Network Working Group J. Schlyter Request for Comments: 4255 OpenSSH Category: Standards Track W. Griffin SPARTA January 2006
Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
Status of This Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes a method of verifying Secure Shell (SSH) host keys using Domain Name System Security (DNSSEC). The document defines a new DNS resource record that contains a standard SSH key fingerprint.
Table of Contents
1. Introduction ....................................................2 2. SSH Host Key Verification .......................................2 2.1. Method .....................................................2 2.2. Implementation Notes .......................................2 2.3. Fingerprint Matching .......................................3 2.4. Authentication .............................................3 3. The SSHFP Resource Record .......................................3 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 5. IANA Considerations .............................................6 6. Normative References ............................................7 7. Informational References ........................................7 8. Acknowledgements ................................................8
The SSH [6] 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 as well as the user authenticating itself to the server.
If a connection is established to a server whose public key is not already known to the client, a fingerprint of the key is presented to the user for verification. If the user decides that 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 accept 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.
In order to distribute the fingerprint using DNS, this document defines a new DNS resource record, "SSHFP", 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].
Upon connection to an 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 match the algorithm and fingerprint of one of the SSHFP resource record(s) returned from DNS, the client MAY accept the identity of the server.
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 [6] 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
Schlyter & Griffin Standards Track [Page 2]
RFC 4255 DNS and SSH Fingerprints January 2006
configurable policy will allow administrators to determine which 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.
A public key verified using this method MUST NOT be trusted if the SSHFP resource record (RR) used for verification was not authenticated by a trusted SIG RR.
Clients that do validate the DNSSEC signatures themselves SHOULD use standard DNSSEC validation procedures.
Clients that do not validate the DNSSEC signatures themselves MUST use a secure transport (e.g., TSIG [9], SIG(0) [10], or IPsec [8]) between themselves and the entity performing the signature validation.
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 [4].
For interoperability reasons, as few fingerprint types as possible should be reserved. The only reason to reserve additional types is to increase security.
The RDATA of 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.:
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 overall security of using SSHFP for SSH host key verification is dependent on the security policies of the SSH host administrator and DNS zone administrator (in transferring the fingerprint), detailed aspects of how verification is done in the SSH implementation, and in the client's diligence in accessing the DNS in a secure manner.
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, SSH host key revocation can be implemented by removing the corresponding SSHFP from DNS.
Schlyter & Griffin Standards Track [Page 5]
RFC 4255 DNS and SSH Fingerprints January 2006
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 [11] 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.
This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any Intellectual Property Rights 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; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA).