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Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Standards Track                          March 12, 2011
Expires: September 13, 2011


                 Specifying That a Server Supports TLS
                    draft-hoffman-server-has-tls-04

Abstract

   A server that hosts applications that can be run with or without TLS
   may want to communicate with clients whether the server is hosting an
   application only using TLS or also hosting the application without
   TLS.  Many clients have a policy to try to set up a TLS session but
   fall back to insecure if the TLS session cannot be set up.  If the
   server can securely communicate whether or not it can fall back to
   insecure tells such a client whether or not they should even try to
   set up an insecure session with the server.  This document describes
   the use cases for this type of communication and a secure method for
   communicating that information.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on September 13, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


1.  Introduction

   Most client-server applications standardized in the IETF have two
   modes: an insecure mode that involves no authentication or integrity
   protection, and a secure mode that requires (at a minimum) that the
   client authenticate the server and set up a communication channel
   with integrity protection.  In most cases, the secure mode is
   achieved by starting a TLS [RFC5246] session and, when successful,
   running the insecure mode inside of it.

   People within the IETF and application developers have historically
   had widely varying views on what a client should and should not do
   about the two modes.  Phrases like "assured security" and "client
   flexibility" are used, often without clear definition.  Deployed
   clients and servers from different vendors act differently for the
   two modes, often relegating the control of the two modes to
   "advanced" configuration options (if such control is given to the
   user at all).

   Section 2 of this document lays out the choices for clients and
   servers for handling the two modes in different circumstances, and
   gives specific semantics for each type of client and server.  Section
   3 gives a protocol for a domain owner to specify whether they offer
   one or both modes for any given application, and to specify a client
   policy preference.  Section 4 defines how to implement various
   policies using the protocol.  Using the protocol given here, a server
   can completely specify what it offers and allows a client to reliably
   choose which mode it wants to use.

   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 [RFC2119].


2.  Security Options for Clients and Servers

   This section describes the different types of clients and servers
   that deal with insecure protocols that can be secured by wrapping the
   protocol in TLS.  It also describes the types of security policies
   that those clients and servers can embody.  It explicitly does not
   argue that one policy is better than another in any particular



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   environment; instead, it assumes that the server operator and the
   client implementor (and, hopefully, the human operating the client)
   can make that decision themselves if given the proper tools.

   This discussion assumes a client-server protocol that is defined for
   an insecure fashion, and is also defined for a secure fashion that
   uses a TLS session for security.  For example, "HTTP run over port
   80" and "HTTP-in-TLS run over port 443" would meet this definition;
   "SMTP without STARTTLS" and "SMTP with STARTTLS" (see [RFC3207])
   would also meet this definition.  Some peer-to-peer protocols might
   meet this definition if the startup actions resemble the typical
   client-server interaction, but this discussion makes no extra attempt
   to cover such protocols.

   Given a particular client application configuration, there are three
   interesting types of clients:

      Insecure Only (CIO) -- The client is configured to only attempt
      communication for the application in its insecure form.  For
      example, a POP client might be configured to only try insecure POP
      on port 110.

      Secure Only (CSO) -- The client is configured to only attempt
      communication for the application in its secure, TLS-wrapped form.
      For example, a POP client might be configured to only try secure
      POP on port 995.

      Allows Fallback From Secure to Insecure (CFB) -- The client is
      configured to attempt communication for the application in its
      secure, TLS-wrapped form, but if it fails to set up a TLS session,
      the client will attempt to attempt communication to the same
      server using the insecure form.  This configuration may be offered
      for reasons such as if the client doesn't trust the CA that the
      server uses to identify itself.

   Given a particular server configuration, there are three interesting
   types of servers:

      Insecure Only (SIO) -- The server responds without TLS on the main
      port for the application.  A host for a web server that only
      responds to HTTP requests on port 80 is an example of this.

      Secure Only (SSO) -- The server responds using TLS on the TLS-
      specific port for the application.  For example, a host for a web
      server only responds to HTTP requests on port 443.  Alternately,
      if the application supports in-band security update (such as
      STARTTTLS for SMTP), the server responds on the normal port, tries
      to establish a TLS session, and does not proceed with the protocol



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      if a TLS session cannot be established.

      Serves Both Secure and Insecure (SSB) -- The server responds
      without TLS on the main port for the application *and* responds
      using TLS on the TLS-specific port for the application, such as
      both ports 80 and 443 for HTTP.  Alternately, if the application
      supports in-band security update (such as STARTTTLS for SMTP), the
      server responds on the normal port, tries to establish a TLS
      session, and proceeds with the normal protocol if a TLS session
      cannot be established.

   It is expected that client configuration will be per-host.  That is,
   a client that is CSO for some hosts might be CFB for other hosts.
   The server configuration, of course, applies to all clients accessing
   it.

   In this taxonomy, a CIO can always communicate with an SIO and SSB.
   A CSO can communicate with an SSO, and can communicate with an SSB as
   long as the TLS session is set up successfully.  A CFB can
   communicate with an SIO, an SSO, and an SSB.

   Given this, a host that only allows clients to use the secure form of
   a protocol MUST only be configured to be SSO; a client that wants to
   only communicate with a server securely MUST only be configured to be
   CSO.

   Note that a host might want to serve both insecure and secure form of
   a protocol, but wants clients to only use the secure form.  For
   example, the insecure form might immediately do an upgrade to the
   secure form, or it might do a protocol-based redirection to a server
   doing the secure form.  Such a host will want to be able to indicate
   that, even if it has both secure and insecure ports for a protocol
   open, it wants clients that can be configured as CFB or CSO to only
   be configured as CSO.

   This taxonomy exposes a problem with the way that clients and servers
   interact today: a CIO that starts an insecure communication with a
   server, or a CFB that falls back to insecure communication with a
   server, has no idea whether the site they wish to communication with
   even hosts an insecure server.  The server might be configured to be
   any of SIO or SSO or SSB, but the client cannot tell.  If a CIO or
   CFB client knows ahead of time that a host did not support insecure
   communication, the client would not even start communication because
   it would either just waste time waiting for a timeout, or it would
   communicate with an impostor.

   Note that the protocol described here is not a discovery protocol.
   It is perfectly reasonable for a server to be running a service in an



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   insecure fashion, a secure fashion, or both, without using the
   protocol that is described here.  The purpose of the protocol is to
   let a client find out, securely, whether a particular server protocol
   is being run securely, not whether it is being run at all.


3.  The HASTLS Resource Record

   The HASTLS resource record type, whose value is TBD1, lists an
   insecure/secure port pair that is served on the host named by the
   domain name for the application and protocol given in the query.  It
   only applies to applications that are secured with TLS, not to
   applications that have insecure and secure versions that use some
   other security protocol.

   The presentation format is:

   _appname._protoname.hostname IN HASTLS ins-port sec-port pol-pref

   The application name ("appname") and protocol name ("protoname")
   being queried are the same as are used in the SRV RRtype described in
   [RFC2782] The insecure port number (called "ins-port"), the secure
   port number (called "sec-port"), and the client policy preference
   (called "pol-pref") are each two-octet positive integers.

   If a server does not offer one of the the two services, that service
   is indicated by port 0.  For protocols that use in-band signaling for
   security upgrades, "ins-port" and "sec-port" have the same value.  A
   HASTLS record MUST NOT have both the "ins-port" and "sec-port" set to
   0.

   A query for a particular application MAY return more than one HASTLS
   resource record, and conformant clients MUST be able to process
   multiple responses from a single query.  For example, a site that
   offers HTTP on both port 80 and port 8080 might return two records,
   one for port 80 and its secure counterpart (if any), and one for
   protport 8080 and its secure counterpart (if any).

   The HASTLS record is not useful for service discovery.  Clients MUST
   NOT make any assumptions about an application for which there is not
   a HASTLS record; the lack of a HASTLS record for a particular
   application says nothing about whether or not the service is offered
   on the host on a specific port.

   The client policy preference octet specifies the host's preference
   for client policy.  It has two possible values:





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      0 -- The host's administrator has no client policy preference for
      this protocol.

      1 -- If the client could be configured as either CFB or CSO for
      this protocol, and the host's administrator was able to configure
      the client for the protocol, that administrator would configure
      the client as CSO.  Stated another way, the host's administrator
      does not want any CFB client to access the host for this protocol.

   Note that specifying 1 for the client policy preference when a host
   does not support a protocol securely makes no sense, but it also does
   no harm.  Further, for a host that is SSO, both policy preferences
   have an identical result.

   For example, the server at www.example.com offers SMTP both securely
   and insecurely.  The host's SMTP administrator has a client policy
   preference that CFB clients not access the host.  The HASTLS record
   would look like:

   _smtp._tcp.www.example.com IN HASTLS 25 25 1

   Another example is the server at www.example.com offering HTTP only
   securely.  The resulting HASTLS records could be either:

   _http._tcp.www.example.com IN HASTLS 0 443 0
       or
   _http._tcp.www.example.com IN HASTLS 0 443 1

   [[ NEED TO ADD: wire format. ]]


4.  Implementing Policy with HASTLS

   Servers that have a policy to declare the server as SIO, SSO, or SSB
   can use HASTLS to announce that policy for each application it
   serves.  A server whose policy is that it is an SIO would set the
   ins-port to a non-zero number and the sec-port to 0.  A server whose
   policy is that it is an SSO would set the ins-port to 0 and the sec-
   port to a non-zero number.  A server whose policy is that it is an
   SSB would set both the ins-port and sec-port to a non-zero number.

   The conformance requirements for a client using the HASTLS record
   depend on the policy configured for the client or the server:

   o  A client communicating with a server that has set its client
      policy preference to 1 MUST NOT try to communicate insecurely with
      that server, even if the server has set the ins-port to a non-zero
      number; this is the equivalent of temporarily setting its policy



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      for the server to CSO for that application.  This temporary policy
      based on the server's client policy preference overrides any
      others for the server.

   o  A client whose policy is that it is a CIO MUST NOT try to
      communicate insecurely with a server that has the ins-port set to
      0.

   o  A client whose policy is that it is a CSO MUST only try to
      communicate securely with a server that has the sec-port set to a
      non-zero number.

   o  A client whose policy is that it is a CSO MUST NOT try to
      communicate with the server if an ins-port value is given.

   o  A client whose policy is that it is a CFB MUST NOT try to
      communicate securely with a server that has the sec-port set to 0.

   o  A client whose policy is that it is a CFB MUST NOT try to
      communicate insecurely with a server that has the ins-port set to
      0.

   o  A client whose policy is that it is a CFB trying to communicate
      with a server whose sec-port is set to a non-zero number SHOULD
      first try to communicate securely over the secure port unless it
      knows from other sources that the TLS session will not be set up
      properly.


5.  IANA Considerations

   This document requests that IANA allocate a new DNS resource record
   type called HASTLS from the data types range; it will have the value
   TBD1.

   Submission template:















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   A.    Submission Date: Date of this document
   B.    Submission Type: New RRTYPE
   C.    Contact Information for submitter: Author of this document
   D.    Motivation for the new RRTYPE application: Contents of
         this document
   E.    Description of the proposed RR type: Contents of this document
   F.    What existing RRTYPE or RRTYPEs come closest to filling that
         need and why are they unsatisfactory: None are even close to
         that given in this document.
   G.    What mnemonic is requested for the new RRTYPE (optional):
         HASTLS
   H.    Does the requested RRTYPE make use of any existing IANA
         Registry or require the creation of a new IANA sub-registry in
         DNS Parameters: No
   I.    Does the proposal require/expect any changes in DNS
         servers/resolvers that prevent the new type from being
         processed as an unknown RRTYPE (see [RFC3597]): No
   J.    Comments: None


6.  Security Considerations

   In order to prevent a man-in-the-middle (MITM) attack where the
   attacker can change DNS responses, the data in the HASTLS record
   needs to be received securely by a DNS requester, such as through
   validated DNSSEC.  At this time, there is no common method for a
   application client to know whether or not the data it receives from
   the DNS has been protected with DNSSEC unless that application has a
   validating resolver, or has an API to the operating system and the
   operating system has a validating resolver.

   Some MITM attacks can do a partial impersonation of the secure server
   if the client does not have good indications when an SSL connection
   is established (see, especially, the "sslstrip" attack described in
   various places in 2009).  This protocol cannot protect against all
   such attacks because the attack is actually on the user interface,
   not the security of the TLS connection.

   If the HASTLS information is received by the client system without
   security, an attacker could change the HASTLS information to fool the
   client into thinking that a host provides insecure application
   services and/or does not provide secure application services.  Thus,
   cryptographic protection of the contents of the HASTLS information
   (such as with DNSSEC) is mandatory.


7.  References




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7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

7.2.  Informative References

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.


Appendix A.  Protocols That Have Both Second Ports and Upgrade Paths

   There is a significant open issue with the current proposal: some
   protocols (hopefully few) have multiple ways of using TLS.  One that
   was mentioned is HTTP, which has both "start TLS on TCP port 443" and
   "use STARTTLS on TCP port 80 as described in RFC 2817".  POP and IMAP
   have the same issue.

   One possibility is for this protocol to say "always use the second
   port" or "always use in-stream upgrade".  This would give the client
   unambiguous instructions, but would not work for servers that do not
   implement the specified option.

   Another possibility is to have the HASTLS response indicate which way
   the secure version is supported.  This is more flexible than the
   first proposal, but also more complicated for the client.  It also is
   possibly overkill if there are only two or three protocols of
   interest.

   This issue needs to be resolved before the proposal is finished.


Author's Address

   Paul Hoffman
   VPN Consortium

   Email: paul.hoffman@vpnc.org





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