commit ca1dea9b8a88e736afbc169a94f66960b722e95c Author: David Goulet <dgoulet@torproject.org> Date: Thu Jun 6 11:35:15 2019 -0400 prop305: Initial import of proposal 305 Hidden service denial of service defenses parameters within the ESTABLISH_INTRO cell. Closes #30790 Signed-off-by: David Goulet <dgoulet@torproject.org> --- proposals/000-index.txt | 2 + .../305-establish-intro-dos-defense-extention.txt | 197 +++++++++++++++++++++ 2 files changed, 199 insertions(+) diff --git a/proposals/000-index.txt b/proposals/000-index.txt index 78d83d2..728c53d 100644 --- a/proposals/000-index.txt +++ b/proposals/000-index.txt @@ -225,6 +225,7 @@ Proposals by number: 302 Hiding onion service clients using padding [ACCEPTED] 303 When and how to remove support for protocol versions [DRAFT] 304 Extending SOCKS5 Onion Service Error Codes [ACCEPTED] +305 ESTABLISH_INTRO Cell DoS Defense Extension [DRAFT] Proposals by status: @@ -235,6 +236,7 @@ Proposals by status: 294 TLS 1.3 Migration 300 Walking Onions: Scaling and Saving Bandwidth 303 When and how to remove support for protocol versions + 305 ESTABLISH_INTRO Cell DoS Defense Extension NEEDS-REVISION: 212 Increase Acceptable Consensus Age [for 0.2.4.x+] 219 Support for full DNS and DNSSEC resolution in Tor [for 0.2.5.x] diff --git a/proposals/305-establish-intro-dos-defense-extention.txt b/proposals/305-establish-intro-dos-defense-extention.txt new file mode 100644 index 0000000..e1ef600 --- /dev/null +++ b/proposals/305-establish-intro-dos-defense-extention.txt @@ -0,0 +1,197 @@ +Filename: 305-establish-intro-dos-defense-extention.txt +Title: ESTABLISH_INTRO Cell DoS Defense Extension +Author: David Goulet, George Kadianakis +Created: 06-June-2019 +Status: Draft + +0. Abstract + + We propose introducing a new cell extension to the onion service version 3 + ESTABLISH_INTRO cell in order for a service operator to send directives to + the introduction point. + +1. Introduction + + The idea behind this proposal is to provide a way for a service operator to + give to the introduction points Denial of Service (DoS) defense parameters + through the ESTABLISH_INTRO cell. + + We are currently developing onion service DoS defenses at the introduction + point layer which for now has consensus parameter values for the defenses' + knobs. This proposal would allow the service operator more flexibility for + tuning these knobs and/or future parameters. + +2. ESTABLISH_INTRO Cell DoS Extention + + We introduce a new extention to the ESTABLISH_INTRO cell. The EXTENSIONS + field will be leveraged and a new protover will be introduced to reflect + that change. + + As a reminder, this is the content of an ESTABLISH_INTRO cell (taken from + rend-spec-v3.txt section 3.1.1): + + AUTH_KEY_TYPE [1 byte] + AUTH_KEY_LEN [2 bytes] + AUTH_KEY [AUTH_KEY_LEN bytes] + N_EXTENSIONS [1 byte] + N_EXTENSIONS times: + EXT_FIELD_TYPE [1 byte] + EXT_FIELD_LEN [1 byte] + EXT_FIELD [EXT_FIELD_LEN bytes] + HANDSHAKE_AUTH [MAC_LEN bytes] + SIG_LEN [2 bytes] + SIG [SIG_LEN bytes] + + We propose a new EXT_FIELD_TYPE value: + + [01] -- DOS_PARAMETERS. + + If this flag is set, the extension should be used by the + introduction point to learn what values the denial of service + subsystem should be using. + + The EXT_FIELD content format is: + + N_PARAMS [1 byte] + N_PARAMS times: + PARAM_TYPE [1 byte] + PARAM_VALUE [8 byte] + + The PARAM_TYPE proposed values are: + + [01] -- DOS_INTRODUCE2_RATE_PER_SEC + The rate per second of INTRODUCE2 cell relayed to the service. + + [02] -- DOS_INTRODUCE2_BURST_PER_SEC + The burst per second of INTRODUCE2 cell relayed to the service. + + The PARAM_VALUE size is 8 bytes in order to accomodate 64bit values + (uint64_t). It MUST match the specified limit for the following PARAM_TYPE: + + [01] -- Min: 0, Max: 2147483647 + [02] -- Min: 0, Max: 2147483647 + + A value of 0 means the defense is disabled. If the rate per second is set + to 0 (param 0x01) then the burst value should be ignored. And vice-versa, + if the burst value is 0 (param 0x02), then the rate value should be + ignored. In other words, setting one single parameter to 0 disables the + INTRODUCE2 rate limiting defense. + + The burst can NOT be smaller than the rate. If so, the parameters should be + ignored by the introduction point. + + The maximum is set to INT32_MAX meaning (2^31 - 1). Our consensus + parameters are capped to that limit and these parameters happen to be also + consensus parameters as well hence the common limit. + + Any valid value does have precedence over the network wide consensus + parameter. + + This will increase the payload size by 21 bytes: + + This extension type and length is 2 extra bytes, the N_EXTENSIONS field + is always present and currently set to 0. + + Then the EXT_FIELD is 19 bytes because one parameter is 9 bytes so for + two parameters, it is 18 bytes plus 1 byte for the N_PARAMS for a total + of 19. + + The ESTABLISH_INTRO v3 cell currently uses 134 bytes for its payload. With + this increase, 343 bytes remain unused (498 maximum payload size minus 155 + bytes new payload). + +3. Protocol Version + + We introduce a new protocol version in order for onion service that wants + to specifically select introduction points supporting this new extension. + But also, it should be used to know when to send this extension or not. + + The new version for the "HSIntro" protocol is: + + "5" -- support ESTABLISH_INTRO cell DoS parameters extension for onion + service version 3 only. + +4. Configuration Options + + We also propose new torrc options in order for the operator to control + those values passed through the ESTABLISH_INTRO cell. + + "HiddenServiceEnableIntroDoSDefense 0|1" + + If this option is set to 1, the onion service will always send to an + introduction point, supporting this extension (using protover), the + denial of service defense parameters regardless if the consensus + enables them or not. The values are taken from + HiddenServiceEnableIntroDoSRatePerSec and + HiddenServiceEnableIntroDoSBurstPerSec torrc option. + (Default: 0) + + "HiddenServiceEnableIntroDoSRatePerSec N sec" + + Controls the introduce rate per second the introduction point should + impose on the introduction circuit. The default values are only used + if the consensus param is not set. + (Default: 25, Min: 0, Max: 4294967295) + + "HiddenServiceEnableIntroDoSBurstPerSec N sec" + + Controls the introduce burst per second the introduction point should + impose on the introduction circuit. The default values are only used + if the consensus param is not set. + (Default: 200, Min: 0, Max: 4294967295) + + They respectively control the parameter type 0x01 and 0x02 in the + ESTABLISH_INTRO cell detailed in section 2. + + The default values of the rate and burst are taken from ongoing anti-DoS + implementation work [1][2]. They aren't meant to be defined with this + proposal. + +5. Security Considerations + + Using this new extension leaks to the introduction point the service's tor + version. This could in theory help any kind of de-anonymization attack on a + service since at first it partitions it in a very small group of running + tor. + + Furthermore, when the first tor version supporting this extension will be + released, very few introduction points will be updated to that version. + Which means that we could end up in a situation where many services want to + use this feature and thus will only select a very small subset of relays + supporting it overloading them but also making it an easier vector for an + attacker that whishes to be the service introduction point. + + For the above reasons, we propose a new consensus parameter that will + provide a "go ahead" for all service out there to start using this + extension only if the introduction point supports it. + + "HiddenServiceEnableIntroDoSDefense?" + + If set to 1, this makes tor start using this new proposed extension + if available by the introduction point (looking at the new protover). + + This parameter should be switched on when a majority of relays have + upgraded to a tor version that supports this extension for which we believe + will also give enough time for most services to move to this new stable + version making the anonymity set much bigger. + + We believe that there are services that do not care about anonymity on the + service side and thus could benefit from this feature right away if they + wish to use it. + +5. Discussions + + One possible new avenue to explore is for the introduction point to send + back a new type of cell which would tell the service that the DoS defenses + have been triggered. It could include some statistics in the cell which can + ultimately be reported back to the service operator to use those for better + decisions for the parameters. + + But also for the operator to be noticed that their service is under attack + or very popular which could mean time to increase or disable the denial of + service defenses. + +References: + +[1] https://lists.torproject.org/pipermail/tor-dev/2019-May/013837.html +[2] https://trac.torproject.org/15516