Nginx proxy_pass doesn't always work when using a variable for the host

Question

I have 2 location blocks with proxy_pass configs, they look in a simplified form like this:

location ^~ /location1/ {
  set $backend1 https://service1.example.com;
  proxy_pass $backend1;
}

location ^~ /location2/ {
  rewrite /location2/(.*) /new/base/path/$1 break;
  set $backend2 https://service2.example.com;
  proxy_pass $backend2;
}

Now the problem is location1 works fine, but location2 doesn't, it returns 500, and I get this error in the logs

2022/09/26 18:07:42 [error] 25952#25952: *263 invalid URL prefix in "", client: 123.123.123.123, server: www.example.com, request: "GET /location2/my/url HTTP/1.1", host: "www.example.com", referrer: "https://www.example.com/referrer/page"

If I set the value with a variable, like this:

location ^~ /location2/ {
  rewrite /location2/(.*) /new/base/path/$1 break;
  proxy_pass https://service2.example.com;
}

then it works. I need to use a variable though, because it helps forcing nginx re-resolve the DNS (as mentioned in this solution)

What I noticed from the error log line is that the variable $backend2 is empty, which gets clearer if I define it like this

set $backend2 service2.example.com;
proxy_pass https://$backend2;

which will out put an error like this instead:

2022/09/26 18:07:42 [error] 25952#25952: *263 invalid URL prefix in "https://", client: 123.123.123.123, server: www.example.com, request: "GET /location2/my/url HTTP/1.1", host: "www.example.com", referrer: "https://www.example.com/referrer/page"

So what could I be missing?

MMSs · Accepted Answer · 2022-09-30T16:04:50.603

I finally found the solution for this problem, that I only realised after posting it.

The problem for me was defining the variable in the location block. Defining it outside the location block was sufficient to resolve the issue.

set $backend2 https://service2.example.com;
location ^~ /location2/ {
  rewrite /location2/(.*) /new/base/path/$1 break;
  proxy_pass $backend2;
}

No clue how did it work in other location blocks though.

Nginx proxy_pass doesn't always work when using a variable for the host

1 Answers1