Spring Security Getting the client IP without using headers - spring

Question

My application sits behind a caching server that acts as a proxy. I'd like to limit access to the application so that it can only be accessed via the proxy. I first tried using a RemoteAddrFilter, however this filters based on the X-Forwarded-For header which is the IP address of the user connecting to the proxy, not the IP address of the proxy (This header is forwarded instead of being overwritten by the proxy). I decided to write my own filter based off of the RemoteAddrFilter:
public class IpFilter extends RequestFilter {
#Override
protected Log getLogger() {
return null;
}
private void sendErrorWhenNotHttp(ServletResponse response) throws IOException {
response.setContentType("text/plain");
response.getWriter().write(sm.getString("http.403"));
response.getWriter().flush();
}
#Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
if (/*check IP*/) {
filterChain.doFilter(servletRequest, servletResponse);
} else if (servletResponse instanceof HttpServletResponse) {
((HttpServletResponse) servletResponse).sendError(denyStatus);
} else {
sendErrorWhenNotHttp(servletResponse);
}
}
}
This gives me much more control over what can be accessed, however I'm unsure what to put into this if statement.
From what I can tell, given the parameters exposed here, the only accessible IP addresses are via headers, which do not represent the IP address of the proxy.
How can I get the IP address of the proxy?