HTTP Status Codes Error 502: Bad Gateway

Identify: does your log match?

HTTP/1.1 502 Bad Gateway
Server: nginx/1.27.2

2026/10/22 13:07:44 [error] 31#31: *1842 connect() failed (111: Connection refused) while connecting to upstream, client: 172.18.0.1, server: api.example.com, request: "GET /v1/orders HTTP/1.1", upstream: "http://172.18.0.5:3000/v1/orders", host: "api.example.com"

1. High

   Upstream app down or wrong port

   The app crashed, is restarting, or listens on a different port than proxy_pass/target group points to.

2. High

   Keep-alive timeout race

   Node.js closes idle sockets after 5s by default while an ALB keeps them 60s. The LB reuses a socket the app just closed and gets a reset.

3. Low

   Response headers too large

   Big cookies or headers exceed proxy_buffer_size, so nginx rejects the upstream response as invalid.

Fix

const server = app.listen(3000); // keepAliveTimeout defaults to 5s

const server = app.listen(3000, '0.0.0.0');
// ALB idle timeout is 60s: keep sockets open longer than that
server.keepAliveTimeout = 65_000;
server.headersTimeout = 66_000;

Verify: reproduce it on purpose

$ docker run --rm -d -p 8080:80 --name p nginx:alpine sh -c "echo 'server{listen 80;location /{proxy_pass http://127.0.0.1:9;}}' > /etc/nginx/conf.d/default.conf && nginx -g 'daemon off;'" && sleep 1 && curl -i localhost:8080; docker rm -f p

Prevent

• Add a /healthz endpoint and an LB health check.
• Graceful shutdown: stop accepting, drain, then exit.
• Alert on upstream 5xx separately from app 5xx.