Filename: 126-geoip-fetching.txt Title: Getting GeoIP data and publishing usage summaries Version: $Revision$ Last-Modified: $Date$ Author: Roger Dingledine Created: 2007-11-24 Status: Open 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: "205500992","208605279","US","USA","UNITED STATES" http://ip-to-country.webhosting.info/node/view/5 Similarly, the maxmind GeoLite Country database is also about 500KB compressed. http://www.maxmind.com/app/geolitecountry 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: 206.124.149.146,Bellevue,WA,US,47.6051,-122.1134 http://www.maxmind.com/app/geolitecity There are other databases out there, like http://www.hostip.info/faq.html http://www.webconfs.com/ip-to-city.php 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 sanitized versions. Goal #1b: Tor bridges collect IP-to-country user stats and publish sanitized versions. Goal #2a: Vidalia learns IP-to-city stats for Tor relays, for better mapping. 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 6 below. 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' command: "GETINFO router-annotations/id/" or "GETINFO router-annotations/name/" which would respond with something like @downloaded-at 2007-11-29 08:06:38 @source "128.31.0.34" @purpose bridge [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. 4.3. Recommendation 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 same URLs. 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? GETINFO ip-to-country/128.31.0.34 250+ip-to-country/128.31.0.34="US","USA","UNITED STATES" 6.1. Other interfaces Robert Hogan has also suggested a GETINFO relays-by-country/cn as well as torrc options for ExitCountryCodes, EntryCountryCodes, ExcludeCountryCodes, etc. 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 anonymity leaks? 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 on that. 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 hours pass. 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. Other ideas?