[tor-dev] Update: Proposal 237 - All relays are directory servers

[Matthew Finkel]

Hi All,

Here's an updated version of prop 237. I rewrote some parts
for clarity and brought it inline with the current implementation
for #12538.

Updated branch on prop237_update in my torspec repo.

Thanks!
Matt
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Filename: 237-directory-servers-for-all.txt
Title: All relays are directory servers
Author: Matthew Finkel
Created: 29-Jul-2014
Status: Open
Target: 0.2.7.x

Overview:

      This proposal aims at simplying how users interact directly with
  the Tor network by turning all relays into directory servers (also
  known as directory caches), too.  Currently an operator has the
  options of running a relay, a directory server, or both.  With the
  acceptance (and implementation) of this proposal the options will be
  simplified by having (nearly) all relays cache and serve directory
  documents, without additional configuration.

Motivation:

      Fetching directory documents and descriptors is not always a
  simple operation for a client. This is especially true and potentially
  dangerous when the client would prefer querying its guard but its
  guard is not a directory server. When this is the case, the client
  must choose and query a distinct directory server. At best this should
  not be necessary and at worst, it seems, this adds another position
  within the network for profiling and partitioning users. With the
  orthogonally proposed move to clients using a single guard, the
  resulting benefits could be reduced by clients using distinct
  directory servers. In addition, in the case where the client does not
  use guards, it is important to have the largest possible amount of
  diversity in the set of directory servers. In a network where (almost)
  every relay is a directory server, the profiling and partitioning
  attack vector is reduced to the guard (for clients who use them),
  which is already in a privileged position for this. In addition, with
  the increased set size, relay descriptors and documents are more
  readily available and it diversifies the providers.

Design:

      The changes needed to achieve this should be simple. Currently all
  relays download and cache the majority of relay documents in any case,
  so the slight increased memory usage from downloading all of them should
  have minimal consequences. There will be necessary logical changes in
  the client, router, and directory code.

      Currently directory servers are defined as such if they advertise
  having an open directory port. We can no longer assume this is true. To
  this end, we will introduce a new server descriptor line.

  	"tunnelled-dir-server" NL

      The presence of this line indicates that the relay accepts
  tunnelled directory requests. For a relay that implements this
  proposal, this line MUST be added to its descriptor if it does not
  advertise a directory port, and the line MAY be added if it also
  advertises an open directory port. In addition to this, relays will
  now download and cache all descriptors and documents listed in the
  consensus, regardless of whether they are deemed useful or usable,
  exactly like the current directory server behavior. All relays will
  also accept directory requests when they are tunnelled over a
  connection established with a BEGIN_DIR cell, the same way these
  connections are already accepted by bridges and directory servers with
  an open DirPort.

      Directory Authorities will now assign the V2Dir flag to a server if
  it supports a version of the directory protocol which is useful to
  clients and it has at least an open directory port or it has an open
  and reachable OR port and advertises "tunnelled-dir-server" in its
  server descriptor.

      Clients choose a directory by using the current criteria with the
  additional criterion that a server only needs the V2Dir status flag
  instead of requiring an open DirPort.

Security Considerations and Implications:

      Currently all directory servers are explicitly configured. This is
  necessary because they must have a configured and reachable external
  port. However, within Tor, this requires additional configuration and
  results in a reduced number of directory servers in the network. As a
  consequence, this could allow an adversary to control a non-negligable
  fraction of the servers. By increasing the number of directory servers
  in the network the likelihood of selecting one that is malicious is
  reduced. Also, with this proposal, it will be more likely that a
  client's entry guard is also a directory server (as alluded to in
  Proposal 207). However, the reduced anonymity set created when the
  guard does not have, or is unwilling to distribute, a specific
  document still exists. With the increased diversity in the available
  servers, the impact of this should be reduced.

      Another question that may need further consideration is whether we
  trust bad directories to be good guards and exits.

Specification:

  	The version 3 directory protocol specification does not
  currently document the use of directory guards. This spec should be
  updated to mention the preferred use of directory guards during
  directory requests. In addition, the new criteria for assigning the
  V2Dir flag should be documented.

Impact on local resources:

      Should relays attempt to download documents from another mirror
  before asking an authority? All relays, with minor exceptions, will
  now contact the authorities for documents, but this will not scale
  well and will partition users from relays.

      If all relays become directory servers, they will choose to
  download all documents, regardless of whether they are useful, in case
  another client does want them. This will have very little impact on
  the most relays, however on memory constrained relays (BeagleBone,
  Raspberry Pi, and similar), every megabyte allocated to directory
  documents is not available for new circuits. For this reason, a new
  configuration option will be introduced within Tor for these systems,
  named DirCache, which the operator may choose to set as 0, thus
  disabling caching of directory documents and denying client directory
  requests.

Future Considerations:

      Should the DirPort be deprecated at some point in the future?

      Write a proposal requiring that a relay must have the V2Dir flag
  as a criterion for being a guard.

      Is V2Dir a good name for this? It's the name we currently use, but
  that's a silly reason to continue using it.
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