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Revise proposal 171 from start to finish

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The big semantic change is to make the IsolateFoo options exist on a
per-client-port basis.
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Nick Mathewson 2010-11-29 14:29:47 -05:00
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doc/spec/proposals/171-separate-streams.txt
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Filename: 171-separate-streams-by-port-or-host.txt
Title: Separate streams across circuits by destination port or destination host
Author: Robert Hogan, Jacob Appelbaum, Damon McCoy
Filename: 171-separate-streams.txt
Title: Separate streams across circuits by connection metadata
Author: Robert Hogan, Jacob Appelbaum, Damon McCoy, Nick Mathewson
Created: 21-Oct-2008
Modified: 30-Aug-2010
Status: Draft
Modified: 7-Dec-2010
Status: Open
Summary:
We propose a new set of options to isolate unrelated streams from one
another, putting them on separate circuits so that semantically
unrelated traffic is not inadvertently made linkable.
Motivation:
Streams are currently attached to circuits without regard to their content,
destination host, or destination port. We propose three options,
IsolateBySOCKSUser, IsolateStreamsByPort and IsolateStreamsByHost to change the
default behavior.
Currently, Tor attaches regular streams (that is, ones not carrying
rendezvous or directory traffic) to circuits based only on whether Tor
circuit's current exit node supports the destination, and whether the
circuit has been dirty (that is, in use) for too long.
The contents of some streams will always have revealing plain text information;
these streams should be treated differently than other streams that may or may
not have unencrypted PII content. DNS, with the exception of DNSCurve, is
always unencrypted. It is reasonable to assume that other protocols may exist
that have a similar issue and may cause user concern. It is also the case that
we must balance network load issues and stream privacy. The Tor network will not
currently scale to one circuit per application connection nor should it anytime
soon.
This means that traffic that would otherwise be unrelated sometimes
gets sent over the same circuit, allowing the exit node to link such
streams with certainty, and allowing other parties to link such
streams probabilistically.
Circuits are currently created with a few constraints and are rotated within
a reasonable time window. This allows a rogue exit node to correlate all
streams on a given circuit.
Older versions of onion routing tried to address this problem by
sending every stream over a separate circuit; performance issues made
this unfeasible. Moreover, in the presence of a localized adversary,
separating streams by circuits increases the odds that, for any given
linked set of streams, at least one will go over a compromised
circuit.
Therefore we ought to look for ways to allow streams that ought to be
linked to travel over a single circuit, while keeping streams that
ought not be linked isolated to separate circuits.
Discussion:
Let's call a series of inherently-linked streams (like a set of
streams downloading objects from the same webpage, or a browsing
session where the user requests several related webpages) a "Session".
"Sessions" are a necessarily a fuzzy concept. While users typically
consider some activities as wholly unrelated to each other ("My IM
session has nothing to do with my web browsing!"), the boundaries
between activities are sometimes hard to determine. If I'm reading
lolcats in one browser tab and reading about treatments for an
embarrassing disease in another, those are probably separate sessions.
If I search for a forum, log in, read it for a while, and post a few
messages on unrelated topics, that's probably all the same session.
So with the proviso that no automated process can identify sessions
100% accurately, let's see which options we have available.
Generally, all the streams on a session come from a single
application. Unfortunately, isolating streams by application
automatically isn't feasible, given the lack of any nice
cross-platform way to tell which local process originated a given
connection. (Yes, lsof works. But a quick review of the lsof code
should be sufficient to scare you away from thinking there is a
portable option, much less a portable O(1) option.) So instead, we'll
have to use some other aspect of a Tor request as a proxy for the
application.
Generally, traffic from separate applications is not in the same
session.
With some applications (IRC, for example), each stream is a session.
Some applications (most notably web browsing) can't be meaningfully
split into sessions without inspecting the traffic itself and
maintaining a lot of state.
How well do ports correspond to sessions? Early versions of this
proposal focused on using destination ports as a proxy for
application, since a connection to port 22 for SSH is probably not in
the same session as one to port 80. This only works with some
applications better than others, though: while SSH users typically
know when they're on port 22 and when they aren't, a web browser can
be coaxed (though img urls or any number of releated tricks) into
connecting to any port at all. Moreover, when Tor gets a DNS lookup
request, it doesn't know in advance which port the resulting address
will be used to connect to.
So in summary, each kind of traffic wants to follow different rules,
and assuming the existence of a web browser and a hostile web page or
exit node, we can't tell one kind of traffic from another by simply
looking at the destination:port of the traffic.
Fortunately, we're not doomed.
Design:
We propose two options for isolation of streams that lessen the observability
and linkability of the Tor client's traffic.
When a stream arrives at Tor, we have the following data to examine:
1) The destination address
2) The destination port (unless this a DNS lookup)
3) The protocol used by the application to send the stream to Tor:
SOCKS4, SOCKS4A, SOCKS5, or whatever local "transparent proxy"
mechanism the kernel gives us.
4) The port used by the application to send the stream to Tor --
that is, the SOCKSListenAddress or TransListenAddress that the
application used, if we have more than one.
5) The SOCKS username and password, if any.
6) The source address and port for the application.
IsolateStreamsByPort will take a list of ports or optionally the keyword 'All'
in place of a port list. The use of the keyword 'All' will ensure that all
application connections attached to streams will be isolated to separate
circuits by port number.
We propose to use 3, 4, and 5 as a backchannel for applications to
tell Tor about different sessions. Rather than running only one
SOCKSPort, a Tor user who would prefer better session isolation should
run multiple SOCKSPorts/TransPorts, and configure different
applications to use separate ports. Applications that support SOCKS
authentication can further be separated on a single port by their
choice of username/password. Streams sent to separate ports or using
different authentication information should never be sent over the
same circuit. We allow each port to have its own settings for
isolation based on destination port, destination address, or both.
IsolateStreamsByHost will take a boolean value. When enabled, all application
connections, regardless of port number will be isolated with separate circuits
per host. If this option is enabled, we should ensure that the client has a
reasonable number of pre-built circuits to ensure perceived performance. This
should also intentionally limit the total number of circuits a client will
build to ten circuits to prevent abuse and load on the network. This is a
trade-off of performance for anonymity. Tor will issue a warning if a client
encounters this limit.
Handling DNS can be a challenge. We can get hostnames by one of three
means:
IsolateBySOCKSUser will take a boolean value. When enabled, all application
connections, regardless of port number will be isolated with separate circuits
per SOCKS username. This options ensures that any two streams that were created
with different SOCKS usernames will be sent over different circuits. The empty
username will be treated as its own username different from all other usernames.
A) A SOCKS4a request, or a SOCKS5 request with a hostname. This
case is handled trivially using the rules above.
B) A RESOLVE request on a SOCKSPort. This case is handled using the
rules above, except that port isolation can't work to isolate
RESOLVE requests into a proper session, since we don't know which
port will eventually be used when we connect to the returned
address.
C) A request on a DNSPort. We have no way of knowing which
address/port will be used to connect to the requested address.
Security implications:
When B or C is required but problematic, we could favor the use of
AutomapHostsOnResolve.
It is believed that the proposed changes will improve the anonymity for end
user stream privacy. The end user will no longer link all streams at a single
exit node during a given time window.
Interface:
There is a possible attack where a hostile web page possibly in collusion with
an exit node contains image links for images at (say) "evil.example.com:53" and
"evil.example.com:31337", and thereby (if they're lucky) correlate port-80
circuits with port-53 and port-31337 circuits.
We propose that {SOCKS,Natd,Trans,DNS}ListenAddr be deprecated in
favor of an expanded {SOCKS,Natd,Trans,DNS}Port syntax:
ClientPortLine = OptionName SP (Addr ":")? Port (SP Options?)
OptionName = "SOCKSPort" / "NatdPort" / "TransPort" / "DNSPort"
Addr = An IPv4 address / an IPv6 address surrounded by brackets.
If optional, we default to 127.0.0.1
Port = An integer from 1 through 65535 inclusive
Options = Option
Options = Options SP Option
Option = IsolateOption / GroupOption
GroupOption = "SessionGroup=" UINT
IsolateOption = OptNo ("IsolateDestPort" / "IsolateDestAddr" /
"IsolateSOCKSUser"/ "IsolateClientProtocol" /
"IsolateClientAddr") OptPlural
OptNo = "No" ?
OptPlural = "s" ?
SP = " "
UINT = An unsigned integer
All options are case-insensitive.
The "IsolateSOCKSUser" and "IsolateClientAddr" options are on by
default; "NoIsolateSOCKSUser" and "NoIsolateClientAddr" respectively
turn them off. The IsolateDestPort and IsolateDestAddr and
IsolateClientProtocol options are off by default. NoIsolateDestPort and
NoIsolateDestAddr and NoIsolateClientProtocol have no effect.
Given a set of ClientPortLines, streams must NOT be placed on the same
circuit if ANY of the following hold:
* They were sent to two different client ports, unless the two
client ports both specify a "SessionGroup" option with the same
integer value.
* At least one was sent to a client port with the IsolateDestPort
active, and they have different destination ports.
* At least one was sent to a client port with IsolateDestAddr
active, and they have different destination addresses.
* At least one was sent to a client port with IsolateClientProtocol
active, and they use different protocols (where SOCKS4, SOCKS4a,
SOCKS5, TransPort, NatdPort, and DNS are the protocols in question)
* At least one was sent to a client port with IsolateSOCKSUser
active, and they have different SOCKS username/password values
configurations. (For the purposes of this option, the
username/password pair of ""/"" is distinct from SOCKS without
authentication, and both are distinct from any non-SOCKS client's
non-authentication.)
* At least one was sent to a client port with IsolateClientAddr
active, and they came from different client addresses. (For the
purpose of this option, any local interface counts as the same
address. So if the host is configured with addresses 10.0.0.1,
192.0.32.10, and 127.0.0.1, then traffic from those addresses can
leave on the same circuit, but traffic to from 10.0.0.2 (for
example) could not share a circuit with any of them.)
These rules apply regardless of whether the streams are active at the
same time. In other words, if the rules say that streams A and B must
not be on the same circuit, and stream A is attached to circuit X,
then stream B must never be attached to stream X, even if stream A is
closed first.
Alternative Interface:
We're cramming a lot onto one line in the design above. Perhaps
instead it would be a better idea to have grouped lines of the form:
StreamGroup 1
SOCKSPort 9050
TransPort 9051
IsolateDestPort 1
IsolateClientProtocol 0
EndStreamGroup
StreamGroup 2
SOCKSPort 9052
DNSPort 9053
IsolateDestAddr 1
EndStreamGroup
This would be equivalent to:
SOCKSPort 9050 SessionGroup=1 IsolateDestPort NoIsolateClientProtocol
TransPort 9051 SessionGroup=1 IsolateDestPort NoIsolateClientProtocol
SOCKSPort 9052 SessionGroup=2 IsolateDestAddr
DNSPort 9053 SessionGroup=2 IsolateDestAddr
But it would let us extend range of allowed options later without
having client port lines group without bound. For example, we might
give different circuit building parameters to different session
groups.
Example of use:
Suppose that we want to use a web browser, an IRC client, and a SSH
client all at the same time. Let's assume that we want web traffic to
be isolated from all other traffic, even if the browser makes
connections to ports usually used for IRC or SSH. Let's also assume
that IRC and SSH are both used for relatively long-lived connections,
and we want to keep all IRC/SSH sessions separate from one another.
In this case, we could say:
SOCKSPort 9050
SOCKSPort 9051 IsolateDestAddr IsolateDestPort
We would then configure our browser to use 9050 and our IRC/SSH
clients to use 9051.
Advanced example of use, #2:
Suppose that we have a bunch of applications, and we launch them all
using torsocks, and we want to keep each applications isolated from
one another. We just create a shell script, "torlaunch":
#!/bin/bash
export TORSOCKS_USERNAME="$1"
exec torsocks $@
And we configure our SOCKSPort with IsolateSOCKSUser.
Or if we're on Linux and we want to isolate by application invocation,
we would change the TORSOCKS_USERNAME line to:
export TORSOCKS_USERNAME="`cat /proc/sys/kernel/random/uuid`"
Advanced example of use, #2:
Now suppose that we want to achieve the benefits of the first example
of use, but we are stuck using transparent proxies. Let's suppose
this is Linux.
TransPort 9090
TransPort 9091 IsolateDestAddr IsolateDestPort
DNSPort 5353
AutomapHostsOnResolve 1
Here we use the iptables --cmd-owner filter to distinguish which
command is originating the packets, directing traffic from our irc
client and our SSH client to port 9091, and directing other traffic to
9090. Using AutomapHostsOnResolve will confuse ssh in its default
configuration; we'll need to find a way around that.
Security Risks:
Disabling IsolateClientAddr is a pretty bad idea.
Setting up a set of applications to use this system effectively is a
big problem. It's likely that lots of people who try to do this will
mess it up. We should try to see which setups are sensible, and see
if we can provide good feedback to explain which streams are isolated
how.
Performance Risks:
This proposal will result in clients building many more circuits than
they do today. To avoid accidentally hammering the network, we should
have in-process limits on the maximum circuit creation rate and the
total maximum client circuits.
Specification:
The Tor client circuit selection process is not entirely specified. Any client
circuit specification must take these changes into account.
Compatibility:
The proposed changes should not create any compatibility issues. New Tor clients
will be able to take advantage of this without any modification to the network.
Implementation:
It is further proposed that IsolateStreamsByPort will be enabled by default
for port 22, 53, and port 80.
It is further proposed that IsolateStreamsByHost will be disabled by default.
The Tor client circuit selection process is not entirely specified.
Any client circuit specification must take these changes into account.
Implementation notes:
The implementation of this option may want to consider cases where the same
exit node is shared by two or more circuits and IsolateStreamsByPort is in
force. Since the purpose of the option is to reduce the opportunity of Exit
Nodes to attack traffic from the same source on multiple ports, the
implementation may need to ensure that circuits reserved for the exclusive use
of given ports do not share the same exit node.
The more obvious ways to implement the "find a good circuit to attach
to" part of this proposal involve doing an O(n_circuits) operation
every time we have a stream to attach. We already do such an
operation, so it's not as if we need to hunt for fancy ways to make it
O(1). What will be harder is implementing the "launch circuits as
needed" part of the proposal. Still, it should come down to "a simple
matter of programming."
Circuits should not be shared by unique clients. Tor should check to ensure
that peer IP addresses are identical when they connect to the SOCKS listener or
the TransPort listener before sharing a circuit. If the addresses are not
identical, Tor should ensure that the circuits are not shared.
The SOCKS4 spec has the client provide authentication info when it
connects; accepting such info is no problem. But the SOCKS5 spec has
the client send a list of known auth methods, then has the server send
back the authentication method it chooses. We'll need to update the
SOCKS5 implementation so it can accept user/password authentication if
it's offered.
Performance and scalability notes:
If we use the second syntax for describing these options, we'll want
to add a new "section-based" entry type for the configuration parser.
Not a huge deal; we already have kludged up something similar for
hidden service configurations.
It is further proposed that IsolateStreamsByPort will be enabled by default for
all ports after a reasonable assessment is performed. Specifically, we should
determine the impact this option has on Tor clients and the Tor network.
Opening circuits for predicted ports has the potential to get a little
more complicated; we can probably get away with the existing
algorithm, though, to see where its weak points are and look for
better ones.
Perhaps we can get our next-gen HTTP proxy to communicate browser tab
or session into to tor via authentication, or have torbutton do it
directly. More design is needed here, though.
Alternative designs:
The implementation of this option may want to consider cases where the
same exit node is shared by two or more circuits and
IsolateStreamsByPort is in force. Since one possible use of the option
is to reduce the opportunity of Exit Nodes to attack traffic from the
same source on multiple ports, the implementation may need to ensure
that circuits reserved for the exclusive use of given ports do not
share the same exit node. On the other hand, if our goal is only that
streams should be unlinkable, deliberately shunting them to different
exit nodes is unnecessary and slightly counterproductive.
Earlier versions of this design included a mechanism to isolate
_particular_ destination ports and addresses, so that traffic sent to,
say, port 22 would never share a port with any traffic *not* sent to
port 22. You can achieve this here by having all applications that
send traffic to one of these ports use a separate SOCKSPort, and
then setting IsolateDestPorts on that SOCKSPort.
Lingering questions:
I suspect there are issues remaining with DNS and TransPort users, and
that my "just use AutomapHostsOnResolve" suggestion may be
insufficient.