BIND 9.x Security Technical Implementation Guide

Limiting the number of concurrent sessions reduces the risk of denial of service (DoS) to the DNS implementation. Name servers do not have direct user connections but accept client connections for queries. Original restriction on client connections should be high enough to prevent a self-imposed DoS, after which the connections are monitored and fine-tuned to best meet the organization's specific requirements. 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 only be made 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 primary zone server and result in DoS to legitimate users. Primary name servers should be configured to limit the hosts from which they will accept dynamic updates. Additionally, the number of concurrent clients, especially TCP clients, needs to be kept to a level that does not risk placing the system in a DoS state.

Limiting the number of concurrent sessions reduces the risk of denial of service (DoS) to the DNS implementation. Name servers do not have direct user connections but accept client connections for queries. Original restriction on client connections should be high enough to prevent a self-imposed DoS, after which the connections are monitored and fine-tuned to best meet the organization's specific requirements. 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 only be made 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 primary zone server and result in DoS to legitimate users. Primary name servers should be configured to limit the hosts from which they will accept dynamic updates. Additionally, the number of concurrent clients, especially TCP clients, needs to be kept to a level that does not risk placing the system in a DoS state.

Limiting the number of concurrent sessions reduces the risk of denial of service (DoS) to the DNS implementation. Name servers do not have direct user connections but accept client connections for queries. Original restriction on client connections should be high enough to prevent a self-imposed denial of service, after which the connections are monitored and fine-tuned to best meet the organization's specific requirements. 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 only be made 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 Primary zone server and result in DoS to legitimate users. Primary name servers should be configured to limit the hosts from which they will accept dynamic updates. Additionally, the number of concurrent clients, especially TCP clients, needs to be kept to a level that does not risk placing the system in a DoS state.

Limiting the number of concurrent sessions reduces the risk of denial of service (DoS) to the DNS implementation. Name servers do not have direct user connections but accept client connections for queries. Original restriction on client connections should be high enough to prevent a self-imposed denial of service, after which the connections are monitored and fine-tuned to best meet the organization's specific requirements. Primary name servers also make outbound connections 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 only be made 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 primary zone server and result in DoS to legitimate users. Primary name servers should be configured to limit the hosts from which they will accept dynamic updates. Additionally the number of concurrent clients, especially TCP clients, needs to be kept to a level that does not risk placing the system in a DoS state.

Auditing and logging are key components of any security architecture. It is essential for security personnel to know what is being performed on the system, where an event occurred, when an event occurred, and by whom the event was triggered, to compile an accurate risk assessment. Logging the actions of specific events provides a means to investigate an attack, recognize resource utilization or capacity thresholds, or to simply identify an improperly configured DNS implementation. Without log records that aid in the establishment of what types of events occurred and when those events occurred, there is no traceability for forensic or analytical purposes, and the cause of events is severely hindered.

Without establishing when events occurred, it is impossible to establish, correlate, and investigate the events relating to an incident. Associating event types with detected events in the application and audit logs provides a means of investigating an attack, recognizing resource utilization or capacity thresholds, or identifying an improperly configured application. To compile an accurate risk assessment and provide forensic analysis, it is essential for security personnel to know when events occurred (date and time).

Without establishing where events occurred, it is impossible to establish, correlate, and investigate the events relating to an incident. Associating information about where the event occurred within the application provides a means of investigating an attack, recognizing resource utilization or capacity thresholds, or identifying an improperly configured application. To compile an accurate risk assessment and provide forensic analysis, it is essential for security personnel to know where events occurred, such as application components, modules, session identifiers, filenames, host names, and functionality.

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 OS/NDM, but the configuration to trigger the auditing is controlled by the DNS server. The list of audited events is the set of events for which audits are to be generated. This set of events is typically a subset of the list of all events for which the system is capable of generating audit records. The DOD has defined the list of events for which the application will provide an audit record generation capability as the following: (i) Successful and unsuccessful attempts to access, modify, or delete privileges, security objects, security levels, or categories of information (e.g., classification levels); (ii) Access actions, such as successful and unsuccessful logon attempts, privileged activities or other system-level access, starting and ending time for user access to the system, concurrent logons from different workstations, successful and unsuccessful accesses to objects, all program initiations, and all direct access to the information system; and (iii) All account creation, modification, disabling, and termination actions. The DOD has defined the data which the application will provide an audit record generation capability for an event as the following: (i) Establish the source of the event; (ii) The outcome of the event; and (iii) Identify the application itself as the source of the event. Without establishing the source of the event, it is impossible to establish, correlate, and investigate the events leading up to an outage or attack. Associating information about the source of the event within the application provides a means of investigating an attack, recognizing resource utilization or capacity thresholds, or identifying an improperly configured application. Without information about the outcome of events, security personnel cannot make an accurate assessment about whether an attack was successful or if changes were made to the security state of the system. Event outcomes can include indicators of event success or failure and event-specific results (e.g., the security state of the information system after the event occurred). As such, they also provide a means to measure the impact of an event and help authorized personnel to determine the appropriate response. 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. The DNS server should be configured to generate audit records whenever a self-test fails. The OS/NDM is responsible for generating notification messages related to this audit record. If authorized individuals do not have the ability to modify auditing parameters in response to a changing threat environment, the organization may not be able to effectively respond, and important forensic information may be lost. This requirement enables organizations to extend or limit auditing as necessary to meet organizational requirements. Auditing that is limited to conserve information system resources may be extended to address certain threat situations. In addition, auditing may be limited to a specific set of events to facilitate audit reduction, analysis, and reporting. Organizations can establish time thresholds in which audit actions are changed, for example, near real-time, within minutes, or within hours. In addition to logging where events occur within the application, the application must also produce audit records that identify the application itself as the source of the event. To compile an accurate risk assessment and provide forensic analysis, it is essential for security personnel to know the source of the event, particularly in the case of centralized logging. In the case of centralized logging, the source would be the application name accompanied by the host or client name. Satisfies: SRG-APP-000089-DNS-000004, SRG-APP-000098-DNS-000009, SRG-APP-000099-DNS-000010, SRG-APP-000275-DNS-000040, SRG-APP-000226-DNS-000032

The private key in the ZSK key pair must be protected from unauthorized access. If possible, the private key should be stored offline (with respect to the internet-facing, DNSSEC-aware name server) in a physically secure, nonnetwork-accessible machine along with the zone file primary copy. This strategy is not feasible in situations in which the DNSSEC-aware name server has to support dynamic updates. To support dynamic update transactions, the DNSSEC-aware name server (which usually is a primary authoritative name server) has to have both the zone file primary copy and the private key corresponding to the zone-signing key (ZSK-private) online to immediately update the signatures for the updated RRsets. Failure to protect the private ZSK opens it to being maliciously obtained and opens the DNS zone to being populated with invalid data. The integrity of the DNS zone would be compromised, leading to a loss of trust whether a DNS response has originated from an authentic source, the response is complete and has not been tampered with during transit.

The private key in the KSK key pair must be protected from unauthorized access. The private key must be stored offline (with respect to the internet-facing, DNSSEC-aware name server) in a physically secure, nonnetwork-accessible machine along with the zone file primary copy. Failure to protect the private KSK may have significant effects on the overall security of the DNS infrastructure. A compromised KSK could lead to an inability to detect unauthorized DNS zone data resulting in network traffic being redirected to a rogue site.

Weak permissions of a TSIG key file could allow an adversary to modify the file, thus defeating the security objective.

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 that is 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 RR and used to authenticate an entire DNS message. The process of authenticating the source of a message and its integrity through hash-based message authentication codes (HMAC) is specified through a set of DNS specifications known collectively as TSIG. The sender of the message uses the HMAC function to generate a MAC and sends this MAC along with the message to the receiver. The receiver, who shares the same secret key, uses the key and HMAC function used by the sender to compute the MAC on the received message. The receiver then compares the computed MAC with the received MAC; if the two values match, it provides assurance that the message has been received correctly and that the sender belongs to the community of users sharing the same secret key. Thus, message source authentication and integrity verification are performed in a single process. 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 that is 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 RR and used to authenticate an entire DNS message.

Incorrect ownership of a TSIG key file could allow an adversary to modify the file, thus defeating the security objective.

Incorrect ownership of a TSIG key file could allow an adversary to modify the file, thus defeating the security objective.

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 outside of a firewall must 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 RRs pertaining to hosts with public services (web servers that serve external web pages or provide B2C services, mail servers, etc.). The other set, called internal name servers, must be located within the firewall. They must be configured so they are not reachable from outside and therefore provide naming services exclusively to 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 RRs pertaining to hosts with public services (web servers that serve external web pages or provide B2C services, mail servers, etc.). The other set, called internal name servers, must be located within the firewall. They must be configured so they are not reachable from outside and therefore provide naming services exclusively to internal clients.

BIND 9.x has implemented an option to use "view" statements to allow for split DNS architecture to be configured on a single name server. If the split DNS architecture is improperly configured, there is a risk that internal IP addresses and host names could leak into the external view of the DNS server. Allowing private IP space to leak into the public DNS system may provide a person with malicious intent the ability to footprint the network and identify potential attack targets residing on the private 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. All of the name servers that do appear in the zone database as designated name servers get their zone data from the hidden primary via a zone transfer request. In effect, all visible name servers are actually secondary servers. This prevents potential attackers from targeting the primary name server because its IP address may not appear in the zone database.

To ensure that RRs associated with a query are really missing in a zone file and have not been removed in transit, the DNSSEC mechanism provides a means for authenticating the nonexistence of an RR. It generates a special RR called an NSEC (or NSEC3) RR that lists the RRTypes associated with an owner name as well as the next name in the zone file. It sends this special RR, along with its signatures, to the resolving name server. By verifying the signature, a DNSSEC-aware resolving name server can determine which authoritative owner name exists in a zone and which authoritative RRTypes exist at those owner names.

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, nonnetwork-accessible machine along with the zone file primary copy. This strategy is not feasible in situations in which the DNSSEC-aware name server has to support dynamic updates. To support dynamic update transactions, the DNSSEC-aware name server (which usually is a primary authoritative name server) has to have both the zone file primary copy and the private key corresponding to the zone-signing key (ZSK-private) online to immediately update the signatures for the updated RRsets. The private key corresponding to the key-signing key (KSK-private) can still be kept offline.

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 that is generated by most key generation utilities used with DNSSEC is Base64 encoded. A TSIG is a string used to generate the message authentication hash stored in a TSIG 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 also can 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. A TSIG is a string used to generate the message authentication hash stored in a TSIG 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 also can 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. A TSIG is a string used to generate the message authentication hash stored in a TSIG RR and used to authenticate an entire DNS message. Weak permissions could allow an adversary to modify the file(s), thus defeating the security objective.

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 in 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.

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, nonnetwork-accessible machine along with the zone file primary copy. This strategy is not feasible in situations in which the DNSSEC-aware name server has to support dynamic updates. To support dynamic update transactions, the DNSSEC-aware name server (which usually is a primary authoritative name server) has to have both the zone file primary copy and the private key corresponding to the zone-signing key (ZSK-private) online to immediately update the signatures for the updated RRsets. The private key corresponding to the key-signing key (KSK-private) can still be kept offline.

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, nonnetwork-accessible machine along with the zone file primary copy. This strategy is not feasible in situations in which the DNSSEC-aware name server has to support dynamic updates. To support dynamic update transactions, the DNSSEC-aware name server (which usually is a primary authoritative name server) has to have both the zone file primary copy and the private key corresponding to the zone-signing key (ZSK-private) online to immediately update the signatures for the updated RRsets. The private key corresponding to the key-signing key (KSK-private) can still be kept offline.

If remote servers to which DOD DNS servers send queries are controlled by entities outside of the U.S. government the possibility of a DNS attack is increased. The Enterprise Recursive Service (ERS) provides the ability to apply enterprise-wide policy to all recursive DNS traffic that traverses the NIPRNet-to-Internet boundary. All recursive DNS servers on the NIPRNet must be configured to exclusively forward DNS traffic traversing NIPRNet-to-Internet boundary to the ERS anycast IPs. Organizations need to carefully configure any forwarding that is being used by their caching name servers. They should only configure "forwarding of all queries" to servers within the DOD. Systems configured to use domain-based forwarding should not forward queries for mission critical domains to any servers that are not under the control of the U.S. government.

It is important to maintain the integrity of a zone file. The serial number of the SOA record is used to indicate to secondary name server that a change to the zone has occurred and a zone transfer should be performed. The serial number used in the SOA record provides the DNS administrator a method to verify the integrity of the zone file based on the serial number of the last update and ensure that all secondary servers are using the correct zone file. When a primary name server notices that the serial number of a zone has changed, it sends a special announcement to all of the secondary name servers for that zone. The primary name server determines which servers are the secondaries for the zone by looking at the list of NS records in the zone and taking out the record that points to the name server listed in the MNAME field of the zone's SOA record as well as the domain name of the local host. When a secondary name server receives a NOTIFY announcement for a zone from one of its configured primary name servers, it responds with a NOTIFY response. The response tells the primary that the secondary received the NOTIFY announcement so that the primary can stop sending it NOTIFY announcements for the zone. Then the secondary proceeds just as if the refresh timer for that zone had expired: it queries the primary name server for the SOA record for the zone that the primary claims has changed. If the serial number is higher, the secondary transfers the zone. The secondary should next issue its own NOTIFY announcements to the other authoritative name servers for the zone. The idea is that the primary may not be able to notify all of the secondary name servers for the zone itself, since it is possible some secondaries cannot communicate directly with the primary (they use another secondary as their primary). Older BIND 8 secondaries do not send NOTIFY messages unless explicitly configured to do so.

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 worse, 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. DNSSEC ensures that the answer received when querying for name resolution actually comes from a trusted name server. Since DNSSEC is still far from being globally deployed external to DOD, and many resolvers either have not been updated or do not support DNSSEC, maintaining cached zone data separate from authoritative zone data mitigates the gap until all DNS data is validated with DNSSEC. Since DNS forwarding of queries can be accomplished in some DNS applications without caching locally, DNS forwarding is the method to be used when providing external DNS resolution to internal clients. Satisfies: SRG-APP-000383-DNS-000047, SRG-APP-000246-DNS-000035

It is important to maintain the integrity of a zone file. The serial number of the SOA record is used to indicate to secondary name server that a change to the zone has occurred and a zone transfer should be performed. The serial number used in the SOA record provides the DNS administrator a method to verify the integrity of the zone file based on the serial number of the last update and ensure that all secondary servers are using the correct zone file. When a primary name server notices that the serial number of a zone has changed, it sends a special announcement to all of the secondary name servers for that zone. The primary name server determines which servers are the secondaries for the zone by looking at the list of NS records in the zone and taking out the record that points to the name server listed in the MNAME field of the zone's SOA record as well as the domain name of the local host. When a secondary name server receives a NOTIFY announcement for a zone from one of its configured primary name servers, it responds with a NOTIFY response. The response tells the primary that the secondary received the NOTIFY announcement so that the primary can stop sending it NOTIFY announcements for the zone. Then the secondary proceeds just as if the refresh timer for that zone had expired: it queries the primary name server for the SOA record for the zone that the primary claims has changed. If the serial number is higher, the secondary transfers the zone. The secondary should issue its own NOTIFY announcements to the other authoritative name servers for the zone. The idea is that the primary may not be able to notify all of the secondary name servers for the zone itself, since it is possible some secondaries cannot communicate directly with the primary (they use another secondary as their primary). Older BIND 8 secondaries do not send NOTIFY messages unless explicitly configured to do so.

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.

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. The DNS architecture needs to maintain one name server whose zone records are correct and the cache is not poisoned. In this effort, the authoritative name server may not forward queries; one of the ways to prevent this is to delete the root hints file. 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 either be configured to return a referral to the root servers or to refuse to answer the query. The requirement 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 for its intended purpose of answering authoritatively for its zone.

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. Failure to separate internal and external roles in DNS may lead to address space that is private (e.g., 10.0.0.0/24) or is otherwise concealed by some form of Network Address Translation from leaking into the public DNS system. Allowing private IP space to leak into the public DNS system may provide a person with malicious intent the ability to footprint the network and identify potential attack targets residing on the private network.

Poorly constructed NS records pose a security risk because they create conditions under which an adversary might be able to provide the missing authoritative name services that are improperly specified in the zone file. The adversary could issue bogus responses to queries that clients would accept because they learned of the adversary's name server from a valid authoritative name server, one that need not be compromised for this attack to be successful. The list of secondary servers must remain current with any changes to the zone architecture that would affect the list of secondaries. If a secondary server has been retired or is not operational but remains on the list, an adversary might have a greater opportunity to impersonate that secondary without detection, rather than if the secondary were actually online. For example, the adversary may be able to spoof the retired secondary's IP address without an IP address conflict, which would not be likely to occur if the true secondary were active.

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.

OS configuration practices as issued by the U.S. Computer Emergency Response Team (US CERT) and the National Institute of Standards and Technology's (NIST's) National Vulnerability Database (NVD), based on identified vulnerabilities that pertain to the application profile into which the name server software fits should be always followed. In particular, hosts that run the name server software should not provide any other services and therefore should be configured to respond to DNS traffic only. In other words, the only allowed incoming ports/protocols to these hosts should be 53/udp and 53/tcp. Outgoing DNS messages should be sent from a random port to minimize the risk of an attacker guessing the outgoing message port and sending forged replies.

The BIND STIG was written to incorporate capabilities and features provided in BIND version 9.9.x. However, security vulnerabilities in BIND are identified and then addressed on a regular, ongoing basis. Therefore, the product must be maintained at the latest stable versions to address vulnerabilities that are subsequently identified and can then be remediated via product updates. Failure to run a version of BIND that has the capability to implement all of the required security features and provide services compliant with the DNS RFCs can have a severe impact on the security posture of a DNS infrastructure. Without the required security in place, a DNS implementation is vulnerable to many types of attacks and could be used as a launching point for further attacks on the organizational network that is using the DNS implementation.

Providing out-of-band (OOB) management is the best first step in any management strategy. No production traffic resides on an OOB network. The biggest advantage to implementation of an OOB network is providing support and maintenance to the network that has become degraded or compromised. During an outage or degradation period, the in-band management link may not be available.

Configuring hosts that run a BIND 9.x implementation to only accept DNS traffic on a DNS interface allows a system to be configured to segregate DNS traffic from all other host traffic. The TCP/IP stack in DNS hosts (stub resolver, caching/resolving/recursive name server, authoritative name server, etc.) could be subjected to packet flooding attacks (such as SYNC and smurf), resulting in disruption of communication. The use of a dedicated interface for DNS traffic allows for these threats to be mitigated by creating a means to limit what types of traffic can be processed using a host-based firewall solution.

Hosts that run the name server software must not provide any other services. Unnecessary services running on the DNS server can introduce additional attack vectors, leading to the compromise of an organization's DNS architecture.

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 because 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.

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 because 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.

It is important to maintain the integrity of a zone file. The serial number of the SOA record is used to indicate to secondary name server that a change to the zone has occurred and a zone transfer should be performed. The serial number used in the SOA record provides the DNS administrator a method to verify the integrity of the zone file based on the serial number of the last update and ensure that all secondary servers are using the correct zone file.

The use of CNAME records for exercises, tests, or zone-spanning aliases should be temporary (e.g., to facilitate a migration). 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. Furthermore, in the case of an authoritative name server, this information is promulgated throughout the enterprise to caching servers and thus compounds the vulnerability.

If a name server were able to 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. Nevertheless, 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 (CDN) or cloud computing platforms. In the case of third-party CDNs or cloud offerings, an approved mission need must be demonstrated.

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, it should be noted that 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.

Configuration settings are the set of parameters that can be changed that affect the security posture and/or functionality of the system. Security-related parameters are those parameters impacting the security state of the application, including the parameters required to satisfy other security control requirements. Configuring the DNS server implementation to follow organizationwide security implementation guides and security checklists ensures compliance with federal standards and establishes a common security baseline across DOD that reflects the most restrictive security posture consistent with operational requirements.

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 because 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.

Configuring hosts that run a BIND 9.x implementation to only accept DNS traffic on a DNS interface allows a system firewall to be configured to limit the allowed incoming ports/protocols to 53/tcp and 53/udp. Sending outgoing DNS messages from a random port minimizes the risk of an attacker guessing the outgoing message port and sending forged replies. The TCP/IP stack in DNS hosts (stub resolver, caching/resolving/recursive name server, authoritative name server, etc.) could be subjected to packet flooding attacks (such as SYNC and smurf), resulting in disruption of communication. By implementing a specific set of firewall rules that limit accepted traffic to the interface, these risk of packet flooding and other TCP/IP based attacks is reduced.

Auditing and logging are key components of any security architecture. It is essential for security personnel to know what is being performed on the system, where an event occurred, when an event occurred, and by whom the event was triggered to compile an accurate risk assessment. Logging the actions of specific events provides a means to investigate an attack, recognize resource utilization or capacity thresholds, or identify an improperly configured DNS system. If auditing is not comprehensive, it will not be useful for intrusion monitoring, security investigations, and forensic analysis. The DNS server must audit all failed attempts at server authentication through DNSSEC and TSIG. The actual auditing is performed by the OS/NDM, but the configuration to trigger the auditing is controlled by the DNS server. Failing to act on the validation errors may result in the use of invalid, corrupted, or compromised information. The validation of bindings can be achieved, for example, by the use of cryptographic checksums. Validations must be performed automatically. 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. Satisfies: SRG-APP-000350-DNS-000044, SRG-APP-000089-DNS-000005, SRG-APP-000504-DNS-000074, SRG-APP-000504-DNS-000082, SRG-APP-000474-DNS-000073

To prevent unauthorized connection of devices, unauthorized transfer of information, or unauthorized tunneling (i.e., embedding of data types within data types), organizations must disable or restrict unused or unnecessary physical and logical ports/protocols on information systems. Applications are capable of providing a wide variety of functions and services. Some of the functions and services provided by default may not be necessary to support essential organizational operations. Additionally, it is sometimes convenient to provide multiple services from a single component (e.g., email and web services); however, doing so increases risk over limiting the services provided by any one component. To support the requirements and principles of least functionality, the application must support the organizational requirements by providing only essential capabilities and limiting the use of ports, protocols, and/or services to only those required, authorized, and approved to conduct official business or to address authorized quality of life issues.

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.

With any network service, there is the potential that an attacker can exploit a vulnerability within the program that allows the attacker to gain control of the process and even run system commands with that control. One possible defense against this attack is to limit the software to particular quarantined areas of the file system, memory, or both. This effectively restricts the service so that it will not have access to the full file system. If such a defense were in place, even if an attacker gained control of the process, the attacker would be unable to reach other commands or files on the system. This approach often is referred to as a padded cell, jail, or sandbox. All of these terms allude to the fact that the software is contained in an area where it cannot harm itself or others. A more technical term is a chroot(ed) directory structure. BIND must be configured to run in a padded cell or chroot(ed) directory structure.

Any host that can query a resolving name server has the potential to poison the server's name cache or take advantage of other vulnerabilities that may be accessed through the query service. The best way to prevent this type of attack is to limit queries to internal hosts, which need to have this service available to them. To guard against poisoning, name servers authoritative for .mil domains must 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 DoS is a condition when a resource is not available for legitimate users. When this occurs, the organization either cannot accomplish its mission or must operate at degraded capacity. A DoS attack against the DNS infrastructure has the potential to cause a DoS to all network users. As the DNS is a distributed backbone service of the internet, various forms of amplification attacks resulting in DoS, while using the DNS, are still prevalent on the internet today. Some potential DoS flooding attacks against the DNS include malformed packet flood, spoofed source addresses, and distributed DoS. Without the DNS, users and systems would not have the ability to perform simple name to IP resolution. Configuring the DNS implementation to defend against cache poisoning, employing increased capacity and bandwidth, building redundancy into the DNS architecture, using DNSSEC, limiting and securing recursive services, DNS black holes, etc., may reduce the susceptibility to some flooding types of DoS attacks.

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. 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 a number of 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). An example means to indicate the security status of child subspaces is using delegation signer (DS) resource records 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.

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 in 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 1 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.

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 the DNSSEC keys 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.

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 the DNSSEC keys 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.

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 the DNSSEC keys 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.

DNS software administrators require DNS transaction logs for a wide variety of reasons including troubleshooting, intrusion detection, and forensics. Ensuring that the DNS transaction logs are recorded on the local system will provide the capability needed to support these actions.

DNS software administrators require DNS transaction logs for a wide variety of reasons including troubleshooting, intrusion detection, and forensics. Ensuring that the DNS transaction logs are recorded on the local system will provide the capability needed to support these actions.

DNS software administrators require DNS transaction logs for a wide variety of reasons including troubleshooting, intrusion detection, and forensics. Ensuring that the DNS transaction logs are recorded on the local system will provide the capability needed to support these actions. Sending DNS transaction data to the null channel would cause a loss of important data.

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.

QNAME minimization limits the amount of information sent in DNS queries to intermediate nameservers, improving privacy by reducing the potential for DNS leak. It modifies the flow of DNS queries to reveal only what is necessary for the current server to find the next one in the resolution chain.

The fetches-per-zone option in BIND 9.x is a configuration parameter that controls the maximum number of simultaneous iterative queries a recursive resolver can send to a single authoritative server for a specific domain. This helps protect authoritative servers from being overwhelmed by queries, especially during a denial-of-service (DoS) attack.

The fetches-per-server option in BIND 9.x configures a limit on the number of outstanding requests (fetches) allowed for a single DNS server. This rate-limiting mechanism helps protect the BIND 9.x server from being overwhelmed by excessive requests to a specific server, particularly when that server is slow or unresponsive.

DNS cookies can help prevent spoofing and cache poisoning attacks by verifying the identity of both the client and server. They do this by including a cryptographic identifier (the cookie) in DNS messages, which can be verified in future messages. This makes it difficult for an attacker to learn the cookie values and thus spoof them.

Limiting the number of concurrent sessions reduces the risk of denial of service (DoS) to the DNS implementation. Name servers do not have direct user connections but accept client connections for queries. Original restriction on client connections should be high enough to prevent a self-imposed denial of service, after which the connections are monitored and fine-tuned to best meet the organization's specific requirements. Primary name servers also make outbound connections 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 primary zone server and result in DoS to legitimate users. Primary name servers should be configured to limit the hosts from which they will accept dynamic updates. Additionally, the number of concurrent clients, especially TCP clients, must be kept to a level that does not risk placing the system in a DoS state.