How to Configure Nginx proxy_pass Slashes Correctly to Avoid Double‑Slash Errors

1. Basic Rules

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

1) Use Nginx forwarding and 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;
    }
}

Accessing the original address directly is very simple.

2) Use Nginx forwarding with a prefix and 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/;
    }
}

Here a prefix is added to the request, but the forwarded request must not contain the prefix, so the trailing "/" after the address needs to be removed.

2. Slash Cases Comparison

Using the service address http://127.0.0.1:8080/api/user/getById, the access address is http://127.0.0.1/api/user/getById. The following cases illustrate how the presence or absence of slashes in location and proxy_pass affect the final URL.

1) Neither location nor proxy_pass has a trailing slash

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

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

2) location has a trailing slash, proxy_pass does not

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

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

4) Both location and proxy_pass have trailing slashes

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

Resulting proxy URL: http://127.0.0.1:8080/user/getById (incorrect – missing "api" segment).

5) location without slash, proxy_pass adds "api"

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

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

6) location with slash, proxy_pass adds "api"

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

Resulting proxy URL: http://127.0.0.1:8080/apiuser/getById (incorrect – missing slash between "api" and "user").

7) location without slash, proxy_pass adds "api/"

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

Resulting proxy URL: http://127.0.0.1:8080/api//user/getById (incorrect – double slash causes backend authentication failures).

8) location with slash, proxy_pass adds "api/"

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

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

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