared keys for TSIG and private keys for SIG(0) and must protect the integrity of DNS information. There is no need to protect the confidentiality of DNS information because it is accessible by all devices that can contact the server.

If zone information has not been validated in more than a year, there is no assurance that it is still valid. If invalid records are in a zone, an adversary could potentially use their existence for improper purposes. A standard operating procedure detailing this process can resolve this requirement.

Applications and application developers must take steps to ensure users cannot use an authorized application to launch DoS attacks against other systems and networks. For example, applications may include mechanisms that throttle network traffic, so users are not able to generate unlimited network traffic via the application. Limiting system resources that are allocated to any user to a bare minimum may also reduce the ability of users to launch some DoS attacks.

In the case of application DoS attacks, care must be taken when designing the application to ensure it makes the best use of system resources. SQL queries have the potential to consume large amounts of CPU cycles if they are not tuned for optimal performance. Web services containing complex calculations requiring large amounts of time to complete can bog down if too many requests for the service are encountered within a short period of time.

Information can be unintentionally or maliciously disclosed or modified during preparation for transmission, including, for example, during aggregation, at protocol transformation points, and during packing/unpacking. These unauthorized disclosures or modifications compromise the confidentiality or integrity of the information.

Information can be unintentionally or maliciously disclosed or modified during preparation for transmission, including, for example, during aggregation, at protocol transformation points, and during packing/unpacking. These unauthorized disclosures or modifications compromise the confidentiality or integrity of the information.

Use of weak or untested encryption algorithms undermines the purposes of using encryption to protect data. The application must implement cryptographic modules adhering to the higher standards approved by the federal government since this provides assurance they have been tested and validated. The choice of digital signature algorithm will be based on recommended algorithms in well-known standards. NIST's Digital Signature Standard (DSS) (FIPS186) provides three algorithm choices: - Digital Signature Algorithm (DSA). - RSA. - Elliptic Curve DSA (ECDSA). Of these three algorithms, RSA and DSA are more widely available and considered candidates of choice for DNSSEC. Both RSA and DSA have comparable signature generation speeds, but DSA is much slower for signature verification. RSA is the recommended algorithm for this guideline. RSA with SHA-1 is currently the only cryptographic algorithm mandated to be implemented with DNSSEC, although other algorithm suites (i.e., RSA/SHA-256, ECDSA) are also specified. It can be expected that name servers and clients will be able to use the RSA algorithm at a minimum. It is suggested that at least one ZSK for a zone use the RSA algorithm. NIST's Secure Hash Standard (SHS) (FIPS 180-3) specifies SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512 as approved hash algorithms to be used as part of the algorithm suite for generating digital signatures using the digital signature algorithms in NIST's DSS (FIPS186). It is expected that there will be support for Elliptic Curve Cryptography in the DNSSEC. The migration path for USG DNSSEC operation will be to ECDSA (or similar) from RSA/SHA-1 and RSA/SHA-256 before 30 September 2015.

DNS zone data for which a Windows 2022 DNS Server is authoritative should represent the network for which it is responsible. If a Windows 2022 DNS Server hosts zone records for other networks or environments, the records could become invalid or stale or be redundant/conflicting with a DNS server truly authoritative for the other network environment.

Failing to an unsecure condition negatively impacts application security and can lead to system compromise. Failure conditions include, for example, loss of communications among critical system components or between system components and operational facilities. Fail-safe procedures include, for example, alerting operator personnel and providing specific instructions on subsequent steps to take (e.g., do nothing, reestablish system settings, shutdown processes, restart the system, or contact designated organizational personnel). If a component such as the DNSSEC or TSIG/SIG(0) signing capabilities were to fail, the DNS server should shut itself down to prevent continued execution without the necessary security components in place. Transactions such as zone transfers would not be able to work correctly in this state.

Each newer version of the name server software, especially the BIND software, generally is devoid of vulnerabilities found in earlier versions because it has design changes incorporated to address those vulnerabilities. The vulnerabilities have been exploited (i.e., some form of attack was launched), and sufficient information has been generated with respect to the nature of those exploits. It makes good business sense to run the latest version of name server software because theoretically it is the safest version. In some installations, it may not be possible to switch to the latest version of name server software immediately. If the version of the name server software is revealed in queries, this information may be used by attackers looking for a specific version of the software that has a discovered weakness. To prevent information about which version of name server software is running on a system, name servers should be configured to refuse queries for its version information.

Several types of resource records (RRs) in the DNS are meant to convey information to humans and applications about the network, hosts, or services. These RRs include the Responsible Person (RP) record, the Host Information (HINFO) record, the Location (LOC) record, and the catch-all text string resource record (TXT) (RFC1035). Although these record types are meant to provide information to users in good faith, they also allow attackers to gain knowledge about network hosts before attempting to exploit them. For example, an attacker may query for HINFO records, looking for hosts that list an operating system or platform known to have exploits. Therefore, great care should be taken before including these record types in a zone. They are best left out completely. More careful consideration should be taken with the TXT resource record type. A DNS administrator will have to decide if the data contained in a TXT RR constitutes an information leak or is a necessary piece of information. For example, several authenticated email technologies use TXT RRs to store email sender policy information such as valid email senders for a domain. These judgments will have to be made on a case-by-case basis. A DNS administrator should take care when including HINFO, RP, TXT, LOC, or other RR types that could divulge information that would be useful to an attacker or the external view of a zone if using split DNS. RRs such as HINFO and TXT provide information about software name and versions (e.g., for resources such as web servers and mail servers) that will enable the well-equipped attacker to exploit the known vulnerabilities in those software versions and launch attacks against those resources.

Predictable failure prevention requires organizational planning to address system failure issues. If components key to maintaining systems security fail to function, the system could continue operating in an insecure state. The organization must be prepared, and the application must support requirements that specify if the application must alarm for such conditions and/or automatically shut down the application or the system. This can include conducting a graceful application shutdown to avoid losing information. Automatic or manual transfer of components from standby to active mode can occur, for example, upon detection of component failures. If a component such as the DNSSEC or TSIG/SIG(0) signing capabilities were to fail, the DNS server should shut itself down to prevent continued execution without the necessary security components in place. Transactions such as zone transfers would not be able to work correctly in this state.

Security function is defined as the hardware, software, and/or firmware of the information system responsible for enforcing the system security policy and supporting the isolation of code and data on which the protection is based. Security functionality includes but is not limited to establishing system accounts, configuring access authorizations (i.e., permissions, privileges), setting events to be audited, and setting intrusion detection parameters. Without verification, security functions may not operate correctly, and this failure may go unnoticed. Notifications provided by information systems include, for example, electronic alerts to system administrators, messages to local computer consoles, and/or hardware indications, such as lights. The DNS server should perform self-tests, such as at server startup, to confirm that its security functions are working properly.

Security function is defined as the hardware, software, and/or firmware of the information system responsible for enforcing the system security policy and supporting the isolation of code and data on which the protection is based. Security functionality includes but is not limited to establishing system accounts, configuring access authorizations (i.e., permissions, privileges), setting events to be audited, and setting intrusion detection parameters. Without verification, security functions may not operate correctly, and this failure may go unnoticed. Notifications provided by information systems include, for example, electronic alerts to system administrators, messages to local computer consoles, and/or hardware indications, such as lights. The DNS server should perform self-tests, such as at server startup, to confirm that its security functions are working properly.

Security function is defined as the hardware, software, and/or firmware of the information system responsible for enforcing the system security policy and supporting the isolation of code and data on which the protection is based. Security functionality includes but is not limited to establishing system accounts, configuring access authorizations (i.e., permissions, privileges), setting events to be audited, and setting intrusion detection parameters. Notifications provided by information systems include messages to local computer consoles and/or hardware indications, such as lights. If anomalies are not acted on, security functions may fail to secure the system. The DNS server does not have the capability of shutting down or restarting the information system. The DNS server can be configured to generate audit records when anomalies are discovered, and the operating system/network device manager can then trigger notification messages to the system administrator based on the presence of those audit records.

Security function is defined as the hardware, software, and/or firmware of the information system responsible for enforcing the system security policy and supporting the isolation of code and data on which the protection is based. Security functionality includes but is not limited to establishing system accounts, configuring access authorizations (i.e., permissions, privileges), setting events to be audited, and setting intrusion detection parameters. If personnel are not notified of failed security verification tests, they will not be able to take corrective action, and the unsecure condition(s) will remain. Notifications provided by information systems include messages to local computer consoles and/or hardware indications, such as lights. The DNS server should be configured to generate audit records whenever a self-test fails. The operating system/network device manager is responsible for generating notification messages related to this audit record.

To enable zone transfer (requests and responses) through authenticated messages, it is necessary to generate a key for every pair of name servers. The key also can be used for securing other transactions, such as dynamic updates, DNS queries, and responses. The binary key string generated by most key generation utilities used with DNSSEC is Base64 encoded. TSIG is a string used to generate the message authentication hash stored in a TSIG Resource Record (RR) and used to authenticate an entire DNS message.

If unauthorized personnel use maintenance tools, they may accidentally or intentionally damage or compromise the system. The act of managing systems and applications includes the ability to access sensitive application information, such as system configuration details, diagnostic information, user information, and potentially sensitive application data. Nonlocal maintenance and diagnostic activities are conducted by individuals communicating through an external network (e.g., the internet) or an internal network. Local maintenance and diagnostic activities are carried out by individuals physically present at the information system or information system component and not communicating across a network connection. Typically, strong authentication requires authenticators that are resistant to replay attacks and employ multifactor authentication. Strong authenticators include, for example, public key infrastructure (PKI) where certificates are stored on a token protected by a password, passphrase, or biometric. This requirement applies to hardware/software diagnostic test equipment or tools. This requirement does not cover hardware/software components that may support information system maintenance, yet are a part of the system (e.g., the software implementing "ping", "ls", or "ipconfig" or the hardware and software implementing the monitoring port of an Ethernet switch). Lack of authentication enables anyone to gain access to the network or possibly a network element that provides opportunity for intruders to compromise resources within the network infrastructure. Network access control mechanisms interoperate to prevent unauthorized access and enforce the organization's security policy. Authorization for access to any network element requires an individual account identifier that has been approved, assigned, and configured on an authentication server. Authentication of all administrator accounts for all privilege levels must be accomplished using two or more factors that include the following: (i) something the user knows (e.g., password/PIN); (ii) something the user has (e.g., cryptographic identification device, token); or (iii) something the user is (e.g., biometric).

Failure to a known state can address safety or security in accordance with the mission/business needs of the organization. Failure to a known secure state helps prevent a loss of confidentiality, integrity, or availability in the event of a failure of the information system or a component of the system. Preserving application state information helps to facilitate application restart and return to the operational mode of the organization with less disruption to mission-essential processes.

Failure to provide logical access restrictions associated with changes to application configuration may have significant effects on the overall security of the system. When dealing with access restrictions pertaining to change control, any changes to the hardware, software, and/or firmware components of the information system and/or application can have significant effects on the overall security of the system. Accordingly, only qualified and authorized individuals should be allowed to obtain access to application components for the purposes of initiating changes, including upgrades and modifications. Logical access restrictions include, for example, controls that restrict access to workflow automation, media libraries, abstract layers (e.g., changes implemented into third-party interfaces rather than directly into information systems), and change windows (e.g., changes occur only during specified times, making unauthorized changes easy to discover). If the name server software is run as a privileged user (e.g., root in Unix systems), any break-in into the software can have disastrous consequences in terms of resources resident in the name server platform. Specifically, a hacker who breaks into the software acquires unrestricted access and therefore can execute any commands or modify or delete any files. It is necessary to run the name server software as a nonprivileged user with access restricted to specified directories to contain damages resulting from break-in.

The private keys in the KSK and ZSK key pairs must be protected from unauthorized access. If possible, the private keys should be stored offline (with respect to the internet-facing, DNSSEC-aware name server) in a physically secure, non-network-accessible machine along with the zone file master copy. This strategy is not feasible in situations in which the DNSSEC-aware name server must support dynamic updates. To support dynamic update transactions, the DNSSEC-aware name server (which usually is a primary authoritative name server) must have both the zone file master copy and the private key corresponding to the zone signing key (ZSK-private) online to immediately update the signatures for the updated Resource Record Sets. The private key corresponding to the key signing key (KSK-private) can still be kept offline.

Protection of log data includes ensuring log data is not accidentally lost or deleted. Backing up audit records to a different system or onto media separate from the system being audited on a defined frequency helps to ensure the audit records will be retained in the event of a catastrophic system failure. This helps to ensure a compromise of the information system being audited does not also result in a compromise of the audit records. This requirement applies only to applications that have a native backup capability for audit records. Operating system backup requirements cover applications that do not provide native backup functions.

Instead of having the same set of authoritative name servers serve different types of clients, an enterprise could have two different sets of authoritative name servers. One set, called external name servers, can be located within a DMZ; these would be the only name servers that are accessible to external clients and would serve resource records (RRs) pertaining to hosts with public services (web servers that serve external web pages or provide business-to-consumer services, mail servers, etc.). The other set, called internal name servers, is to be located within the firewall and should be configured so the servers are not reachable from outside and hence provide naming services exclusively to internal clients.

This setting can prevent someone from sending a denial-of-service attack using the DNS servers. For instance, a bot net can send requests to the DNS server using the IP address of a third computer as the requestor. Without RRL, the DNS servers might respond to all the requests, flooding the third computer.