Secure Shell Working Group J. Schlyter
Internet-Draft Carlstedt Research &
Expires: February 10, 2003 Technology
W. Griffin
Network Associates Laboratories
August 12, 2002
Using DNS to securely publish SSH key fingerprints
draft-ietf-secsh-dns-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document describes a method to verify SSH host keys using
DNSSEC. The document defines a new DNS resource record that contains
a standard SSH key fingerprint.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. SSH Host Key Verification . . . . . . . . . . . . . . . . . 3
2.1 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Fingerprint matching . . . . . . . . . . . . . . . . . . . . 3
2.3 Authentication . . . . . . . . . . . . . . . . . . . . . . . 3
3. The SSHFP resource record . . . . . . . . . . . . . . . . . 4
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
4.1 Backend transport integrity . . . . . . . . . . . . . . . . 5
4.2 Effects on the SSH trust model . . . . . . . . . . . . . . . 5
5. IANA considerations . . . . . . . . . . . . . . . . . . . . 5
References . . . . . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 7
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
Full Copyright Statement . . . . . . . . . . . . . . . . . . 8
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1. Introduction
The SSH [9] 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.
Server authentication is normally done by presenting the fingerprint
of an unknown public key to the user for verification. If the user
decides 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 do verify the
fingerprint out-of-band before accepting the key, the average user
usually blindly accepts the key presented.
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 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 a 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
matches the algorithm and fingerprint of one of the SSHFP resource
record(s) returned from DNS, the client MAY accept the identity of
the server. It is RECOMMENDED that the client ask the user for
confirmation before accepting the identity of the server.
2.2 Fingerprint matching
The public key and the SSHFP resource record are matched together by
comparing algorithm number and fingerprint.
2.3 Authentication
A public key verified using this method MUST only be trusted if the
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SSHFP RR used for verification was authenticated by a trusted SIG RR.
Clients that do not validate the DNSSEC signatures themselves MUST
use a secure transport, e.g. TSIG [6], SIG(0) [7] or IPsec [4],
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 a
SSH public host key that is associated with a Domain Name System
(DNS) name.
The RR type code for the SSHFP RR is TBA.
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.
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
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values are assigned:
Value Fingerprint type
----- ----------------
0 reserved
1 SHA-1
Reserving other types requires IETF consensus. For interoperability
reasons, as few fingerprint types as possible should be reserved.
The only reason to reserve additional types is to increase security.
3.1.3 Fingerprint
The fingerprint is calculated over the public key blob as described
in [10].
3.2 Presentation format of the SSHFP RR
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
4. Security considerations
4.1 Backend transport integrity
Since DNSSEC only protects the integrity of the host key fingerprint
after it is signed by the DNS zone administrator, the fingerprint
must be transfered 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 [8]
between the SSH server and the nameserver.
4.2 Effects on the SSH trust model
... to be written ...
5. IANA considerations
IANA needs to allocate a RR type code for SSHFP from the standard RR
type space.
IANA needs to open a new registry for the SSHFP RR type for public
key algorithms. Defined types are:
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0 is reserved
1 is RSA
2 is DSA
Adding new reservations requires IETF consensus.
IANA needs to open a new registry for the SSHFP RR type for
fingerprint types. Defined types are:
0 is reserved
1 is SHA-1
Adding new reservations requires IETF consensus.
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] Thayer, R., Doraswamy, N. and R. Glenn, "IP Security Document
Roadmap", RFC 2411, November 1998.
[5] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[6] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)", RFC
2845, May 2000.
[7] Eastlake, D., "DNS Request and Transaction Signatures (
SIG(0)s)", RFC 2931, September 2000.
[8] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.
[9] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T J. and S.
Lehtinen, "SSH Transport Layer Protocol", work in progress
draft-ietf-secsh-architecture-12.txt, March 2002.
[10] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T J. and S.
Lehtinen, "SSH Transport Layer Protocol", work in progress
draft-ietf-secsh-transport-14.txt, March 2002.
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Authors' Addresses
Jakob Schlyter
Carlstedt Research & Technology
Stora Badhusgatan 18-20
Goteborg SE-411 21
Sweden
EMail: jakob@crt.se
URI: http://www.crt.se/~jakob/
Wesley Griffin
Network Associates Laboratories
15204 Omega Drive Suite 300
Rockville, MD 20850
USA
EMail: wgriffin@tislabs.com
URI: http://www.nailabs.com/
Appendix A. Acknowledgements
The authors gratefully acknowledges, in no particular order, the
contributions of the following persons:
Bill Sommerfeld
Olafur Gudmundsson
Edward Lewis
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