Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:  nickm
 Type:  defect   | Status:  closed
 Priority:  Very High|  Milestone:  Tor:
 |  0.2.9.x-final
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:  fixed
 Keywords:  029-proposed, dns, 029-backport  |  Actual Points:  .2
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-
Changes (by nickm):

 * status:  merge_ready => closed
 * resolution:   => fixed


Comment:

 This has baked long enough without problems; backporting to 0.2.9

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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:  nickm
 Type:  defect   | Status:
 |  merge_ready
 Priority:  Very High|  Milestone:  Tor:
 |  0.2.9.x-final
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:
 Keywords:  029-proposed, dns,   |  Actual Points:  .2
  TorCoreTeam201609, 029-backport|
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-
Changes (by nickm):

 * keywords:  029-proposed, dns, TorCoreTeam201609, 029-backport, review-
 group-15 => 029-proposed, dns, TorCoreTeam201609, 029-backport
 * milestone:  Tor: 0.3.0.x-final => Tor: 0.2.9.x-final


Comment:

 Squashed, with phw's fix for #19025, as `bug19769_19025_029`.

 Merged `bug19769_19025_029` to master; possible 0.2.9 backport.

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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:  nickm
 Type:  defect   | Status:
 |  merge_ready
 Priority:  Very High|  Milestone:  Tor:
 |  0.3.0.x-final
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:
 Keywords:  029-proposed, dns,   |  Actual Points:  .2
  TorCoreTeam201609, 029-backport|
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-
Changes (by nickm):

 * status:  needs_review => merge_ready


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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:  nickm
 Type:  defect   | Status:
 |  needs_review
 Priority:  Very High|  Milestone:  Tor:
 |  0.3.0.x-final
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:
 Keywords:  029-proposed, dns,   |  Actual Points:  .2
  TorCoreTeam201609, 029-backport|
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-

Comment (by pulls):

 Looks good to me, sorry for the delay.

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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:  nickm
 Type:  defect   | Status:
 |  needs_review
 Priority:  Very High|  Milestone:  Tor:
 |  0.3.0.x-final
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:
 Keywords:  029-proposed, dns,   |  Actual Points:  .2
  TorCoreTeam201609, 029-backport|
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-
Changes (by nickm):

 * status:  accepted => needs_review


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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:  nickm
 Type:  defect   | Status:
 |  accepted
 Priority:  Very High|  Milestone:  Tor:
 |  0.3.0.x-final
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:
 Keywords:  029-proposed, dns,   |  Actual Points:  .2
  TorCoreTeam201609, 029-backport|
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-
Changes (by nickm):

 * owner:   => nickm
 * status:  needs_review => accepted


Comment:

 setting owner

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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:
 Type:  defect   | Status:
 |  needs_review
 Priority:  Very High|  Milestone:  Tor:
 |  0.3.0.x-final
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:
 Keywords:  029-proposed, dns,   |  Actual Points:  .2
  TorCoreTeam201609, 029-backport|
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-

Comment (by nickm):

 My `bug19769_029` branch has been updated, based on feedback and
 corrections from pulls. Please review?

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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:
 Type:  defect   | Status:
 |  needs_review
 Priority:  Very High|  Milestone:  Tor:
 |  0.3.0.x-final
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:
 Keywords:  029-proposed, dns,   |  Actual Points:  .2
  TorCoreTeam201609, 029-backport|
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-
Changes (by nickm):

 * keywords:  029-proposed, dns, TorCoreTeam201609 => 029-proposed, dns,
 TorCoreTeam201609, 029-backport


Comment:

 This is potentially backportable to 0.2.9.

 Potential enhancement: these values could be consensus parameters rather
 than hardcoded #defines.

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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:
 Type:  defect   | Status:
 |  needs_review
 Priority:  Very High|  Milestone:  Tor:
 |  0.3.0.x-final
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:
 Keywords:  029-proposed, dns,   |  Actual Points:  .2
  TorCoreTeam201609  |
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-

Comment (by nickm):

 When we merge this, we should also merge the patch from #19025.

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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:
 Type:  defect   | Status:
 |  needs_review
 Priority:  Very High|  Milestone:  Tor:
 |  0.3.0.x-final
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:
 Keywords:  029-proposed, dns,   |  Actual Points:  .2
  TorCoreTeam201609  |
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-
Changes (by nickm):

 * status:  needs_information => needs_review
 * actualpoints:   => .2


Comment:

 My branch `bug19769_029` implements this.

 phw, pulls: Is this about what you had in mind?  I took some liberties and
 might have messed things up.

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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:
 Type:  defect   | Status:
 |  needs_information
 Priority:  Very High|  Milestone:  Tor:
 |  0.3.0.x-final
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:
 Keywords:  029-proposed, dns,   |  Actual Points:
  TorCoreTeam201609  |
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-

Comment (by nicoo):

 Since pulls asked for feedback from exit operators, here is some based on
 my experience with [https://nos-oignons.net/ Nos oignons].

 Our configuration is [https://nos-oignons.net/wiki-
 admin/Services/DNS/Resolver/ publicly documented], but in French, so here
 is a summary:

 * We use Unbound as a local, DNSSEC-validating resolver on the exit nodes.
   * It obviously only listens locally.
   * We use its `private-address` feature to prevent RFC1918 addresses from
 figuring in results, to mitigate DNS rebinding attacks.
   * We use `hide-{identity,version}`, mostly out of general principle:
 anybody reading our documentation would learn that we run Unbound;
 however, it's unclear to me whether those could be exploited to tie users
 to specific exits being used for DNS resolution (and if that's relevant).
   * We use `harden-short-bufsize` and `harden-large-queries` to make
 Unbound return `SERVFAIL` on edge cases that can be exploited for DoSing
 the resolver.
   * We forward queries for `nos-oignons.{net,org,fr}` directly to our
 authoritative resolver.  This is not especially relevant for the exit, but
 error logs mails and so on will break if the domain fails to resolve.
 * `/etc/resolv.conf` always specifies `search nos-oignons.net` (does
 little-t tor honor that?  that could be awkward) and `127.0.0.1` as the
 first nameserver.
   If a fallback resolver is specified, it is either operated by the
 network hosting the exit node or by a close-by (network-wise) organization
 we have friendly ties to (typically, a non-profit, associative ISP).

 While writing this, I'm realising it might be useful to have “DNS
 resolution best-practices” for exit operators, since this is mostly
 something ''adhoc'' we came up with based on what our sysadmins were doing
 in other places, not something we systematically researched.

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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:
 Type:  defect   | Status:
 |  needs_information
 Priority:  Very High|  Milestone:  Tor:
 |  0.3.0.x-final
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:
 Keywords:  029-proposed, dns,   |  Actual Points:
  TorCoreTeam201609  |
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-
Changes (by nicoo):

 * cc: nicoo (added)


Comment:

 Regarding MIN_DNS_TTL, Microsoft Windows used (still does?) to have a
 minimum TTL of 15 minutes for it's client-side cache, IIRC.
 Given how prevalent that platform is, I guess a 1 minute MIN_DNS_TTL is
 very unlikely to break things.

 Are there plans to allow more values than {MIN,MAX}_DNS_TTL?

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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:
 Type:  defect   | Status:
 |  needs_information
 Priority:  Very High|  Milestone:  Tor:
 |  0.3.0.x-final
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:
 Keywords:  029-proposed, dns,   |  Actual Points:
  TorCoreTeam201609  |
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-

Comment (by pulls):

 == Concretely ==


 {{{
 MIN_DNS_TTL = 5*60;
 MAX_DNS_TTL = 60*60;

 dns_clip_ttl(uint32_t ttl)
 {
   if (ttl < MIN_DNS_TTL)
 return MIN_DNS_TTL;
   else
 return MAX_DNS_TTL;
 }
 }}}


 - Fix #19025 (otherwise ttl above will always be MIN_DNS_TTL).
 - Potentially refactor the DNS caching code to support evictions (while
 doing this, maybe rip out all old client-side DNS caching code?).
 - Add some form of logging to track cache size, usage, and eviction rate.
 - dns_get_expiry_ttl should be the same as dns_clip_ttl above. Please note
 that we are not sure these are the only relevant functions.


 == Given ==

 - For popular websites, caching at exits is highly likely, and DefecTor
 attacks are the same as WF attacks.
 - For unpopular websites, caching and TTLs are moot, since the probability
 of an DNS record being chached is negligible. Caching these records are
 just an extra burden on the exit and in a sense also a risk due to leaking
 recent activity at the exit on compromise. DefecTor attacks will be more
 precise than WF attacks here, and Tor needs WF defenses to mitigate
 (another long-term topic).
 - For long TTLs, for what it is worth, we know from #19025 that the real-
 world impact of ignoring these long TTLs are not a serious issue.
 - For short TTLs, the impact of increasing it is our primary worry since
 we might break something.
 - We want to prevent fine-grained TTLs to protect against tagging attacks.
 - We do not want too high TTLs to have a chance to auto-magically resolve
 DNS cache poisoning.
 - The total size of the cache might be a vector for DoS.
 - The client-side DNS cache remains off.


 == Goals for DefectTor mitigation ==

 - Allow long TTLs to be long(er).
 - For short TTLs, go as far up as we are comfortable to without
 significantly risking breaking things.


 == Proposal ==

 Stage changes: start with repairing the TTL bug #19025 and change clipping
 to 5*60 seconds for MIN_DNS_TTL and 60*60 seconds for MAX_DNS_TTL,
 honoring no intermediate values (see code above). Wait for feedback on
 unexpected breaks. If all manageable, increase MIN_DNS_TTL to 60*60 in a
 future patch, effectively always caching for 60 minutes. If DefecTor
 attacks become a real concern short-term, encourage concerned site owners
 to consider longer TTLs to hit the MAX_DNS_TTL value. Make the cache size
 limited and eviction when full uniformly random. We random to give an
 attacker less control since it can presumably cause evictions at will
 (LIFO is easy to manipulate for an attacker).

 Getting feedback on real-world cache size, usage, and eviction rate from
 exit operators would be useful, so perhaps some form of log output is
 reasonable?

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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:
 Type:  defect   | Status:
 |  needs_information
 Priority:  Medium   |  Milestone:  Tor:
 |  0.3.0.x-final
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:
 Keywords:  029-proposed, dns,   |  Actual Points:
  TorCoreTeam201609  |
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-

Comment (by nickm):

 Throwing this and #19025 into 0.3.0 : see
 https://lists.torproject.org/pipermail/tor-talk/2016-October/042445.html

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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:
 Type:  defect   | Status:
 |  needs_information
 Priority:  Medium   |  Milestone:  Tor:
 |  0.3.0.x-final
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:
 Keywords:  029-proposed, dns,   |  Actual Points:
  TorCoreTeam201609  |
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-
Changes (by nickm):

 * milestone:  Tor: 0.2.??? => Tor: 0.3.0.x-final


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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:
 Type:  defect   | Status:
 |  needs_information
 Priority:  Medium   |  Milestone:  Tor:
 |  0.2.???
Component:  Core Tor/Tor |Version:
 Severity:  Normal   | Resolution:
 Keywords:  029-proposed, dns,   |  Actual Points:
  TorCoreTeam201609  |
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-
Changes (by teor):

 * status:  new => needs_information


Comment:

 It would be nice to fix this, but we need to decide what to do first.

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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:
 Type:  defect   | Status:  new
 Priority:  Medium   |  Milestone:  Tor:
Component:  Core Tor/Tor |  0.2.???
 Severity:  Normal   |Version:
 Keywords:  029-proposed, dns,   | Resolution:
  TorCoreTeam201607  |  Actual Points:
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-
Changes (by pulls):

 * cc: pulls (added)


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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:
 Type:  defect   | Status:  new
 Priority:  Medium   |  Milestone:  Tor:
Component:  Core Tor/Tor |  0.2.???
 Severity:  Normal   |Version:
 Keywords:  029-proposed, dns,   | Resolution:
  TorCoreTeam201607  |  Actual Points:
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-
Changes (by phw):

 * cc: phw (added)


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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:
 Type:  defect   | Status:  new
 Priority:  Medium   |  Milestone:  Tor:
Component:  Core Tor/Tor |  0.2.???
 Severity:  Normal   |Version:
 Keywords:  029-proposed, dns,   | Resolution:
  TorCoreTeam201607  |  Actual Points:
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-

Comment (by pulls):

 We have ongoing research on DNS-based traffic correlation attacks
 (https://nymity.ch/dns-traffic-correlation/) that relates to this. While
 fixing #19025 will help in mitigating attacks to an extent, the most
 important change to consider related to DNS is to also significantly
 increase MIN_DNS_TTL. This is because useful domains for our attacks today
 have low TTLs: about 50% of Alexa top 1M have a useful domain with TTL <=
 60 seconds, and 75% a TTL <= 30 min. Do you think it would be practical to
 have MIN_DNS_TTL set to, say, 30 min? Would too much break?

 If I understand the proposal here in #19769, rounding TTLs between
 [0s,30m) to MIN_DNS_TTL also for exits (?), then this will actually
 benefit an attacker who can observe both entry traffic and DNS requests
 for about 25% of Alexa top 1M (but for the remaining 25% it's an
 improvement together with #19025 over the status quo).

 Sorry if this is the wrong place for this, especially since we don't have
 a paper to share yet.

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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:
 Type:  defect   | Status:  new
 Priority:  Medium   |  Milestone:  Tor:
Component:  Core Tor/Tor |  0.2.???
 Severity:  Normal   |Version:
 Keywords:  029-proposed, dns,   | Resolution:
  TorCoreTeam201607  |  Actual Points:
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-

Comment (by nickm):

 So, clients don't do DNS cacheing by default any more, because of risks
 like this.  Do you think it might make more sense to simply remove client-
 side DNS cacheing entirely?

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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
-+-
 Reporter:  teor |  Owner:
 Type:  defect   | Status:  new
 Priority:  Medium   |  Milestone:  Tor:
Component:  Core Tor/Tor |  0.2.???
 Severity:  Normal   |Version:
 Keywords:  029-proposed, dns,   | Resolution:
  TorCoreTeam201607  |  Actual Points:
Parent ID:   | Points:  0.5
 Reviewer:   |Sponsor:
-+-
Changes (by teor):

 * keywords:  029-proposed => 029-proposed, dns, TorCoreTeam201607


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Re: [tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
--+--
 Reporter:  teor  |  Owner:
 Type:  defect| Status:  new
 Priority:  Medium|  Milestone:  Tor: 0.2.???
Component:  Core Tor/Tor  |Version:
 Severity:  Normal| Resolution:
 Keywords:  029-proposed  |  Actual Points:
Parent ID:| Points:  0.5
 Reviewer:|Sponsor:
--+--

Comment (by teor):

 This would also require a change to torspec to describe the TTL rounding
 at:
 https://gitweb.torproject.org/torspec.git/tree/tor-spec.txt#n1359

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[tor-bugs] #19769 [Core Tor/Tor]: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)

[Tor Bug Tracker & Wiki]

#19769: Round down DNS TTL to the nearest DEFAULT_DNS_TTL (30 minutes)
--+--
 Reporter:  teor  |  Owner:
 Type:  defect| Status:  new
 Priority:  Medium|  Milestone:  Tor: 0.2.???
Component:  Core Tor/Tor  |Version:
 Severity:  Normal|   Keywords:  029-proposed
Actual Points:|  Parent ID:
   Points:  0.5   |   Reviewer:
  Sponsor:|
--+--
 In #19025, we fix a bug that prevented exits sending DNS TTLs to clients
 for IPv4 and IPv6 addresses.

 But we don't want to have too many potential values for these TTLs, to
 avoid tagging attacks.

 So I propose
 * Exits round down (truncate) the TTL received from the DNS server, and
 * Clients round down the TTL received from the Exit,
 to the nearest of:
 * MIN_DNS_TTL (1 minute), or
 * DEFAULT_DNS_TTL (30, 60, 90, 120, 150, 180 minutes)

 MAX_DNS_TTL is 3 hours, so there are only 7 possible values for the TTL.
 I chose to round down because that way, Tor DNS TTLs are only ever shorter
 than the lifetime specified by the DNS server.

 I don't think we need to add noise to the TTL received from either the DNS
 server or Exit. I can't see the value in randomising it, and allowing
 randomisation could hide a tagging attack.

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