Fake Server Name Indication
draft-belyavskiy-fakesni-01

Network Working Group
Internet-Draft                                                       TCI
Intended status: Experimental                          February 20, 2019
Expires: August 24, 2019

                      Fake Server Name Indication
                      draft-belyavskiy-fakesni-01

Abstract

   The document provides a specification of the Fake Server Name
   Indication.  Being implemented, the Fake SNI specification provides a
   way to work around the monitoring solutions without providing any
   additional information to external observers.

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1.  Introduction

   Many DPI solutions use SNI information as a criterion to filter
   connection to various sites.  Though Encrypted SNI makes impossible
   to read the SNI value, there is information [1] that absence of SNI
   looks suspicious itself and all communications are blocked.

   This specification introduces a way to provide a value of SNI treated
   by TLS server as an alias to one of the names known by server but not
   matching the possibly suspicious hostname.

   This specification does not save from DPI solutions but it provides
   one more loophole to cheat them.

2.  Fake SNI design goals

   The solution specified in this document is inspired by the design of
   Encrypted SNI.

   The provider publishes a name matching the target name to be provided
   in the clear text.  This document defines a publication mechanism
   using DNS, but other mechanisms are also possible.

   When a client wants to establish a TLS connection to a domain served
   by a Fake SNI-supporting provider, it replaces the value in
   "server_name" extension in the ClientHello with the value obtained by
   transport.  The provider can then find out the desired name from its
   configuration and either establish the connection with the desired
   host or reject it.

3.  Definitions

      Original name - the hostname of service that is subject to hide.

      Fake name - the hostname specified by server and sent by client to
      indicate intention to connect to host with original name.

4.  Fake SNI indication

   Fake SNI information is published in DNS via TXT RR.  For example,
   the Fake SNI record for domain example.com may look like

   _fakesni.example.com. 60S IN TXT "myfakerecord.com IP"

   where IP address may be omitted.  If present, it MUST match an IP
   address specified in A/AAAA record for the domain.

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   Value specified in the Fake SNI RR MUST NOT match any hostname
   available for the IP address it is valid for.  Fake names for
   different hosts MUST be different.

5.  Server behaviour

   On receiveing the value of known Fake SNI in the TLS ClientHello
   server MUST return the certificate matching the original hostname.
   Otherwise server SHOULD abort the connection.

6.  Client behaviour

   Client MAY use the Fake SNI record as fallback if connecting using
   ESNI is blocked.  In this case client initiates normal TLS connection
   specifying the value from Fake SNI record in the server_name
   extension.  If the certificate received from server does not match
   the original hostname, the client MUST abort the connection.
   Otherwise the client MUST follow the normal process of TLS handshake.

7.  Security considerations

   As Fake SNI can be used in TLS 1.2, it does not provide any problems
   to DPI because in this case the original hostname is available in
   clear text in server certificate.  TLS 1.3 encrypts the Certificate
   message, so it is RECOMMENDED to use Fake SNI together with TLS 1.3.
   To strengthen the protection, it's recommended to obtain _fakesni RR
   via DoT or DoH.

   As DPI solutions are able to obtain the DNS _fakesni records as
   legitimate clients do, it is RECOMMENDED to set reasonable TTL values
   for the _fakesni records.  Also it is RECOMMENDED to use such values
   of fake names that are syntactically correct domain names.  Otherwise
   DPI can recognise the fake names as fake ones.

8.  References

8.1.  URIs

   [1] https://mailarchive.ietf.org/arch/msg/tls/
       WiT3oEh6PO96mm0z28BNMp0YgGs

Author's Address

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   Dmitry Belyavskiy
   Cryptocom LTD
   Kedrova st, 14/2
   Moscow  127083
   RU

   Email: beldmit@gmail.com

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