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+Network Working Group J. Schlyter
+Request for Comments: 4255 OpenSSH
+Category: Standards Track W. Griffin
+ SPARTA
+ January 2006
+
+
+ Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
+
+Status of This Memo
+
+ This document specifies an Internet standards track protocol for the
+ Internet community, and requests discussion and suggestions for
+ improvements. Please refer to the current edition of the "Internet
+ Official Protocol Standards" (STD 1) for the standardization state
+ and status of this protocol. Distribution of this memo is unlimited.
+
+Copyright Notice
+
+ Copyright (C) The Internet Society (2006).
+
+Abstract
+
+ This document describes a method of verifying Secure Shell (SSH) host
+ keys using Domain Name System Security (DNSSEC). The document
+ defines a new DNS resource record that contains a standard SSH key
+ fingerprint.
+
+Table of Contents
+
+ 1. Introduction ....................................................2
+ 2. SSH Host Key Verification .......................................2
+ 2.1. Method .....................................................2
+ 2.2. Implementation Notes .......................................2
+ 2.3. Fingerprint Matching .......................................3
+ 2.4. Authentication .............................................3
+ 3. The SSHFP Resource Record .......................................3
+ 3.1. The SSHFP RDATA Format .....................................4
+ 3.1.1. Algorithm Number Specification ......................4
+ 3.1.2. Fingerprint Type Specification ......................4
+ 3.1.3. Fingerprint .........................................5
+ 3.2. Presentation Format of the SSHFP RR ........................5
+ 4. Security Considerations .........................................5
+ 5. IANA Considerations .............................................6
+ 6. Normative References ............................................7
+ 7. Informational References ........................................7
+ 8. Acknowledgements ................................................8
+
+
+
+
+Schlyter & Griffin Standards Track [Page 1]
+
+RFC 4255 DNS and SSH Fingerprints January 2006
+
+
+1. Introduction
+
+ The SSH [6] protocol provides secure remote login and other secure
+ network services over an insecure network. The security of the
+ connection relies on the server authenticating itself to the client
+ as well as the user authenticating itself to the server.
+
+ If a connection is established to a server whose public key is not
+ already known to the client, a fingerprint of the key is presented to
+ the user for verification. If the user decides that the fingerprint
+ is correct and accepts the key, the key is saved locally and used for
+ verification for all following connections. While some security-
+ conscious users verify the fingerprint out-of-band before accepting
+ the key, many users blindly accept the presented key.
+
+ The method described here can provide out-of-band verification by
+ looking up a fingerprint of the server public key in the DNS [1][2]
+ and using DNSSEC [5] to verify the lookup.
+
+ In order to distribute the fingerprint using DNS, this document
+ defines a new DNS resource record, "SSHFP", to carry the fingerprint.
+
+ Basic understanding of the DNS system [1][2] and the DNS security
+ extensions [5] is assumed by this document.
+
+ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+ "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
+ document are to be interpreted as described in RFC 2119 [3].
+
+2. SSH Host Key Verification
+
+2.1. Method
+
+ Upon connection to an SSH server, the SSH client MAY look up the
+ SSHFP resource record(s) for the host it is connecting to. If the
+ algorithm and fingerprint of the key received from the SSH server
+ match the algorithm and fingerprint of one of the SSHFP resource
+ record(s) returned from DNS, the client MAY accept the identity of
+ the server.
+
+2.2. Implementation Notes
+
+ Client implementors SHOULD provide a configurable policy used to
+ select the order of methods used to verify a host key. This document
+ defines one method: Fingerprint storage in DNS. Another method
+ defined in the SSH Architecture [6] uses local files to store keys
+ for comparison. Other methods that could be defined in the future
+ might include storing fingerprints in LDAP or other databases. A
+
+
+
+Schlyter & Griffin Standards Track [Page 2]
+
+RFC 4255 DNS and SSH Fingerprints January 2006
+
+
+ configurable policy will allow administrators to determine which
+ methods they want to use and in what order the methods should be
+ prioritized. This will allow administrators to determine how much
+ trust they want to place in the different methods.
+
+ One specific scenario for having a configurable policy is where
+ clients do not use fully qualified host names to connect to servers.
+ In this scenario, the implementation SHOULD verify 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.
+
+2.3. Fingerprint Matching
+
+ The public key and the SSHFP resource record are matched together by
+ comparing algorithm number and fingerprint.
+
+ The public key algorithm and the SSHFP algorithm number MUST
+ match.
+
+ A message digest of the public key, using the message digest
+ algorithm specified in the SSHFP fingerprint type, MUST match the
+ SSHFP fingerprint.
+
+2.4. Authentication
+
+ A public key verified using this method MUST NOT be trusted if the
+ SSHFP resource record (RR) used for verification was not
+ authenticated by a trusted SIG RR.
+
+ Clients that do validate the DNSSEC signatures themselves SHOULD use
+ standard DNSSEC validation procedures.
+
+ Clients that do not validate the DNSSEC signatures themselves MUST
+ use a secure transport (e.g., TSIG [9], SIG(0) [10], or IPsec [8])
+ between themselves and the entity performing the signature
+ validation.
+
+3. The SSHFP Resource Record
+
+ The SSHFP resource record (RR) is used to store a fingerprint of an
+ SSH public host key that is associated with a Domain Name System
+ (DNS) name.
+
+ The RR type code for the SSHFP RR is 44.
+
+
+
+
+
+Schlyter & Griffin Standards Track [Page 3]
+
+RFC 4255 DNS and SSH Fingerprints January 2006
+
+
+3.1. The SSHFP RDATA Format
+
+ The RDATA for a SSHFP RR consists of an algorithm number, fingerprint
+ type and the fingerprint of the public host key.
+
+ 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
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | algorithm | fp type | /
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
+ / /
+ / fingerprint /
+ / /
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+3.1.1. Algorithm Number Specification
+
+ This algorithm number octet describes the algorithm of the public
+ key. The following values are assigned:
+
+ Value Algorithm name
+ ----- --------------
+ 0 reserved
+ 1 RSA
+ 2 DSS
+
+ Reserving other types requires IETF consensus [4].
+
+3.1.2. Fingerprint Type Specification
+
+ The fingerprint type octet describes the message-digest algorithm
+ used to calculate the fingerprint of the public key. The following
+ values are assigned:
+
+ Value Fingerprint type
+ ----- ----------------
+ 0 reserved
+ 1 SHA-1
+
+ Reserving other types requires IETF consensus [4].
+
+ For interoperability reasons, as few fingerprint types as possible
+ should be reserved. The only reason to reserve additional types is
+ to increase security.
+
+
+
+
+
+
+
+Schlyter & Griffin Standards Track [Page 4]
+
+RFC 4255 DNS and SSH Fingerprints January 2006
+
+
+3.1.3. Fingerprint
+
+ The fingerprint is calculated over the public key blob as described
+ in [7].
+
+ The message-digest algorithm is presumed to produce an opaque octet
+ string output, which is placed as-is in the RDATA fingerprint field.
+
+3.2. Presentation Format of the SSHFP RR
+
+ The RDATA of the presentation format of the SSHFP resource record
+ consists of two numbers (algorithm and fingerprint type) followed by
+ the fingerprint itself, presented in hex, e.g.:
+
+ host.example. SSHFP 2 1 123456789abcdef67890123456789abcdef67890
+
+ The use of mnemonics instead of numbers is not allowed.
+
+4. Security Considerations
+
+ Currently, the amount of trust a user can realistically place in a
+ server key is proportional to the amount of attention paid to
+ verifying that the public key presented actually corresponds to the
+ private key of the server. If a user accepts a key without verifying
+ 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: jakob@openssh.com
+ URI: http://www.openssh.com/
+
+
+ Wesley Griffin
+ SPARTA
+ 7075 Samuel Morse Drive
+ Columbia, MD 21046
+ USA
+
+ EMail: wgriffin@sparta.com
+ 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
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+ WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
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+
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+Acknowledgement
+
+ Funding for the RFC Editor function is provided by the IETF
+ Administrative Support Activity (IASA).
+
+
+
+
+
+
+
+Schlyter & Griffin Standards Track [Page 9]
+