DNSEXT Working Group Olafur Gudmundsson
<draft-ietf-dnsext-delegation-signer-01.txt><draft-ietf-dnsext-delegation-signer-02.txt> Updates: RFC 1035, RFC 2535, RFC 3008. Delegation Signer record in parent. Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Comments should be sent to the authors or the DNSEXT WG mailing list email@example.com This draft expires on January 15,February 20, 2002. Copyright Notice Copyright (C) The Internet Society (2001). All rights reserved. Abstract One ofThe Delegation Signer (DS) RR set is stored in a delegating (parent) zone at each delegation point, and indicates the biggest problemskeys used in DNS occur when records ofthe same type can appear on both sidesdelegated (child) zone. The main design goal of an delegation. Ifthe contentsDS RR simplify the operation of these sets differs clients can get confused. RFC2535 KEY records followssecure delegations by eliminating the need to store the same model as for NS records,RR in parent and child, as is responsible for the records butdone with the child is responsible forNS RR set and the contents. This document proposes to store a different recordKEY set in the parent that specifies which one of the child's keys are authorizedRFC2535. Secure resolvers need to sign thetake an additional step with DS to verify a child's KEY RR set. This changeOperationally this schema is not backwards compatible with RFC2535 but simplifies DNSSEC operation.much simpler as operation of the two zones at delegation is now decoupled to great extent. 1 - Introduction Familiarity with the DNS system [RFC1035], DNS security extensions [RFC2535] and DNSSEC terminology [RFC3090] is important. When the same data can reside in two administratively different DNS zones sources it is common thatzones, the data frequently gets out of sync. NS record in a zone indicates that therethis name is a delegation at this nameand the NS record lists the authorative servers for the real zone. Based on actual measurements 10-30% of all delegations in the Internet have differing NS sets at parent and child. There are number of reasons for this, including lack of communication between parent and child and bogus name-servers being listed to meet registrar requirements. DNSSEC [RFC2535,RFC3008,RFC3090] specifies that child must have its KEY set signed by the parent to create a verifiable chain of KEYs. There is some debate, where the signed KEY set should reside, parent[Parent] or child[RFC2535]. If the KEY set resides at the child, frequent two way communication is needed between the two parties,parties. First the child needs to transmit the key sets upset to parent and then the parent must send the signed set or signatures downto child. If the KEY set resides at the parent[Parent]parent the communication is reduced having onlyas the child send updatedonly sends changed key sets to parent. DNSSEC[RFC2535] requires that the parent store NULL key set for unsecure children, this complicates resolution process asin many cases as servers for both parent and child need to be queried for KEY set if the [Parent] proposalchild server does not return a KEY set. Storing the KEY record only in the parent zone simplifies this. Furtherthis and allows the elimination of the NULL key set. Another complication of the DNSSEC KEY model is that KEY record is used to store DNS zone keys and public keys for other protocols. This can lead to large key sets at delegation points.There are number of potential problems with this including: 1. KEY set may become quite large if many applications/protocols store their keys at the zone apex. Example ofPossible protocols are IPSEC, HTTP, SMTP, SSH etc.and others that use public key cryptography. 2. Key set may require frequent updates. 3. Probability of compromised/lost keys increases and triggers emergency key rollover.rollover procedures. 4. Parent may refuse sign key sets with NON DNS zone keys. 5. Parent may not have QoS on key changes that meetsmeet the child's expectations.expectations in turnaround time in resigning the key set. Given these and other reasons there is good reason to explore alternatives to using only KEY records to create chain of trust. Some of these problems can be reduced or eliminated by operational rules or protocol changes. To reduce the number of keys at apex, a rule to require applications to store their KEY records at the SRV name for that application is one possibility. Another is to restrict KEY record to DNS keys only and create a new type for all non DNS keys. Third possible solution is to ban the storage of non DNS related keys at zone apex. There are other possible solutions but they are outside the scope of this document. 1.11.2 - Reserved words The key words "MAY","MAY NOT", "MUST", "MUST NOT", "REQUIRED", "RECOMMENDED", "SHOULD", and "SHOULD NOT" in this document are to be interpreted as described in RFC2119. 2 - DS (Delegation KEY Signer) 2.1 - Delegation Signer Record model This document proposes an alternative to the KEY record chain of trust, that uses a special record that can only reside at the parent. This record will identify the key(s) that child will useare allowed to self sign its own KEY set. The chain of trust is now established by verifying the parent KEY set, the DS set from the parent and thenthe KEY set at the child. This is cryptographically equivalent to just using KEY records. Communication between the parent and child is reduced asgreatly reduced, since the parentchild only needs to know ofnotify parent about changes in DNS zone KEY(s) used tokeys that sign theits apex KEY set. If other KEY records are stored at the zone apex, the parent does not need to be awareRRset. Parent is ignorant of them. This approach has the advantage that it minimizes the communication betweenall other keys in the parent and childchild's apex KEY RRset, and the child is the authority for the KEY set withmaintains full control over the contents. This enables each to operateapex KEY set and its content. Child can maintain each zone independent of each other.any policies over its DNS and other KEY usage with minimal impact on parent. Thus if child wants to have frequent key rollover for its DNS keys parent does not need to be aware of it as the child can use one key to only sign its apex KEY set and other keys to sign the other record sets in the zone. The child can just as well use the same key to sign all records in its zone. Another advantage is that thisThis model fits well with slow rolloutroll out of DNSSEC and islands of security model. In the islands of security model someone that trusts "good.example." preconfigurescan preconfigure a key from "good.example." as a trusted keys and from then on trusts any data that is signed by that key or has a chain of trust to that key. If "example." starts advertising DS recordsrecords, "good.example." does not have to change operations, by suspending self-signing. IfDS records can also be used to identify trusted keys instead of KEY records. One further advantage is the information stored in the parent is minimized, as only records for secure delegations are needed. The main disadvantage of this approach is double the number of signaturesthat need to be verified for the each delegationverifying delegations KEY set.set requires twice as many signature verification operations. There is no impact on verifyingthe number of signatures verified for other recordRR sets. 1.2 - Reserved words The key words "MUST", "MUST NOT", "SHOULD", and "MAY" in this document are to be interpreted as described in RFC2119. 2 - DS (Delegation KEY signer) record: 2.12.2 Protocol change A DS recordRR set MUST onlyappear at each secure delegations in the parent zone. The record lists the child's keys that SHOULD sign the child's key set. Insecuredelegation MUST NOT havefrom a secure zone. If a DS record,RR set accompanies the presence of DS record SHOULD be considered a hintNS RR set, the intent is to state that the child might be secure. Resolver MUST only trust KEY records that matchzone is secured. If an NS RR set exists without a DS record. NOTE: It has been suggestedRR set the intent is to state that NULL DS record for insecure childrenthe child zone is better than no record.unsecure. The advantage ispublic keys indicated in the DS RR set are the keys the child has informed the parent, the child allows to have authenticated denialsign the child zone apex KEY RR set. Barring emergency, the intent of child's security status,the drawback is for large delegating zones there will be many NULLDS records. If parent uses NXT records adding NXT recordRR set it to indicate to state the authority sectionchild's zone keyset signing keys. If the child's APEX is not signed by any KEY indicated in the cases when no DS record exists at delegation will give the same result as NULLDS record. WG please comment on which approach is better.RR set than any of number of problems may have occurred, and are described later. Updates RFC2535 sections 2.3.4 and 3.4, as well as RFC3008 section 2.7: Delegating zones MUST NOT store KEY records for delegations. The only records that can appear at delegation in parent are NS, SIG, NXT and DS. Zone MUST self sign its apex KEY set, it SHOULD sign it with a key that corresponds to a DS record in the parent. The KEY used to sign the apex KEY RRset CANMAY sign other RRsets in the zone. If child apex KEY RRset is not signed with one of the keys specified in the DS record the child is locally secure[RFC3090] and SHOULD only be considered secure the resolver has been instructedconfigured to trust the key used, via preconfiguration.used. Authorative server answeringfor a query, that has the OK bit set[OKbit],zone with DS records MUST include the DS setrecords in answers for a delegation, when the additional section if the answerOKbit[okbit] is a referralset in the query and thereif space is space.available in answer. DS records SHOULD have lower priority than address records but higher priority than KEY records. Caching servers SHOULD return the DS record in the additional section under the same condition. 220.127.116.11.1 - Comments on protocol change Over the years there has been various discussions on that the delegation model in DNS is broken as there is no real good way to assert if delegation exists. In RFC2535 version of DNSSEC the authentication of a delegation is the NS bit in the NXT bitmap at the delegation point. Something more explicit is needed and the DS record addresses this for secure delegations. DS record is the first DNS record to bethat can only stored atappear on the upper side of a delegation. NS records appear at both sides as do SIG and NXT. All other records can only appear at the lower side. This will cause some problems as servers authorative for parent may reject DS record even if the server understands unknown types, or not hand them out unless explicitly asked. Similarly a nameserver acting as a authorative for child and as a caching recursive server may never return the DS record. A caching server does not care from which side DS record comes from and thus shoulddoes not have to be changed if it supports unknown types. Different TTL values on the childschild's NS set and parents DS set may cause the DS set to expire before the NS set. In this case an non-DS aware server would ask the child server for the DS set and get NXDOMAIN answer. DS aware server will know to ask thea parent DNS server for the DS record. Secure resolvers need to know about the DS record and how to interpret it. In the worst case, introducing the DS record, doubles the signatures that need to be checked to validate a KEY set. Note: The working group must determine if the tradeoff between more work in resolvers is justified by the operational simplification of this model. The author thinkthinks this is a small price to pay to have a cleaner delegations structure. One argumentargument, put forward is that DNS should be optimized for read when ever possible, and on the face of it adding the DS record makes reading data from DNS more expensive. The operational complexities and legal hurdles thatrelated to KEY records in either parents or children makemay prevent DNSSEC to ever get deployed. 2.2deployment. 2.3 Wire format of DS record The DS (type=TDB) record consists of algorithm, size,key tag and SHA-1 digest of the public key KEY record allowed to sign the child's delegation. 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | key tag | sizealgorithm | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| algorithm+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SHA-1 digest | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | (20 bytes) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | | +-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The key tag is calculated as specified in RFC2535, the size is the size of the public key in bits as specified in the document specifying the algorithm.Algorithm MUST be an algorithm number assigned in the range 1..251. The SHA-1 digest is calculated over the canonical name of the delegation followed by the RDATA of the KEY record. The size of the DS RDATA is 2523 bytes, regardless of thekey size. NOTE: if 160 bits is to large the SHA-1 digest can be shortened but that weakens the overall security of the system. 18.104.22.168.1 Justifications for fields The algorithm and sizekey tag fields are here to allow resolvers to quickly identify the candidate KEY records to examine. Key Tag is to allow quick check if this is a good candidate.The key tag is redundant but providesadds some greater assurance than SHA-1 digest on its own. SHA-1 is a strong cryptographic checksum, it is real hard for attacker to generate a KEY record that has the same SHA-1 digest. Making sure that the KEY record is a valid public key is much harder. Combining the name of the key and the key data as input to the digest provides stronger assurance of the binding. Combining the SHA-1 with the other fields makes the task of the attacker is as hard breaking the public key. Even if someone creates a database of all SHA-1 key hashes seen so far, the addition of the name renders that database useless for attacks against random zones. 2.3 Presentation format of DS record The presentation format of DS record consists of 2 numbers, followed by eithera KEY record that has the same SHA-1 digest. Combining the name of the signature algorithm orkey and the algorithm number. The digest iskey data as input to be presented in hex. 2.4 Justifications for compact formatthe digest provides stronger assurance of the binding. This format allows concise representation of the keys that child will use, thus keeping down the size of the answer for the delegation, reducing the probability of packet overflow. The SHA-1 hash is strong enough to uniquely identify the key. This is similar to the PGP footprint. EachDS record has RDATA size of 25, regardless of the size of the keys, keeping the answers from the parent smaller than if public key was used. The smallest currently defined KEY record RDATA is 70 bytes. Compact DS formatis also betterwell suited to listlists trusted keys for islands of security in configuration files. 2.4 Presentation format of DS record The presentation format of DS record consists of 2 numbers followed by digest presented in hex. 2.5 Transition issues for installed base RFC2535 compliant resolver will assume that all DS secured delegations are locally secure. This is a bad thing, thus it might be necessary for a transition period to support both DS and SIG@Child. The cost is one more signatures in the answer and that early adopters have to use cumbersome communications that DS is supposed to solve. Resolvers will notsolves. This section needs work, it needs list of all cases and find if there are any where resolvers get confused as they will select signatures with the KEY they trust and ignoreor can not determine what the other one.security status of child is. 3 Resolver Example To create a chain of trust resolver goes from trusted KEY to DS to KEY. Assume the key for domain example."example." is trusted. In zone "example." we have example. KEY <stuff> secure.example. DS tag=12345 size=1024 alg=dsatag=10243 alg=3 <foofoo> secure.example. NS ns1.secure.example. NS ns1.secure.example. ssecure.example. NXT NS SIG NXT DS tail.example.unsecure.example. secure.example. SIG(NXT) secure.example. SIG(DS) unsecure.example NS ns1.unsecure.example. unsecure.example NS ns2.unsecure.example. unsecure.example. NXT NS SIG NXT .example. unsecure.example. SIG(NXT) In zone "secure.example." we have secure.example. SOA <soa stuff> secure.example. NS ns1.secure.example. NS ns1.secure.example. secure.example. KEY <tag=12345 size=1024 alg=dsa>alg=3> KEY <tag=54321 size=512 alg=rsa/sha1>alg=5> KEY <tag=32145 size=1024 alg=dsa>alg=3> secure.example. SIG(KEY) <key-tag=12345 size=1024 alg=dsa>alg=3> secure.example. SIG(SOA) <key-tag=54321 size=512 alg=rsa/sha1>alg=5> secure.example. SIG(NS) <key-tag=54321 size=512 alg=rsa/sha1>alg=5> In this example the trusted key for example"example." signs the DS record for "secure.example.", making that a trusted record. The DS record states what key is supposedexpected to sign the KEY recordRRset at secure.example. In this example"secure.example". Here "secure.example." has three different KEY records and the one corresponding to theKEY identified in the DS record signs the KEY set, thus the keyKEY set is validated and trusted. Note that one of the other keys in the keyset actually signs the zone data, and resolvers will trust the signatures as the key appears in the KEY set. This example has only one DS record for the child but there no reason to outlaw multiple DS records. More than one DS record is needed during signing key rollover. It is strongly recommended that the DS set be kept small. Resolver determines the security status of "unsecure.example." by examining the parent size NXT for this name. 3.1 Resolver cost estimates for DS records From a RFC2535 resolver point of view for each delegation followed to chase down an answer one KEY record has to be verified and possibly some other records based on policy, for example the contents of the NS set. Once the resolver gets to the appropriate delegation validating the answer may require verifying one or more signatures. For aA simple A record lookup requires at least N delegations to be verified and 1 RRset. For a DS enabled resolver the cost is 2N+1. For MX record the cost where the target of the MX record is in the same zone as the MX record the costs are N+2 and 2N+2. In the case of negative answer the same holdsratios hold true. Resolver may require an extra query to get the DS record butand this may add to the overall cost of the query, but this is never worse than chasing down NULL KEY records from the parent in RFC2535 DNSSEC. DS adds processing overhead on resolvers, increases the size of delegation answers. DS requiresanswers but much less storage in large delegation zonesthan SIG@Parent. 4 Acknowledgments Number of people have over the last few years contributed number of ideas that are captured in this document. The core idea of using one key to that hasonly the role of signing asign key set, comes from discussions with Bill Manning and Perry Metzger on how to put in a single root key in all resolver that lives for a long time.resolvers. Brian Wellington, Dan Massey, Edward Lewis, Havard Eidnes,Jakob Schlyter, Scott Rosen, Edward Lewis, Dan Massey, Mark Kosters, Olaf Kolman, Miek Gieben, Havard Eidnes, Donald Eastlake 3rd., Randy Bush, Rob Austein, Roy Arens, Scott RosenArends, and others have provided usefulluseful comments. 4 - Security Considerations: This document proposes a change to the validation chain of KEY records in DNS. The change in is not believed to reduce security in the overall system, in RFC2535 DNSSEC child must communicate keys to parent and prudent parents will require some authentication on that handshake. The modified protocol will require same authentication but allows the child to exert more local control over its own KEY set. In the representation of DS record, thereThere is a possibility that an attacker can generate an valid KEY that matches all the checksDS fields thus starting to forge data from the child. This is considered impractical as on average more than 2**802^80 keys must be generated before one is found that will match. DS record is a change to DNSSEC protocol and there is some installed base of implementations, as well as text books on how to set up secured delegations. Implementations that do not understand DS record will not be able to follow the KEY to DS to KEY chain and consider all zone secured that way insecure. 5 - IANA Considerations: IANA needs to allocate RR type code for DS from the standard RR type space. References: [RFC1035] P. Mockapetris, ``Domain Names - Implementation and Specification'', STD 13, RFC 1035, November 1987. [RFC2535] D. Eastlake, ``Domain Name System Security Extensions'', RFC 2535, March 1999. [RFC3008] B. Wellington, ``Domain Name System Security (DNSSEC) Signing Authority'', RFC 3008, November 2000. [RFC3090] E. Lewis `` DNS Security Extension Clarification on Zone Status'', RFC 3090, March 2001. [OKbit] D. Conrad, ``Indicating Resolver Support of DNSSEC'', work in progress <draft-ietf-dnsext-dnssec-okbit-02.txt>, April 2001. [Parent] R. Gieben, T. Lindgreen, ``Parent stores the child's zone KEYs'', work in progress <draft-ietf-dnsext-parent-stores- zones-keys-01.txt>, May 2001. Author Address Olafur Gudmundsson 3826 Legation Street, NW Washington, DC, 20015 USA <firstname.lastname@example.org> Appendix A: Changes from Prior versions Changes from version 01 Deleted KEY size field as it did not contribute anything but complexity. Number of wordsmith changes to make document more readable. The word CAN was used when SHOULD was intended. Deleted section 2.4 "Justifications for compact format" moved relevant text to section 2.2. Reverse alphabetized the acknowledgments section. Reorganized sections 1 and 2 for readability. Changes from version 00 Changed name from DK to DS based on working group comments. Dropped verbose format based on WG comments. Added text about TTL issue/problem in caching servers. Added text about islands of security and clarified the cost impact. Major editing of arguments and some reordering of text for clarity. Added section on transition issues. Full Copyright Statement Copyright (C) The Internet Society (2001). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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