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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?

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6 Answers 6

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To me it looks like the HTTP 1.1 RFC is pretty specific:

The Content-Length entity-header field indicates the size of the entity-body, in decimal number of OCTETs, sent to the recipient or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET.

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Interestingly enough, curl tells you that the body is missing... they may not "know" that the method is a HEAD when reading the reply and assume that the body is missing.
curl --head behaves better than curl -X HEAD.
wi don't get what it says, can you say it simpler?
@deadManN in HEAD request the Content-Length should be the same as it would have been in a GET request even though there's no actual body.
No longer relevant as the RFC 2616 was superseded by RFC 7230-7237. Section 4.3.2 of RFC 7231 states The server SHOULD send the same header fields in response to a HEAD request as it would have sent if the request had been a GET, except that the payload header fields (Section 3.3) MAY be omitted. Payload header fields defined in section 3.3 are Content-Length, Content-Range, Trailer and Transfer-Encoding.
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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 this very website...

telnet stackoverflow.com 80 HEAD / HTTP/1.0 Host:stackoverflow.com HTTP/1.1 200 OK Date: Mon, 11 Jan 2016 10:58:25 GMT Content-Type: text/html; charset=utf-8 Connection: close Set-Cookie: __cfduid=c2eb4742a1e02d89cab0402220736c0bd1452509905; expires=Tue, 10-Jan-17 10:58:25 GMT; path=/; domain=.stackoverflow.com; HttpOnly Cache-Control: public, no-cache="Set-Cookie", max-age=36 Expires: Mon, 11 Jan 2016 10:59:02 GMT Last-Modified: Mon, 11 Jan 2016 10:58:02 GMT Vary: * X-Frame-Options: SAMEORIGIN X-Request-Guid: 487e80bc-3783-4cfd-d883-a3bc84253234 Set-Cookie: prov=8dc24306-c067-45eb-bf5d-cffa855c2b03; domain=.stackoverflow.com; expires=Fri, 01-Jan-2055 00:00:00 GMT; path=/; HttpOnly Server: cloudflare-nginx CF-RAY: 26303c15f8e035a2-LHR

No content-length there.

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8 Comments

Notably, you also won't see Content-Length if it's a Transfer-Encoding: chunked response.
@PaulDixon Could you shed some light on this thread? Thx. stackoverflow.com/questions/34290203/…
A HEAD request to google.com will now (11th of Jan 2016) return Content-Length as well.
ah well, that example had a good 6 year run! Have changed it to use stackoverflow.com :)
I think this example doesn't work anymore either; a HEAD request to stackoverflow.com now returns Content-Length.
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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
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I think you're seeing the Content length of the error message you get for not using HTTP/1.0. If you send a proper 1.0 HEAD request, you do not get a content length. I tried this on a local apache instance also, and again, no content length was returned.
@Paul: Fixed my malformed request. I still get a Content-Length however, as I should. Even when using HTTP/1.0: i.imgur.com/iq9bm.jpg
Yes, the StackOverflow IIS servers do send it. Google doesn't though.
@Paul: Interesting. Google doesn't send it for 200 responses though. I get it for all the other return codes: 301, 302, 400, etc. +1 for finding the proper definition of "SHOULD" :)
How can you add the header in nginx?
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Update: As noted in Géry Ogam's answer, the latest spec (RFC 9110: HTTP Semantics) now states that while Content-Length can be omitted, if a server sends Content-Length, it must send the correct value. So this requirement has been strengthened since RFC 7231.

(What it literally says is that the server MUST NOT send Content-Length unless it has the correct value—which amounts to the same thing.)

Contra the accepted answer, section 4.3.2 of RFC 7231 states:

The server SHOULD send the same header fields in response to a HEAD request as it would have sent if the request had been a GET, except that the payload header fields (Section 3.3)

—which is to say, Content-Length, Content-Range, Trailer, and Transfer-Encoding—

MAY be omitted.

This is even weaker than the note on SHOULD in Paul Dixon's answer:

  1. MAY This word, or the adjective "OPTIONAL", mean that an item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because the vendor feels that it enhances the product while another vendor may omit the same item.

So the real answer is, you don't need to include Content-Length, but if you do, you should give the correct value.

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8

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.

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2

According to RFC 9110 - HTTP Semantics, the latest HTTP specification published in June 2022,

A server MAY send a Content-Length header field in a response to a HEAD request (Section 9.3.2); a server MUST NOT send Content-Length in such a response unless its field value equals the decimal number of octets that would have been sent in the content of a response if the same request had used the GET method.

The server SHOULD send the same header fields in response to a HEAD request as it would have sent if the request method had been GET. However, a server MAY omit header fields for which a value is determined only while generating the content. For example, some servers buffer a dynamic response to GET until a minimum amount of data is generated so that they can more efficiently delimit small responses or make late decisions with regard to content selection. Such a response to GET might contain Content-Length and Vary fields, for example, that are not generated within a HEAD response. These minor inconsistencies are considered preferable to generating and discarding the content for a HEAD request, since HEAD is usually requested for the sake of efficiency.

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