Nginx returns 502 Bad Gateway after upgrading to Ubuntu 24.04

Question

I recently upgraded my DigitalOcean droplet from Ubuntu 22.04 to Ubuntu 24.04 LTS. After the upgrade, nginx started returning 502 Bad Gateway errors for all my sites.

I have checked the following so far:

nginx service is running fine (systemctl status nginx shows active)
PHP-FPM 8.3 is running and listening on the correct socket
The upstream Node.js app responds correctly when I curl localhost:3000 directly
No permission issues on socket files

Here is the relevant error from /var/log/nginx/error.log:

connect() failed (111: Connection refused) while connecting to upstream

My nginx config uses proxy_pass to forward requests to a Node.js application running on port 3000. The app works fine when accessed directly but nginx cannot proxy to it.

I suspect something changed with systemd socket activation or AppArmor profiles in Ubuntu 24.04 Noble Numbat. Has anyone else experienced this issue after upgrading?

KFSys · Answer

Heya, the connect() failed (111: Connection refused) while connecting to upstream error almost always means an issue with the app behind the reverse proxy. Check the error logs of the app. It can be a broken function or something.

Bobby · Answer

Hi there, The error is clear: connection refused on port 3000 means Nginx is reaching the right place but nothing is listening there. Since the app works when you curl it directly, the issue is likely that the app is binding to 127.0.0.1:3000 on one network interface but Nginx is trying to connect to a different one, or the app is not actually running under the same context after the upgrade. A few things to check: Confirm the app is actually listening and on which interface: ss -tlnp | grep 3000 If it shows 127.0.0.1:3000 you are fine. If it shows ::1:3000 (IPv6 only) and your Nginx config uses proxy_pass http://localhost:3000, that could be the issue since localhost resolves differently on Ubuntu 24.04. Try changing your proxy_pass to be explicit: proxy_pass http://127.0.0.1:3000; Also check if the app is actually running after the upgrade. Ubuntu 24.04 changed some systemd defaults and services do not always survive an OS upgrade with their unit files intact: systemctl status your-app journalctl -u your-app --since "1 hour ago" Your AppArmor suspicion is worth checking too: sudo aa-status sudo journalctl | grep apparmor | grep nginx If AppArmor is blocking Nginx from making outbound connections, you will see denials in the journal. The IPv6/localhost issue is the most common cause of this after an Ubuntu upgrade so start there.

alexdo · Answer

Hi there, You already did the right diagnostic work here, the curl test especially, so it’s worth explaining why that test didn’t actually rule out the real cause, since Bobby’s answer already has the correct fix but not the reasoning behind it. curl localhost:3000 succeeding doesn’t mean nginx can reach the same address the same way. Modern curl resolves “localhost” to both ::1 and 127.0.0.1, tries them, and just uses whichever one connects, that’s the Happy Eyeballs behavior curl has shipped with for years. nginx’s proxy_pass doesn’t do that, it resolves “localhost” once and connects to a single address, and on Ubuntu that’s almost always the IPv6 result first since /etc/hosts lists ::1 localhost before 127.0.0.1 localhost. If your Node app is only bound to 127.0.0.1, nginx trying [::1]:3000 gets refused instantly, which matches your error exactly, while curl quietly falls back to whichever address actually works and hides the problem. That also means the AppArmor and systemd socket activation suspicion is probably a dead end, I couldn’t find anything documenting a behavioral change there in 24.04 that would produce this specific error, and the address family mismatch already explains every symptom you listed. Bobby’s fix, hardcoding proxy_pass http://127.0.0.1:3000 ;, is the right one. If you’d rather not hardcode the IP, the other option is having the Node app listen on :: instead of 127.0.0.1, which accepts both address families and would let proxy_pass http://localhost:3000 ; keep working as written. Hope that this helps!

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Nginx returns <b>502 Bad Gateway</b> after upgrading to Ubuntu 24.04

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