How to check for the existence of a response header in Nginx rules?

Question

Setting up the rewriting rules for the request proved to be quite easy in Nginx. For the response, not so much (at least, not for me). I want to strip the Content-Type header from the response if the Content-Length header of the response isn't set. I have the NginxHttpHeadersMoreModule installed, so that should allow me to remove the header, but I can't seem to find a way to check for the existence of the Content-Length header of the response using a rule in Nginx's configuration. Any suggestions on how to do this would be most appreciated!

Someone did warn you that this is generally a bad idea, right? Omitting `Content-Type` in the HTTP response basically gives the recipient [carte blanche to do whatever it wants](http://www.w3.org/Protocols/rfc2616/rfc2616-sec7.html#sec7.2.1), and you may not be able to predict that behavior. — Michael Hampton, Aug 27 '12 at 08:48

score 1 · Answer 1 · edited Sep 01 '16 at 13:39

1

I guess you want something like the following in a location block:

if ($sent_http_content_length ~ '') {
    more_clear_headers Content-Type
    ...
}

See $sent_http_name

(Disclaimer: I haven't tried this; I just dug through the docs because I found the question interesting. This may or may not work)

edited Sep 01 '16 at 13:39

Community

1

answered Feb 14 '12 at 22:03

nickgrim

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`if` only applies to incoming client requests, you can't use upstream response headers in conditions because the response hasn't been received yet. – Martijn Pieters Oct 12 '21 at 12:06

score 1 · Accepted Answer · edited Apr 27 '19 at 06:13

1

Looks like someone asked this on Stack Overflow, and there is a variable for each sent header called $sent_http_my_custom_header.

See https://web.archive.org/web/20140606142058/http://wiki.nginx.org/HttpCoreModule#.24sent_http_HEADER for the details.

Reference: https://stackoverflow.com/questions/12431496/nginx-read-custom-header-from-upstream-server

edited Apr 27 '19 at 06:13

bishop

1,086
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answered Sep 10 '13 at 11:19

Dave

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3

score 0 · Answer 3 · edited May 19 '17 at 21:41

0

There is a variable $content_length that according the documentation is equal to line Content-Length header of request. You probably can do

if ($content_length = 0) {
    do stuff with header
}

I am not sure what value $content_length will assume, or even if it will exist if the header is absent, but you may use that as a starting point to the solution.

edited May 19 '17 at 21:41

M. Schmidt

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answered Jul 30 '10 at 14:34

coredump

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This variable contains the Content-Length of the request, not the response. So unfortunately, it doesn't really help me. – Victor Welling Jul 30 '10 at 14:42
Oh you right... :\ – coredump Jul 30 '10 at 16:38
Right now, documentation link to wiki returns 404, good urls don't change they say :( Also note that nginx says that [if is evil](https://www.nginx.com/resources/wiki/start/topics/depth/ifisevil/) you have been warned ;) – GabLeRoux Apr 14 '17 at 14:54

score 0 · Answer 4 · edited May 19 '17 at 20:13

Well checking for the nonexistence of something is always a problem... but....

I generally telnet to the port nginx is listening on and make a hand crafted http repsonse:

telnet www.stackoverflow.com 80
Trying 69.59.196.211...
Connected to stackoverflow.com.
Escape character is '^]'.
GET /index.html HTTP/1.1
host: www.stackoverflow.com

HTTP/1.1 301 Moved Permanently
Server: nginx
Date: Fri, 30 Jul 2010 14:31:02 GMT
Content-Type: text/html; charset=UTF-8
Connection: keep-alive
Location: http://stackoverflow.com/index.html
Content-Length: 158

<head><title>Document Moved</title></head>
<body><h1>Object Moved</h1>This document may be found <a HREF="http://stackoverflow.com/index.html">here</a></body>

You can also use dump header options to curl or wget. For example wget supports:

   -S
   --server-response
       Print the headers sent by HTTP servers and responses sent by FTP
       servers.

Lastly, custom logging might let you add specific response headers, for example:

log_format up_head '$remote_addr - $remote_user [$time_local]  $request '
  'upstream_http_content_type $upstream_http_content_type';

Oh I am not so sure this is what you wanted... are you looking to verify what you have done works or how to do it? — Kyle Brandt, Jul 30 '10 at 14:48
Sorry, I guess I wasn't clear in my question. I want to know how to configure it, I already know how to see if it works ;) Thank you for your reply though! — Victor Welling, Jul 30 '10 at 14:57

How to check for the existence of a response header in Nginx rules?

4 Answers4