HEAD requests still have content-type when gzip on

[Stephen Dendtler]

When using proxy_pass for HEAD requests, combined with gzip being turned on, nginx is returning body content even when the backend returns no content. This is a bug as no content is supposed to be returned in a HEAD response.

We're consistently seeing 20 bytes of binary data being sent after the HEAD response headers in a separate packet, possibly corresponding to a gzip header for 0 bytes of content.

This seems to be confusing some proxies such as Apache when keepalives are enabled where it sends a HEAD request followed by a GET request. Apache is accepting the content returned on the HEAD request without applying un-gzip methods, and prepending it to the content returned by the GET request as it's part of the same TCP stream. While this is arguably a problem in Apache, if nginx's output conformed to the HTTP spec then this wouldn't be an issue.

Temporary workaround is to add this to the corresponding location definition:

if ($request_method = HEAD ) {

gzip off;

}

Sample gzip config:

gzip on;
gzip_min_length 1000;
gzip_buffers 12 32k;
gzip_http_version 1.0;
gzip_comp_level 6;
gzip_proxied any;
gzip_vary off;
gzip_types text/plain text/html text/css application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript;
gzip_disable "MSIE [1-6].(?!.*SV1)";

Sample request to replicate the issue:

HEAD / HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip, deflate
Connection: Keep-Alive

Headers from the upstream server will be returned (expected), along with 20 bytes of binary data (bug).

[Valentin V. Bartenev]

This patch should fix the problem:

src/http/modules/ngx_http_gzip_filter_module.c

@@ -251 +251 @@
             && r->headers_out.content_encoding->value.len)
         || (r->headers_out.content_length_n != -1
             && r->headers_out.content_length_n < conf->min_length)
-        || ngx_http_test_content_type(r, &conf->types) == NULL
-        || r->header_only)
+        || ngx_http_test_content_type(r, &conf->types) == NULL)
     {
         return ngx_http_next_header_filter(r);
     }
@@ -319 +318 @@
     ngx_chain_t          *cl;
     ngx_http_gzip_ctx_t  *ctx;
 
+    if (r->header_only) {
+        return ngx_http_next_body_filter(r, in);
+    }
+
     ctx = ngx_http_get_module_ctx(r, ngx_http_gzip_filter_module);
 
     if (ctx == NULL || ctx->done) {

[Valentin V. Bartenev]

In 4991/nginx:

(The changeset message doesn't reference this ticket)

[Valentin V. Bartenev]

A better fix was commited as r4991. Thanks for the report.

[Maxim Dounin]

In 5055/nginx:

(The changeset message doesn't reference this ticket)