--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -23,6 +23,8 @@ static ngx_int_t ngx_http_upstream_cache
     ngx_http_upstream_t *u);
 static ngx_int_t ngx_http_upstream_cache_status(ngx_http_request_t *r,
     ngx_http_variable_value_t *v, uintptr_t data);
+static ngx_int_t ngx_http_upstream_cache_key(ngx_http_request_t *r,
+    ngx_http_variable_value_t *v, uintptr_t data);
 static ngx_int_t ngx_http_upstream_cache_last_modified(ngx_http_request_t *r,
     ngx_http_variable_value_t *v, uintptr_t data);
 static ngx_int_t ngx_http_upstream_cache_etag(ngx_http_request_t *r,
@@ -414,6 +416,10 @@ static ngx_http_variable_t  ngx_http_ups
       ngx_http_upstream_cache_status, 0,
       NGX_HTTP_VAR_NOCACHEABLE, 0 },
 
+    { ngx_string("upstream_cache_key"), NULL,
+      ngx_http_upstream_cache_key, 0,
+      NGX_HTTP_VAR_NOCACHEABLE, 0 },
+
     { ngx_string("upstream_cache_last_modified"), NULL,
       ngx_http_upstream_cache_last_modified, 0,
       NGX_HTTP_VAR_NOCACHEABLE|NGX_HTTP_VAR_NOHASH, 0 },
@@ -6004,6 +6010,49 @@ ngx_http_upstream_cache_status(ngx_http_
 
 
 static ngx_int_t
+ngx_http_upstream_cache_key(ngx_http_request_t *r,
+    ngx_http_variable_value_t *v, uintptr_t data)
+{
+    u_char            *p;
+    size_t             len;
+    ngx_str_t         *key;
+    ngx_uint_t         i;
+    ngx_http_cache_t  *c;
+
+    if (r->cache == NULL || r->cache->keys.nelts == 0) {
+        v->not_found = 1;
+        return NGX_OK;
+    }
+
+    c = r->cache;
+
+    len = 0;
+    key = c->keys.elts;
+
+    for (i = 0; i < c->keys.nelts; i++) {
+        len += key[i].len;
+    }
+
+    p = ngx_pnalloc(r->pool, len);
+    if (p == NULL) {
+        return NGX_ERROR;
+    }
+
+    v->len = len;
+    v->valid = 1;
+    v->no_cacheable = 0;
+    v->not_found = 0;
+    v->data = p;
+
+    for (i = 0; i < c->keys.nelts; i++) {
+        p = ngx_cpymem(p, key[i].data, key[i].len);
+    }
+
+    return NGX_OK;
+}
+
+
+static ngx_int_t
 ngx_http_upstream_cache_last_modified(ngx_http_request_t *r,
     ngx_http_variable_value_t *v, uintptr_t data)
 {