Secure Shell Working Group J. Schlyter Internet-Draft Carlstedt Research & Expires:
July 12,September 23, 2003 Technology W. Griffin Network Associates Laboratories January 11,March 25, 2003 Using DNS to securely publish SSH key fingerprints draft-ietf-secsh-dns-02.txtdraft-ietf-secsh-dns-03.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 July 12,September 23, 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. SSH Host Key Verification . . . . . . . . . . . . . . . . . 3 2.1 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2 Implementation notesNotes . . . . . . . . . . . . . . . . . . . . 3 2.3 Fingerprint matchingMatching . . . . . . . . . . . . . . . . . . . . 4 2.4 Authentication . . . . . . . . . . . . . . . . . . . . . . . 4 3. The SSHFP resource recordResource Record . . . . . . . . . . . . . . . . . 4 3.1 The SSHFP RDATA formatFormat . . . . . . . . . . . . . . . . . . . 4 3.1.1 Algorithm number specificationNumber Specification . . . . . . . . . . . . . . . 4 3.1.2 Fingerprint type specificationType Specification . . . . . . . . . . . . . . . 5 3.1.3 Fingerprint . . . . . . . . . . . . . . . . . . . . . . . . 5 3.2 Presentation formatFormat of the SSHFP RR . . . . . . . . . . . . 5 4. Security considerationsConsiderations . . . . . . . . . . . . . . . . . . 5 5. IANA considerationsConsiderations . . . . . . . . . . . . . . . . . . . . 67 Normative References . . . . . . . . . . . . . . . . . . . . 7 Informational References . . . . . . . . . . . . . . . . . . 78 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 8 A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 Intellectual Property and Copyright Statements . . . . . . . 910 1. Introduction The SSH  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  and using DNSSEC  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  and the DNS security extensions  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 . 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 notesNotes Client implementors SHOULD 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. 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 matchingMatching The public key and the SSHFP resource record are matched together by comparing algorithm number and 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. Clients that do not validate the DNSSEC signatures themselves MUST use a secure transport, e.g. TSIG , SIG(0)  or IPsec , between themselves and the entity performing the signature validation. 3. The SSHFP resource recordResource 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 formatFormat 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 specificationNumber 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 specificationType 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 . 3.2 Presentation formatFormat 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 considerationsConsiderations 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. 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 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  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 considerationsConsiderations 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  Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987.  Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999.  Rinne, T., Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T J.T. and S. Lehtinen, "SSH Transport Layer Protocol", workProtocol Architecture", draft-ietf-secsh-architecture-13 (work in progress draft-ietf-secsh-architecture-13.txt,progress), September 2002.  Rinne, T., Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T J.M. and S. Lehtinen, "SSH Transport Layer Protocol", workdraft-ietf-secsh-transport-15 (work in progress draft-ietf-secsh-transport-15.txt,progress), September 2002. Informational References  Thayer, R., Doraswamy, N. and R. Glenn, "IP Security Document Roadmap", RFC 2411, November 1998.  Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington, "Secret Key Transaction Authentication for DNS (TSIG)", RFC 2845, May 2000.  Eastlake, D., "DNS Request and Transaction Signatures ( SIG(0)s)", RFC 2931, September 2000.  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: firstname.lastname@example.org URI: http://www.crt.se/~jakob/ Wesley Griffin Network Associates Laboratories 15204 Omega Drive Suite 300 Rockville, MD 20850 USA EMail: email@example.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 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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