+
+
Secure Shell Working Group J. Schlyter
Internet-Draft Carlstedt Research &
-Expires: May 4, 2003 Technology
+Expires: October 1, 2003 Technology
W. Griffin
Network Associates Laboratories
- November 3, 2002
+ April 2, 2003
Using DNS to securely publish SSH key fingerprints
- draft-ietf-secsh-dns-01.txt
+ draft-ietf-secsh-dns-04.txt
Status of this Memo
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.
+ 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
+ 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://
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
- This Internet-Draft will expire on May 4, 2003.
+ This Internet-Draft will expire on October 1, 2003.
Copyright Notice
- Copyright (C) The Internet Society (2002). All Rights Reserved.
+ 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
+ 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
+ 2.2 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 . . . . . . . . . . . . . . . 4
- 3.1.2 Fingerprint type specification . . . . . . . . . . . . . . . 5
+ 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 . . . . . . . . . . . . 5
- 4. Security considerations . . . . . . . . . . . . . . . . . . 5
- 5. IANA considerations . . . . . . . . . . . . . . . . . . . . 6
- References . . . . . . . . . . . . . . . . . . . . . . . . . 7
+ 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 . . . . . . . . . . . . . . . . . . . . . . 8
- Full Copyright Statement . . . . . . . . . . . . . . . . . . 9
-
+ A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
+ Intellectual Property and Copyright Statements . . . . . . . 10
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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
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
+ 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.
+ 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 [5] to verify the lookup.
+ 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 [5] is assumed by this document.
+ 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
record(s) returned from DNS, the client MAY accept the identity of
the server.
-2.2 Implementation notes
+2.2 Implementation Notes
- Client implementors SHOULD to provide a configurable policy used to
- select the order of methods used to verify a host key and which
- fingerprints to trust ultimately, after user confirmation or not at
- all.
+ 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.
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+ 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
+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 RR used for verification was authenticated by a trusted SIG RR.
+ 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 [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
validation.
-3. The SSHFP resource record
+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
The RR type code for the SSHFP RR is TBA.
-3.1 The SSHFP RDATA format
+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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-3.1.1 Algorithm number specification
+3.1.1 Algorithm Number Specification
This algorithm number octet describes the algorithm of the public
key. The following values are assigned:
-
-
-
-
-
-
-
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-
Value Algorithm name
----- --------------
0 reserved
Reserving other types requires IETF consensus.
-3.1.2 Fingerprint type specification
+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
3.1.3 Fingerprint
The fingerprint is calculated over the public key blob as described
- in [10].
+ 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
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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
host.example. SSHFP 2 1 123456789abcdef67890123456789abcdef67890
-4. Security considerations
+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 key presented is actually the key at 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.
+ 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
-
-
-
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-
-
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
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
+ 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.
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.
+ 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
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 [8]
+ 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.
+ different from other key enrollment situations, e.g. a client 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
type space (type 44 requested).
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.
-References
+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.
+Schlyter & Griffin Expires October 1, 2003 [Page 7]
- [4] Thayer, R., Doraswamy, N. and R. Glenn, "IP Security Document
- Roadmap", RFC 2411, November 1998.
+Internet-Draft DNS and SSH fingerprints April 2003
- [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.
+ [1] Mockapetris, P., "Domain names - concepts and facilities", STD
+ 13, RFC 1034, November 1987.
- [7] Eastlake, D., "DNS Request and Transaction Signatures (
- SIG(0)s)", RFC 2931, September 2000.
+ [2] Mockapetris, P., "Domain names - implementation and
+ specification", STD 13, RFC 1035, November 1987.
- [8] Wellington, B., "Secure Domain Name System (DNS) Dynamic
- Update", RFC 3007, November 2000.
+ [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
+ Levels", BCP 14, RFC 2119, March 1997.
- [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.
+ [4] Eastlake, D., "Domain Name System Security Extensions", RFC
+ 2535, March 1999.
- [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.
+ [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
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+ [7] Thayer, R., Doraswamy, N. and R. Glenn, "IP Security Document
+ Roadmap", RFC 2411, November 1998.
-Internet-Draft DNS and SSH fingerprints November 2002
+ [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
URI: http://www.crt.se/~jakob/
+
+
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Wesley Griffin
Network Associates Laboratories
15204 Omega Drive Suite 300
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+
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
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