On 06/03/2012 23:28, Maxim Dounin wrote:
> Hello!
>
> On Tue, Mar 06, 2012 at 10:32:56PM +0000, Ed W wrote:
>
>> On 06/03/2012 17:47, Maxim Dounin wrote:
>>>> I can't see that it's possible to use something like:
>>>>
>>>> location / {
>>>> proxy_pass http://127.0.0.1:3000;
>>>> error_page 502 @502;
>>>> }
>>>>
>>>> location @502 {
>>>> index /502.html;
>>>> root /var/www;
>>>> internal;
>>>> }
>>> Try something like:
>>>
>>> location @502 {
>>> internal;
>>> root /var/www;
>>> rewrite ^ /502.html break;
>>> }
>>>
>>> This should work at least for GET requests.
>> Perfect - thanks!
>>
>>
>> Now, I have thought about this a little while and I don't quite
>> understand how it works...?
>>
>> The key seems to be the rewrite clause - near as I can see, without
>> this the error request gets reprocessed by the "location /" section,
>> but the rewrite is causing it to terminate that?
>>
>> Can you teach me to understand this please?
> Under normal conditions
>
> location @502 {
> root /var/www;
> }
>
> will behave much like normal static location, as described here:
>
> http://nginx.org/en/docs/http/request_processing.html
>
> That is, request to a "/file", if it happens to hit his location,
> will be processed as follows:
>
> 1. It doesn't ends with '/', hence nginx will try to open a file
> for it,<root> +<uri>. I.e. "/var/www/file" will be opened.
>
> 2. The (1) will likely fail (as there is no such file) and will
> generate 404.
>
> More importantly, request to a "/" will be processed as
> follows:
>
> 1. It ends with "/", so nginx will try to use index module to
> handle it.
>
> 2. Index module will test<root> +<uri> +<index> file, i.e.
> "/var/www/502.html" with your config. If it's found - nginx will
> do internal redirect to "/502.html".
>
> 3. The internal redirect will end up in "location /". This is
> obviously not what you want.
>
> With "rewrite ^ /502.html break;" the following two important
> things happen:

Hi, Thanks for this REALLY clear post. I wonder if there is somewhere
sensible to record this on the wiki - it's very incisive on a subtle
subject!

Just to confirm my understanding, the point 2) above seems to the key -
ie the "index" module is doing a "redirect" *without* a break, hence
adding the explicit redirect WITH a break is what we need to prevent the
cycle. This fits with my observations (of course) because when I added
a location matching "location /502.html" this also prevents the cycle

Thanks for investing quite some time answering this!


Note, the background to this is that I want to bounce any incoming url
to "http://portal/login?redirect_to=$encoded_incoming_url". I have
tried to achieve this using only core nginx, but I can't handle the
corner cases such as incoming url having multiple parameters. I have
worked around using my cgi to generate the redirect. *IF* you believe
that this should be possible using only nginx I would be interested to
re-ask that question - however, I think you are going to confirm that
it's not currently possible?

Many thanks again!

Ed W

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