X-Pingback
Automated backlink notifications between WordPress sites depend on discovering the pingback endpoint. The X-Pingback unofficial response header provides the URL of a site's XML-RPC pingback handler.
Usage
WordPress adds the X-Pingback header to singular
post and page responses where pings are open, a
behavior in place since WordPress 4.4. The header
value points to the site's xmlrpc.php file, which
handles incoming pingback notifications.
The Pingback protocol allows one website to notify
another when linking to its content. When site A
publishes a post containing a link to site B, site A
sends an XML-RPC request to the pingback endpoint
advertised by site B. This creates a backlink
reference on site B, similar to a trackback. The
X-Pingback header is one of two discovery
methods for the endpoint. The other is a <link>
element in the HTML <head>.
The header originates from the
Pingback 1.0 specification,
which predates modern linking and webmention
standards. While the protocol remains functional in
WordPress, many site operators disable pingbacks
due to spam abuse and the security surface exposed
by the xmlrpc.php endpoint. Disabling XML-RPC or
pingbacks in WordPress settings removes this header
from responses.
Values
The value is always an absolute URL pointing to the
site's XML-RPC endpoint. In WordPress installations,
this is the xmlrpc.php file at the site root or
within a subdirectory when WordPress is installed in
a subfolder.
Example
A standard WordPress site includes the pingback
endpoint at the root of the domain. The URL points
to the xmlrpc.php handler where incoming pingback
requests are processed.
X-Pingback: https://example.re/xmlrpc.php
When WordPress is installed in a subdirectory, the endpoint URL reflects the installation path.
X-Pingback: https://example.re/wordpress/xmlrpc.php
A renamed post's old permalink redirecting to the new address shows both headers together. X-Redirect-By identifies WordPress as the redirect source, appearing only on 3xx responses, while X-Pingback advertises the pingback endpoint the single post exposes.
HTTP/1.1 301 Moved Permanently
Location: https://www.example.re/blog/new-title/
Content-Type: text/html; charset=UTF-8
X-Pingback: https://www.example.re/xmlrpc.php
X-Redirect-By: WordPress