Content-Length header with HEAD requests?

Question

The http spec says about the HEAD request:

The HEAD method is identical to GET except that the server MUST NOT return a message-body in the response. The metainformation contained in the HTTP headers in response to a HEAD request SHOULD be identical to the information sent in response to a GET request.

Should the response to a HEAD request contain a Content-Length header? Should it be the value which would be returned on a GET request, even if there is no response body? Or should the Content-Length be 0?

Answer 1

The HTTP-spec at W3C states:

If the new field values indicate that the cached entity differs from the current entity (as would be indicated by a change in Content-Length, ...

Which (to me) means it should hold the "correct" value as you would in a GET response.

Answer 2

Yes, the Content-Length of a HEAD response SHOULD, but not always does (see @Paul's answer) include the Content-Length value of a GET response:

Stack Overflow does:

> telnet stackoverflow.com 80
HEAD / HTTP/1.1
Host: stackoverflow.com


HTTP/1.1 200 OK
Cache-Control: public, max-age=60
Content-Length: 362245                           <--------
Content-Type: text/html; charset=utf-8
Expires: Mon, 04 Oct 2010 11:51:49 GMT
Last-Modified: Mon, 04 Oct 2010 11:50:49 GMT
Vary: *
Date: Mon, 04 Oct 2010 11:50:49 GMT

Google doesn't:

> telnet www.google.com 80
HEAD / HTTP/1.1
Host: www.google.ie


HTTP/1.1 200 OK
Date: Mon, 04 Oct 2010 11:55:36 GMT
Expires: -1
Cache-Control: private, max-age=0
Content-Type: text/html; charset=ISO-8859-1
Server: gws
X-XSS-Protection: 1; mode=block
Transfer-Encoding: chunked

Answer 3

Section 14.13 of the HTTP/1.1 spec detailed the Content-Length header, and says this:

Applications SHOULD use this field to indicate the transfer-length of the message-body, unless this is prohibited by the rules in section 4.4.

The word 'SHOULD' has a very specific meaning in RFCs:

1. SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.

So, you may not always see a Content-Length. Typically you might not see it for any content which is dynamically generated, since that might be too expensive to service an exploratory HEAD request. For example, a HEAD request to Apache for a static file will have a Content-Length, but a request for a PHP script may not.

For example, try Google...

telnet www.google.com 80
HEAD / HTTP/1.0
Host:www.google.co.uk


HTTP/1.0 200 OK
Date: Mon, 04 Oct 2010 11:56:02 GMT
Expires: -1
Cache-Control: private, max-age=0
Content-Type: text/html; charset=ISO-8859-1
Server: gws
X-XSS-Protection: 1; mode=block

No content-length there.