nginx - Proxy to a different backend based on query param

Question

I've got a case where I want to proxy a particular call down to a different backend based on the existence of a query param. The following is sort of what I start with

location ~ ^/abc/xyz/?$ {
    proxy_pass $backend_url;
 }

What I'd like to do is check for a query param foo (or even just the existence of that string anywhere). So I thought I could do this

location ~ ^/abc/xyz/?$ {
    set $backend_url "somelocation"

    if ($request_url ~ .*foo.*) {
       set $backend_url "someotherlocation"
       proxy_pass $backend_url
    }
    proxy_pass $backend_url;
 }

But this doesn't seem to actually proxy to the new location. Am I doing something wrong with my code, or is the whole approach wrong?

Ivan Shatsky · Accepted Answer · 2021-06-23 08:48:11Z

1

I don't know why are you using two proxy_pass directives, this block should do it in a logic you described:

location ~ ^/abc/xyz/?$ {
    set $backend_url "somelocation";
    if ($request_url ~ \?(.*&)?foo(=|&|$)) {
       set $backend_url "otherlocation";
    }
    proxy_pass $backend_url;
 }

I slightly modified your regex to match only request URLs where foo is a query argument name and not a query argument value (or its part). However I'd rather use map directive for this purpose (map block should be placed outside the server block):

map $arg_foo $backend_url {
    ""      somelocation;  # if the value is empty
    default otherlocation; # otherwise
}

server {
    ...
    location ~ ^/abc/xyz/?$ {
        proxy_pass $backend_url;
    }
}

Pay an attention you may need to define a resolver for this configuration to work (some additional info about this can be found here).

answered Jun 23, 2021 at 8:48

Ivan Shatsky

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4 Comments

Ren Over a year ago

Thanks! I put th proxy_pass in there because in the real version of the code, there's a bit more stuff modifying the backend_url (some Lua module). But it might not be needed.

Ren Over a year ago

Re: your second suggestion - does arg_foo do something special (ie. Extract a query param named foo) or do I need to something to populate that?

Ivan Shatsky Over a year ago

Yes, you are right, $arg_name are special nginx variables to extract according query arguments (here is the description). About your first comment - actually every if block is a virtual nested location that's trying to inherit all available directives from the previous level (sometimes including even those than are not normally inherited), so it isn't make a big difference.

Ivan Shatsky Over a year ago

if directive behaviour sometimes extremely unclear. An excellent explanation on how an if location works is made by Yichun Zhang, an author of lua-nginx-module and the OpenResty nginx fork: How nginx "location if" works

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