ULB错误码有哪些？

ULB的错误码基本是遵循HTTP的规范。

客户端错误，返回4XX
400 Bad request：Your browser sent an invalid request.
403 Forbidden：Request forbidden by administrative rules.
408 Request Time-out：Your browser didn't send a complete request in time.
服务器错误，返回5XX
500 Server Error：An internal server error occured.
502 Bad Gateway：The server returned an invalid or incomplete response.
503 Service Unavailable：No server is available to handle this request.
504 Gateway Time-out：The server didn't respond in time.