--- src/http/ngx_http_upstream.h.orig	2017-03-10 18:45:34.000000000 -0600
+++ src/http/ngx_http_upstream.h	2017-03-10 18:46:59.000000000 -0600
@@ -414,6 +414,8 @@
 ngx_int_t ngx_http_upstream_hide_headers_hash(ngx_conf_t *cf,
     ngx_http_upstream_conf_t *conf, ngx_http_upstream_conf_t *prev,
     ngx_str_t *default_hide_headers, ngx_hash_init_t *hash);
+ngx_int_t ngx_http_upstream_header_variable(ngx_http_request_t *r,
+    ngx_http_variable_value_t *v, uintptr_t data);
 
 
 #define ngx_http_conf_upstream_srv_conf(uscf, module)                         \