Proposal: Getting GeoIP data and publishing usage summaries
arma at mit.edu
Mon Dec 3 11:39:18 UTC 2007
Title: Getting GeoIP data and publishing usage summaries
Version: $Revision: 12644 $
Last-Modified: $Date: 2007-12-03 06:18:44 -0500 (Mon, 03 Dec 2007) $
Author: Roger Dingledine
1. Background and motivation
Right now we can keep a rough count of Tor users, both total and by
country, by watching connections to a single directory mirror. Being
able to get usage estimates is useful both for our funders (to
demonstrate progress) and for our own development (so we know how
quickly we're scaling and can design accordingly, and so we know which
countries and communities to focus on more). This need for information
is the only reason we haven't deployed "directory guards" (think of
them like entry guards but for directory information; in practice,
it would seem that Tor clients should simply use their entry guards
as their directory guards; see also proposal 125).
With the move toward bridges, we will no longer be able to track Tor
clients that use bridges, since they use their bridges as directory
guards. Further, we need to be able to learn which bridges stop seeing
use from certain countries (and are thus likely blocked), so we can
avoid giving them out to other users in those countries.
Right now we already do GeoIP lookups in Vidalia: Vidalia draws relays
and circuits on its 'network map', and it performs anonymized GeoIP
lookups to its central servers to know where to put the dots. Vidalia
caches answers it gets -- to reduce delay, to reduce overhead on
the network, and to reduce anonymity issues where users reveal their
knowledge about the network through which IP addresses they ask about.
But with the advent of bridges, Tor clients are asking about IP
addresses that aren't in the main directory. In particular, bridge
users inform the central Vidalia servers about each bridge as they
discover it and their Vidalia tries to map it.
Also, we wouldn't mind letting Vidalia do a GeoIP lookup on the client's
own IP address, so it can provide a more useful map.
Finally, Vidalia's central servers leave users open to partitioning
attacks, even if they can't target specific users. Further, as we
start using GeoIP results for more operational or security-relevant
goals, such as avoiding or including particular countries in circuits,
it becomes more important that users can't be singled out in terms of
their IP-to-country mapping beliefs.
2. The available GeoIP databases
There are at least two classes of GeoIP database out there: "IP to
country", which tells us the country code for the IP address but
no more details, and "IP to city", which tells us the country code,
the name of the city, and some basic latitude/longitude guesses.
A recent ip-to-country.csv is 3421362 bytes. Compressed, it is 564252
bytes. A typical line is:
Similarly, the maxmind GeoLite Country database is also about 500KB
The maxmind GeoLite City database gives more finegrained detail like
geo coordinates and city name. Vidalia currently makes use of this
information. On the other hand it's 16MB compressed. A typical line is:
There are other databases out there, like
that want more attention, but for now let's assume that all the db's
are around this size.
3. What we'd like to solve
Goal #1a: Tor relays collect IP-to-country user stats and publish
Goal #1b: Tor bridges collect IP-to-country user stats and publish
Goal #2a: Vidalia learns IP-to-city stats for Tor relays, for better
Goal #2b: Vidalia learns IP-to-country stats for Tor relays, so the user
can pick countries for her paths.
Goal #3: Vidalia doesn't do external lookups on bridge relay addresses.
Goal #4: Vidalia resolves the Tor client's IP-to-country or IP-to-city
for better mapping.
Goal #5: Reduce partitioning opportunities where Vidalia central
servers can give different (distinguishing) responses.
4. Solution overview
Our goal is to allow Tor relays, bridges, and clients to learn enough
GeoIP information so they can do local private queries.
4.1. The IP-to-country db
Directory authorities should publish a "geoip" file that contains
IP-to-country mappings. Directory caches will mirror it, and Tor clients
and relays (including bridge relays) will fetch it. Thus we can solve
goals 1a and 1b (publish sanitized usage info). Controllers could also
use this to solve goal 2b (choosing path by country attributes). It
also solves goal 4 (learning the Tor client's country), though for
huge countries like the US we'd still need to decide where the "middle"
should be when we're mapping that address.
The IP-to-country details are described further in Sections 5 and
4.2. The IP-to-city db
In an ideal world, the IP-to-city db would be small enough that we
could distribute it in the above manner too. But for now, it is too
large. Here's where the design choice forks.
Option A: Vidalia should continue doing its anonymized IP-to-city
queries. Thus we can achieve goals 2a and 2b. We would solve goal
3 by only doing lookups on descriptors that are purpose "general"
(see Section 4.2.1 for how). We would leave goal 5 unsolved.
Option B: Each directory authority should keep an IP-to-city db,
lookup the value for each router it lists, and include that line in
the router's network-status entry. The network-status consensus would
then use the line that appears in the majority of votes. This approach
also solves goals 2a and 2b, goal 3 (Vidalia doesn't do any lookups
at all now), and goal 5 (reduced partitioning risks).
Option B has the advantage that Vidalia can simplify its operation,
and the advantage that this consensus IP-to-city data is available to
other controllers besides just Vidalia. But it has the disadvantage
that the networkstatus consensus becomes larger, even though most of
the GeoIP information won't change from one consensus to the next. Is
there another reasonable location for it that can provide similar
consensus security properties?
4.2.1. Controllers can query for router annotations
Vidalia needs to stop doing queries on bridge relay IP addresses.
It could do that by only doing lookups on descriptors that are in
the networkstatus consensus, but that precludes designs like Blossom
that might want to map its relay locations. The best answer is that it
should learn the router annotations, with a new controller 'getinfo'
"GETINFO router-annotations/id/<OR identity>" or
"GETINFO router-annotations/name/<OR nickname>"
which would respond with something like
@downloaded-at 2007-11-29 08:06:38
[We could also make the answer include the digest for the router in
question, which would enable us to ask GETINFO router-annotations/all.
Is this worth it? -RD]
Then Vidalia can avoid doing lookups on descriptors with purpose
"bridge". Even better would be to add a new annotation "@private true"
so Vidalia can know how to handle new purposes that we haven't created
yet. Vidalia could special-case "bridge" for now, for compatibility
with the current 0.2.0.x-alphas.
My overall recommendation is that we should implement 4.1 soon
(e.g. early in 0.2.1.x), and we can go with 4.2 option A for now,
with the hope that later we discover a better way to distribute the
IP-to-city info and can switch to 4.2 option B.
Below we discuss more how to go about achieving 4.1.
5. Publishing and caching the GeoIP (IP-to-country) database
Each v3 directory authority should put a copy of the "geoip" file in
its datadirectory. Then its network-status votes should include a hash
of this file (Recommended-geoip-hash: %s), and the resulting consensus
directory should specify the consensus hash.
There should be a new URL for fetching this geoip db (by "current.z"
for testing purposes, and by hash.z for typical downloads). Authorities
should fetch and serve the one listed in the consensus, even when they
vote for their own. This would argue for storing the cached version
in a better filename than "geoip".
Directory mirrors should keep a copy of this file available via the
We assume that the file would change at most a few times a month. Should
Tor ship with a bootstrap geoip file? An out-of-date geoip file may
open you up to partitioning attacks, but for the most part it won't
be that different.
There should be a config option to disable updating the geoip file,
in case users want to use their own file (e.g. they have a proprietary
GeoIP file they prefer to use). In that case we leave it up to the
user to update his geoip file out-of-band.
[XXX Should consider forward/backward compatibility, e.g. if we want
to move to a new geoip file format. -RD]
6. Controllers use the IP-to-country db for mapping and for path building
Down the road, Vidalia could use the IP-to-country mappings for placing
on its map:
- The location of the client
- The location of the bridges, or other relays not in the
networkstatus, on the map.
- Any relays that it doesn't yet have an IP-to-city answer for.
Other controllers can also use it to set EntryNodes, ExitNodes, etc
in a per-country way.
To support these features, we need to export the IP-to-country data
via the Tor controller protocol.
Is it sufficient just to add a new GETINFO command?
6.1. Other interfaces
Robert Hogan has also suggested a
as well as torrc options for ExitCountryCodes, EntryCountryCodes,
7. Relays and bridges use the IP-to-country db for usage summaries
Once bridges have a GeoIP database locally, they can start to publish
sanitized summaries of client usage -- how many users they see and from
what countries. This might also be a more useful way for ordinary Tor
relays to convey the level of usage they see, which would allow us to
switch to using directory guards for all users by default.
But how to safely summarize this information without opening too many
7.1 Attacks to think about
First, note that we need to have a large enough time window that we're
not aiding correlation attacks much. I hope 24 hours is enough. So
that means no publishing stats until you've been up at least 24 hours.
And you can't publish follow-up stats more often than every 24 hours,
or people could look at the differential.
Second, note that we need to be sufficiently vague about the IP
addresses we're reporting. We are hoping that just specifying the
country will be vague enough. But a) what about active attacks where
we convince a bridge to use a GeoIP db that labels each suspect IP
address as a unique country? We have to assume that the consensus GeoIP
db won't be malicious in this way. And b) could such singling-out
attacks occur naturally, for example because of countries that have
a very small IP space? We should investigate that.
7.2. Granularity of users
Do we only want to report countries that have a sufficient anonymity set
(that is, number of users) for the day? For example, we might avoid
listing any countries that have seen less than five addresses over
the 24 hour period. This approach would be helpful in reducing the
singling-out opportunities -- in the extreme case, we could imagine a
situation where one blogger from the Sudan used Tor on a given day, and
we can discover which entry guard she used.
But I fear that especially for bridges, seeing only one hit from a
given country in a given day may be quite common.
As a compromise, we should start out with an "Other" category in
the reported stats, which is the sum of unlisted countries; if that
category is consistently interesting, we can think harder about how
to get the right data from it safely.
But note that bridge summaries will not be made public individually,
since doing so would help people enumerate bridges. Whereas summaries
from normal relays will be public. So perhaps that means we can afford
to be more specific in bridge summaries? In particular, I'm thinking the
"other" category should be used by public relays but not for bridges
(or if it is, used with a lower threshold).
Even for countries that have many Tor users, we might not want to be
too specific about how many users we've seen. For example, we might
round down the number of users we report to the nearest multiple of 5.
My instinct for now is that this won't be that useful.
7.3 Other issues
Another note: we'll likely be overreporting in the case of users with
dynamic IP addresses: if they rotate to a new address over the course
of the day, we'll count them twice. So be it.
7.4. Where to publish the summaries?
We designed extrainfo documents for information like this. So they
should just be more entries in the extrainfo doc.
But if we want to publish summaries every 24 hours (no more often,
no less often), aren't we tried to the router descriptor publishing
schedule? That is, if we publish a new router descriptor at the 18
hour mark, and nothing much has changed at the 24 hour mark, won't
the new descriptor get dropped as being "cosmetically similar", and
then nobody will know to ask about the new extrainfo document?
One solution would be to make and remember the 24 hour summary at the
24 hour mark, but not actually publish it anywhere until we happen to
publish a new descriptor for other reasons. If we happen to go down
before publishing a new descriptor, then so be it, at least we tried.
7.5. What if the relay is unreachable or goes to sleep?
Even if you've been up for 24 hours, if you were hibernating for 18
of them, then we're not getting as much fuzziness as we'd like. So
I guess that means that we need a 24-hour period of being "awake"
before we'll willing to publish a summary. A similar attack works if
you've been awake but unreachable for the first 18 of the 24 hours. As
another example, a bridge that's on a laptop might be suspended for
some of each day.
This implies that some relays and bridges will never publish summary
stats, because they're not ever reliably working for 24 hours in
a row. If a significant percentage of our reporters end up being in
this boat, we should investigate whether we can accumulate 24 hours of
"usefulness", even if there are holes in the middle, and publish based
What other issues are like this? It seems that just moving to a new
IP address shouldn't be a reason to cancel stats publishing, assuming
we were usable at each address.
7.6. IP addresses that aren't in the geoip db
Some IP addresses aren't in the public geoip databases. In particular,
I've found that a lot of African countries are missing, but there
are also some common ones in the US that are missing, like parts of
Comcast. We could just lump unknown IP addresses into the "other"
category, but it might be useful to gather a general sense of how many
lookups are failing entirely, by adding a separate "Unknown" category.
We could also contribute back to the geoip db, by letting bridges set
a config option to report the actual IP addresses that failed their
lookup. Then the bridge authority operators can manually make sure
the correct answer will be in later geoip files. This config option
should be disabled by default.
7.7 Bringing it all together
So here's the plan:
24 hours after starting up (modulo Section 7.5 above), bridges and
relays should construct a daily summary of client countries they've
seen, including the above "Unknown" category (Section 7.6) as well.
Non-bridge relays lump all countries with less than K (e.g. K=5) users
into the "Other" category (see Sec 7.2 above), whereas bridge relays are
willing to list a country even when it has only one user for the day.
Whenever we have a daily summary on record, we include it in our
extrainfo document whenever we publish one. The daily summary we
remember locally gets replaced with a newer one when another 24
7.8. Some forward secrecy
How should we remember addresses locally? If we convert them into
country-codes immediately, we will count them again if we see them
again. On the other hand, we don't really want to keep a list hanging
around of all IP addresses we've seen in the past 24 hours.
Step one is that we should never write this stuff to disk. Keeping it
only in ram will make things somewhat better. Step two is to avoid
keeping any timestamps associated with it: rather than a rolling
24-hour window, which would require us to remember the various times
we've seen that address, we can instead just throw out the whole list
every 24 hours and start over.
We could hash the addresses, and then compare hashes when deciding if
we've seen a given address before. We could even do keyed hashes. Or
Bloom filters. But if our goal is to defend against an adversary
who steals a copy of our ram while we're running and then does
guess-and-check on whatever blob we're keeping, we're in bad shape.
We could drop the last octet of the IP address as soon as we see
it. That would cause us to undercount some users from cablemodem and
DSL networks that have a high density of Tor users. And it wouldn't
really help that much -- indeed, the extent to which it does help is
exactly the extent to which it makes our stats less useful.
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