nginx rewrite should be a loop

Question

Is chatGPT bullsh!ttn me, or am I missing something obvious?

This is my question:

I have an nginx location block like this:

    location / {
      location ~ \.html {
        # First send it back to the start with a changed format
        rewrite /(.*) /html/$1  break;
        # Then proxy that to the S3
        proxy_pass https://MYS3.s3.eu-west-1.amazonaws.com;
      }
    }

It should match locations ending in .html and rewrite them to have the request_uri prepended with /html/ - then proxy that request to the s3 bucket.

This works, but I don't understand why. Why would the rewritten url with the prepended /html/ not also be rewritten again and again in a loop by the same directive?

ChatGPT answered with this:

The rewrite directive in nginx is processed in a specific order within the location block.

When a rewrite is executed, the modified URL is processed again from the beginning of the location block. However, the request goes through the location block only once per request.

In your example, the request URI is rewritten in the first rewrite directive to prepend /html/ to the original URI. Then, the newly modified URI is matched against the location block again. However, this time it doesn't match the location ~ .html directive because the modified URI now begins with /html/.

Therefore, the execution flow moves forward to the proxy_pass directive and the request is proxied to the specified S3 bucket without entering into an infinite loop of rewrites.

In summary, the rewritten URL is not further rewritten because it matches a different location block within the same location context.

I just don't understand why the rewrite directive would not happen every time, since the new URI still has '.html' in it? And chatGPT is confusing me with the idea that

The newly modified URI is matched against the location block again ... this time it doesn't match the location ~ .html directive

Can't both match and not match?

Anyone here who can shine a light for me :-)

Answer 1

You have break flag in your rewrite directive. Documentation says that it work as the break directive:

Stops processing the current set of ngx_http_rewrite_module directives.

If a directive is specified inside the location, further processing of the request continues in this location.

Let's assume the request url was /some/file.html. So, first nginx rewrites it to /html/some/file.html and then act as if there was a break directive. That means nginx stop executing any ngx_http_rewrite_module directive (though there is no any in your case) and process the request in current location, that means it will use proxy_pass and will not not try to look for new location (that is the part where ChatGPT is wrong).