draft-ietf-secsh-dns-01.txt   draft-ietf-secsh-dns-02.txt 
Secure Shell Working Group J. Schlyter Secure Shell Working Group J. Schlyter
Internet-Draft Carlstedt Research & Internet-Draft Carlstedt Research &
Expires: May 4, 2003 Technology Expires: July 12, 2003 Technology
W. Griffin W. Griffin
Network Associates Laboratories Network Associates Laboratories
November 3, 2002 January 11, 2003
Using DNS to securely publish SSH key fingerprints Using DNS to securely publish SSH key fingerprints
draft-ietf-secsh-dns-01.txt draft-ietf-secsh-dns-02.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract Abstract
This document describes a method to verify SSH host keys using This document describes a method to verify SSH host keys using
DNSSEC. The document defines a new DNS resource record that contains DNSSEC. The document defines a new DNS resource record that contains
a standard SSH key fingerprint. a standard SSH key fingerprint.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
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2.3 Fingerprint matching . . . . . . . . . . . . . . . . . . . . 4 2.3 Fingerprint matching . . . . . . . . . . . . . . . . . . . . 4
2.4 Authentication . . . . . . . . . . . . . . . . . . . . . . . 4 2.4 Authentication . . . . . . . . . . . . . . . . . . . . . . . 4
3. The SSHFP resource record . . . . . . . . . . . . . . . . . 4 3. The SSHFP resource record . . . . . . . . . . . . . . . . . 4
3.1 The SSHFP RDATA format . . . . . . . . . . . . . . . . . . . 4 3.1 The SSHFP RDATA format . . . . . . . . . . . . . . . . . . . 4
3.1.1 Algorithm number specification . . . . . . . . . . . . . . . 4 3.1.1 Algorithm number specification . . . . . . . . . . . . . . . 4
3.1.2 Fingerprint type specification . . . . . . . . . . . . . . . 5 3.1.2 Fingerprint type specification . . . . . . . . . . . . . . . 5
3.1.3 Fingerprint . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1.3 Fingerprint . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2 Presentation format of the SSHFP RR . . . . . . . . . . . . 5 3.2 Presentation format of the SSHFP RR . . . . . . . . . . . . 5
4. Security considerations . . . . . . . . . . . . . . . . . . 5 4. Security considerations . . . . . . . . . . . . . . . . . . 5
5. IANA considerations . . . . . . . . . . . . . . . . . . . . 6 5. IANA considerations . . . . . . . . . . . . . . . . . . . . 6
References . . . . . . . . . . . . . . . . . . . . . . . . . 7 Normative References . . . . . . . . . . . . . . . . . . . . 7
Informational References . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 8
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
Full Copyright Statement . . . . . . . . . . . . . . . . . . 9 Intellectual Property and Copyright Statements . . . . . . . 9
1. Introduction 1. Introduction
The SSH [9] protocol provides secure remote login and other secure The SSH [5] protocol provides secure remote login and other secure
network services over an insecure network. The security of the network services over an insecure network. The security of the
connection relies on the server authenticating itself to the client. connection relies on the server authenticating itself to the client.
Server authentication is normally done by presenting the fingerprint Server authentication is normally done by presenting the fingerprint
of an unknown public key to the user for verification. If the user 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 decides the fingerprint is correct and accepts the key, the key is
saved locally and used for verification for all following saved locally and used for verification for all following
connections. While some security-conscious users do verify the connections. While some security-conscious users do verify the
fingerprint out-of-band before accepting the key, the average user fingerprint out-of-band before accepting the key, the average user
usually blindly accepts the key presented. usually blindly accepts the key presented.
The method described here can provide out-of-band verification by 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] looking up a fingerprint of the server public key in the DNS [1][2]
and using DNSSEC [5] to verify the lookup. and using DNSSEC [4] to verify the lookup.
In order to distribute the fingerprint using DNS, this document In order to distribute the fingerprint using DNS, this document
defines a new DNS resource record to carry the fingerprint. defines a new DNS resource record to carry the fingerprint.
Basic understanding of the DNS system [1][2] and the DNS security Basic understanding of the DNS system [1][2] and the DNS security
extensions [5] is assumed by this document. extensions [4] is assumed by this document.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [3]. document are to be interpreted as described in RFC 2119 [3].
2. SSH Host Key Verification 2. SSH Host Key Verification
2.1 Method 2.1 Method
Upon connection to a SSH server, the SSH client MAY look up the SSHFP 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 resource record(s) for the host it is connecting to. If the
algorithm and fingerprint of the key received from the SSH server algorithm and fingerprint of the key received from the SSH server
matches the algorithm and fingerprint of one of the SSHFP resource matches the algorithm and fingerprint of one of the SSHFP resource
record(s) returned from DNS, the client MAY accept the identity of record(s) returned from DNS, the client MAY accept the identity of
the server. the server.
2.2 Implementation notes 2.2 Implementation notes
Client implementors SHOULD to provide a configurable policy used to Client implementors SHOULD provide a configurable policy used to
select the order of methods used to verify a host key and which select the order of methods used to verify a host key and which
fingerprints to trust ultimately, after user confirmation or not at fingerprints to trust ultimately, after user confirmation or not at
all. all.
One specific scenario for having a configurable policy is where
clients use unqualified 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 2.3 Fingerprint matching
The public key and the SSHFP resource record are matched together by The public key and the SSHFP resource record are matched together by
comparing algorithm number and fingerprint. comparing algorithm number and fingerprint.
2.4 Authentication 2.4 Authentication
A public key verified using this method MUST only be trusted if the A public key verified using this method MUST only be trusted if the
SSHFP RR used for verification was authenticated by a trusted SIG RR. SSHFP RR used for verification was authenticated by a trusted SIG RR.
Clients that do not validate the DNSSEC signatures themselves MUST Clients that do not validate the DNSSEC signatures themselves MUST
use a secure transport, e.g. TSIG [6], SIG(0) [7] or IPsec [4], use a secure transport, e.g. TSIG [8], SIG(0) [9] or IPsec [7],
between themselves and the entity performing the signature between themselves and the entity performing the signature
validation. validation.
3. The SSHFP resource record 3. The SSHFP resource record
The SSHFP resource record (RR) is used to store a fingerprint of a 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 SSH public host key that is associated with a Domain Name System
(DNS) name. (DNS) name.
The RR type code for the SSHFP RR is TBA. The RR type code for the SSHFP RR is TBA.
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0 reserved 0 reserved
1 SHA-1 1 SHA-1
Reserving other types requires IETF consensus. For interoperability Reserving other types requires IETF consensus. For interoperability
reasons, as few fingerprint types as possible should be reserved. reasons, as few fingerprint types as possible should be reserved.
The only reason to reserve additional types is to increase security. The only reason to reserve additional types is to increase security.
3.1.3 Fingerprint 3.1.3 Fingerprint
The fingerprint is calculated over the public key blob as described The fingerprint is calculated over the public key blob as described
in [10]. in [6].
3.2 Presentation format of the SSHFP RR 3.2 Presentation format of the SSHFP RR
The presentation format of the SSHFP resource record consists of two The presentation format of the SSHFP resource record consists of two
numbers (algorithm and fingerprint type) followed by the fingerprint numbers (algorithm and fingerprint type) followed by the fingerprint
itself presented in hex, e.g: itself presented in hex, e.g:
host.example. SSHFP 2 1 123456789abcdef67890123456789abcdef67890 host.example. SSHFP 2 1 123456789abcdef67890123456789abcdef67890
4. Security considerations 4. Security considerations
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Another dependency is on the implementation of DNSSEC itself. As Another dependency is on the implementation of DNSSEC itself. As
stated in Section 2.4, we mandate the use of secure methods for 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 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 is especially important if SSHFP is to be used as a basis for host
key rollover and/or revocation, as described above. key rollover and/or revocation, as described above.
Since DNSSEC only protects the integrity of the host key fingerprint Since DNSSEC only protects the integrity of the host key fingerprint
after it is signed by the DNS zone administrator, the fingerprint after it is signed by the DNS zone administrator, the fingerprint
must be transferred securely from the SSH host administrator to the must be transferred securely from the SSH host administrator to the
DNS zone administrator. This could be done manually between the DNS zone administrator. This could be done manually between the
administrators or automatically using secure DNS dynamic update [8] administrators or automatically using secure DNS dynamic update [10]
between the SSH server and the nameserver. We note that this is no between the SSH server and the nameserver. We note that this is no
different from other key enrollment situations, e.g. a client different from other key enrollment situations, e.g. a client
sending a certificate request to a certificate authority for signing. sending a certificate request to a certificate authority for signing.
5. IANA considerations 5. IANA considerations
IANA needs to allocate a RR type code for SSHFP from the standard RR IANA needs to allocate a RR type code for SSHFP from the standard RR
type space (type 44 requested). type space (type 44 requested).
IANA needs to open a new registry for the SSHFP RR type for public IANA needs to open a new registry for the SSHFP RR type for public
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Adding new reservations requires IETF consensus. Adding new reservations requires IETF consensus.
IANA needs to open a new registry for the SSHFP RR type for IANA needs to open a new registry for the SSHFP RR type for
fingerprint types. Defined types are: fingerprint types. Defined types are:
0 is reserved 0 is reserved
1 is SHA-1 1 is SHA-1
Adding new reservations requires IETF consensus. Adding new reservations requires IETF consensus.
References Normative References
[1] Mockapetris, P., "Domain names - concepts and facilities", STD [1] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987. 13, RFC 1034, November 1987.
[2] Mockapetris, P., "Domain names - implementation and [2] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[4] Thayer, R., Doraswamy, N. and R. Glenn, "IP Security Document [4] Eastlake, D., "Domain Name System Security Extensions", RFC
Roadmap", RFC 2411, November 1998.
[5] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999. 2535, March 1999.
[6] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington, [5] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T J. and S.
Lehtinen, "SSH Transport Layer Protocol", work in progress
draft-ietf-secsh-architecture-13.txt, September 2002.
[6] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T J. and S.
Lehtinen, "SSH Transport Layer Protocol", work in progress
draft-ietf-secsh-transport-15.txt, September 2002.
Informational References
[7] Thayer, R., Doraswamy, N. and R. Glenn, "IP Security Document
Roadmap", RFC 2411, November 1998.
[8] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)", RFC "Secret Key Transaction Authentication for DNS (TSIG)", RFC
2845, May 2000. 2845, May 2000.
[7] Eastlake, D., "DNS Request and Transaction Signatures ( [9] Eastlake, D., "DNS Request and Transaction Signatures (
SIG(0)s)", RFC 2931, September 2000. SIG(0)s)", RFC 2931, September 2000.
[8] Wellington, B., "Secure Domain Name System (DNS) Dynamic [10] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000. 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-13.txt, September 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-15.txt, September 2002.
Authors' Addresses Authors' Addresses
Jakob Schlyter Jakob Schlyter
Carlstedt Research & Technology Carlstedt Research & Technology
Stora Badhusgatan 18-20 Stora Badhusgatan 18-20
Goteborg SE-411 21 Goteborg SE-411 21
Sweden Sweden
EMail: jakob@crt.se EMail: jakob@crt.se
URI: http://www.crt.se/~jakob/ URI: http://www.crt.se/~jakob/
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contributions of the following persons: contributions of the following persons:
Martin Fredriksson Martin Fredriksson
Olafur Gudmundsson Olafur Gudmundsson
Edward Lewis Edward Lewis
Bill Sommerfeld Bill Sommerfeld
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