body filter redirect

[Maxim Dounin]

Hello!

On Mon, Jul 12, 2021 at 12:47:46PM -0700, Dk Jack wrote:

> Hi,
> In my module, I am trying to take actions based on the content of the body.
> To accomplish this, I am capturing the body using body file as shown in the
> example below:
> 
> http://mdounin.ru/hg/ngx_http_catch_body_filter_module/file/tip/ngx_http_catch_body_filter_module.c
> 
> This is working well. I can inspect the contents of the body and take
> different actions like rejecting requests etc. However, one of the actions
> I want to do is redirect the request based on the body content i.e if the
> content contains some string etc, then redirect the request i.e change the
> upstream/location etc. My question is, is it even possible to redirect a
> request in the body filter stage? Is there another way to do body
> capture/filter and still be able to redirect the request after inspecting
> the body? Any help, suggestions or sample code is appreciated. Thanks.

It is generally too late to do any actions like "change the 
upstream/location etc" based on the body content, as the request 
body is usually read only when the location is already known and 
the upstream name is already obtained.  Also consider 
"proxy_request_buffering off;" - reading the request body implies 
that nginx is sending the request body to the backend as well.  
That is, you can't really do more than modifying the request body 
or rejecting the request completely.

You may, however, provide some variables from your body request 
body filter - and, if it's possible in your particular 
configuration, use these variables to do some conditional 
processing.

Note thought that conditional processing needs to happen after the 
request body is read.  As such, it is usually quite limited, 
especially when using standard modules, since the request body is 
usually read last.

For example, with proxy the request body is read after the 
variables in the "proxy_pass" directive are evaluated, so even 
with "proxy_request_buffering on;" (the default) you cannot use 
the variables from your request body filter module to change the 
upstream.  You can, however, use it to control headers in the 
upstream request by using "proxy_set_header" with appropriate 
variables (again, assuming buffered request body reading - for 
obvious reasons this won't work with "proxy_request_buffering 
off;").

More flexibility can be achieved by reading the request body 
earlier - for example, you can do this from your own module, or 
with embedded scripting such as perl or njs.

-- 
Maxim Dounin
http://mdounin.ru/