Mastering Nginx proxy_pass: How Slash Placement Affects URL Forwarding

1. Basic Rules

Assume the backend address is: http://127.0.0.1:8080/api/user/getById?id=123 Then:

1) Using Nginx forwarding, access via

http://127.0.0.1/api/user/getById?id=123

server {
    listen 80;
    server_name 127.0.0.1;

    location /api/ {
        proxy_pass http://127.0.0.1:8080;
    }
}

Directly using the original address works fine.

2) Using Nginx forwarding with a prefix, access via

http://127.0.0.1/test/api/user/getById?id=123

server {
    listen 80;
    server_name 127.0.0.1;

    location /test/ {
        proxy_pass http://127.0.0.1:8080/;
    }
}

The request adds a prefix, but the forwarded request must not contain the prefix; the trailing “/” is the key point.

2. Slash ("/") Cases Comparison

Using backend address http://127.0.0.1:8080/api/user/getById and front‑end address http://127.0.0.1/api/user/getById, the following combinations illustrate how the presence or absence of slashes in location and proxy_pass affect the final proxied URL.

1) Neither location nor proxy_pass ends with a slash

location /api {
    proxy_pass http://127.0.0.1:8080;
}

Resulting URL: http://127.0.0.1:8080/api/user/getById (correct).

2) Location ends with a slash, proxy_pass does not

location /api/ {
    proxy_pass http://127.0.0.1:8080;
}

Resulting URL: http://127.0.0.1:8080/api/user/getById (correct).

3) Location without slash, proxy_pass with trailing slash

location /api {
    proxy_pass http://127.0.0.1:8080/;
}

Resulting URL: http://127.0.0.1:8080//user/getById (incorrect – double slash).

4) Both location and proxy_pass end with a slash

location /api/ {
    proxy_pass http://127.0.0.1:8080/;
}

Resulting URL: http://127.0.0.1:8080/user/getById (incorrect – missing “api”).

5) Location without slash, proxy_pass includes “api”

location /api {
    proxy_pass http://127.0.0.1:8080/api;
}

Resulting URL: http://127.0.0.1:8080/api/user/getById (correct).

6) Location with slash, proxy_pass includes “api”

location /api/ {
    proxy_pass http://127.0.0.1:8080/api;
}

Resulting URL: http://127.0.0.1:8080/apiuser/getById (incorrect – missing slash).

7) Location without slash, proxy_pass includes “api/”

location /api {
    proxy_pass http://127.0.0.1:8080/api/;
}

Resulting URL: http://127.0.0.1:8080/api//user/getById (incorrect – double slash).

8) Location with slash, proxy_pass includes “api/”

location /api/ {
    proxy_pass http://127.0.0.1:8080/api/;
}

Resulting URL: http://127.0.0.1:8080/api/user/getById (correct).

In summary, the presence of trailing slashes in location and proxy_pass dramatically changes the forwarded URL; choose the configuration that matches your desired path structure.