Remote CDM files from an HTTP Server

NetcdfFile can open remote files served over HTTP, if the HTTP server allows range requests for a resource, indicated by the Accept-Ranges header (optional), and returns a Content-Length header (required). Therefore, the server must not return the data as "chunked" transfer-coding, such as gzip. Note this must happen for both the GET and the HEAD HTTP requests.

Performance will be strongly affected by the file format and the data access pattern. This is because each request for data will incur one or more file access requests. Each file access request becomes an HTTP request, incurring the cost of one round-trip message to the server. Generally, the netcdf-3 format (not netcdf-4) is most likely to give good performance, especially when reading large amounts of non-record data per request. Use OPeNDAP or CDMRemote for more consistent performance.

The THREDDS Data Server enables range requests by default. You may have to configure other HTTP servers. Following are our notes on how to do so.

Enabling Accept-Ranges response header in Apache HTTP Server

  1. Make sure mod_headers is either compiled into the httpd binary (from the commandline run: $APACHE_HOME/bin/httpd -l & look for mod_headers) or loaded as a DSO module (look in httpd.conf for mod_headers).
  2. Add the following directive to apply to the web content you need (e.g., it can be at the server-level, vhost-level, <Directory></Directory>, and .htaccess): Header set Accept-Ranges bytes Example:
    <Directory "/opt/local_htdocs">
       Options FollowSymLinks Indexes
       AllowOverride None
       Order allow,deny
       Allow from all
       Header set Accept-Ranges bytes
  3. Restart apache
  4. (optional): you can test the result by using the Live Http Headers extension in firefox.

For more information see


  1. Add Live HTTP Headers plug-in to Firefox.
  2. Try a URL in firefox: (this file is probably timed off, but you can go to to find a current one) You should get something like the following in the Live Headers window:
    GET /thredds/fileServer/station/metar/ HTTP/1.1
       User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US; rv: Gecko/2009011913 Firefox/3.0.6 (.NET CLR 3.5.30729)
       Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
       Accept-Language: en-us,en;q=0.5
       Accept-Encoding: gzip,deflate
       Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
       Keep-Alive: 300
       Connection: keep-alive
    HTTP/1.x 200 OK
       Server: Apache-Coyote/1.1
       Last-Modified: Mon, 09 Feb 2009 20:56:13 GMT
       Accept-Ranges: bytes
       Content-Type: application/x-netcdf
       Content-Length: 44414600
       Date: Mon, 09 Feb 2009 20:56:14 GMT

    In particular you should see these two headers:

     Accept-Ranges: bytes
     Content-Length: 12345
  3. Try the same thing with the server you want to test.