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Network Working Group                                        D. EastlakeRequest for Comments: 2541                                           IBMCategory: Informational                                       March 1999DNS Security Operational ConsiderationsStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1999).  All Rights Reserved.Abstract   Secure DNS is based on cryptographic techniques.  A necessary part of   the strength of these techniques is careful attention to the   operational aspects of key and signature generation, lifetime, size,   and storage.  In addition, special attention must be paid to the   security of the high level zones, particularly the root zone.  This   document discusses these operational aspects for keys and signatures   used in connection with the KEY and SIG DNS resource records.Acknowledgments   The contributions and suggestions of the following persons (in   alphabetic order) are gratefully acknowledged:         John Gilmore         Olafur Gudmundsson         Charlie KaufmanEastlake                     Informational                      [Page 1]

RFC 2541        DNS Security Operational Considerations       March 1999Table of Contents   Abstract...................................................1   Acknowledgments............................................11. Introduction............................................22. Public/Private Key Generation...........................23. Public/Private Key Lifetimes............................24. Public/Private Key Size Considerations..................34.1 RSA Key Sizes..........................................34.2 DSS Key Sizes..........................................45. Private Key Storage.....................................46. High Level Zones, The Root Zone, and The Meta-Root Key..57. Security Considerations.................................5   References.................................................6   Author's Address...........................................6   Full Copyright Statement...................................71. Introduction   This document describes operational considerations for the   generation, lifetime, size, and storage of DNS cryptographic keys and   signatures for use in the KEY and SIG resource records [RFC 2535].   Particular attention is paid to high level zones and the root zone.2. Public/Private Key Generation   Careful generation of all keys is a sometimes overlooked but   absolutely essential element in any cryptographically secure system.   The strongest algorithms used with the longest keys are still of no   use if an adversary can guess enough to lower the size of the likely   key space so that it can be exhaustively searched.  Technical   suggestions for the generation of random keys will be found in [RFC   1750].   Long term keys are particularly sensitive as they will represent a   more valuable target and be subject to attack for a longer time than   short period keys.  It is strongly recommended that long term key   generation occur off-line in a manner isolated from the network via   an air gap or, at a minimum, high level secure hardware.3. Public/Private Key Lifetimes   No key should be used forever.  The longer a key is in use, the   greater the probability that it will have been compromised through   carelessness, accident, espionage, or cryptanalysis.  Furthermore, ifEastlake                     Informational                      [Page 2]

RFC 2541        DNS Security Operational Considerations       March 1999   key rollover is a rare event, there is an increased risk that, when   the time does come to change the key, no one at the site will   remember how to do it or operational problems will have developed in   the key rollover procedures.   While public key lifetime is a matter of local policy, these   considerations imply that, unless there are extraordinary   circumstances, no long term key should have a lifetime significantly   over four years.  In fact, a reasonable guideline for long term keys   that are kept off-line and carefully guarded is a 13 month lifetime   with the intent that they be replaced every year.  A reasonable   maximum lifetime for keys that are used for transaction security or   the like and are kept on line is 36 days with the intent that they be   replaced monthly or more often.  In many cases, a key lifetime of   somewhat over a day may be reasonable.   On the other hand, public keys with too short a lifetime can lead to   excessive resource consumption in re-signing data and retrieving   fresh information because cached information becomes stale.  In the   Internet environment, almost all public keys should have lifetimes no   shorter than three minutes, which is a reasonable estimate of maximum   packet delay even in unusual circumstances.4. Public/Private Key Size Considerations   There are a number of factors that effect public key size choice for   use in the DNS security extension.  Unfortunately, these factors   usually do not all point in the same direction.  Choice of zone key   size should generally be made by the zone administrator depending on   their local conditions.   For most schemes, larger keys are more secure but slower.  In   addition, larger keys increase the size of the KEY and SIG RRs.  This   increases the chance of DNS UDP packet overflow and the possible   necessity for using higher overhead TCP in responses.4.1 RSA Key Sizes   Given a small public exponent, verification (the most common   operation) for the MD5/RSA algorithm will vary roughly with the   square of the modulus length, signing will vary with the cube of the   modulus length, and key generation (the least common operation) will   vary with the fourth power of the modulus length.  The current best   algorithms for factoring a modulus and breaking RSA security vary   roughly with the 1.6 power of the modulus itself.  Thus going from a   640 bit modulus to a 1280 bit modulus only increases the verification   time by a factor of 4 but may increase the work factor of breaking   the key by over 2^900.Eastlake                     Informational                      [Page 3]

RFC 2541        DNS Security Operational Considerations       March 1999   The recommended minimum RSA algorithm modulus size is 704 bits which   is believed by the author to be secure at this time.  But high level   zones in the DNS tree may wish to set a higher minimum, perhaps 1000   bits, for security reasons.  (Since the United States National   Security Agency generally permits export of encryption systems using   an RSA modulus of up to 512 bits, use of that small a modulus, i.e.   n, must be considered weak.)   For an RSA key used only to secure data and not to secure other keys,   704 bits should be adequate at this time.4.2 DSS Key Sizes   DSS keys are probably roughly as strong as an RSA key of the same   length but DSS signatures are significantly smaller.5. Private Key Storage   It is recommended that, where possible, zone private keys and the   zone file master copy be kept and used in off-line, non-network   connected, physically secure machines only.  Periodically an   application can be run to add authentication to a zone by adding SIG   and NXT RRs and adding no-key type KEY RRs for subzones/algorithms   where a real KEY RR for the subzone with that algorithm is not   provided. Then the augmented file can be transferred, perhaps by   sneaker-net, to the networked zone primary server machine.   The idea is to have a one way information flow to the network to   avoid the possibility of tampering from the network.  Keeping the   zone master file on-line on the network and simply cycling it through   an off-line signer does not do this.  The on-line version could still   be tampered with if the host it resides on is compromised.  For   maximum security, the master copy of the zone file should be off net   and should not be updated based on an unsecured network mediated   communication.   This is not possible if the zone is to be dynamically updated   securely [RFC 2137]. At least a private key capable of updating the   SOA and NXT chain must be on line in that case.   Secure resolvers must be configured with some trusted on-line public   key information (or a secure path to such a resolver) or they will be   unable to authenticate.  Although on line, this public key   information must be protected or it could be altered so that spoofed   DNS data would appear authentic.Eastlake                     Informational                      [Page 4]

RFC 2541        DNS Security Operational Considerations       March 1999   Non-zone private keys, such as host or user keys, generally have to   be kept on line to be used for real-time purposes such as DNS   transaction security.6. High Level Zones, The Root Zone, and The Meta-Root Key   Higher level zones are generally more sensitive than lower level   zones.  Anyone controlling or breaking the security of a zone thereby   obtains authority over all of its subdomains (except in the case of   resolvers that have locally configured the public key of a   subdomain).  Therefore, extra care should be taken with high level   zones and strong keys used.   The root zone is the most critical of all zones.  Someone controlling   or compromising the security of the root zone would control the   entire DNS name space of all resolvers using that root zone (except   in the case of resolvers that have locally configured the public key   of a subdomain). Therefore, the utmost care must be taken in the   securing of the root zone. The strongest and most carefully handled   keys should be used.  The root zone private key should always be kept   off line.   Many resolvers will start at a root server for their access to and   authentication of DNS data.  Securely updating an enormous population   of resolvers around the world will be extremely difficult.  Yet the   guidelines insection 3 above would imply that the root zone private   key be changed annually or more often and if it were staticly   configured at all these resolvers, it would have to be updated when   changed.   To permit relatively frequent change to the root zone key yet   minimize exposure of the ultimate key of the DNS tree, there will be   a "meta-root" key used very rarely and then only to sign a sequence   of regular root key RRsets with overlapping time validity periods   that are to be rolled out. The root zone contains the meta-root and   current regular root KEY RR(s) signed by SIG RRs under both the   meta-root and other root private key(s) themselves.   The utmost security in the storage and use of the meta-root key is   essential.  The exact techniques are precautions to be used are   beyond the scope of this document.  Because of its special position,   it may be best to continue with the same meta-root key for an   extended period of time such as ten to fifteen years.7. Security Considerations   The entirety of this document is concerned with operational   considerations of public/private key pair DNS Security.Eastlake                     Informational                      [Page 5]

RFC 2541        DNS Security Operational Considerations       March 1999References   [RFC 1034]   Mockapetris, P., "Domain Names - Concepts and                Facilities", STD 13,RFC 1034, November 1987.   [RFC 1035]   Mockapetris, P., "Domain Names - Implementation and                Specifications", STD 13,RFC 1035, November 1987.   [RFC 1750]   Eastlake, D., Crocker, S. and J. Schiller, "Randomness                Requirements for Security",RFC 1750, December 1994.   [RFC 2065]   Eastlake, D. and C. Kaufman, "Domain Name System                Security Extensions",RFC 2065, January 1997.   [RFC 2137]   Eastlake, D., "Secure Domain Name System Dynamic                Update",RFC 2137, April 1997.   [RFC 2535]   Eastlake, D., "Domain Name System Security Extensions",RFC 2535, March 1999.   [RSA FAQ]    RSADSI Frequently Asked Questions periodic posting.Author's Address   Donald E. Eastlake 3rd   IBM   65 Shindegan Hill Road, RR #1   Carmel, NY 10512   Phone:   +1-914-276-2668(h)            +1-914-784-7913(w)   Fax:     +1-914-784-3833(w)   EMail:   dee3@us.ibm.comEastlake                     Informational                      [Page 6]

RFC 2541        DNS Security Operational Considerations       March 1999Full Copyright Statement   Copyright (C) The Internet Society (1999).  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.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Eastlake                     Informational                      [Page 7]