Hello!

On Tue, Nov 12, 2013 at 07:55:28PM +0100, Jan Algermissen wrote:

> > Hello!
> >
> > On Tue, Nov 12, 2013 at 03:28:48PM +0100, Jan Algermissen wrote:
> >
> > > Maxim,
> > >
> > > a while ago you replied to my question below.
> > >
> > > Since yesterday I am trying to get hold of the proxy_host
> > > variable set by the proxy module but without success, maybe you
> > > can help a little with the code.
> > >
> > > From the source I understood that the proxy module sets the
> > > proxy_host var during NGINX startup. I guess, that happens on a
> > > per-location basis, depending on encountering a proxy_pass
> > > directive.
> > >
> > > Correct?
> >
> > Not exactly. Variables only exists during requests processing.
> > That is, the $proxy_host variable isn't set during nginx startup,
> > but instead its value becomes known when proxy module starts
> > working with a request.
>
> Ok. Sorry - I saw in the proxy code that
> ngx_http_proxy_set_vars() is called inside the proxy_pass
> directive handler. So I assumed they get set on startup.

It's called to cache appropriate values in location a
configuration (plcf->vars) if there are no variables in
proxy_pass. These values are later used to initialize run-time
data in ctx->vars once proxy starts handling a request (again, if
there are no variables in proxy_pass).

> > > I should then be able to access the proxy_host variable during
> > > the authentication phase (this is where my module sits).
> > >
> > > Correct?
> >
> > No, see above. The $proxy_host variable value is not known till
> > proxy started to work (and, if variables are used in proxy_pass,
> > evaluated its parameter).
>
> But the value of the proxy_host will never change after startup,
> is that right? I mean, it will always be the value of the
> proxy_host directve, or? At least when I log the var, it always
> is.

In a simple configuration with proxy_pass without variables - it's
always the same in a given location, yes. It can be anything in a
configuration like

proxy_pass $backend;

though.

> Anyhow, do you have any suggestion how I best go about adding a
> request header that is based on the proxy_host variable value.
>
> (Background: I need to change the Authorization header, so that
> request signature actually uses the right target host - see my
> original question)

Write a configuration like this:

proxy_pass http://backend.example.com;
proxy_set_header Authorization $your_module_variable;

In your variable's get handler obtain $proxy_host variable, do
needed calculations, and return a value you want to use in a
request to an upstream server.

--
Maxim Dounin
http://nginx.org/en/donation.html

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