参考：
https://blog.csdn.net/zzhongcy/article/details/89090193?spm=1001.2101.3001.6661.1&utm_medium=distribute.pc_relevant_t0.none-task-blog-2%7Edefault%7ECTRLIST%7Edefault-1-89090193-blog-120278315.pc_relevant_multi_platform_whitelistv1&depth_1-utm_source=distribute.pc_relevant_t0.none-task-blog-2%7Edefault%7ECTRLIST%7Edefault-1-89090193-blog-120278315.pc_relevant_multi_platform_whitelistv1&utm_relevant_index=1
https://www.cnblogs.com/ellisonzhang/p/15222022.html
错误信息：
| 错误信息 | 错误说明 |
| upstream prematurely（过早的） closed connection | 请求uri的时候出现的异常，是由于upstream还未返回应答给用户时用户断掉连接造成的，对系统没有影响，可以忽略 |
| recv() failed (104: Connection reset by peer) | （1）服务器的并发连接数超过了其承载量，服务器会将其中一些连接Down掉； （2）客户关掉了浏览器，而服务器还在给客户端发送数据； （3）浏览器端按了Stop |
| (111: Connection refused) while connecting to upstream | 用户在连接时，若遇到后端upstream挂掉或者不通，会收到该错误 |
| (111: Connection refused) while reading response header from upstream | 用户在连接成功后读取数据时，若遇到后端upstrream挂掉或者不通，会收到该错误 |
| (111: Connection refused) while sending request to upstream | Nginx和upstream连接成功后发送数据时，若遇到后端upstream挂掉或者不通，会收到该错误 |
| (110: Connection timed out) while connecting to upstream | nginx连接后面的upstream时超时 |
| (110: Connection timed out) while reading upstream | nginx读取来自upstream的响应时超时 |
| (110: Connection timed out) while reading response header from upstream | nginx读取来自upstream的响应头时超时 |
| (104: Connection reset by peer) while connecting to upstream | upstream发送了RST，将连接重置 |
| upstream sent invalid header while reading response header from upstream | upstream发送的响应头无效 |
| upstream sent no valid HTTP/1.0 header while reading response header from upstream | upstream发送的响应头无效 |
| client intended to send too large body | 用于设置允许接受的客户端请求内容的最大值，默认值是1M，client发送的body超过了设置值 |
