properly parse x-forwarded-host

rootfs/etc/nginx/lua/lua_ingress.lua

@@ -68,6 +68,16 @@ local function redirect_host()
   return host_port[1];
 end
 
+local function parse_x_forwarded_host()
+  local hosts, err = ngx_re_split(ngx.var.http_x_forwarded_host, ",")
+  if err then
+    ngx.log(ngx.ERR, string_format("could not parse variable: %s", err))
+    return ""
+  end
+
+  return hosts[1]
+end
+
 function _M.init_worker()
   randomseed()
 end
@@ -96,10 +106,7 @@ function _M.rewrite(location_config)
 
     -- Obtain best http host
     if ngx.var.http_x_forwarded_host then
-      -- TODO(elvinefendi) https://github.com/kubernetes/ingress-nginx/issues/3790 can
-      -- be fixed here by splitting the value of ngx.var.http_x_forwarded_host by ','
-      -- and taking the first portion
-      ngx.var.best_http_host = ngx.var.http_x_forwarded_host
+      ngx.var.best_http_host = parse_x_forwarded_host()
     end
   end
 

test/e2e/settings/forwarded_headers.go

@@ -54,6 +54,7 @@ var _ = framework.IngressNginxDescribe("X-Forwarded headers", func() {
 				return strings.Contains(server, "server_name forwarded-headers")
 			})
 
+		By("ensuring single values are parsed correctly")
 		resp, body, errs := gorequest.New().
 			Get(f.GetURL(framework.HTTP)).
 			Set("Host", host).
@@ -70,6 +71,22 @@ var _ = framework.IngressNginxDescribe("X-Forwarded headers", func() {
 		Expect(body).Should(ContainSubstring(fmt.Sprintf("x-forwarded-proto=myproto")))
 		Expect(body).Should(ContainSubstring(fmt.Sprintf("x-forwarded-port=1234")))
 		Expect(body).Should(ContainSubstring(fmt.Sprintf("x-forwarded-for=1.2.3.4")))
+
+		By("ensuring that first entry in X-Forwarded-Host is used as the best host")
+		resp, body, errs := gorequest.New().
+			Get(f.GetURL(framework.HTTP)).
+			Set("Host", host).
+			Set("X-Forwarded-Port", "1234").
+			Set("X-Forwarded-Proto", "myproto").
+			Set("X-Forwarded-For", "1.2.3.4").
+			Set("X-Forwarded-Host", "myhost.com, another.host,example.net").
+			End()
+
+		Expect(errs).Should(BeEmpty())
+		Expect(resp.StatusCode).Should(Equal(http.StatusOK))
+		Expect(body).Should(ContainSubstring(fmt.Sprintf("host=myhost.com")))
+		Expect(body).Should(ContainSubstring(fmt.Sprintf("x-forwarded-host=myhost.com, another.host,example.net")))
+
 	})
 	It("should not trust X-Forwarded headers when setting is false", func() {
 		host := "forwarded-headers"