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I'm getting 502 bad gateway on an nginx with the following errors:

[error] 1679#1679: *13 connect() failed (111: Connection refused) while connecting to upstream, client: 46.176.252.229, server: dodeka-designers.com, request: "GET / HTTP/1.1", upstream: "http://127.0.0.1:3000/", host: "dodeka-designers.com"

This is my /etc/nginx/sites-available/default content. Does anyone have any idea what the issue could be?

server 
 listen 80;
 server_name dodeka-designers.com www.dodeka-designers.com;
 location ~ /.well-known 
 allow all;
 

 location / 
 proxy_pass http://localhost:3000;
 proxy_http_version 1.1;
 proxy_set_header Upgrade $http_upgrade;
 proxy_set_header Connection 'upgrade';
 proxy_set_header Host $host;
 proxy_cache_bypass $http_upgrade;
 client_max_body_size 50M;
 

 location /back 
 if ($request_method = 'OPTIONS') 
 add_header 'Access-Control-Allow-Origin' '*';
 add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
 add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
 add_header 'Access-Control-Max-Age' 1728000;
 add_header 'Content-Type' 'text/plain charset=UTF-8';
 add_header 'Content-Length' 0;
 return 200;
 
 if ($request_method = 'POST') 
 add_header 'Access-Control-Allow-Origin' '*';
 add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
 add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
 add_header 'Access-Control-Expose-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
 
 if ($request_method = 'GET') 
 add_header 'Access-Control-Allow-Origin' '*';
 add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
 add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
 add_header 'Access-Control-Expose-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
 
 rewrite ^/back(.*)$ $1 break;
 root /var/www/html;

 client_max_body_size 50M;
 proxy_http_version 1.1;
 proxy_connect_timeout 3600;
 proxy_send_timeout 3600;
 proxy_read_timeout 3600;
 send_timeout 3600;
 proxy_set_header Upgrade $http_upgrade;
 proxy_set_header Connection 'upgrade';
 proxy_set_header Host $host;
 proxy_cache_bypass $http_upgrade;

 index index.php index.html index.htm;
 

 location ~ .php$ 

 rewrite ^/back(.*)$ $1 break;
 root /var/www/html;

 include snippets/fastcgi-php.conf;
 fastcgi_pass unix:/run/php/php7.0-fpm.sock;
 fastcgi_split_path_info ^(.+.php)(/.+)$;

 client_max_body_size 50M;
 proxy_http_version 1.1;
 proxy_connect_timeout 3600;
 proxy_send_timeout 3600;
 proxy_read_timeout 3600;
 send_timeout 3600;
 fastcgi_read_timeout 3600;
 proxy_set_header Upgrade $http_upgrade;
 proxy_set_header Connection 'upgrade';
 proxy_set_header Host $host;
 proxy_cache_bypass $http_upgrade;
 



 listen 443 ssl; # managed by Certbot
ssl_certificate /etc/letsencrypt/live/dodeka-designers.com/fullchain.pem; # managed by Certbot
ssl_certificate_key /etc/letsencrypt/live/dodeka-designers.com/privkey.pem; # managed by Certbot
 include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
 ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot
 

You are running a reverse proxy that is trying to connect to another service on 'http://localhost:3000', so this error is not on nginx since this other service on localhost:3000 is not answering nginx requests or it is not answering it properly to be handle by the reverse proxy (for example, it is producing a redirection).

If you are able to connect to port 3000 via telnet for example (command to execute this test: telnet localhost 3000), check if it is speaking in plain text or if it is encripted (https...) - because you configured nginx to connect to this other service as http only, if you are not able to connect to port 3000, then you must determine what service you have in this port that should be fixed, nginx has nothing to do with that.

You did not expect nginx to be a **reverse proxy?** Them you need to edit its configuration to remove the "proxy" lines.