[tor-dev] Proposal 205: Remove global client-side DNS caching
nickm at freehaven.net
Fri Jul 20 22:27:22 UTC 2012
Title: Remove global client-side DNS caching
Author: Nick Mathewson
Created: 20 July 2012
This proposal suggests that, for reasons of security, we move
client-side DNS caching from a global cache to a set of per-circuit
This will break some things that used to work. I'll explain how to
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 18.104.22.168 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 22.214.171.124, 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.
2.1. 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. 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
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