[tor-dev] Proposal 274: A Name System API for Tor Onion Services

Arthur D. Edelstein arthuredelstein at gmail.com
Fri Oct 7 20:29:50 UTC 2016

Hi George,

A nice proposal!

A question that occurs to me is how the name plugin will interact with
the network. Presumably it will make requests over Tor, either
periodically to update its local database, or just-in-time, whenever a
RESOLVE request is made. If name resolution requires a just-in-time
connection, what circuit should this connection use? Right now, Tor
Browser uses a separate circuit for each URL bar domain ("first
party") to avoid linking visits to different websites. And thus DNS
resolution happens on each first-party's circuit. So I think something
similar needs to happen with name plugins, to avoid exposing all
resolution requests to a single exit node. Does this need to be part
of the API somehow?


On Fri, Oct 7, 2016 at 1:06 PM, George Kadianakis <desnacked at riseup.net> wrote:
> Hello list,
> we've been busy discussing secure name systems for the past few months,
> and how we could integrate them with Tor.
> We had a few people ask us for precise interfaces, so here is an attempt
> for a modular name system API. It's largely based on the PT API, which
> has been pretty successful.
> The proposal was brainstormed on a Sunday morning in Seattle between
> Yawning, David and me. I wrote the actual proposal contents so any
> innacuracies, impossibilities and errors are mine :)
> If people like it and are interested in implementing it, we can draft
> out an implementation plan for any person who wants to go for it. But
> for now, please consider this proposal text as a draft that is meant to
> be improved from this list's feedback.
> Have fun and looking forward to your comments (and code).
> ---
> Filename: 274-naming-layer-api.txt
> Title: A Name System API for Tor Onion Services
> Author: George Kadianakis, Yawning Angel, David Goulet
> Created: 04-Oct-2016
> Status: Draft
>   Table Of Contents:
>                         1. Introduction
>                                 1.1. Motivation
>                                 1.2. Design overview and rationale
>                         2. System Specification
>                                 2.1. System overview [SYSTEMOVERVIEW]
>                                 2.2. System Illustration
>                                 2.3. System configuration [TORRC]
>                                         2.3.1. Tor name resolution logic
>                                 2.4. Name system initialization [INITPROTOCOL]
>                                 2.5. Name resolution using NS API
>                                         2.5.1. Message format
>                                         2.5.2. RESOLVED status codes
>                                         2.5.3. Further name resolution behavior
>                                 2.6. Cancelling a name resolution request
>                                 2.7. Launching name plugins [INITENVVARS]
>                                 2.8. Name plugin workflow [NSBEHAVIOR]
>                                         2.8.1. Name plugin shutdown [NSSHUTDOWN]
>                                 2.9. Further details of stdin/stdout communication
>                                         2.9.1. Message Format
>                         3. Discussion
>                                 3.1. Using second-level domains instead of tld
>                                 3.2. Name plugins handling all tlds '*'
>                                 3.3. Deployment strategy
>                                 3.4. Miscellaneous discussion topics
>                         4. Acknowledgements
>                         A.1: Example communication Tor <-> name plugin [PROTOEXAMPLE]
>                         A.2: Example plugins [PLUGINEXAMPLES]
> 1. Introduction
>    This proposal specifies a modular API for integrating name systems with Tor.
> 1.1. Motivation
>    Tor onion service addresses are decentralized and self-authenticated but
>    they are not human-memorable (e.g. 3g2upl4pq6kufc4m.onion). This is a source
>    of poor usability, since Internet users are familiar with the convenient
>    naming of DNS and are not used to addresses being random text.
>    In particular, onion addresses are currently composed of 16 random base32
>    characters, and they look like this:
>                       3g2upl4pq6kufc4m.onion
>                       vwakviie2ienjx7t.onion
>                       idnxcnkne4qt76tg.onion
>                       vwakviie2ienjx6t.onion
>    When Proposal 224 get deployed, onion addresses will become even
>    bigger: 53 base32 characters. That's:
>         llamanymityx4fi3l6x2gyzmtmgxjyqyorj9qsb5r543izcwymle.onion
>         lfels7g3rbceenuuqmpsz45z3lswakqf56n5i3bvqhc22d5rrsza.onion
>         odmmeotgcfx65l5hn6ejkaruvai222vs7o7tmtllszqk5xbysola.onion
>         qw3yvgovok3dsarhqephpu2pkiwzjdk2fgdfwwf3tb3vgzxr5kba.onion
>    Over the years Tor users have come up with various ad-hoc ways of handling
>    onion addresses. For example people memorize them, or use third-party
>    centralized directories, or just google them everytime.
>    We believe that the UX problem of non-human-memorable problems is not
>    actually solved with the above ad-hoc solutions and remains a critical
>    usability barrier that prevents onion services from being used by a wider
>    audience.
> 1.2. Design overview and rationale
>    During the past years there has been lots of research on secure naming and
>    various such systems have been proposed (e.g. GNS, Namecoin, etc.).
>    Turns out securely naming things is a very hard research problem, and hence
>    none of the proposed systems is a clear winner: all of them carry various
>    trade-offs. Furthermore, none of the proposed systems has seen widespread use
>    so far, which makes it even harder to pick a single project.
>    Given the plenitude of options, one approach to decide which system
>    is best is to make various decent name systems available and let the
>    Tor community and the sands of time pick the winner. Also, it might
>    be that there is no single winner, and perhaps different specialized
>    name system should be used in different situations. We believe that
>    by getting secure name systems actually get utilized by real users,
>    the whole field will mature and existing systems will get battle-hardened.
>    Hence, the contribution of this proposal is a modular Name System API
>    (NSA) that allows developers to integrate their own name systems in
>    Tor. The interface design is name-system-agnostic, and it's heavily
>    based on the pluggable transports API (proposal 180). It should be
>    flexible enough to accommodate all sorts of name systems (see [PLUGINEXAMPLES]).
> 2. System Specification
>    A developer that wants to integrate a name system with Tor needs to first
>    write a wrapper that understands the Tor Name System API (NS API). Using the
>    Name System API, Tor asks the name system to perform name queries, and
>    receives the query results. The NS API works using environment variables and
>    stdin/stdout communication. It aims to be portable and easy to implement.
> 2.1. System overview [SYSTEMOVERVIEW]
>    Here is an overview of the Tor name system:
>    Alice, a Tor user, can activate various name systems by editing her
>    torrc file and specifying which tld each name system is responsible
>    for. For this section, let's consider a simple fictional name system,
>    unicorn, which magically maps domains with the .corn tld to the
>    correct onion address. Here it is:
>        OnionNamePlugin 0 .corn   /usr/local/bin/unicorn
>    After Alice enables the unicorn plugin, she attempts connecting to
>    elephantforum.corn. Tor will intercept the SOCKS request, and use the
>    executable at /usr/local/bin/unicorn to query the unicorn name system
>    for elephantforum.corn. Tor communicates with the unicorn plugin
>    using the Tor NS API through which name queries and their results can
>    be transported using stdin/stdout.
>    If elephantforum.corn corresponds to an onion address in the unicorn
>    name system, unicorn should return the onion address to Tor using the
>    Tor NS API. Tor must then internally rewrite the elephantforum.corn
>    address to the actual onion address, and initiate a connection to it.
> 2.2. System Illustration
>    Here is a diagram illustrating how the Tor Name System API works. The name
>    system used in this example is GNS, but there is nothing GNS-specific here
>    and GNS could be swapped for any other name system (like hosts files, or
>    Namecoin).
>    The example below illustrates how a user who types debian.zkey in their Tor
>    browser gets redirected to sejnfjrq6szgca7v.onion after Tor consults the GNS
>    network.
>    Please revisit this illustration after reading the rest of the proposal.
>        |                                    $~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~$
>        |  1.                                $          4. GNS magic!!              $
>        |  User: SOCKS CONNECT to            $ debian.zkey -> sejnfjrq6szgca7v.onion$
>        |        http://debian.zkey/         $~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~$~~~~~~~$
>        |                                                                   $
>  +-----|-----------------------------------------+                         $
>  |+----v-----+     2.                 +---------+|       3.                $
>  ||Tor       |     debian.zkey        |Tor      ||       debian.zkey     +-$-------+
>  ||Networking------------------------->Naming   ------------------------->         |
>  ||Submodule |                        |Submodule||  Tor Name System API  |  GNS    |
>  ||          <-------------------------         <-------------------------  wrapper|
>  ||          | 6.                     |         ||5.                     |         |
>  |+----|-----+ sejnfjrq6szgca7v.onion +---------+|sejnfjrq6szgca7v.onion +---------+
>  +-----|-----------------------------------------+
>        |  7.
>        |  Tor: Connect to
>        |       http://sejnfjrq6szgca7v.onion/
>        v
> 2.3. System configuration [TORRC]
>    As demonstrated in [SYSTEMOVERVIEW], a Tor user who wants to use a name
>    system has to edit their configuration file appropriately. Here is the torrc
>    line format:
>        OnionNamePlugin <priority> <tld> <path>
>    where <priority> is a positive integer denoting the priority with which this
>    name plugin should be consulted. <tld> is a string which restricts the scope
>    of this plugin to a particular tld.  Finally, <path> is a filesystem path to
>    an executable that speaks the Tor Name System API and can act as an
>    intermediary between Tor and the name system.
>    For example here is a snippet from a torrc file:
>        OnionNamePlugin 0 .hosts      /usr/local/bin/local-hosts-file
>        OnionNamePlugin 1 .zkey       /usr/local/bin/gns-tor-wrapper
>        OnionNamePlugin 2 .bit        /usr/local/bin/namecoin-tor-wrapper
>        OnionNamePlugin 3 .scallion   /usr/local/bin/community-hosts-file
> 2.3.1. Tor name resolution logic
>    When Tor receives a SOCKS request to an address that has a name
>    plugin assigned to it, it needs to perform a query for that address
>    using that name plugin.
>    If there are multiple name plugins that correspond to the requested
>    address, Tor queries all relevant plugins sorted by their priority
>    value, until one of them returns a successful result. If two plugins
>    have the same priority value, Tor MUST abort.
>    If all plugins fail to successfuly perform the name resolution, Tor SHOULD
>    default to using the exit node for name resolution.
>    XXX or not?  because of leaks?
> 2.4. Name system initialization [INITPROTOCOL]
>    When Tor finds OnionNamePlugin lines in torrc, it launches and initializes
>    their respective executables.
>    When launching name plugins, Tor sets various environment variables to pass
>    data to the name plugin (e.g. NS API version, state directory, etc.). More
>    information on the environment variables at [INITENVVARS].
>    After a name plugin initializes and parses all needed environment
>    variables, it communicates with Tor using its stdin/stdout.
>    The first line that a name plugin sends to stdout signifies that it's ready
>    to receive name queries. This line looks like this:
>    where VERSION is the Tor NS API protocol version that the plugin supports,
>    STATUS_CODE is an integer status code, and STATUS_MSG is an optional string
>    error message. STATUS_CODE value 0 is reserved for "success", and all other
>    integers are error codes.
>    See [PROTOEXAMPLE] for an example of this protocol.
> 2.5. Name resolution using NS API
>    Here is how actual name resolution requests are performed in NS API.
> 2.5.1. Message format
>    When Tor receives a SOCKS request to an address with a tld that has a name
>    plugin assigned to it, Tor performs an NS API name query for that address.
>    Tor does this by printing lines on the name plugin stdout as follows:
>    where QUERY_ID is a unique integer corresponding to this query, and
>    NAME_STRING is the name to be queried.
>    When the name plugin completes the name resolution, it prints the following
>    line in its stdout:
>    where QUERY_ID is the corresponding query ID and STATUS_CODE is an integer
>    status code. RESULT is the resolution result (an onion address) or an error
>    message if the resolution was not succesful.
>    See [PROTOEXAMPLE] for an example of this protocol.
>    XXX Should <RESULT> be optional in the case of failure?
> 2.5.2. RESOLVED status codes
>    Name plugins can deliver the following status codes:
>    0 -- The name resolution was successful.
>    1 -- Name resolution generic failure.
>    2 -- Name tld not recognized.
>    3 -- Name not registered.
>    4 -- Name resolution timeout exceeded.
>    XXX add more status codes here as needed
> 2.5.3. Further name resolution behavior
>    Tor and name plugins MAY cache name resolution results in memory as
>    needed. Caching results on disk should be avoided.
>    Tor SHOULD abort (or cancel) an ongoing name resolution request, if it takes
>    more than NAME_RESOLUTION_TIMEOUT seconds.
>    Tor MUST validate that the resolution result is a valid .onion name.
>    XXX should we also accept IPs and regular domain results???
>    XXX perhaps we should make sure that results are not names that need
>        additional name resolution to avoid endless loops. e.g. imagine
>        some sort of loop like this:
>         debian.zkey -> debian-bla.zkey -> debian.zkey -> etc.
> 2.6. Cancelling a name resolution request
>    Tor might need to cancel an ongoing name resolution request
>    (e.g. because a timeout passed, or the client is not interested in
>    that address anymore). In this case, Tor sends the following line to
>    the plugin stdout as follows:
>    to which the name plugin, after performing the cancellation, SHOULD
>    answer with:
> 2.7. Launching name plugins [INITENVVARS]
>    As described in [INITPROTOCOL], when Tor launches a name plugin, it sets
>    certain environment variables. At a minimum, it sets (in addition to the
>    normal environment variables inherited from Tor):
>     "TOR_NS_STATE_LOCATION" -- A filesystem directory path where the
>        plugin should store state if it wants to.  This directory is not
>        required to exist, but the plugin SHOULD be able to create it if
>        it doesn't.  The plugin MUST NOT store state elsewhere.
>       Example: TOR_NS_STATE_LOCATION=/var/lib/tor/ns_state/
>     "TOR_NS_PROTO_VERSION" -- To tell the plugin which versions of this
>        configuration protocol Tor supports. Future versions will give a
>        comma-separated list.  Plugins MUST accept comma-separated lists
>        containing any version that they recognize, and MUST work correctly even
>        if some of the versions they don't recognize are non-numeric.  Valid
>        version characters are non-space, non-comma printing ASCII characters.
>       Example: TOR_NS_PROTO_VERSION=1,1a,2,4B
>     "TOR_NS_PLUGIN_OPTIONS" -- Specifies configuration options for this
>        name plugin as a semicolon-separated list of k=v strings with
>        options that are to be passed to the plugin.
>        Colons, semicolons, equal signs and backslashes MUST be escaped with a
>        backslash.
>        If there are no arguments that need to be passed to any of the
>        plugins, "TOR_NS_PLUGIN_OPTIONS" MAY be omitted.
>        For example consider the following options for the "banana" name plugin:
>          TOR_NS_PLUGIN_OPTIONS=timeout=5;url=https://bananacake.com
>          Will pass to banana the parameters 'timeout=5' and
>          'url=https://bananacake.com'.
>        XXX Do we like this option-passing interface? Do we have any lessons
>            from our PT experiences?
>    XXX Add ControlPort/SocksPort environment variables.
>    See [PROTOEXAMPLE] for an example of this environment
> 2.8. Name plugin workflow [NSBEHAVIOR]
>    Name plugins follow the following workflow:
>      1) Tor sets the required environment values and launches the name plugin
>         as a sub-process (fork()/exec()). See [INITENVVARS].
>      2) The name plugin checks its environment, and determines the supported NS
>         API versions using the env variable TOR_NS_PROTO_VERSION.
>         2.1) If there are no compatible versions, the name plugin writes
>              an INIT message with a failure status code as in
>              [INITPROTOCOL], and then shuts down.
>      3) The name plugin parses and handles the rest of the environment values.
>         3.1) If the environment variables are malformed, or otherwise
>              invalid, the name plugin writes an INIT message with a
>              failure status code as in [INITPROTOCOL], and then shuts
>              down.
>      4) After the name plugin completely initializes, it sends a successful
>         INIT message to stdout as in [INITPROTOCOL]. Then it continues
>         monitoring its stdin for incoming RESOLVE messages.
>      6) When the name plugin receives a RESOLVE message, it performs the name
>         resolution and replies with the appropriate RESOLVED message.
>      7) Upon being signaled to terminate by the parent process [NSSHUTDOWN], the
>         name plugin gracefully shuts down.
> 2.8.1. Name plugin shutdown [NSSHUTDOWN]
>    To ensure clean shutdown of all name plugins when Tor shuts down, the
>    following rules apply for name plugins:
>    Name plugins MUST handle OS specific mechanisms to gracefully terminate
>    (e.g. SIGTERM).
>    Name plugins SHOULD monitor their stdin and exit gracefully when it is
>    closed.
> 2.9. Further details of stdin/stdout communication
> 2.9.1. Message Format
>    Tor communicates with its name plugins by writing NL-terminated lines to
>    stdout.  The line metaformat is
>       <Line> ::= <Keyword> <OptArgs> <NL>
>       <Keyword> ::= <KeywordChar> | <Keyword> <KeywordChar>
>       <KeyWordChar> ::= <any US-ASCII alphanumeric, dash, and underscore>
>       <OptArgs> ::= <Args>*
>       <Args> ::= <SP> <ArgChar> | <Args> <ArgChar>
>       <ArgChar> ::= <any US-ASCII character but NUL or NL>
>       <SP> ::= <US-ASCII whitespace symbol (32)>
>       <NL> ::= <US-ASCII newline (line feed) character (10)>
>    Tor MUST ignore lines with keywords that it doesn't recognize.
> 3. Discussion
> 3.1. Using second-level domains instead of tld
>    People have suggested that users should try to connect to reddit.zkey.onion
>    instead of reddit.zkey. That is, we should always preserve .onion as the
>    tld, and only utilize second-level domains for naming.
>    The argument for this is that this way users cannot accidentally leak
>    addresses to DNS, as the .onion domain is reserved by RFC 7686.
>    The counter-argument here is that this might be confusing to users since
>    they are not used to the second-level domain being special (e.g. co.uk).
>    Also, what happens when someone registers a 16-character name, that matches
>    the length of a vanilla onion address?
>    We should consider the concerns here and take the right decision.
> 3.2. Name plugins handling all tlds '*'
>    In [TORRC], we assigned a single tld to each name plugin.  Should we also
>    accept catch-all tlds using '*'? I'm afraid that this way a name system
>    could try to resolve even normal domains like reddit.com .
>    Perhaps we trust the name plugin itself, but maybe the name system
>    network could exploit this? Also, the catch-all tld will probably
>    cause some engineering complications in this proposal (as it did for PTs).
> 3.3. Deployment strategy
>    We need to devise a deployment strategy that will allow us to improve
>    the UX of our users as soon as possible, but without taking hasty,
>    sloppy or uneducated decisions.
>    For starters, we should make it easy for developers to write wrappers around
>    their secure name systems. We should develop libraries that speak the NS API
>    protocol and can be used to quickly write wrappers. Similar libraries were quite
>    successful during pluggable transport deployment; see pyptlib and goptlib.
>    In the beginning, name plugins should be third-party applications that can
>    be installed by interested users manually or through package managers. Users
>    will also have to add the appropriate OnionNamePlugin line to their torrc.
>    This will be a testing phase, and also a community-growing phase.
>    After some time, and when we get a better idea of how name plugins
>    work for the community, we can start considering how to make them
>    more easily usable.  For example, we can start by including some name
>    plugins into TBB in an optional opt-in fashion. We should be careful
>    here, as people have real incentives for attacking name systems and
>    we should not put our users unwillingly in danger.
> 3.4. Miscellaneous discussion topics
>    1. The PT spec tries hard so that a single executable can expose multiple
>       PTs. In this spec, it's clear that each executable is a single name
>       plugin. Is this OK or a bad idea? Should we change interfaces so that
>       each name plugin has an identifier, and then use that identifier for
>       things?
>    2. Should we make our initialization protocol _identical_ to the PT API
>       initialization protocol? That is, use ENV-ERROR etc. instead of INT?
>    3. Does it make sense to support reverse queries, from .onion to names? So
>       that people auto-learn the names of the onions they use?
> 4. Acknowledgements
>    Proposal details discussed during Tor hackfest in Seattle between
>    Yawning, David and me. Thanks to Lunar and indolering for more
>    discussion and feedback.
> Appendix A.1: Example communication Tor <-> name plugin [PROTOEXAMPLE]
>    Environemnt variables:
>      TOR_NS_STATE_LOCATION=/var/lib/tor/ns_state
>      TOR_NS_PLUGIN_OPTIONS=timeout=5;cache=/home/user/name_cache
>    Messages between Tor and the banana name plugin:
>      Name plugin (banana) -> Tor:
>         INIT 1 0
>      Tor -> Name plugin (banana):
>         RESOLVE 1 daewonskate.banana
>      Name plugin (banana) -> Tor:
>         RESOLVED 1 0 jqkscnkne4qt91iq.onion
>      Tor -> Name plugin (banana):
>         RESOLVE 1 architecturedirect.zkey
>      Name plugin (banana) -> Tor (banana):
>         RESOLVE 1 2 "zkey not recognized tld"
>      Tor -> Name plugin (banana):
>         RESOLVE 1 origamihub.banana
>      Name plugin (banana) -> Tor (banana):
>         RESOLVE 1 2 wdxfpaxar4dg12vd.onion
> Appendix A.2: Example plugins [PLUGINEXAMPLES]
>    Here are a few examples of name plugins for brainstorming:
>    a) Simplest plugin: A local hosts file. Basically a local petname system
>       that maps names to onion addresses.
>    b) A remote hosts file. A centralized community hosts file that people trust.
>    c) Multiple remote hosts files. People can add their own favorite community
>       hosts file.
>    d) Multiple remote hosts files with notaries and reputation
>       trust. Like moxie's convergence tool but for names.
>    e) GNS
>    f) OnioNS
>    g) Namecoin/Blockstart
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