Network Working Group                                        D. Eastlake
Request for Comments: 2541                                           IBM
Category: Informational                                       March 1999


                DNS Security Operational Considerations

Status 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 Kaufman

Table of Contents

   Abstract...................................................1
   Acknowledgments............................................1
   1. Introduction............................................2
   2. Public/Private Key Generation...........................2
   3. Public/Private Key Lifetimes............................2
   4. Public/Private Key Size Considerations..................3
   4.1 RSA Key Sizes..........................................3
   4.2 DSS Key Sizes..........................................4
   5. Private Key Storage.....................................4
   6. High Level Zones, The Root Zone, and The Meta-Root Key..5
   7. Security Considerations.................................5
   References.................................................6
   Author's Address...........................................6
   Full Copyright Statement...................................7

1. 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, if

   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.

   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.

   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 in section 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.

References

   [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.com

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