ffsync backend listens to IPv4 only, NGinx forwards to IPv4 or IPv6

[ebouaziz]

Describe the bug

Some request apparently fail to reach the backend ffsync server, producing this kind of errors in NGINX logs:

2023/01/16 20:05:32 [error] 227067#227067: *90587 connect() failed (111: Connection refused) while connecting to upstream, client: 2a01:xxx:xxx:xxx:xxx:xxx:x:xxx, server: xyz.ynh.fr, request: "POST /ffsync/storage/1.5/1/storage/tabs?batch=true&commit=true HTTP/2.0", upstream: "http://[::1]:6001/storage/1.5/1/storage/tabs?batch=true&commit=true", host: "xyz.ynh.fr"

Context

• i7 PC at home
• YunoHost version: 11.1.0.2
• I have access to my server: Through SSH
• Are you in a special context or did you perform some particular tweaking on your YunoHost instance?: no

Steps to reproduce

• tail -f /var/log/nginx/xxx.ynh.fr-error.log
• make firefox sync with server

Expected behavior

No errors in logs

Logs

see above

Theory

• sync server is bound to 127.0.0.1:6001
• ffsync's proxy_pass is set to http://localhost:6001/;
• I think that localhost is sometimes resolved to [::1] like shown in logs

I changed proxy_pass to http://127.0.0.1:6001/; and the error does not show anymore.

Maybe it's possible to make the sync server listen on both IPv4 and IPv6 but I'm not sure of the appropriate syntax in the configuration file. Using localhost in synserver.ini does not help, ffsync still logs [INFO] Listening at: http://127.0.0.1:6001.