Re: [tor-bugs] #21050 [Metrics/Onionoo]: Onionoo serves invalid cache headers

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#21050: Onionoo serves invalid cache headers
-+--
 Reporter:  lukechilds   |  Owner:  metrics-team
 Type:  defect   | Status:  closed
 Priority:  Medium   |  Milestone:
Component:  Metrics/Onionoo  |Version:
 Severity:  Normal   | Resolution:  wontfix
 Keywords:   |  Actual Points:
Parent ID:   | Points:
 Reviewer:   |Sponsor:
-+--
Changes (by karsten):

 * status:  new => closed
 * resolution:   => wontfix


Comment:

 I was able to reproduce this issue: I got responses with `max-age=2400`
 and an `Age` going up from `0` to `2405` depending on when I made the
 request.  After that last response I got a new response with `Age`
 starting at `0` again.

 But unless this breaks clients somehow, which I assume is not the case, I
 don't think this is worth investigating.  We only run into this case at
 most once every 5 minutes per requested resource and for all clients
 requesting that resource.  It's cheaper to provide more caching resources
 than hunt down and fix this bug.  It's not even clear that this is a bug
 in the Java part of Onionoo or in the specific Varnish configuration used
 here.  It might be a Varnish bug.  But who knows.

 I'm closing this ticket.  Feel free to re-open this ticket if this issue
 breaks clients.  And thanks for reporting anyway!

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Re: [tor-bugs] #21050 [Metrics/Onionoo]: Onionoo serves invalid cache headers

[Tor Bug Tracker & Wiki]

#21050: Onionoo serves invalid cache headers
-+--
 Reporter:  lukechilds   |  Owner:  metrics-team
 Type:  defect   | Status:  new
 Priority:  Medium   |  Milestone:
Component:  Metrics/Onionoo  |Version:
 Severity:  Normal   | Resolution:
 Keywords:   |  Actual Points:
Parent ID:   | Points:
 Reviewer:   |Sponsor:
-+--

Comment (by lukechilds):

 Sorry, where I said

 > the Date header wasn't updated so the max-age was calculated from an old
 date.

 I meant age not max-age

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[tor-bugs] #21050 [Metrics/Onionoo]: Onionoo serves invalid cache headers

[Tor Bug Tracker & Wiki]

#21050: Onionoo serves invalid cache headers
-+--
 Reporter:  lukechilds   |  Owner:  metrics-team
 Type:  defect   | Status:  new
 Priority:  Medium   |  Milestone:
Component:  Metrics/Onionoo  |Version:
 Severity:  Normal   |   Keywords:
Actual Points:   |  Parent ID:
   Points:   |   Reviewer:
  Sponsor:   |
-+--
 Onionoo serves invalid cache headers the first time you receive a 304. All
 304 responses after that are ok until the cache max-age is reached. Then
 the next 304 response will have invalid headers and after that it's ok
 again until the new max-age is reached etc.

 They are invalid because the Date header isn't reset once the max-age is
 reached and the Age header carries on incrementing. This results in an age
 that is higher than the max-age, which means the headers are saying the
 content has already expired before it's been sent.

 Example:

 

 Make a request at 09:10:32

 {{{
 Response code: 200
 Date: Wed, 21 Dec 2016 09:10:32 GMT
 Age: 0
 Cache-Control: public, max-age=300
 Last-Modified: Wed, 21 Dec 2016 08:23:36 GMT
 }}}

 AKA This content is from 09:10:32 (now) and you can cache it until
 09:15:32 (5 minutes)

 

 Now lets make a request at 09:15:41, it's been more than 5 minutes so
 we'll set the If-Modified-Since header to see if our cached version is
 still up to date

 {{{
 Response code: 304
 Date: Wed, 21 Dec 2016 09:10:32 GMT
 Age: 309
 Cache-Control: public, max-age=300
 Last-Modified: Wed, 21 Dec 2016 08:23:36 GMT
 }}}

 AKA This content is from 09:15:41 (now) and you can cache it until
 09:15:32 (9 seconds ago).

 So the headers are saying it was out of date 9 seconds before the response
 was sent. It wasn't, it's still fresh, it's just the Date header wasn't
 updated so the max-age was calculated from an old date.

 

 If we make another request with If-Modified-Since set at 09:15:54 (13
 seconds later) everything catches up:

 {{{
 Response code: 304
 Date: Wed, 21 Dec 2016 09:15:41 GMT
 Age: 13
 Cache-Control: public, max-age=300
 Last-Modified: Wed, 21 Dec 2016 08:23:36 GMT
 }}}

 AKA This content is from 09:15:54 (now) and you can cache it until
 09:20:41 (4:47 minutes)

 Notice how the Date header is now correctly set as the date of our
 previous request with the age header calculated from that.

 

 This is technically not abiding by the HTTP protocol:

 https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html
 > 14.9 Cache-Control
 >
 > The Cache-Control general-header field is used to specify directives
 that MUST be obeyed by all caching mechanisms along the request/response
 chain.

 In the second request in the example above the max-age in the Cache-
 Control headers is 300 and the age is 309 which means that Onionoos
 caching mechanism isn't obeying the Cache-Control header.

 The end result of this is that the first time a 304 response is sent it
 will never be cached by a client even though the content is still fresh.
 In the above example 3 HTTP requests needed to be made, if the headers
 were set correctly only 2 requests would have been required. Fixing this
 should reduce the amount of traffic that hits the Onionoo server.

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