This is the first of two torspec proposals to help Tor work with Sepcial-Use TLDs, like the GNU Name system or NameCoin. The second part will be an anycast facility. - Jeff
Filename: xxx-special-use-tld-support.txt Title: Special-Use TLD Support Author: Jeffrey Burdges Created: 20 Sept 2015 Status: Draft Implemented-In: ?
Abstract
Suppose Special-Use TLDs in Tor via external Domain Name System (DNS) suppliers, such as the GNU Name System and NameCoin.
Background
Special-use TLD supplier software integrates with the host operating system's DNS layer so that other software resolves the special-use TLD identically to standard DNS TLDs. On Linux for example, a Special-Use TLD package could create a plugin for the Name Service Switch (NSS) subsystem of the GNU C Library.
Tor cannot safely use the local system's own DNS for name resolution, as doing so risks deanonmizing a user through their DNS queries. Instead Tor does DNS resolution at a circut's exit relay. It follows that Tor users cannot currently use special-use TLDs packages in a safe manor.
In addition, there are projects to add public key material to DNS, like TLSA records and DNSSEC, that necessarily go beyond NSS.
Design
We denote by N an abstract name service supplier package. There are two steps required to integrate N safely with Tor :
Of course, N must be modified so as to (a) employ Tor for it's own traffic and (b) to use Tor in a safe way. We deem this step outside the scope of the present document since it concerns modifications to N that depend upon N's design. We caution however that peer-to-peer technologies are famous for sharing unwanted information and producing excessively distinctive traffic profiles, making (b) problematic. Another proposal seeks to provide rudementary tools to asist with (a).
We shall instead focus on modifying Tor to route some-but-not-all DNS queries to N. For this, we propose a NameService configuration option that tells Tor where to obtain the DNS record lying under some specific TLD.
Anytime Tor resolves a DNS name ending in an Special-Use TLD appearing in an NameService configuration line then Tor makes an RPC request for the name record using given UNIX domain socket or address and port.
We should allow CNAME records to refer to .onion domains, and to regular DNS names, but care must be taken in handling CNAME records that refer to Special-Use TLDs handled by NameSerice lines. Tor should reject CNAME records that refer to the .exit domains.
Configuration
We propose two Tor configuration options :
NameSubstitution [.]source_dnspath [.]target_dnspath NameService [.]dnspath socketspec [noncannonical] [timeout=num] [-- service specific options]
We require that socketspec be either the path to a UNIX domain socket or an address of the form IP:port. We also require that that each *dnspath be a string conforming to RFC 952 and RFC 1123 sec. 2.1. In other words, a dnsspec consists of a series of labels separated by periods . with each label of up to 63 characters consisting of the letters a-z in a case insensitive mannor, the digits 0-9, and the hyphen -, but hyphens may not appear at the beginning or end of labels.
NameSubstitution rules are applied only to DNS query strings provided by the user, not CNAME results. If a trailing substring of a query matches source_dnspath then it is replaced by target_dnspath.
NameService rules route matching query to to appropriate name service supplier software. If a trailing substring of a query matches dnspath, then a query is sent to the socketspec using the RPC protcol descrived below. Of course, NameService rules are applied only after all the NameSubstitution rules.
There is no way to know in advance if N handles cahcing itself, much less if it handles caching in a way suitable for Tor. Ideally, we should demands that N return an approporaite expiration time, which Tor can respect without harming safety or performance. If this proves problematic, then configuration options could be added to adjust Tor's caching behavior.
Seconds is the unit for the timeout option, which defaults to 60 and applies only to the name service supplier lookup. Tor DNS queries, or attempts to contact .onion addresses, that result from CNAME records should be given the full timeout alloted to standard Tor DNS queries, .onion lookups, etc.
Any text following -- is passed verbatim to the name service suppllier as service specific options, according to the RPC protocol described below.
Control Protocol
An equivalent of NameService and NameSubstitution should be added to the Tor control protocol, so that multiple users using the same Tor daemon can have different name resolution rules.
RPC protocol
We require an RPC format that communicates two values, first any service specific options give on the NameService line, and second the query name itself of course. We might however discuss if there are any standardized flags, distinct from these options, and whether they should be communicated separately.
In principle, Tor could make due with simply receiving a strong in return. We recommend however that Tor expect a return format as or more powerful than full DNS queries. In particular, we should endever to return TLSA records at the same time as the underlying DNS record, so that Tor Browser can utilize that key material. The GNS Record format used by the GNU Name System addresses this and other issues, so it should be taken as a candidate. See : https://github.com/GNUnet/gnunet/blob/gnunet/src/include/gnunet_gnsrecord_li...
Sepcial-use tLD suppliers should internally process CNAME records that fall into their own domains, but they should return CNAME records to Tor that refer to .onion or .exit domains, or to normal DNS names. Initially, Tor should issue an error if it recieves a CNAME record that matches an NameService line. If however that NameService line contains the noncannonical option, then CNAME records should instead bypass it, and use Tor's DNS system.
At present, alternative DNS packages should not pass CNAME records between themselves, despite speaking the same RPC protocol, as this creates unknown risks. As such forwarding can be done most safely by Tor itself, the Tor Project reserves the right to forward CNAME records between NameService lines in the future. Applications should therefore not depend upon the above error being returned.
Variations
Tor would conceivably benefit from externalizing its own DNS handling as a separate process. This might however require that Tor have the ability to start name service suppliers. A fuller consideration of this might alter our design of the NameService configuration option.
Acknowledgments
Based on extensive discussions with Christian Grothoff and George Kadianakis.