[tor-dev] Proposal 205: Remove global client-side DNS caching

[Nick Mathewson]

Filename: 205-local-dnscache.txt
Title: Remove global client-side DNS caching
Author: Nick Mathewson
Created: 20 July 2012
Status: Open


0. Overview

   This proposal suggests that, for reasons of security, we move
   client-side DNS caching from a global cache to a set of per-circuit
   caches.

   This will break some things that used to work.  I'll explain how to
   fix them.

1. Background and Motivation

   Since the earliest Tor releases, we've kept a client-side DNS
   cache.  This lets us implement exit policies and exit enclaves --
   if we remember that www.mit.edu is 18.9.22.169 the first time we
   see it, then we can avoid making future requests for www.mit.edu
   via any node that blocks net 18.  Also, if there happened to be a
   Tor node at 18.9.22.169, we could use that node as an exit enclave.

   But there are security issues with DNS caches.  A malicious exit
   node or DNS server can lie.  And unlike other traffic, where the
   effect of a lie is confined to the request in question, a malicious
   exit node can affect the behavior of future circuits when it gives
   a false DNS reply.  This false reply could be used to keep a client
   connecting to an MITM'd target, or to make a client use a chosen
   node as an exit enclave for that node, or so on.

   With IPv6, tracking attacks will become even more possible: A
   hostile exit node can give every client a different IPv6 address
   for every hostname they want to resolve, such that every one of
   those addresses is under the attacker's control.

   And even if the exit node is honest, having a cached DNS result can
   cause Tor clients to build their future circuits distinguishably:
   the exit on any subsequent circuit can tell whether the client knew
   the IP for the address yet or not.  Further, if the site's DNS
   provides different answers to clients from different parts of the
   world, then the client's cached choice of IP will reveal where it
   first learned about the website.

   So client-side DNS caching needs to go away.

2. Design

2.1. The basic idea

   I propose that clients should cache DNS results in per-circuit DNS
   caches, not in the global address map.

2.2. What about exit policies?

   Microdescriptor-based clients have already dropped the ability to
   track which nodes declare which exit policies, without much ill
   effect.  As we go forward, I think that remembering the IP address
   of each request so that we can match it to exit policies will be
   even less effective, especially if proposals to allow AS-based exit
   policies can succeed.

2.3. What about exit enclaves?

   Exit enclaves are already borken.  They need to move towards a
   cross-certification solution where a node advertises that it can
   exit to a hostname or domain X.Y.Z, and a signed record at X.Y.Z
   advertises that the node is an enclave exit for X.Y.Z.  That's
   out-of-scope for this proposal, except to note that nothing
   proposed here keeps that design from working.

2.4. What about address mapping?

   Our current address map algorithm is, more or less:

     N = 0
     while  N < MAX_MAPPING && exists map[address]:
         address = map[address]
         N = N + 1
     if N == MAX_MAPPING:
         Give up, it's a loop.

   Where 'map' is the union of all mapping entries derived from the
   controller, the configuration file, trackhostexits maps,
   virtual-address maps, DNS replies, and so on.

   With this design, the DNS cache will not be part of the address
   map.  That means that entries in the address map which relied on
   happening after the DNS cache entries can no longer work so well.
   These would include:

       A) Mappings from an IP address to a particular exit, either
          manually declared or inserted by TrackHostExits.
       B) Mappings from IP addresses to other IP addresses.
       C) Mappings from IP addresses to hostnames.

   We can try to solve these by introducing an extra step of address
   mapping after the DNS cache is applied.  In other words, we should
   apply the address map, then see if we can attach to a circuit.  If
   we can, we try to apply that circuit's dns cache, then apply the
   address map again.


2.5. What about the performance impact?

   That all depends on application behavior.

   If the application continues to make all of its requests with the
   hostname, there shouldn't be much trouble.  Exit-side DNS caches and
   exit-side DNS will avoid any additional round trips across the Tor
   network; compared to that, the time to do a DNS resolution at the
   exit node *should* be small.

   That said, this will hurt performance a little in the case where
   the exit node and its resolver don't have the answer cached, and it
   takes a long time to resolve the hostname.


   If the application is doing "resolve, then connect to an IP", see
   2.6 below.

2.6. What about DNSPort?

   If the application is doing its own DNS caching, they won't get
   much security benefit from here.

   If the application is doing a resolve before each connect, there
   will be a performance hit when the resolver is using a circuit that
   hadn't previously resolved the address.

   Also, DNSPort users: AutomapHostsOnResolve is your friend.

3. Alternate designs and future directions

3.1. Why keep client-side DNS caching at all?

   A fine question!  I am not sure it actually buys us anything any
   longer, since exits also have DNS caching.  Shall we discuss that?
   It would sure simplify matters.

3.2. The impact of DNSSec

   Once we get DNSSec support, clients will be able to verify whether
   an exit's answers are correctly signed or not.  When that happens,
   we could get most of the benefits of global DNS caching back,
   without most of the security issues, if we restrict it to
   DNSSec-signed answers.