Hello!
On Thu, Mar 11, 2010 at 09:17:03AM -0600, Ryan Malayter wrote:
> I'm trying to add Cache-Control headers to an ill-behaved backend
> application (that I don't control). For many requests, the backend
> sets Cache-Control headers with an empty value (an RFC violation, I
> know, and a bug report has been filed, but resolution is likely going
> to take months from the vendor). Nginx 0.7.62 using default Ubuntu
> package from Karmic.
>
> I need to avoid using the "more Headers" module if at all possible for
> administrative reasons (standard packages are much easier to deal with
> administratively and make life much easier during audits).
>
> So, I am trying to use proxy_hide_header and variables to
> conditionally set the header. The problem is that using
> $upstream_http_cache_control in a "set" or "if" statement always seems
> to evaluate to the empty string. However, I can add a customer header
> which shows the value passed from the upstream correctly if is not
> empty.
>
> Here is the relevant portion of my config (I have tried many variants
> of positive and negative logic, without success):
>
> location / {
> proxy_pass http://backend;
> proxy_set_header Host $host;
> proxy_read_timeout 900;
> proxy_redirect default;
> proxy_hide_header Pragma;
> proxy_hide_header Cache-Control;
>
> set $mycc $upstream_http_cache_control;
> if ($mycc = "") {
> set $mycc "no-cache";
> }
As rewrite module directives ("if", "set") are executed during
rewrite phase of request processing it will always see empty value
of $upstream_http_cache_control variable (it's not yet available
as no headers got from upstream yet).
> add_header "Cache-Control" $mycc;
> add_header "X-Upstream-Cache-Control" $upstream_http_cache_control;
> add_header "X-Upstream-Expires" $upstream_http_expires;
> }
>
> With this configuration, I always get "Cache-Control: no-cache"
> headers, even when "X-Upstream-Cache-Control" shows that the upstream
> did pass a non-empty Cache-Control header. An Example response:
This is expected behaviour.
[...]
> Is there any way do do this using default nginx modules?
You may try to do this with embedded perl, specifically perl_set.
Something like this should work:
perl_set $mycc 'sub {
return shift->variable("upstream_http_cache_control") || "no-cache";
}';
Maxim Dounin
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