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From Sudheer Vinukonda <sudheervinuko...@yahoo.com>
Subject Re: ERR_PROXY_DENIED on DELETE requests? (Grafana behind ATS)
Date Fri, 04 Nov 2016 20:16:52 GMT
I don't believe ATS caches responses to POST by default (there's a setting to override it if
someone needs it). Delete OTOH affects the cache as well as Origin.

I do agree with your view that (other than being slightly on the conservative side in terms
of being vulnerable against DOS attacks), there's no strong technical reason to not allow
DELETE by default. But, again, this is only a "default" and is modifiable. The awareness aspect
of it in terms of documentation and/or ease of identifying/troubleshooting etc may probably
need to be improved.

- Sudheer

> On Nov 4, 2016, at 12:05 PM, Jered Floyd <jered@convivian.com> wrote:
> 
> 
> How does this differ from POST requests?  POST is also an unsafe request method that
requires cache invalidation.
> 
> Can I not remove a popular object by executing a POST request against it just as easily
as a DELETE?
> 
> --Jered
> 
> ----- On Nov 4, 2016, at 2:38 PM, Bryan Call <bcall@apache.org> wrote:
> Yes, the main reason we block the DELETE method is to guard against a DOS attack.  Most
people use ATS as a reverse proxy and rely on it to cache responses.  It would be very easy
for someone to come along and remove popular objects in cache repeatedly and DOS the proxy
and origin servers.
> 
> I would have been open to allowing the DELETE method by default if origins normally send
back a non-200 response for DELETE method requests unless they are strictly allowed to handle
it, but that doesn’t seem to be the case.
> 
> -Bryan
> 
> 
> On Nov 3, 2016, at 7:47 PM, Sudheer Vinukonda <sudheervinukonda@yahoo.com> wrote:
> 
> My understanding is that the defaults chosen are defensive in nature in terms of shielding
the Origin servers. For example, blocking delete helps to protect Origin Servers from a possible
DOS attack, as otherwise each and every delete request must be proxied across and could bring
down the Origin.
> 
> - Sudheer
> 
> On Nov 3, 2016, at 7:21 PM, Jered Floyd <jered@convivian.com> wrote:
> 
> 
> Bryan,
> 
> Unfortunately, I don't think I know enough about the cache use case to be helpful here;
I use ATS solely as a reverse proxy.
> 
> It appears to me than many RESTful applications make use of the full suite of HTTP verbs.
 GET/POST/PUT/DELETE are the CRUD primitives, and need to be proxied from clients to origin
servers unhindered.  In the reverse-proxy use case, I don't see how cache modifications can
be an in-band operation, and thus must be relayed to a service endpoint specifically on the
proxy server.  How common is it for DELETE requests to be sent to ATS for cache control reasons
by the content owner, versus having items fall out due to expiry or LRU replacement?
> 
> RFC 7231 section 4.3.5 says, "The DELETE method requests that the origin server remove
the association between the target resource and its current functionality," and also "Responses
to the DELETE method are not cacheable. If a DELETE request passes through a cache that has
one or more stored responses for the effective request URI, those stored responses will be
invalidated (see Section 4.4 of [RFC7234])." 
> 
> I haven't gone through it and RFC 7234 in detail, but that seems pretty clear that client
requests should be proxied, and that the cache should be invalidated. (Perhaps only on a 2xx
response?)
> 
> As I said, I don't know how often DELETE is used for operational cache management so
this may not be practical, but it seems to me that blocking certain methods (PUT, DELETE,
etc) is wholly inappropriate in ATS' role as a proxy server.  Perhaps someone closer to the
game can comment? (Should this move to the dev list?)
> 
> --Jered
> 
> ----- On Nov 3, 2016, at 5:30 PM, Bryan Call <bcall@apache.org> wrote:
> The problem with not denying it by default is someone can come by and delete objects
out of the cache.  Do you have any ideas on making this better?  Unfortunately origins like
httpd will respond back with 200 responses on the DELETE methods by default (using php in
my test), so we can’t rely on the origins response code to know if/when to delete the cached
object.  Right now we don’t make sure the origin responses back with a 200 response before
we delete the object from cache.  Maybe that should be changed?  I am not an expert on webdav,
so any input would be helpful.
> 
> -Bryan
> 
> 
> 
> 
> On Nov 2, 2016, at 2:49 PM, Jered Floyd <jered@convivian.com> wrote:
> 
> 
> Sudheer,
> 
> Aha! 
> 
> Thank you; that also resolves a long-standing issue I've had with CalDAV entry modification.
 This is perhaps a questionable default...
> 
> --Jered
> 
> ----- On Nov 2, 2016, at 5:44 PM, Sudheer Vinukonda <sudheervinukonda@yahoo.com>
wrote:
> The default traffic server install blocks DELETE method from anywhere outside of the
localhost. 
> 
> You can modify it as needed for your env. 
> 
> https://docs.trafficserver.apache.org/en/latest/admin-guide/files/ip_allow.config.en.html?highlight=ip_allow#ip-allow-config
> 
> Thanks,
> 
> Sudheer
> 
> 
> 
> From: Jered Floyd <jered@convivian.com>
> To: users@trafficserver.apache.org 
> Sent: Wednesday, November 2, 2016 2:37 PM
> Subject: ERR_PROXY_DENIED on DELETE requests? (Grafana behind ATS)
> 
> 
> Hello fellow ATS users,
> 
> I just ran into a bit of a head-scratcher that I bet someone here knows the answer to.
> 
> I recently set up a Grafana install behind ATS 6.2.0, and have found that I can't delete
dashboards, un-star things, or anything else involving the DELETE verb.  When I access the
origin server directly there are no problems.  When going through ATS, the operation results
in a "403 Access Denied" in the error popup.
> 
> ATS logs show instances like:
> 1478122046.588 0 [my client IP] ERR_PROXY_DENIED/403 198 DELETE http://[my origin server]:3000/api/user/stars/dashboard/2
- DIRECT/- text/html
> 
> Why is ATS refusing to proxy these requests?
> 
> I'm going to go dig into the source right now but perhaps someone has a quicker answer?
> 
> Thanks,
> --Jered
> 
> 
> 
> 
> 
> 
> 

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