    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

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    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

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    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.

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    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.

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    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.

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    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].

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    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.

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    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/

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    RFC 4255                DNS and SSH Fingerprints            January 2006

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       Copyright (C) The Internet Society (2006).

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