Upper limit values for DNS

Introduction There are parameters in the DNS protocol that do not have clear upper limits. For example, the number of alias levels using CNAME Resource records, the number of name servers, the number of resource records in an RRSet, the number of delegation levels using unrelated name server names, and the number of DNSKEYs for each domain name. If a protocol is implemented without considering the upper limit, it may become vulnerable to DoS attacks, and several attack methods have been proposed. This draft proposes reasonable upper limits for DNS protocols.

Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. Many of the specialized terms used in this document are defined in DNS Terminology .

Problem Statement There are parameters in the DNS protocol that do not have clear upper limits. For example, the number of Resource Records in an RRSet, the number of alias levels using CNAME Resource records, and the number of delegation levels using unrelated name server names. If a protocol is implemented without considering the upper limit, it may become vulnerable to DoS attacks. In recent years, DNS vulnerabilities research have been actively progressed and many vulnerabilities have been made public. Each time a vulnerability is discovered, upper limits on the execution time, number of attempts, and size are added to DNS software implementations. If we define upper limits for some parameters in advance and treat anything that exceeds them as an error, we can reduce the need to respond reactively. Authoritative servers are expected to enter an error state when they read zone files, receive zone transfers, or receive dynamic updates that contains RRsets that exceed the upper limit. Full-service resolvers can prevent malfunction by treating a name resolution error as responses from authoritative servers that exceed the upper limit.

Recent upper limit values in implementations Number of Resource Records in an RRSet BIND 9 introduced 'max-records-per-type' parameter, and the default is 100. CVE-2024-1737 "BIND's database will be slow if a very large number of RRs exist at the same name" was reported and BIND 9.18.28 implemented the limit. Number of RRSIGs/DNSKEYs/DSs in a RRSet KeyTrap is a vulnerability caused by the fact that there is no upper limit on the number of DNSKEY, DS, or RRSIG resource records. Unbound introduced the maximum number of RRSIG validations for an RRset (MAX_VALIDATE_RRSIGS) as 8, and the maximum allowed digest match failures per DS, for DNSKEYs with the same properties (MAX_DS_MATCH_FAILURES) as 4. Number of alias levels using CNAME Resource records Unbound and BIND 9 introduced 'max-query-restarts' parameter, and the default is 11. (Hard limit on the number of times Unbound is allowed to restart a query upon encountering a CNAME record.)

Possible upper limits

Possible upper limit items Number of Resource Records in a RRSet Number of NS Resource Records in a delegation Number of DS Resource Records in a delegation Number of glue RRs in a delegation Number of DNSKEY Resource Records in a DNSKEY RRSet Number of RRSIG RRs for each name and type Number of levels of unrelated only delegations Number of alias levels using CNAME Resource records

Packet size limits There were comments that there are size limitations even if no precise upper limits are set. The DNS packet format has an upper limit of 65535 octets, so an RRset cannot exceed that size. However, the size 65535 is large, attackers use this upper limit to carry out resource-wasting attacks. Also, the upper limit size of a single resource record is 65535 octets minus DNS header size because RDLENGTH is 16 bits. Section 4.2.1 UDP usage of limits the UDP message size 512. The size of a DNS response that can be sent using unfragmented UDP is about 1400 octets.

Upper limit concept Best Current Practice documents should allow for values that are currently in widespread use. However, apparent anomalies may be excluded. It is desirable to determine the upper limit values by conducting extensive measurements on the Internet and excluding obvious errors or malicious errors, and to set the value that has the least impact. For this reason, this document specifies desirable upper-limit values and upper limit values that should result in an error if exceeded. Some DNS RFCs define upper limit values.

Number of Resource Records in a RRSet Since there are 13 root name servers and 13 name servers for com and net TLDs, the maximum number of NS RR in an NS RRSet should be larger than or equal to 13. Since there are 13 name servers for root, com, net and they have both IPv4 and IPv6 addresses, 26 glue records in a delegation should be allowed. In recent years, there have been cases where many TXT resource records have been set at the zone apex. Many services seem to request their designated authentication tokens written as TXT records at the zone apex to verify domain name registrants. However, there seem to be cases where authentication tokens are only added, as there seems to be no procedure for deleting them once they have been set. It is necessary to standardize and deploy , and to write TXT records not at the zone apex, but application-specific undercore prefix labels.

Number of alias levels using CNAME/DNAME Many resolver implementations can resolve over 10 CNAME aliases. Unbound and BIND 9 introduced 'max-query-restarts' parameter, and the default is 11. However, a stub resolver that receives a response containing multiple CNAME aliases must find the final A, AAAA Resource record that corresponds to the CNAME in each application. To avoid this complexity, the recommended number of CNAME chains is 1. CNAME/DNAME aliases with more than three levels are too complicated.

Number of RRSIGs/DNSKEYs/DSs in a RRSet KeyTrap is a vulnerability caused by the fact that there is no upper limit on the number of DNSKEY, DS, or RRSIG resource records. If there were upper limits on these, the damage could be mitigated. Therefore, considering the DNSKEY rollover and the multi-signer model, the maximum number of DNSKEYs for both KSK and ZSK may be 6. The maximum number of DS RRs in a DS RRSet may be 3. The number of RRSIG RRs for each owner name and type pair may be 6. Unbound introduced the maximum number of RRSIG validations for an RRset (MAX_VALIDATE_RRSIGS) as 8.

Number of delegation levels using unrelated name server names states that all in-domain glue records are attached to the delegation response. Therefore, using in-domain name server names for DNS delegation minimizes name resolution costs. Unrelated (or, rarely sibling) name server names are used/required for DNS hosting services. However, using unrelated name server names increases the name resolution costs and may increase the likelihood of name resolution errors. For some domain names, there are multiple layers of dependence on unrelated name server names when resolving the name. If information on unrelated name server names is not in the cache, such as immediately after the recursive resolver is started, the recursive resolver must perform A/AAAA name resolution for the unrelated name server names. If the name server for the domain name that holds the name server name also has only unrelated name server names, the resolver must also perform A/AAAA name resolution for those unrelated name server names. If the time required for name resolution exceeds one second, the stub resolver may treat the name resolution as having timed out, and users operating a browser, etc., may become impatient and reload the page. Even if the first name resolution takes too long, the A/AAAA RRs for the name server names are cached for each TTL time, so name resolution from the second time onwards can be performed in a short time, and there is thought to be little actual harm, but if the TTL time expires, it will be necessary to start over with name resolution for the unrelated name server names, and the name resolution time will be extended again. Frequent use of unrelated name server names can cause unstable name resolution, such as a significant difference between the time it takes to resolve a name and the time it takes to resolve a name, or sometimes the name resolution time is so long that it is treated as a timeout. This section proposes to use in-domain name servers as much as possible for name resolution of unrelated name server names to reduce the name resolution costs. Furthermore, there are cases where cyclic dependencies in delegation occur, settings that depend on sibling glue, and cases where the sibling glue disappears or some name servers stop responding, making it impossible to resolve names. pointed out attacks and countermeasures that use increased load due to cyclic dependencies. Many cyclic delegations are likely due to misconfigurations. To avoid complex name resolutions and misconfigurations, it is better to avoid using unrelated name server names as much as possible. Unrelated name server names SHOULD be hosted by a domain name with at least one in-domain name server name. In other words, DNS providers SHOULD have at least one in-domain nameserver for their domain names. From ICANN's Centralized Zone Data Service (CZDS), an investigation into the dependencies of second-level domain names within the five largest TLDs (com, net, org, info, biz) revealed that less than 10% were unclassifiable (dependence on other TLDs or errors), around 0.3% were resolvable by in-domain name server names, 64% were dependent on unrelated name server names at only the first level, 26% were dependent on unrelated name server names at two levels, and less than 1% were dependent on unrelated name server names at three levels or more. Therefore, if delegation using only unrelated name server names were permitted up to two levels, name resolution would be successful for 99% of domain names.

Recommendation of DNS upper limit values Name Desirable upper limit hard limit protocol limit implementation limit DNS message size 1400 65535 UDP/DNS message size (without EDNS) 512 UDP/DNS message size (with EDNS) 1400 (RFC9715) 1232 (DNS Flag Day) number of RRs in a RRSet 13 100 (BIND 9) number of NS RRs in a delegation 13 13 root/TLDs NS number of glue RRs in a delegation 26 26 TLD glue number of DS RRs in a delegation 3 number of DNSKEY RRs 6 number of RRSIG RRs for each name and type 2 8 8 (Unbound) number of levels of unrelated only delegations 1 (2) number of CNAME/DNAME chains 1 11 11 (Unbound, BIND 9) DNS software is expected to make these items configurable parameters that operators can control. Recursive resolvers SHOULD respond with a name resolution error (Server Failure) if it receives a response from an authoritative server that exceeds the hard limits. Authoritative servers SHOULD be in an error state if they find RRSets that exceed the hard limits when they load zone files, receive zone data by zone transfers, or receive DNS Updates.

IANA Considerations This document requests no IANA actions.

Security Considerations