502 Bad Gateway (nginx): debug the upstream in order

TL;DR

A 502 from nginx means nginx itself is fine; the thing behind it is not. Read the error log line first, because the phrase after "while connecting to upstream" names the fix: Connection refused means the backend is down or on the wrong port, Connection timed out means firewall or routing, "upstream sent too big header" means raise proxy buffers, "no live upstreams" means every backend in the group failed. Fix the named cause, reload nginx, confirm with curl.

The error

The response body nginx serves (your users see this):

[html]
[head][title]502 Bad Gateway[/title][/head]
[body]
[center][h1]502 Bad Gateway[/h1][/center]
[hr][center]nginx/1.26.0[/center]
[/body]
[/html]

The log line that tells you which fix to apply:

2026/10/04 06:00:00 [error] 1234#0: *5 connect() failed (111: Connection refused) while connecting to upstream, client: 203.0.113.10, server: example.com, request: "GET / HTTP/1.1", upstream: "http://YOUR-BACKEND:8080/", host: "example.com"

Steps

1. Get the exact upstream error from the log

Run:

tail -n 100 /var/log/nginx/error.log | grep -i upstream

Expected output: one of the four phrases below. Do not guess from the 502 page alone; the page is identical for all four causes.

2. "connect() failed (111: Connection refused)" means the backend is not listening

The backend is down, crashed, or bound to a different port or interface. On the backend host, run:

ss -ltnp | grep 8080

Expected output: a LISTEN row for your app on the port nginx proxies to. If there is no row, start or restart the backend and check its own logs. If there is a row bound to the loopback address only, the app listens on loopback while nginx connects over the container or machine network; either bind the app to all interfaces or point proxy_pass at loopback.

3. "connect() failed (110: Connection timed out)" means packets never arrived

Usually a firewall, security group, or wrong host. From the nginx host, bypass nginx and hit the backend directly:

curl -m 5 http://YOUR-BACKEND:8080/health

Expected output: the backend's normal response. If curl hangs or fails, fix the path first: check security group rules, host firewalls, and that the hostname in proxy_pass resolves to the right IP with getent hosts YOUR-BACKEND.

4. "upstream sent too big header while reading response header from upstream" means proxy buffers are too small

The backend returned headers larger than nginx's default buffers (common with big cookies or JWTs). Add to the location block:

proxy_buffer_size 16k;
proxy_buffers 4 16k;

Then validate and reload:

nginx -t && nginx -s reload

Expected output:

nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
nginx: configuration file /etc/nginx/nginx.conf test is successful

5. "no live upstreams while connecting to upstream" means every backend in the group failed checks

All members of the upstream block are marked down. Test each backend individually with the curl from step 3. Then review the upstream block: max_fails and fail_timeout may be marking backends down on transient blips. Do not just raise the thresholds; find which backends are actually unhealthy.

6. Confirm the fix

curl -s -o /dev/null -w "%{http_code}" https://example.com/

Expected output: 200. If you still get 502, repeat step 1; there may be a second, different upstream error.

When to use

• Browser or curl against your site returns 502 and the Server header says nginx.
• nginx sits in front of an app server, API, Gunicorn, Node, PHP-FPM, or another reverse proxy.
• The 502 appeared right after a deploy, a backend restart, or a network change.

When NOT to use

• The 502 comes from Traefik, Caddy, HAProxy, Envoy, or a cloud load balancer; their error shapes and fix knobs differ.
• You get 503 (backend says unavailable) or 504 (backend too slow); those are different failure classes.
• The response is generated by nginx itself (a location returning an error page directly). Then there is no upstream to debug.
• The upstream is a unix socket (proxy_pass to unix:/...). The log phrases differ; check socket file permissions instead.

Tool and version compatibility

nginx 1.18 and newer; the log phrases and directives are identical through 1.26 and 1.28. Paths shown are Debian/Ubuntu defaults; on RHEL-likes the error log is also /var/log/nginx/error.log. In Docker, replace the log path with docker logs [nginx-container] and run the backend checks inside the backend container or from a shell with network access to it.

Variant phrasings

• "nginx 502"
• "bad gateway nginx reverse proxy"
• "upstream connect() failed 111"
• "nginx 502 proxy_pass"
• "nginx bad gateway after deploy"

Root cause

nginx is a middleman: it accepts the client connection, opens a second connection to the upstream named in proxy_pass, and relays. A 502 is nginx reporting that the second connection failed or the response on it was malformed. The failure is always on the nginx-to-backend leg, never on the client-to-nginx leg, which is why client-side retries and cache clears never help.

Edge cases

• 502 right after a deploy: the old backend shut down before nginx drained its connections. Use graceful shutdown (SIGQUIT to nginx workers, connection draining on the backend) or a rolling deploy.
• Keepalive to upstream: if proxyhttpversion 1.1 and keepalive are set but the backend closes idle connections sooner, requests die mid-flight. Align keepalive_timeout on both sides.
• Backend returns a malformed status line (some embedded servers do): nginx logs "upstream sent invalid header". The fix is on the backend, not nginx.
• Intermittent 502 under load only: check backend connection limits and the upstream keepalive pool size; refusal under burst is a capacity problem, not a config problem.