Why does location matching change when using proxy_pass in nginx?

Question

Simple nginx config

server {
  listen 0.0.0.0:80;

  server_name localhost 127.0.0.1;

  location /en-us/ {
    return 301 http://www.lego.com;
  }
}

Here's what happens when I curl that config without a trailing slash on /en-us.

curl -v http://127.0.0.1/en-us -o /dev/null
# HTTP/1.1 404 Not Found

I expect that 404. The location block is using a prefix match, and so it makes sense to me that /en-us is not matching that block.

Here's where I get confused...

Same config, but with a proxy_pass

Here I use the same config, but instead of returning a 301 I do a proxy_pass.

server {
  listen 0.0.0.0:80;

  server_name localhost 127.0.0.1;

  location /en-us/ {
    proxy_pass http://www.lego.com;
  }
}

Now watch what happens when I run the same curl.

curl -v http://127.0.0.1/en-us -o /dev/null
# HTTP/1.1 301 Moved Permanently

That 301 on /en-us makes no sense to me. It's redirecting to Location: http://127.0.0.1/en-us/.

In the first config, /en-us seemed to not match my location block (404). In the second config, /en-us did (301).

Why do the same location rules seem to match differently between my two configs?
Why would that second config result in a 301 to add a trailing slash?

I'm using nginx version: nginx/1.4.6 (Ubuntu)

That explains it perfectly. Thank you! "If a location is defined by a prefix string that ends with the slash character, and requests are processed by one of proxy_pass, fastcgi_pass, ... then the special processing is performed ... a permanent redirect with the code 301 will be returned to the requested URI with the slash appended ..." — Will Haley
– Will Haley, Commented Mar 25, 2016 at 0:49

Will Haley · Accepted Answer · 2017-11-14 05:00:24Z

0

The documentation explains things clearly, as is pointed out in the comments.

If a location is defined by a prefix string that ends with the slash character, and requests are processed by one of proxy_pass, fastcgi_pass, ... then the special processing is performed ... a permanent redirect with the code 301 will be returned to the requested URI with the slash appended ...

Special processing is done for trailing slashes on a proxy_pass

answered Nov 14, 2017 at 5:00

Will Haley

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