[tor-bugs] #20132 [Core Tor/Tor]: Let large client deployments use a local directory cache

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#20132: Let large client deployments use a local directory cache
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     Reporter:  teor          |      Owner:
         Type:  enhancement   |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.2.???
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-hs, tor2web, rsos
Actual Points:                |  Parent ID:
       Points:  1             |   Reviewer:
      Sponsor:                |
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 One of the things that concerns me about large tor client farms is that
 they download a ~1.5MB consensus per client per hour.

 This is a particular concern with large deployments of bridges, hidden
 services (particularly with OnionBalance and/or single onion services),
 and Tor2web.

 One way to work around this issue is to set up a number of local Tor
 directory caches (unadvertised relays) on the machines hosting the Tor
 client instances. Then the clients can be told to use these directory
 caches to retrieve their directory documents.

 Ideally, each client should be configured with a few caches in the same
 data center, just in case one goes down.

 It would really help to have a client option for this in Tor, but there is
 a tradeoff - compromise that relay, and you own all the clients.

 For Tor2web and Single Onion Services, this almost works already using
 EntryNodes, but we disable EntryGuards in order to turn off path bias
 detection. Also, Single Onion Services use 3-hop paths for HSDir posts,
 and we want Tor2web to use 3-hop paths for HSDir fetches to avoid denial
 of service (#20104).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/20132>
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