Prevent Wordpress from sending Cache-control http header

Question

My site sits on a server that uses Varnish as a (powerful) caching engine. Unfortunately, it seems wordpress is busting the Varnish cache by sending a cache-control http header. If I curl -I domain.com I get:

HTTP/1.1 200 OK
Server: Apache/2.4.10
X-Powered-By: PHP/5.4.4-14+deb7u14
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Set-Cookie: PHPSESSID=e00738aoughg407ljm270kj0l6; path=/
Content-Type: text/html; charset=UTF-8
Accept-Ranges: bytes
Date: Fri, 31 Oct 2014 21:44:16 GMT
Connection: keep-alive
Via: 1.1 varnish
Age: 0

I have other wordpress sites hosted on this server, which work correctly with the Varnish server, so I'm pretty sure the problem is caused by this specific installation. Here is what I tried:

• disable all plugins. Empty Varnish cache, then curl -I: same result.
• looked through all my theme files. Nothing suspicious.

Do you have any other idea as to what may cause the problem?

Answer 1

Thanks to @chrisguitarguy's answer, you can control the http headers sent by Wordpress via the "send_headers" hook. Here is the function I added to my theme's functions.php file, and that solved the issue with the Varnish server.

function varnish_safe_http_headers() {
    header( 'X-UA-Compatible: IE=edge,chrome=1' );
    session_cache_limiter('');
    header("Cache-Control: public, s-maxage=120");
  if( !session_id() )
  {
    session_start();
  }
}
add_action( 'send_headers', 'varnish_safe_http_headers' );

Answer 2

You can hook into wp_headers and remove the cache control headers. WordPress generally doesn't send Cache-Control except for admin area or ajax requests, however.

add_filter('wp_headers', 'wpse167128_nocache');
function wpse167128_nocache($headers)
{
    unset($headers['Cache-Control']);
    return $headers;
}