Possible to set proxy_pass as a variable derived from the query_string or header param NGINX proxy?

Question

I have a situation where I'd like to proxy_pass to a backend server, with the url for the server being dynamic and derived from either a url param value or from a header param value. Not sure that is even possible, still a bit of a novice with NGINX proxies.

I had thought that $arg_name was the generic way to get the value for a query string parameter in NGINX, so if I have a URL like ....?proxy=/proxy1, then $arg_proxy should have the value of "/proxy1" ?

What I am aiming for is something like this:

set $proxy  "http:/${arg_proxy}:8042";

location /backend/ {

    proxy_pass $proxy;
    rewrite /backend(.*) $1 break;
    proxy_set_header Host $http_host;
    proxy_request_buffering off;
    proxy_max_temp_file_size 0;
    client_max_body_size 0;
}

I tried that, and a few other variations with settting the variable. It doesn't crash the server, but it is also acting like the value for $arg_proxy is blank, even though it is not, or isn't getting read.

invalid URL prefix in "http:/:8042"

It would be nice to alternatively extract the proxy value from a header param, because there are cases where that would also be helfpul.

Addendum:

I tried something like this using a Cookie:

map $cookie_TEST $proxy_url {

volatile;
default     "";
"pacs-1"    "pacs-1";
"pacs-2"    "pacs-2";
"pacs-3"    "pacs-3";
}

upstream pacs-1 {

server pacs-1:8042;
keepalive 32;
keepalive_requests 1024;
keepalive_time 1h;
keepalive_timeout 600s;
}

upstream pacs-2 {

server pacs-2:8042;
keepalive 32;
keepalive_requests 1024;
keepalive_time 1h;
keepalive_timeout 600s;
}

upstream pacs-3 {

server pacs-3:8042;
keepalive 32;
keepalive_requests 1024;
keepalive_time 1h;
keepalive_timeout 600s;
}

But then in the proxy block:

    if ($proxy_url = 'pacs-1') {
        rewrite /orthanc(.*) $1 break;
        proxy_pass http://pacs-1;
    }
    if ($proxy_url = 'pacs-2') {
        rewrite /orthanc(.*) $1 break;
        proxy_pass http://pacs-2;
    }
    
    if ($proxy_url = 'pacs-3') {
        rewrite /orthanc(.*) $1 break;
        proxy_pass http://pacs-3;
    }

Seems like there should be a way without using if's, which is a bad idea really, but it seems to "work" so far with what I am trying.

SilentEntity · Accepted Answer · 2022-10-26 20:48:58Z

1

Args are uri specific , try using the below example:

location /search {
    set $proxxy "www.google.com";
    if ($args) {
        set $proxxy $arg_proxy;
    }
    if ($proxxy){
        proxy_pass https://$proxxy;
    }
    proxy_pass https://www.google.com;
}

one can't access access args independent of the request.

answered Oct 26, 2022 at 20:48

SilentEntity

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1 Comment

SScotti Over a year ago

Yeah. I think it might have actually been working, for the first request, but not for subsequent ones because the query string param isn't there for .css, .js, subsequent requests, although the referrer has it. If it possible to grab something from my session (Using PHP), that would be better actually. Is that possible, I guess via a Cookie ?

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