| Internet-Draft | The DNS SIG(0) RR | March 2025 |
| Eastlake & Stenstam | Expires 4 September 2025 | [Page] |
This document specifies use of the Domain Name System (DNS) SIG Resource Record (RR) to provide a public key based authentication of DNS requests and transactions. Such a resource record, because it has a "type covered" field of zero, is frequently called a "SIG(0)". This document obsoletes RFC 2931.¶
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This document specifies use of the Domain Name System (DNS) SIG Resource Record (RR) to provide a public key based method to authenticate DNS requests and transactions. Such a resource record, because it has a "type covered" field of zero, is frequently called a "SIG(0)". This document obsoletes [RFC2931].¶
General familiarity with DNS terminology [RFC9499] is assumed in this document.¶
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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
SIG(0) provides authentication and integrity protection for DNS transactions and requests that is not provided by the DNSSEC RRSIG RR specified in [RFC4034]. The authenticated data origin services of DNSSEC [RFC4035] either provide protected data resource records (RRs) or authenticatable denial of their existence. Those services provide no protection for¶
Transaction authentication provides some cryptographic assurance to a resolver that it is at least getting the DNS response message from the server and that the received message is in response to the request it sent. This is accomplished by adding either a TSIG RR [RFC8945] or, as specified herein, a SIG(0) RR, at the end of the additional information section of the response which authenticates the concatenation of the corresponding resolver request and the server's response.¶
Requests can also be authenticated by including a SIG(0) RR at the end of the request's additional information section. Authenticating requests serves little function in DNS servers that predate the specification of dynamic update except to enable more rigorous enforcement of restrictions on which resolvers can make what requests of the server. The method of signing requests specified herein is for authenticating dynamic update requests [RFC3007], TKEY requests [rfc2930bis], or other requests specified in the future that require authentication.¶
Depending on the request authority it is sought to establish, the private keys used in public key based request security belongs to the host composing the DNS request message or other entity composing the request or to a zone to be affected by the request or other private keys. The corresponding public key(s) can be stored in and retrieved from the DNS for verification as KEY RRs with a protocol byte of 255 (ANY).¶
A TSIG [RFC8945] RR can also be used for request and transaction authentication. There are significant differences between TSIG and SIG(0).¶
Because TSIG involves secret keys available at both the requester and server the presence of such a key can imply that the other party understands TSIG and likely has the same key installed. Furthermore, TSIG uses keyed hash authentication codes which are relatively inexpensive to compute. Thus, it is common to authenticate requests with TSIG and to authenticate responses with TSIG if the corresponding request is authenticated.¶
SIG(0) on the other hand, uses public key authentication, where the public keys can be stored in DNS as KEY RRs and a private key is stored at the signer. Existence of such a KEY RR does not necessarily imply that SIG(0) is implemented or enabled. In addition, SIG(0) involves relatively expensive public key cryptographic operations that should be minimized and the verification of a SIG(0) involves obtaining and verifying the corresponding KEY which can be an expensive operation. Indeed, a policy of using SIG(0) on all requests and verifying it before responding would, for some configurations, lead to a deadly embrace with the attempt to obtain and verify the KEY needed to authenticate the request SIG(0) resulting in additional requests accompanied by a SIG(0) leading to further requests accompanied by a SIG(0), etc. Furthermore, omitting SIG(0)s when not required on requests halves the number of public key operations required by the transaction.¶
For these reasons, SIG(0)s SHOULD only be used on requests when necessary to authenticate that the requester has some required privilege or identity. SIG(0)s on transactions are defined in such a way as to not require a SIG(0) on the corresponding request and still provide transaction protection. Which SIG(0)s are authenticated by the server or required to be authenticated by the requester SHOULD be a local configuration option.¶
The structure of SIG resource records (RRs) is the same as the structure of the RRSIG RR in [RFC4034] Section 4 except as provided below.¶
The type of the SIG RR is 24. For SIG(0) RRs, the owner name, class, TTL, and original TTL, are meaningless. The TTL fields MUST be zero and the CLASS field MUST be 255 (ANY), and these fields are ignored on receipt. A TTL of zero decreases the risk that a DNS implementation that does not understand SIG(0) will cache such an RR. To conserve space, the owner name SHOULD be root (a single zero octet).¶
The inception and expiration times in SIG(0)s are for the purpose of resisting replay attacks. They should be set to form a time bracket such that messages received outside that bracket can be ignored. In IP networks, this time bracket should not normally extend further than 5 minutes into the past and 5 minutes into the future.¶
For all transaction SIG(0)s, there MUST be a KEY RR associated with the signer name with the public key corresponding to the private key used to calculate the signature. For general transaction authentication and integrity, the signer field is a name of the originating host; for example, the host domain name used may be the inverse IP address mapping name for an IP address of the host if the relevant KEY is stored there. For SIG(0)s on requests requiring authentication, the signer relates to the authority being requested such as authority to update a zone.¶
When SIG(0) authentication on a response is desired, that SIG RR MUST be considered the highest priority of any additional information for inclusion in the response. If the SIG(0) RR cannot be added without causing the message to be truncated, the server MUST alter the response so that a SIG(0) can be included, set the TC bit, and return RCODE 0 (NOERROR). The client should at this point retry the request using TCP.¶
A DNS request may be optionally signed by including one SIG(0) at the end of the request additional information section. Such a SIG RR is distinguished by having a "type covered" field of zero. It is calculated for and signs the "data" consisting of¶
That is¶
data = RDATA | ( request without SIG(0) )¶
where "|" is concatenation and RDATA is the RDATA of the SIG(0) being calculated omitting the signature itself.¶
Similarly, a SIG(0) can be used to secure a response and the request that produced it. Such a transaction signature is calculated by using a "data" consisting of¶
data = RDATA | full request | ( response without SIG(0) )¶
where "|" is concatenation and RDATA is the RDATA of the SIG(0) being calculated less the signature itself.¶
Verification of a response SIG(0) by the requesting resolver shows that the query and response were not tampered with in transit, that the response corresponds to the intended query, and that the response comes from the queried server.¶
In the case of a DNS message via TCP, a SIG(0) on the first data packet is calculated with "data" as above and for each subsequent packet, it is calculated as follows:¶
data = RDATA | ( DNS payload without SIG(0) ) | previous packet¶
where "|" is concatenations, RDATA is as above, and previous packet is the previous DNS payload including DNS header and SIG(0) but not the TCP/IP header. As an alternative, TSIG may be used after, if necessary, setting up a key with TKEY [rfc2930bis].¶
Except where needed to authenticate an update, TKEY, or similar privileged request, servers are not required to check a request SIG(0).¶
Requests and responses can either have a TSIG RR or a SIG(0) RR but not both.¶
If a SIG RR is at the end of the additional information section of a response and has a type covered of zero, it is a transaction signature covering the response and the request that produced the response.¶
If the time a SIG(0) on a request is received is outside the interval indicated by the inception and expiration times in the SIG(0), the BADTIME error is returned.¶
If a response's SIG(0) check succeeds, such a transaction authentication SIG does NOT directly authenticate the validity of any data RRs in the message. However, it authenticates that they were sent by the queried server and have not been altered in transit. (Only an RRSIG RR [RFC4034] signed by the zone or a key tracing its authority to the zone or to resolver configuration can directly authenticate data RRs, depending on resolver policy.) If a resolver or server does not implement transaction and/or request SIG(0)s, it MUST ignore them without error where they are optional and treat them as failing where they are required.¶
Since SIG(0) does not provide for an Original ID field as provided in TSIG [RFC8945] and does not provide an error field as provided in TSIG and TKEY [rfc2930bis], the Original ID of a request with a SIG(0) may be provided, and errors are returned in an EDNS(0) RR as an option [RFC6891] with option code TBD.¶
+0 (MSB) +1 (LSB)
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: | OPTION-CODE = TBD |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | OPTION-LENGTH = 4 |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
4: | Original ID |
+- -+
6: | Error return value |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
A server acting as a forwarding server of a DNS message SHOULD check for the existence of a SIG(0) record. If it cannot verify the SIG(0), the server MUST forward the message unchanged including the SIG(0). If the SIG(0) passes all checks and verifies, the forwarding server MUST, if possible, include a SIG(0) or TSIG of its own to the destination or the next forwarder. If no transaction security is available to the destination and the message is a query, and if the corresponding response has the AD flag (see [RFC4035]) set, the forwarder MUST clear the AD flag before adding the SIG(0) to the response and returning the result to the system from which it received the query.¶
A forwarded SIG(0) is not verifiable unless the original transaction ID is preserved by, for example,¶
Private keys used to create SIG(0) RRs are very sensitive information and all available steps should be taken to protect them on every host on which they are stored. Such hosts may need to be physically protected. If they are multi-user machines, great care should be taken so that unprivileged users have no access to private keying material.¶
The inclusion of the SIG(0) inception and expiration time under the signature improves resistance to replay attacks.¶
More?¶
IANA is requested to assign an EDNS OPT number in the "DNS EDNS0 Option Codes (OPT)" Registry as follows:¶
| Value | Name | Status | Reference |
|---|---|---|---|
| TBD | SIGZERO | Standard | [this document] |
RFC Editor: Please delete this section before publication.¶
The comments and suggestions of the following are gratefully acknowledged:¶
tbd¶
The comments and suggestions of the following persons were incorporated into [RFC2931], which was the previous version of this document, and are gratefully acknowledged:¶
Olafur Gudmundsson, Ed Lewis, Erik Nordmark, Brian Wellington.¶