nginx代理502，日志报错connect() failed (111: Connection refused) while connecting to upstream

环境：

在centos7服务器，docker部署的nginx，同服务器下部署的应用

现象：

nginx使用localhost代理同服务器上的应用，访问返回502错误，nginx日志如下

2024/08/09 07:37:44 [error] 68#68: *16653 connect() failed (111: Connection refused) while connecting to upstream, client: 10.19.89.212, server: localhost, request: "GET /gateway-manager/api/v1/gateway/key/profiles HTTP/1.1", upstream: "http://127.0.0.1:8001/api/v1/gateway/key/profiles", host: "172.30.4.30:81"
2024/08/09 07:37:44 [error] 68#68: *16653 connect() failed (111: Connection refused) while connecting to upstream, client: 10.19.89.212, server: localhost, request: "GET /gateway-manager/api/v1/gateway/key/profiles HTTP/1.1", upstream: "http://127.0.0.1:8001/api/v1/gateway/key/profiles", host: "172.30.4.30:81"
10.19.89.212 - - [09/Aug/2024:07:37:44 +0000] "GET /gateway-manager/api/v1/gateway/key/profiles HTTP/1.1" 502 559 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/122.0.6261.95 Safari/537.36"
2024/08/09 07:38:38 [error] 68#68: *16656 connect() failed (111: Connection refused) while connecting to upstream, client: 10.19.89.212, server: localhost, request: "GET /gateway-manager/api/v1/gateway/key/profiles HTTP/1.1", upstream: "http://127.0.0.1:8001/api/v1/gateway/key/profiles", host: "172.30.4.30:81"
10.19.89.212 - - [09/Aug/2024:07:38:38 +0000] "GET /gateway-manager/api/v1/gateway/key/profiles HTTP/1.1" 502 559 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/122.0.6261.95 Safari/537.36"

确认问题过程：

1. 提示Connection refused是连接被拒绝，进入容器模拟请求。

   [root@S-ZW-HLWJ-3 nginx]# docker exec -it nginx /bin/bash
   root@1752081aa212:/# curl http://127.0.0.1:8001/api/v1/gateway/key/profiles
   curl: (7) Failed to connect to 127.0.0.1 port 8001: Connection refused
   root@1752081aa212:/# 

2. docker容器访问宿主机服务不通，涉及容器和宿主机是否网络隔离，确认容器网络模式。

   [root@S-ZW-HLWJ-3 nginx]# docker inspect -f '{{ .HostConfig.NetworkMode }}' nginx
   default
   # default是bridge（访问宿主机需要绑定端口），container（访问宿主机需要绑定端口），Host（通过宿主机网络通信），None（无法通信）
   # 因为容器未绑定端口所以无法在容器内通过127.0.0.1:8001访问到宿主机部署的服务。（如果不使用docker部署就不会有这个问题了，用Host网络模式也不会有这个问题）

解决办法

1. 修改nginx.conf代理地址为宿主机IP（其他方法：1修改容器网络模式或者增加端口映射，2不使用docker部署nginx）

   server {
           listen       81;
           ...
   		# 修改localhost为服务器IP
   		location /gateway-manager/api/{
   			# proxy_pass http://localhost:8001/api/;
   			proxy_pass http://172.30.4.1:8001/api/;
   		}
           ...
   }

2. 重新加载配置，测试

   # 验证nginx配置文件格式是否正确
   [root@S-ZW-HLWJ-3 ~]# docker exec nginx nginx -t
   nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
   nginx: configuration file /etc/nginx/nginx.conf test is successful
   
   # 重新加载配置文件
   [root@S-ZW-HLWJ-3 ~]# docker exec nginx nginx -s reload
   2024/08/09 08:53:21 [notice] 64#64: signal process started
   
   # 使用curl命令访问代理后地址测试
   [root@S-ZW-HLWJ-3 ~]# curl http://172.30.4.30:81/gateway-manager/api/v1/gateway/key/profiles
   {"code":"00000","message":"OK","data":[{"id":"271ClMkmYfg","projectName":"jiangk"}],"ok":true}
   [root@S-ZW-HLWJ-3 ~]#