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Secure Shell Working Group J. Schlyter
Internet-Draft Carlstedt Research &
Expires: October 1, 2003 Technology
W. Griffin
Network Associates Laboratories
April 2, 2003
Using DNS to securely publish SSH key fingerprints
draft-ietf-secsh-dns-04.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http://
www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on October 1, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). 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 Implementation Notes . . . . . . . . . . . . . . . . . . . . 3
2.3 Fingerprint Matching . . . . . . . . . . . . . . . . . . . . 4
2.4 Authentication . . . . . . . . . . . . . . . . . . . . . . . 4
3. The SSHFP Resource Record . . . . . . . . . . . . . . . . . 4
3.1 The SSHFP RDATA Format . . . . . . . . . . . . . . . . . . . 4
3.1.1 Algorithm Number Specification . . . . . . . . . . . . . . . 5
3.1.2 Fingerprint Type Specification . . . . . . . . . . . . . . . 5
3.1.3 Fingerprint . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2 Presentation Format of the SSHFP RR . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . 7
Normative References . . . . . . . . . . . . . . . . . . . . 8
Informational References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 8
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . 10
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1. Introduction
The SSH [5] 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 verify the
fingerprint out-of-band before accepting the key, many users blindly
accepts 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 [4] 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 [4] 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.
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 [5] 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
configurable policy will allow administrators to determine which
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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
SSH FP fingerprint.
2.4 Authentication
A public key verified using this method MUST only be trusted if the
SSHFP resource record (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 [8], SIG(0) [9] or IPsec [7],
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.
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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
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 [6].
The message-digest algorithm is presumed to produce an opaque octet
string output which is placed as-is in the RDATA fingerprint field.
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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
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 approach suggested here shifts the burden of key checking from
each user of a machine to the key checking performed by the
administrator of the DNS recursive server used to resolve the host
information. Hopefully, by reducing the number of times that keys
need to be verified by hand, each verification is performed more
completely. Furthermore, by requiring an administrator do the
checking, the result may be more reliable than placing this task in
the hands of an application user.
The overall security of using SSHFP for SSH host key verification is
dependent on detailed aspects of how verification is done in SSH
implementations. 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,
we can implement SSH host key revocation by removing the
corresponding SSHFP from DNS.
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
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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 [10]
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 needs to allocate a RR type code for SSHFP from the standard RR
type space (type 44 requested).
IANA needs to open a new registry for the SSHFP RR type for public
key algorithms. Defined types are:
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.
Normative References
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[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] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[5] Rinne, T., Ylonen, T., Kivinen, T. and S. Lehtinen, "SSH
Protocol Architecture", draft-ietf-secsh-architecture-13 (work
in progress), September 2002.
[6] Rinne, T., Ylonen, T., Kivinen, T., Saarinen, M. and S.
Lehtinen, "SSH Transport Layer Protocol",
draft-ietf-secsh-transport-15 (work in progress), 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
2845, May 2000.
[9] Eastlake, D., "DNS Request and Transaction Signatures (
SIG(0)s)", RFC 2931, September 2000.
[10] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.
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/
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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:
Martin Fredriksson
Olafur Gudmundsson
Edward Lewis
Bill Sommerfeld
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