Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
RFC 4255

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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