| Network Working Group | |
| Internet-Draft | TCI |
| Intended status: Experimental | March 6, 2019 |
| Expires: September 7, 2019 |
Fake Server Name Indication
draft-belyavskiy-fakesni-02
The document provides a specification of the Fake Server Name Indication. Being implemented, the Fake SNI specification provides a way to cheat the monitoring solutions without providing any additional information to external observers.
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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 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.
The solution specified in this document is inspired by the design of Encrypted SNI. It is fully-compatible with current TLS specifications. As it does not make much sense to use it with TLS 1.0-1.2, where the original host name will be provided unencrypted in the certificate, in case of TLS 1.3 the certificate is delivered encrypted.
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.
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. Specifying IP address for a specific fake name may help in case when a service is hosted using more than one CDN.
The fake name specified in the Fake SNI RR MUST identify the original hostname it is valid for. Fake names for different hosts on the same IP address MUST be different to distinguish the original names.
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.
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.
Depending on the DPI modus operandi it may make sense to provide a valid fake name (e.g. from deep-level subdomain) resolving to the same IPs as original hostname does. If DPI tries to resolve the fake name, such behaviour will make distinguishing between real and fake names difficult.
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.
The current version of the specification is available at GitHub repository.