rfc4255.txt

来自「非常好的dns解析软件」· 文本 代码 · 共 508 行 · 第 1/2 页

   the fingerprint with something learned through a secured channel, the
   connection is vulnerable to a man-in-the-middle attack.

   The overall security of using SSHFP for SSH host key verification is
   dependent on the security policies of the SSH host administrator and
   DNS zone administrator (in transferring the fingerprint), detailed
   aspects of how verification is done in the SSH implementation, and in
   the client's diligence in accessing the DNS in a secure manner.

   One such aspect is in which order fingerprints are looked up (e.g.,
   first checking local file and then SSHFP).  We note that, in addition
   to protecting the first-time transfer of host keys, SSHFP can
   optionally be used for stronger host key protection.

      If SSHFP is checked first, new SSH host keys may be distributed by
      replacing the corresponding SSHFP in DNS.

      If SSH host key verification can be configured to require SSHFP,
      SSH host key revocation can be implemented by removing the
      corresponding SSHFP from DNS.







Schlyter & Griffin          Standards Track                     [Page 5]

RFC 4255                DNS and SSH Fingerprints            January 2006


   As stated in Section 2.2, we recommend that SSH implementors provide
   a policy mechanism to control the order of methods used for host key
   verification.  One specific scenario for having a configurable policy
   is where clients use unqualified host names to connect to servers.
   In this case, we recommend that SSH implementations check the host
   key against a local database before verifying the key via the
   fingerprint returned from DNS.  This would help prevent an attacker
   from injecting a DNS search path into the local resolver and forcing
   the client to connect to a different host.

   A different approach to solve the DNS search path issue would be for
   clients to use a trusted DNS search path, i.e., one not acquired
   through DHCP or other autoconfiguration mechanisms.  Since there is
   no way with current DNS lookup APIs to tell whether a search path is
   from a trusted source, the entire client system would need to be
   configured with this trusted DNS search path.

   Another dependency is on the implementation of DNSSEC itself.  As
   stated in Section 2.4, we mandate the use of secure methods for
   lookup and that SSHFP RRs are authenticated by trusted SIG RRs.  This
   is especially important if SSHFP is to be used as a basis for host
   key rollover and/or revocation, as described above.

   Since DNSSEC only protects the integrity of the host key fingerprint
   after it is signed by the DNS zone administrator, the fingerprint
   must be transferred securely from the SSH host administrator to the
   DNS zone administrator.  This could be done manually between the
   administrators or automatically using secure DNS dynamic update [11]
   between the SSH server and the nameserver.  We note that this is no
   different from other key enrollment situations, e.g., a client
   sending a certificate request to a certificate authority for signing.

5.  IANA Considerations

   IANA has allocated the RR type code 44 for SSHFP from the standard RR
   type space.

   IANA has opened a new registry for the SSHFP RR type for public key
   algorithms.  The defined types are:

      0 is reserved
      1 is RSA
      2 is DSA

   Adding new reservations requires IETF consensus [4].






Schlyter & Griffin          Standards Track                     [Page 6]

RFC 4255                DNS and SSH Fingerprints            January 2006


   IANA has opened a new registry for the SSHFP RR type for fingerprint
   types.  The defined types are:

      0 is reserved
      1 is SHA-1

   Adding new reservations requires IETF consensus [4].

6.  Normative References

   [1]   Mockapetris, P., "Domain names - concepts and facilities", STD
         13, RFC 1034, November 1987.

   [2]   Mockapetris, P., "Domain names - implementation and
         specification", STD 13, RFC 1035, November 1987.

   [3]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [4]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434, October
         1998.

   [5]   Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "DNS Security Introduction and Requirements", RFC 4033, March
         2005.

         Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "Resource Records for the DNS Security Extensions", RFC 4034,
         March 2005.

         Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "Protocol Modifications for the DNS Security Extensions", RFC
         4035, March 2005.

   [6]   Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
         Protocol Architecture", RFC 4251, January 2006.

   [7]   Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
         Transport Layer Protocol", RFC 4253, January 2006.

7.  Informational References

   [8]   Thayer, R., Doraswamy, N., and R. Glenn, "IP Security Document
         Roadmap", RFC 2411, November 1998.






Schlyter & Griffin          Standards Track                     [Page 7]

RFC 4255                DNS and SSH Fingerprints            January 2006


   [9]   Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
         Wellington, "Secret Key Transaction Authentication for DNS
         (TSIG)", RFC 2845, May 2000.

   [10]  Eastlake 3rd, D., "DNS Request and Transaction Signatures
         ( SIG(0)s )", RFC 2931, September 2000.

   [11]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
         Update", RFC 3007, November 2000.

8.  Acknowledgements

   The authors gratefully acknowledge, in no particular order, the
   contributions of the following persons:

      Martin Fredriksson

      Olafur Gudmundsson

      Edward Lewis

      Bill Sommerfeld

Authors' Addresses

   Jakob Schlyter
   OpenSSH
   812 23rd Avenue SE
   Calgary, Alberta  T2G 1N8
   Canada

   EMail: [e-mail]
   URI:   http://www.openssh.com/


   Wesley Griffin
   SPARTA
   7075 Samuel Morse Drive
   Columbia, MD  21046
   USA

   EMail: [e-mail]
   URI:   http://www.sparta.com/








Schlyter & Griffin          Standards Track                     [Page 8]

RFC 4255                DNS and SSH Fingerprints            January 2006


Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM 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.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   [e-mail].

Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).







Schlyter & Griffin          Standards Track                     [Page 9]