Hello!

On Fri, Dec 18, 2020 at 06:37:57PM +0000, Jeffrey Knight wrote:

> Hello !
>
> I'm trying to get up a reverse proxy where my users can pass in a url of the form
>
> https://my.server.com?https://some.other.server.com
>
> and it'll proxy to it.
>
> It works perfectly with this configuration with the proxy_pass target hard coded:
>
> ```
> server {
> server_name my.server.com;
>
> listen [::]:443 ssl ipv6only=on; # managed by Certbot
> listen 443 ssl; # managed by Certbot
>
> location / {
> proxy_pass https://some.other.server.com;
> add_header Cache-Control "public, max-age=3";
>
> add_header 'Access-Control-Allow-Origin' "$http_origin";
> add_header 'X-Frame-Options' "ALLOW FROM $http_origin";
> add_header 'Access-Control-Allow-Credentials' 'true';
> add_header 'Vary' 'Origin';
> }
> }
> ```
>
> testing with:
> > curl -X POST https://my.server.com -H "Content-Type: application/json" -d "{\"id\": \"123\"}"
>
> But if I swap out the proxy_pass target with a variable, I'm getting a 502 Bad Gateway.
>
> ```
> server {
> server_name my.server.com;
>
> listen [::]:443 ssl ipv6only=on; # managed by Certbot
> listen 443 ssl; # managed by Certbot
>
> location / {
> proxy_pass $args;
> add_header Cache-Control "public, max-age=3";
>
> add_header 'Access-Control-Allow-Origin' "$http_origin";
> add_header 'X-Frame-Options' "ALLOW FROM $http_origin";
> add_header 'Access-Control-Allow-Credentials' 'true';
> add_header 'Vary' 'Origin';
> }
> }
> ```
>
> Testing with:
> > curl -X POST https://my.server.com?https://some.other.server.com -H "Content-Type: application/json" -d "{\"id\": \"123\"}"

The 502 error returned by nginx implies there is a relevant
message in the error log at the "error" level. What's in the
error log?

[...]

> [1] https://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_pass

I suspect the relevant quote from this link is:

: Parameter value can contain variables. In this case, if an address
: is specified as a domain name, the name is searched among the
: described server groups, and, if not found, is determined using a
: resolver.

And you don't have resolver defined in your configuration. But
the error log should know better.

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