March 2016 - tor-dev - lists.torproject.org

Proposal 259: New Guard Selection Behaviour
by George Kadianakis 04 Apr '16

04 Apr '16

Hello, proposal 259 has evolved plenty since the last time it was posted on [tor-dev]. All the technical discussion can be found in various threads, and here is a thread about the proposal itself as it is now. The current proposal is pretty much done, and it's currently being implemented for testing. We imagine that during testing we might have to slightly alter the algorithm and its parameters to improve performance and security. Here are some behaviors that are still unspecified in the current proposal based on our discussion yesterday: - What exactly should be done if a circuit has restrictred requirements? (e.g. it needs a Fast/Stable guard, but our top guard is not Fast/Stable) - How exactly should directory guards work? Should the guard lists be initialized only with directory guards? Or should we just initialize our guard lists from the set of all guards, and just skip non-V2Dir guards whenever we need to make a directory circuit? FWIW, there are currently 2076 guards, out of which 1659 support V2Dir (i.e. they are directory guards). - How should the algorithm work wrt ReachableAddresses? I wonder how the current algorithm work wrt ReachableAddresses and if it's behavior is good. --- Filename: 259-guard-selection.txt Title: New Guard Selection Behaviour Author: Isis Lovecruft, George Kadianakis, [Ola Bini] Created: 2015-10-28 Status: Draft §1. Overview Tor uses entry guards to prevent an attacker who controls some fraction of the network from observing a fraction of every user's traffic. If users chose their entries and exits uniformly at random from the list of servers every time they build a circuit, then an adversary who had (k/N) of the network would deanonymize F=(k/N)^2 of all circuits... and after a given user had built C circuits, the attacker would see them at least once with probability 1-(1-F)^C. With large C, the attacker would get a sample of every user's traffic with probability 1. To prevent this from happening, Tor clients choose a small number of guard nodes (currently 3). These guard nodes are the only nodes that the client will connect to directly. If they are not compromised, the user's paths are not compromised. But attacks remain. Consider an attacker who can run a firewall between a target user and the Tor network, and make many of the guards they don't control appear to be unreachable. Or consider an attacker who can identify a user's guards, and mount denial-of-service attacks on them until the user picks a guard that the attacker controls. In the presence of these attacks, we can't continue to connect to the Tor network unconditionally. Doing so would eventually result in the user choosing a hostile node as their guard, and losing anonymity. This proposal outlines a new entry guard selection algorithm, which addresses the following concerns: - Heuristics and algorithms for determining how and which guard(s) is(/are) chosen should be kept as simple and easy to understand as possible. - Clients in censored regions or who are behind a fascist firewall who connect to the Tor network should not experience any significant disadvantage in terms of reachability or usability. - Tor should make a best attempt at discovering the most appropriate behaviour, with as little user input and configuration as possible. §2. Design Alice, an OP attempting to connect to the Tor network, should undertake the following steps to determine information about the local network and to select (some) appropriate entry guards. In the following scenario, it is assumed that Alice has already obtained a recent, valid, and verifiable consensus document. The algorithm is divided into four components such that the full algorithm is implemented by first invoking START, then repeatedly calling NEXT while adviced it SHOULD_CONTINUE and finally calling END. For an example usage see §A. Appendix. Several components of NEXT can be invoked asynchronously. SHOULD_CONTINUE is used for the algorithm to be able to tell the caller whether we consider the work done or not - this can be used to retry primary guards when we finally are able to connect to a guard after a long network outage, for example. This algorithm keeps track of the unreachability status for guards in state global to the system, so that repeated runs will not have to rediscover unreachability over and over again. However, this state does not need to be persisted permanently - it is purely an optimization. The algorithm expects several arguments to guide its behavior. These will be defined in §2.1. The goal of this algorithm is to strongly prefer connecting to the same guards we have connected to before, while also trying to detect conditions such as a network outage or a network environment that blocks most ports. The way it does this is by keeping track of how many guards we have exposed ourselves to, and if we have connected to too many we will fall back to only retrying the ones we have already tried. The algorithm also decides on sample set that should be persisted - in order to minimize the risk of an attacker forcing enumeration of the whole network by triggering rebuilding of circuits. §2.1. The START algorithm In order to start choosing an entry guard, use the START algorithm. This takes four arguments that can be used to fine tune the workings: USED_GUARDS This is a list that contains all the guards that have been used before by this client. We will prioritize using guards from this list in order to minimize our exposure. The list is expected to be sorted based on priority, where the first entry will have the highest priority. SAMPLED_UTOPIC_GUARDS This is a set that contains all guards that should be considered for connection under utopic conditions. This set should be persisted between runs. It will be filled in by the algorithm if it's empty, or if it contains less than SAMPLE_SET_THRESHOLD guards after winnowing out older guards. It should be filled by using NEXT_BY_BANDWIDTH with UTOPIC_GUARDS as an argument. SAMPLED_DYSTOPIC_GUARDS This is a set that contains all guards that should be considered for connection under dystopic conditions. This set should be persisted between runs. It will be filled in by the algorithm if it's empty, or if it contains less than SAMPLE_SET_THRESHOLD guards after winnowing out older guards. It should be filled by using NEXT_BY_BANDWIDTH with DYSTOPIC_GUARDS as an argument. EXCLUDE_NODES A set of nodes that we should not consider using as a guard. N_PRIMARY_GUARDS The number of guards we should consider our primary guards. These guards will be retried more frequently and will take precedence in most situations. By default the primary guards will be the first N_PRIMARY_GUARDS guards from USED_GUARDS. DIR If this argument is set, we should only consider guards that can be directory guards. If not set, we will consider all guards. The primary work of START is to initialize the state machine depicted in §2.2. The initial state of the machine is defined by: GUARDS This is a set of all guards from the consensus, without EXCLUDE_NODES and potentially filtered if DIR is set. UTOPIC_GUARDS This is a set of all guards to use under utopic conditions. It will primarily be used to fill in SAMPLED_UTOPIC_GUARDS. This set will be initialized to be the same as GUARDS. DYSTOPIC_GUARDS This is a set of all guards to use under dystopic conditions (usually when we are subject to a firewall that restricts the ports we can connect to). It will primarily be used to fill in SAMPLED_DYSTOPIC_GUARDS. This set will be initialized to be the subset of GUARDS that listen to ports that are allowed by dystopic conditions. REMAINING_UTOPIC_GUARDS This is a running set of the utopic guards we have not yet tried to connect to. It should be initialized to be SAMPLED_UTOPIC_GUARDS without USED_GUARDS. REMAINING_DYSTOPIC_GUARDS This is a running set of the dystopic guards we have not yet tried to connect to. It should be initialized to be SAMPLED_DYSTOPIC_GUARDS without USED_GUARDS. STATE A variable that keeps track of which state in the state machine we are currently in. It should be initialized to STATE_PRIMARY_GUARDS. PRIMARY_GUARDS This list keeps track of our primary guards. These are guards that we will prioritize when trying to connect, and will also retry more often in case of failure with other guards. It should be initialized by calling algorithm NEXT_PRIMARY_GUARD repeatedly until PRIMARY_GUARDS contains N_PRIMARY_GUARDS elements. §2.2. The NEXT algorithm The NEXT algorithm is composed of several different possibly flows. The first one is a simple state machine that can transfer between four different states. Every time NEXT is invoked, it will resume at the state where it left off previously. In the course of selecting an entry guard, a new consensus can arrive. When that happens we need to update the data structures used, but nothing else should change. Before jumping in to the state machine, we should first check if it was at least PRIMARY_GUARDS_RETRY_INTERVAL minutes since we tried any of the PRIMARY_GUARDS. If this is the case, and we are not in STATE_PRIMARY_GUARDS, we should save the previous state and set the state to STATE_PRIMARY_GUARDS. §2.2.1. The STATE_PRIMARY_GUARDS state Return each entry in PRIMARY_GUARDS in turn. For each entry, if it was not possible to connect to it and mark the entry as unreachable [XXX defining "was not possible to connect" as "entry is not live" according to current definition of "live entry guard" in tor source code, seems to improve success rate on the flaky network scenario. See: https://github.com/twstrike/tor_guardsim/issues/1#issuecomment-187374942] If all entries have been tried, restore the previous state and go there. If there is no previous state, transition to STATE_TRY_UTOPIC. §2.2.2. The STATE_TRY_UTOPIC state Return each entry in USED_GUARDS that is not in PRIMARY_GUARDS in turn. For each entry, if it was not possible to connect to it and mark the entry as unreachable. Return each entry from REMAINING_UTOPIC_GUARDS using NEXT_BY_BANDWIDTH. For each entry, if it was not possible to connect to it, remove the entry from REMAINING_UTOPIC_GUARDS and mark it as unreachable. If no entries remain in REMAINING_UTOPIC_GUARDS, transition to STATE_TRY_DYSTOPIC. §2.2.3. The STATE_TRY_DYSTOPIC state Return each entry from REMAINING_DYSTOPIC_GUARDS using NEXT_BY_BANDWIDTH. For each entry, if it was not possible to connect to it, remove the entry from REMAINING_DYSTOPIC_GUARDS and mark it as unreachable. If no entries remain in REMAINING_DYSTOPIC_GUARDS, transition to STATE_PRIMARY_GUARDS. §2.2.5. ON_NEW_CONSENSUS First, ensure that all guard profiles are updated with information about whether they were in the newest consensus or not. If a guard is not included in the newest consensus, the guard is considered bad. If any PRIMARY_GUARDS have become bad, remove the guard from PRIMARY_GUARDS. Then ensure that PRIMARY_GUARDS contain N_PRIMARY_GUARDS entries by repeatedly calling NEXT_PRIMARY_GUARD. If any guards in USED_GUARDS have switched from being bad to being non-bad, add it back in the place it should have been in PRIMARY_GUARDS if it had been non-bad when populating PRIMARY_GUARDS. If this results in PRIMARY_GUARDS being larger than N_PRIMARY_GUARDS, truncate the list to be N_PRIMARY_GUARDS entries long. §2.3. The SHOULD_CONTINUE algorithm This algorithm takes as an argument a boolean indicating whether the circuit was successfully built or not. After the caller have tried to build a circuit with a returned guard, they should invoke SHOULD_CONTINUE to understand if the algorithm is finished or not. SHOULD_CONTINUE will always return true if the circuit failed. If the circuit succeeded, SHOULD_CONTINUE will always return false, unless the guard that succeeded was the first guard to succeed after INTERNET_LIKELY_DOWN_INTERVAL minutes - in that case it will set the state to STATE_PRIMARY_GUARDS and return true. §2.4. The END algorithm The goal of this algorithm is simply to make sure that we keep track of successful connections made. This algorithm should be invoked with the guard that was used to correctly set up a circuit. Once invoked, this algorithm will mark the guard as used, and make sure it is in USED_GUARDS, by adding it at the end if it was not there. §2.5. Helper algorithms These algorithms are used in the above algorithms, but have been separated out here in order to make the flow clearer. NEXT_PRIMARY_GUARD - Return the first entry from USED_GUARDS that is not in PRIMARY_GUARDS and that is in the most recent consensus. - If USED_GUARDS is empty, use NEXT_BY_BANDWIDTH with REMAINING_UTOPIC_GUARDS as the argument. NEXT_BY_BANDWIDTH - Takes G as an argument, which should be a set of guards to choose from. - Return a randomly select element from G, weighted by bandwidth. §3. Consensus Parameters, & Configurable Variables This proposal introduces several new parameters that ideally should be set in the consensus but that should also be possible to set or override in the client configuration file. Some of these have proposed values, but for others more simulation and trial needs to happen. PRIMARY_GUARDS_RETRY_INTERVAL In order to make it more likely we connect to a primary guard, we would like to retry the primary guards more often than other types of guards. This parameter controls how many minutes should pass before we consider retrying primary guards again. The proposed value is 3. SAMPLE_SET_THRESHOLD In order to allow us to recognize dystopic situations or a completely unreachable network, we would like to avoid connecting to too many guards before switching modes. We also want to avoid exposing ourselves to too many nodes in a potentially hostile situation. This parameter, expressed as a fraction, determines the number of guards we should keep as the sampled set of the only guards we will consider connecting to. It will be used as a fraction for both the utopic and the dystopic sampled set. If we assume there are 1900 utopic guards and of them there are 500 dystopic guards, a setting of 0.02 means we will have a sample set of 38 utopic guards and 10 dystopic guards. This limits our total exposure. Proposed value is 0.02. INTERNET_LIKELY_DOWN_INTERVAL The number of minutes since we started trying to find an entry guard before we should consider the network down and consider retrying primary guards before using a functioning guard found. Proposed value 5. §4. Security properties and behavior under various conditions Under normal conditions, this algorithm will allow us to quickly connect and use guards we have used before with high likelihood of working. Assuming the first primary guard is reachable and in the consensus, this algorithm will deterministically always return that guard. Under dystopic conditions (when a firewall is in place that blocks all ports except for potentially port 80 and 443), this algorithm will try to connect to 2% of all guards before switching modes to try dystopic guards. Currently, that means trying to connect to circa 40 guards before getting a successful connection. If we assume a connection try will take maximum 10 seconds, that means it will take up to 6 minutes to get a working connection. When the network is completely down, we will try to connect to 2% of all guards plus 2% of all dystopic guards before realizing we are down. This means circa 50 guards tried assuming there are 1900 guards in the network. In terms of exposure, we will connect to a maximum of 2% of all guards plus 2% of all dystopic guards, or 3% of all guards, whichever is lower. If N is the number of guards, and k is the number of guards an attacker controls, that means an attacker would have a probability of 1-(1-(k/N)^2)^(N * 0.03) to have one of their guards selected before we fall back. In real terms, this means an attacker would need to control over 10% of all guards in order to have a larger than 50% chance of controlling a guard for any given client. In addition, since the sampled set changes slowly (the suggestion here is that guards in it expire every month) it is not possible for an attacker to force a connection to an entry guard that isn't already in the users sampled set. §A. Appendix: An example usage In order to clarify how this algorithm is supposed to be used, this pseudo code illustrates the building of a circuit: OPEN_CIRCUIT: context = ALGO_CHOOSE_ENTRY_GUARD_START(used_guards, sampled_utopic_guards=[], sampled_dystopic_guards=[], exclude_nodes=[], n_primary_guards=3, dir=false, guards_in_consensus) while True: entryGuard = ALGO_CHOOSE_ENTRY_GUARD_NEXT(context) circuit = composeCircuitAndConnect(entryGuard) if not SHOULD_CONTINUE(isSuccessful(circuit)): ALGO_CHOOSE_ENTRY_GUARD_END(context, entryGuard) return circuit -*- coding: utf-8 -*-

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[::]/8 is marked as private network, why?
by Sebastian Hahn 04 Apr '16

04 Apr '16

Hey, I've been wondering about the private_nets const in src/or/policies. It was added in a96c0affcb4cda1a2e0d83d123993d10efc6e396 but Nick doesn't remember why, and I'm hoping someone has an idea (maybe teor, who I've CCed here, who documented this in a later commit?). If nobody knows why we do this I think we should remove it as likely incorrect. Cheers and thanks Sebastian

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A few ideas about improved design/modularity in Tor
by Nick Mathewson 04 Apr '16

04 Apr '16

(Hi! Here's a document I've been poking at , on and off, for a while. There's a lot more to say here, but I think it's ready to share with tor-dev for comment. Thanks!) ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ +++ 0. PRELIMINARIES. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ============== WHY? If you've ever looked at even slightly serious software engineering methodology, you've seen some discussion of modularity: the idea that programs should be divided into a number of smaller modules; that complexity should be hidden within modules and not in the interfaces between them; that which modules can call which other modules should be limited; and other little rules like that. We'd like modules isolated from one another for these reasons: * Make them simpler to test * Minimize what is allowed to touch what * Make tor easier to understand * Make tor easier to hack on * Let us write parts of tor in other languages. * Make sandboxing more powerful This is actually several problems all in one! - OO/Isolation/Modularity design that doesn't actually split into multiple processes, but makes the callgraph much much simpler. - Multiprocess sandbox framework with RPC implementing the above abstraction - Finding the right stuff to carve out into separate sandboxes - Finding the right stuff to carve into separate modules. ============ Steps/phases: Over the past 6-8 months we've gone a lot of work to simplify Tor's callgraph in preparation for this work. We've got our callgraph simplified to a pretty extreme degree, for example. Here are some of our next steps, some of which can be done in parallel. 1. Figure out which modules should be able to call each others. Isolate them based on their ability to do so. Refactor until they actually work that way. - In some cases we will need to split - hide big piles of modules behind single entry points. - separate src/common and src/or into separate parts. - Add tools to enforce this separation - Add tools to separate Tor functions safely and not too error-prone. (See sections 1 and 3 below.) 2. Add backend abstractions as needed to minimize module coupling. These should be abstractions that are friendly to in- and multi-process implementations. We will need at least: - publish/subscribe{,/acknowledge}. (See https://en.wikipedia.org/wiki/Publish%E2%80%93subscribe_pattern. The 'acknowledge' abstraction allows the publisher to wait for every subscriber to acknowledge receipt. More on this in section 4 below.) - handles (We need frequently need handles to objects where the handles need to be persistent while the objects themselves can go away. We use several patterns for this now. We should just have a handle pattern for this instead. This would need separate implementations for in-process and cross-process access. See section 2 below for more information.) - Need a way to mark functions as "internal to module". We're going to have a larger number of multi-file modules. Right now, our only restriction for making functions less visible is "static", and the fact that we don't allow src/common to call src/or. Both of these should be cleaned up. 3. Write abstractions for splitting code into multiple processes that talk over IPC. (See section 5 below) ========================= Metrics We can measure our success in a number of ways: * How large are the strongly connected components among modules calling another? Right now the largest SCC is 52 C files. * How many layer/module violations can we find? This needs to be an automated process. * How many exported functions from each module can we remove? * How much can we remove from the main linux sandbox code? How many functions can we isolate to purpose-built modules? ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ +++ 0. THE MODULES ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ========================= Modules in src/or LOWEST LEVEL These should only get called from higher-level components. They shouldn't call anything of a higher level. Types and data structures. - part of configuration. - parts of statefile - routerset - policy (some) - torcert Various backends and utilities. - confparse - geoip - fp_pair? - reasons - replaycache - buffers Nodelist data structures, excluding routerinfo. - nodelist - microdesc - networkstatus - routerparse.c (parts) SLIGHTLY HIGHER These should only get called from higher-level components, and only call lower-level components. When they currently call a higher-level piece, they should use a callback or a publish/subscribe API or something. Nothing in here should really be tor-specific. mainloop: - connections (common, genericparts) - main (nonspecific parts) - config (maybe) - workqueue - scheduler - periodic - cpuworker (parts) - hibernate ? (Parts?) HIGHER THAN THAT These modules are where we start making Tor do Tor. They should only call lower-level modules onionrouting: - channels - onion* - cpuworker (parts) - connection_or - connection_edge - circuitlist - command - relay - parts of router.c - transports HIGHER STILL These modules can call downwards, though they should really be made as independent as possible from each other and from one another. We should enumerate which of these can call which others; mostly, they should leave one another alone. hidden services: - rend* controller: - control.c auxiliary: - rephist authority: - dirvote - dirserver (parts) - dirauth - dircollate - keypin relay: - ext_orport - parts of router.c - routerkeys.c cache: - parts of directory - dirserver (parts) - routerlist (parts) - routerparse.c (parts) exit: - dns client: - dnsserv - most circuitbuild, circuituse stuff - circpathbias - circuitstats directory: - directory - dirserver - HIGHEST LEVEL Here's the parts of the application that know all the other parts, and call down. Nothing else calls up to these modules. application (highest level) - Main (parts) - parts of config - parts of statefile - ntmain - status ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ +++ 2. INFRASTRUCTURE: HANDLES. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ======================== Handles in C * We need a light-weight handle mechanism. (A mechanism where we can have a reference to X that can still live even when X dies, where lookups of X through the reference are safe even when X is freed, and where lookups of X through the reference are FAST.) * It's okay if the things being handled need to be marked as 'handleable'. * When the handle is in a separate process, we probably need a table-based implementation: * Unique handle (64/96 bit int) for each thingie; hashtable to look up by handle. * When the reference is local, can get as fast as pointer lookup. * Threadsafety can be important but is not always required. For an example implementation, see #18362. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ +++ 3. INFRASTRUCTURE: ENFORCING ISOLATION. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ === Making isolation happen. It'll do us no good to declare that modules are isolated if they actually aren't. Once we have defined the allowed call patterns, we need to make sure that calls that fall outside of those patterns aren't permitted. We can build something pretty simple out of 'nm', I believe. See ticket #18617. This is a work in progress. === Moving functions around. I'm working on a tool to move functions from one file to another in a deterministic way based on annotations in the original file. This lets us version our movement, and have the movement happen deterministically so that the patches are easier to verify. If we integrate it with our callgraph-examination tools above, we can make sure that our movement plans actually make the code more conformant with our modularity plans. See ticket #18618. This is a work in progress. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ +++ 4. INFRASTRUCTURE: PUBLISH/SUBSCRIBE ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Many of our modularity violations happen because when some property changes, or some event occurs, we need to inform a wide variety of other modules. The publish/subscribe{,/acknowledge} pattern, and the similar observer pattern, were made for this. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ +++ 5. ADVANCED INFRASTRUCTURE: MULTIPROCESS SANDBOXING ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ======================== Multiprocess abstraction / design We'll consider multiprocess for these cases: * Easier/stronger/more-portable sandboxing. * Writing subcomponents in languages whose runtime/link rules don't play so nicely with C. * Enforcing modularity when modularity delivers security. For sandboxing, here are some high-risk privileges we should make sure that other code doesn't have. * Filesystem calls -- anything that can open a file. * Exec -- anything that can launch a process. * Access to identity or signing keys. * Invoking getaddrinfo() [because it uses a fairly huge segment of the underlying operating system] And here is some higher-risk code that we could most safely isolate from the main modules. * Anything that parses an object. * Consensus diff processing. Our basic abstractions are those described above (pub/sub, handles, hooks), with the addition of: * Blocking RPC * Capabilities???? * message queues. Controversial: * We should NOT require that most of this be blindingly fast. Correct is perfectly adequate. Fast is only needed in the critical path. Here's the general architecture I have in mind: * At the highest level, have a launcher/supervisor process whose only job is to start the other processes, monitor them, and notice if they go down. * Additionally, have a communications-hub process whose role is to pass messages between other processes, performing access control on the messages to make sure that each message is only sent by a process that is allowed to send it, to a process that it is allowed to send to. Processes may only communicate with the hub. * Messages should use a simple RPC format. I vote for protobufs or capnproto. They sure have a speed/simplicity tradeoff though. * pipes or socketpairs should get used on Unix. Windows can use windows Pipes, which aren't quite FDs. Windows will need to have its pipes in a different thread from the socket-based libevent loop (if there is one.) * We need a way to pass fds or handles back and forth. Both Windows and Unix can do this. (via DuplicateHandle() on Windows and sendmsg() on PF_UNIX sockets on unix.) The protocol needs to handle this explicitly. Here's the development plan I would suggest: * In parallel, work on improving Tor's modularity as the earlier sections of this document suggest. Unless we get more modular, we won't be able to make anything actually separate besides the privilege-restriction sandboxing parts above (like isolating keys and FS access). * Research what exactly windows and osx allow. * Write the top-level supervisor process and hub process. * Ensure that the APIs we expose can be written in C in a fairly mechanism-agnostic way, so that we can migrate to this architecture with minimal additional effort. * Move privilege-restriction sandboxing parts into subprocesses. And here is an alternative plan: * Investigate the chromium sandbox code; see if it can be extracted, or how much of it can be extracted. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ +++ A. APPENDIX. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ === progress so far I've gotten these done so far: * The starts of a build-time callgraph enforcement mechanism using nm and readelf. * A handle-based-reference module. (See ticket #18362, but cpunks doesn't like it.) * A draft refactoring tool that lets you annotate code with what should move where, so that you can deterministicly try the code movement and see what happens, and so you can see what effects it will have on the module-level callgraph.

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(no subject)
by Nick Mathewson 28 Mar '16

28 Mar '16

Hi, folks! The Tor git master branch is now 0.2.9.0-alpha-dev. Development on 0.2.8.x continues in the branch "maint-0.2.8". I expect that 0.2.8.2-alpha will be the last 0.2.8.x-alpha, and then we can move on to rc and stable. We now return you to your semiregularly semischeduled Tor development. yrs, -- Nick

1 0

Re: [tor-dev] Should we disable the collection of some stats published in extra-infos?
by Karsten Loesing 28 Mar '16

28 Mar '16

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 15/01/16 23:00, Rob Jansen wrote: > Hello, Hi Rob, I'm moving this discussion from metrics-team@ to tor-dev@, because I think it's relevant for little-t-tor devs who are not subscribed to metrics-team@. Hope you don't mind. > I was recently reviewing the statistics that Tor allows relays to > collect and report to the dir servers [1], which then get published > in extra-info documents [2]. Most of this can be enabled by simply > setting a torrc option. There are quite a few statistics that I > feel should not be collected. I'm wondering if the original purpose > for collecting many of these statistics still exists, and if we > still feel that the privacy compromises that were made when the > collection was implemented are still valid in most cases. > > Here are the stats I am most worried about, and why: > > [unique ips per country code] *-ips (there are many of these, e.g. > "entry-ips") Usually this involves storing individual user IP > addresses in memory (in order to track uniqueness) over some period > of time (usually 24 hours), sometimes for longer than the user > would have otherwise been known to Tor (if a user's session is 1 > hour, Tor could remember the IP for at most 23 additional hours). > This is reported, e.g., per entry; there are many cases in the data > where it is very likely that only one user is connecting to a guard > from a given country (because it is rounded up to 8). Users in > small countries have the greatest risk (intersection attacks become > really easy). I agree that might just lose these statistics. We used them in the past as first approximation to counting users, but obviously that only works as long as clients only connect to a single relay. The only place where we're still using them is in a workaround for estimating bridge users. See #15469 for more details and #8786 for something we'd have to implement before taking these statistics out. > [exit statistics by port number] exit-kibibytes-written > exit-kibibytes-read exit-streams-opened Tor is classifying its > traffic into ports, which could uniquely identify the application > being used by the client. They also track bandwidth usage per port > (and per exit); again, this is bad for those using a random or > unique looking ports (that a given exit does not see very often) > because it could be used to create a fingerprint. Intersection > attacks become easier with this information. Agreed, I can see us dropping these statistics, too. We're currently not using them. But also see my suggestion below. > The less problematic stats: > > [circuit-based cell statistics] cell-processed-cells > cell-queued-cells cell-time-in-queue cell-circuits-per-decile This > provides queue timings and number of cells being processed at a > relay. The number of cells can be used to compute bandwidth of > circuits. It may be possible to launch some attacks that create > several circuits with the intent of moving which decile buckets > some legitimate circuits get placed into, but this is less > worrisome of an attack than the others. I'm less worried about this one. But, suggestion below. > Should Tor still be collecting these things? Should Tor disable the > collection of these statistics until we have a more > privacy-preserving way to collect and aggregate them? > > The good news is that privacy-preserving techniques exist that can > reduce information leakage. I'm developing a tool based on the > secret-sharing variant of PrivEx [3] to collect some of these types > of statistics while providing privacy guarantees. We are currently > using it to collect only those stats that are useful for producing > Tor traffic models. A great advantage of this tool is that the > various counters that we store during the collection phase get > noise added and are randomized during initialization; only the > aggregates are ever known and revealed by the aggregation server, > limiting the information that is lost if a relay is compromised. > This is a large improvement over the current collection method, > which only adds noise before publication and reveals statistics on > a per-relay basis. Suggestion: How about we evaluate these statistics published by relays in the past years to see if there are other benefits or risks we didn't think of, and then we decide whether to leave them in, modify them, or take them out? The reason is that I'd want to avoid removing this code only to realize shortly after that we overlooked a good reason for keeping it. These statistics are being collected for years now, and it might take another year or so for relays to upgrade to stop collecting them. So what's another month. Thanks for (re-)starting this discussion! > All the best, Rob All the best, Karsten > [1] https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt [2] > https://collector.torproject.org/recent/relay-descriptors/extra-infos/ > > [3] www.cypherpunks.ca/~iang/pubs/privex-ccs14.pdf > > > > _______________________________________________ metrics-team > mailing list metrics-team(a)lists.torproject.org > https://lists.torproject.org/cgi-bin/mailman/listinfo/metrics-team > -----BEGIN PGP SIGNATURE----- Comment: GPGTools - http://gpgtools.org iQEcBAEBAgAGBQJWnffFAAoJEJD5dJfVqbCrSREH/1G3nZDwC7a8FGFEySgQX3MZ n1dlMcylDU6kypgGMEYs2KmMRwhNYmCVFn6fRJgrFN/KgJ0oZJG1pRcglnIFwNwH 5VUWDZp3+frhY5jBbJ3JXqA4Exhi/0mUYCTRwfcFQ/JOyzjlcFbLRW/rkqFHn62H wLidaVeagiJ+TI/T8zgwtqQjTDSZrZPpmlxvlO57D1bGhW1ZPJVbOeUzuudhNpRS WVu//MH51juCzcML32MeiMV6wWYYOm1irKZ8lZHVCPbsL98qoh3ewtO6fb62THIa sWcQZlwMQdcC0+509PmMeYwkRn+40MUwkk84/glyP57dfpgtOwzZCohL3yQRQ1s= =F6F+ -----END PGP SIGNATURE-----

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guide to start developing tor
by RAJAT MODI 28 Mar '16

28 Mar '16

hi i am new to tor and want to contribute to tis development .i have already browsed the volunterr page on your website .However i am unable to decide from where should i start ,Kindly guide me for the same . -- RAJAT MODI

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Remember: Network Team/tor core dev meeting Monday at new time!
by Nick Mathewson 28 Mar '16

28 Mar '16

Hello! Tomorrow on Monday, we will be having our meeting on the new time: 1700 UTC (1pm eastern, 10am Pacific). The patch party will be Tuesday at 2300 UTC. If you like hacking Tor, set an alarm and stop on by! Peace, -- Nick

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Sibyls: introducing AS-level sign-up rate limits, per relay caps, family-level caps
by nusenu 26 Mar '16

26 Mar '16

Hi, there is a strict limit on the number of relays that one can run on a single IP address to make Sybil attacks harder, but there are no limits on sign-up rates. Such sign-up rate limits would prevent many of the known and simple Sibyls of the past months and probably reduce the manual actions required by dir auth ops to mitigate them (but I've little inside on that part). In the past 8 months the proposed AS sign-up rate limit thresholds (see below) triggered on ~27 events [some-examples]. I consider all of them to be true-positives, but 15 (vs total: >750) likely unrelated relays signed up on the same ASes during these events. AS-level sign-up rate limit thresholds --------------------------------------- max new relays per 24 hours per AS per OS family: 9 (default); value for OVH/ONLINE/Digital Ocean ASes: 14 I deduced to these threshold values by watching the past 8 months of OrNetRadar emails while keeping false-positives at a minimum (at the price of more false-negatives). How does a well behaving operator (aka Brian) add more than 9 relays per AS per day? All relays in an effective family should be treated as "1 new relay". (In my opinion a reason to keep the family [0] functionality and implement prop242). denial of service risk To prevent trivial dos attacks where an attacker with a single IP generates several new relay fingerprints until the entire AS is blocked from adding new relays for a few hours these relays should come from distinct IP addresses. Pros: + automatically enforced + less manual action required by dir auth ops (maybe some dir auths can commend on that.) + address more (and smaller) Sibyls (I assume that many small Sibyls are currently ignored due to risk/effort trade-offs) Cons: - maxmind AS-IP mapping db becomes crucial and needs to be in sync across dir auths - false-positives - non-dir auths will not notice if an AS run into this limit unless they publish some kind of transparency reports - dir auths must keep more state (not sure if they know all existing FPs of the past already) Feasibility Initially the idea was to provide immediate feedback to the relay that tries to publish its descriptor to the dir auth via the HTTP response (an important point), but since descriptors are not uploaded to a single central dir auth that does not work I guess. So before thinking on how one would actually achieve that or something similar I wanted to gather the general opinion about something like this. single relay caps ------------------ 250000 consensus weight (to prevent [1]) family-level caps ------------------ max cw fraction: 2.5% [2] max. exit probability: 10% [3] guard probability: 5% [4] AS-level caps: --------------- max cw fraction/guard/exit probability: 15% [5] AS12876 (ONLINE SAS) and AS16276 (OVH) are already past that value, so if you do not want to change the game for them you would have to set it to ~20%. [0] https://trac.torproject.org/projects/tor/ticket/6676 [1] https://lists.torproject.org/pipermail/tor-talk/2015-December/039696.html https://trac.torproject.org/projects/tor/ticket/17810#comment:8 [2] https://raw.githubusercontent.com/ornetstats/stats/master/o/main_operators_… [3] https://raw.githubusercontent.com/ornetstats/stats/master/o/main_exit_opera… [4] https://raw.githubusercontent.com/ornetstats/stats/master/o/main_guard_oper… [5] https://compass.torproject.org [some-examples] http://article.gmane.org/gmane.network.onion-routing.ornetradar/1135 http://article.gmane.org/gmane.network.onion-routing.ornetradar/1132 http://article.gmane.org/gmane.network.onion-routing.ornetradar/1100 http://article.gmane.org/gmane.network.onion-routing.ornetradar/1030 http://article.gmane.org/gmane.network.onion-routing.ornetradar/939 http://article.gmane.org/gmane.network.onion-routing.ornetradar/943 http://article.gmane.org/gmane.network.onion-routing.ornetradar/898 http://article.gmane.org/gmane.network.onion-routing.ornetradar/856 http://article.gmane.org/gmane.network.onion-routing.ornetradar/813

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[GSoC 2016] Roster - Gamification for Tor Operators (Sean Saito)
by Sean Saito 26 Mar '16

26 Mar '16

Hi! I have finished and submitted my final proposal for GSoC yesterday. This summer I would like to complete Roster, a Tor Project initiative I’ve been working on since last year. It is basically a social incentivization scheme to gamify operating a Tor relay. All the details are included in the Google doc, and, while the deadline for edits has passed, I would appreciate any comments and feedback. https://docs.google.com/document/d/1wOBUhOPAAWTLfQIt4ChKixLMgIrZlO3YBQ-eixh… Thanks a lot! Best, Sean

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Tor 0.2.8 and Fallback Directory Mirrors
by Tim Wilson-Brown - teor 26 Mar '16

26 Mar '16

Dear Tor App Developers, Just a heads' up that tor 0.2.8 includes a fallback directory mirrors feature, where tor clients bootstrap from a set of hard-coded long-lived directory mirrors. This makes bootstrap more reliable, and makes it harder to block tor. The wiki has an entry with more details. [0] We're currently trying to finalise a list of around 100 fallback directory mirrors by the time tor 0.2.8-stable is released. [1] If we have this many mirrors, I am confident we won't overwhelm any of the mirrors when an app like Orbot, Tor Browser, or Tails switches to tor 0.2.8. (Are there any other tor apps with millions of users that I've missed?) If this feature does cause problems, or if your app needs to bootstrap only from the authorities (Tails time syncing? [2]), the fallback directory mirrors feature can be disabled using the tor config option "UseDefaultFallbackDirs 0". Tim [0]: https://trac.torproject.org/projects/tor/wiki/doc/FallbackDirectoryMirrors <https://trac.torproject.org/projects/tor/wiki/doc/FallbackDirectoryMirrors> [1]: https://trac.torproject.org/projects/tor/ticket/17158 <https://trac.torproject.org/projects/tor/ticket/17158> [2]: https://tails.boum.org/contribute/design/Time_syncing/ <https://tails.boum.org/contribute/design/Time_syncing/> Tim Wilson-Brown (teor) teor2345 at gmail dot com PGP 968F094B teor at blah dot im OTR CAD08081 9755866D 89E2A06F E3558B7F B5A9D14F

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