DNSIND Working Group

This document proposes a method for performing secure Domain Name System (DNS) dynamic updates. The method described here is intended Expires July 2000 [Page 1] INTERNET-DRAFT Simple Secure Dynamic Update January 2000 to be flexible and useful while requiring as few changes to the protocol as possible. The authentication of the dynamic update message is separate from later DNSSEC validation of the data. Secure communication based on authenticated requests and transactions is used to provide authorization. 1 Introduction This document defines a means to secure dynamic updates of the Domain Name System (DNS), allowing only authorized sources to make changes to a zone’s contents. The existing unsecured dynamic update operations form the basis for this work. Familiarity with the DNS system [RFC1034, RFC1035] and dynamic update [RFC2136] is helpful and is assumed by this document. In addition, knowledge of DNS security extensions [RFC2535], SIG(0) transaction security [RFC2535], and TSIG transaction security [TSIG] is recommended. This document updates portions of RFC 2535, in particular section 3.1.2. This document obsoletes RFC 2137, an alternate proposal for secure dynamic update, due to implementation experience. 1.1 Overview of DNS Dynamic Update DNS dynamic update defines a new DNS opcode and a new interpretation of the DNS message if that opcode is used. An update can specify insertions or deletions of data, along with prerequisites necessary for the updates to occur. All tests and changes for a DNS update request are restricted to a single zone, and are performed at the primary server for the zone. The primary server for a dynamic zone must increment the zone SOA serial number when an update occurs or before the next retrieval of the SOA. 1.2 Overview of DNS Transaction Security Exchanges of DNS messages which include TSIG [TSIG] or SIG(0) [RFC2535] records allow two DNS entities to authenticate DNS requests and responses sent between them. A TSIG MAC (message authentication code) is derived from a shared secret, and a SIG(0) is generated from a private key whose public counterpart is stored in DNS. In both cases, a record containing the message signature/MAC is included as the final resource record in a DNS message. Keyed hashes, used in TSIG, are inexpensive to calculate and verify. Public key encryption, as used in SIG(0), is more scalable as the public keys are stored in DNS. Expires July 2000 [Page 2] INTERNET-DRAFT Simple Secure Dynamic Update January 2000 1.3 Comparison of data authentication and message authentication DNSSEC SIG records can be used to protect the integrity of individual RRs or RRsets in an update message. However, this cannot sufficiently protect the dynamic update request. SIG records do not cover the message header, which includes record counts. Therefore, it is possibly to maliciously insert or remove RRsets without causing a verification failure. A SIG record can be used to protect the contents of the zone section (an SOA record). Including such a SIG record in the zone section violates the dynamic update protocol. If SIG records were used to protect the prerequisite section, it would be impossible to determine whether the SIGs themselves were a prerequisite or simply used for validation. In the update section, signing requests to add an RRset is straightforward, and this signature could be permanently used to protect the data, as specified in [RFC2535]. However, if an RRset is deleted, there is no data for a SIG to cover. Requiring SIGs in the zone, prerequisite, and update sections might be a feasible solution. Multiple signatures would be generated and verified for each update, though, which requires considerable processing time. Message based authentication, using TSIG or SIG(0), avoids all of these problems. Only one signature/MAC is generated for the entire message, and it protects the integrity of the message header and all sections, as well as having the advantage that only one verification is performed. 1.4 Data and message signatures As specified in [signing-auth], the DNSSEC validation process performed by a resolver MUST NOT process any non-zone keys unless local policy dictates otherwise. When performing secure dynamic update, all zone data modified in a signed zone MUST be signed by a relevant zone key. This completely disassociates authentication of an update request from authentication of the data itself. The primary usefulness of host and user keys, with respect to DNSSEC, is to authenticate messages, including dynamic updates. Thus, host and user keys MAY be used to generate SIG(0) records to authenticate updates and MAY be used in the TKEY [TKEY] process to generate TSIG shared secrets. In both cases, no SIG records generated by non-zone keys will be used in a DNSSEC validation process unless local policy dictates. Expires July 2000 [Page 3] INTERNET-DRAFT Simple Secure Dynamic Update January 2000 Authentication of data, once it is present in DNS, only involves DNSSEC zone keys and signatures generated by them. 1.5 Signatory strength [RFC2535, section 3.1.2] defines the signatory field of a key as the final 4 bits of the flags field, but does not define its value. This proposal leaves this field undefined. Updating [RFC2535], this field SHOULD be set to 0 in KEY records, and MUST be ignored. 2 Authentication TSIG or SIG(0) records MUST be included in all secure dynamic update messages. This allows the server to verifiably determine the originator of a message. If the message contains authentication in the form of a SIG(0), the identity of the sender (that is, the principal) is the owner of the KEY RR that generated the SIG(0). If the message contains a TSIG generated by a statically configured shared secret, the principal is the same as or derived from the shared secret name. If the message contains a TSIG generated by a dynamically configured shared secret, the principal is the same as the one that authenticated the TKEY process; if the TKEY process was unauthenticated, no information is known about the principal, and the associated TSIG shared secret MUST NOT be used for secure dynamic update. SIG(0) signatures SHOULD NOT be generated by zone keys, since transactions are initiated by a host or user, not a zone. DNSSEC SIG records (other than SIG(0)) MAY be included in an update message, but MUST NOT be used to authenticate the update request. If an update fails because it is signed with an unauthorized key, the server MUST indicate failure by returning a message with RCODE REFUSED. Other TSIG, SIG(0), or dynamic update errors are returned as specified in the appropriate protocol description. Expires July 2000 [Page 4] INTERNET-DRAFT Simple Secure Dynamic Update January 2000 3 Policy All policy is configured by the zone administrator and enforced by the zone’s primary name server. Policy dictates the authorized actions that an authenticated principal can take. Policy checks are based on the principal and the desired action, where the principal is derived from the message signing key and applied to dynamic update messages signed with that key. The server’s policy defines criteria which determine if the key used to sign the update is permitted to perform the requested updates. By default, a principal MUST NOT be permitted to make any changes to zone data; any permissions MUST be enabled though configuration. The policy is fully implemented in the primary zone server’s configuration for several reasons. This removes limitations imposed by encoding policy into a fixed number of bits (such as the KEY RR’s signatory field). Policy is only relevant in the server applying it, so there is no reason to expose it. Finally, a change in policy or a new type of policy should not affect the DNS protocol or data format, and should not cause interoperability failures. 3.1 Standard policies Implementations SHOULD allow access control policies to use the principal as an authorization token, and MAY also allow policies to grant permission to a signed message regardless of principal. A common practice would be to restrict the permissions of a principal by domain name. That is, a principal could be permitted to add, delete, or modify entries corresponding to one or more domain names. Implementations SHOULD allow per-name access control, and SHOULD provide a concise representation of the principal’s own name, its subdomains, and all names in the zone. Additionally, a server SHOULD restrict updates by RR type, so that a principal could add, delete, or modify specific record types at certain names. Implementations SHOULD allow per-type access control, and SHOULD provide concise representations of all types and all ‘‘user’’ types, where a user type is defined as one that does not affect the operation of DNS itself. 3.1.1 User types User types include all data types except SOA, NS, SIG, and NXT. SOA and NS SHOULD NOT be modified by normal users, since these types create or Expires July 2000 [Page 5] INTERNET-DRAFT Simple Secure Dynamic Update January 2000 modify delegation points. The addition of SIG records can lead to attacks resulting in additional workload for resolvers, and the deletion of SIG records could lead to extra work for the server if the zone SIG was deleted. Note that these records are not forbidden, but not recommended for normal users. NXT records MUST NOT be created, modified, or deleted by dynamic update, as their update may cause instability in the protocol. This is an update to RFC 2136. Issues concerning updates of KEY records are discussed in the Security Considerations section. 3.2 Additional policies Users are free to implement any policies. Policies may be as specific or general as desired, and as complex as desired. They may depend on the principal or any other characteristics of the signed message. 4 Interaction with DNSSEC An authorized update request MAY include SIG records with each RRset. Since SIG records (except SIG(0) records) MUST NOT be used for authentication of the update message, they are not required. If the updated zone is secured, the data affected by an update operation MUST be secured by one or more SIG records. For each RRset, if the update includes a valid signature by a zone key, this signature SHOULD be reused. Otherwise, the server MUST generate SIG records with one or more zone keys (of which the private components MUST be online). If multiple zone keys are online and an RRset requires a signature, a SIG MUST be generated by at least one of the zone keys. If a principal is authorized to add SIG records and there are SIG records in the request, the following rules are applied. If the SIG was generated by a zone key for the relevant zone, verification is attempted (the public key must be available if the determination that it is a zone key was made). If successful, the SIG is retained; otherwise, the SIG is dropped. Otherwise, the SIG is retained without verification, since it is considered immaterial to the DNSSEC validation process. The server MAY examine SIG records and drop SIGs with a temporal validity period in the past. At the completion of the update process, each updated RRset must be signed in accordance with the zone’s signing policy; the SIGs must either be included in the update or generated by the server. The server MUST also, if necessary, generate a new SOA record and new NXT records, and sign these with the appropriate zone keys. NXT records Expires July 2000 [Page 6] INTERNET-DRAFT Simple Secure Dynamic Update January 2000 are explicitly forbidden. SOA updates are allowed, since themaintenance of SOA parameters is outside of the scope of the DNSprotocol.5 Security considerationsThis document requires that a zone key and possibly other cryptographicsecret material be held in an on-line, network-connected host, mostlikely a name server. This material is at the mercy of host security toremain a secret. Exposing this secret puts DNS data at risk ofmasquerade attacks. The data at risk is that in both zones served bythe machine and delegated from this machine.Allowing updates of KEY records may lead to undesirable results, since aprincipal may be allowed to insert a public key without holding theprivate key, and possibly masquerade as the key owner.6 AcknowledgementsThe author would like to thank the following people for review andinformative comments (in alphabetical order):Donald EastlakeOlafur GudmundssonAndreas GustafssonBob HalleyStuart KwanEd Lewis 7 References[RFC1034] P. Mockapetris, ‘‘Domain Names Concepts and Facilities,’’RFC 1034, ISI, November 1987.[RFC1035] P. Mockapetris, ‘‘Domain Names Implementation andSpecification,’’ RFC 1035, ISI, November 1987.[RFC2136] P. Vixie (Ed.), S. Thomson, Y. Rekhter, J. Bound ‘‘DynamicUpdates in the Domain Name System,’’ RFC 2136, ISC & Bellcore& Cisco & DEC, April 1997.[RFC2137] D. Eastlake ‘‘Secure Domain Name System Dynamic Update,’’ RFC2137, CyberCash, April 1997.[RFC2535] D. Eastlake, ‘‘Domain Name System Security Extensions,’’ RFC2065, IBM, March 1999. Expires July 2000[Page 7] INTERNET-DRAFTSimple Secure Dynamic UpdateJanuary 2000 [TSIG] P. Vixie (Ed.), O. Gudmundsson, D. Eastlake, B. Wellington‘‘Secret Key Transaction Signatures for DNS (TSIG),’’ draft-ietf-dnsind-tsig-13.txt, ISC & NAILabs & IBM & NAILabs,December 1999.[TKEY] D. Eastlake ‘‘Secret Key Establishment for DNS (TKEY RR),’’draft-ietf-dnsind-tkey-03.txt, IBM, December 1999.[signing-auth]B. Wellington ‘‘Domain Name System Security (DNSSEC) SigningAuthority,’’ draft-ietf-dnsext-signing-auth-00.txt, NAILabs,January 2000.8 Author’s Address Brian WellingtonNAILabsNetwork Associates3060 Washington Road (Rt. 97)Glenwood, MD 21738+1 443 259 2369 9 Full Copyright StatementCopyright (C) The Internet Society (2000). All Rights Reserved.This document and translations of it may be copied and furnished toothers, and derivative works that comment on or otherwise explain it orassist in its implmentation may be prepared, copied, published anddistributed, in whole or in part, without restriction of any kind,provided that the above copyright notice and this paragraph are includedon all such copies and derivative works. However, this document itselfmay not be modified in any way, such as by removing the copyright noticeor references to the Internet Society or other Internet organizations,except as needed for the purpose of developing Internet standards inwhich case the procedures for copyrights defined in the InternetStandards process must be followed, or as required to translate it intolanguages other than English.The limited permissions granted above are perpetual and will not berevoked by the Internet Society or its successors or assigns.This document and the information contained herein is provided on an "ASIS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASKFORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT Expires July 2000[Page 8] INTERNET-DRAFTSimple Secure Dynamic UpdateJanuary 2000 LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOTINFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY ORFITNESS FOR A PARTICULAR PURPOSE." 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