IPv6 stuff seems to have broken Syncthing

[jdaviescoates]

The IPv6 woes continue... this time it's syncthing:

Logs:

Apr 15 17:52:53 2025/04/15 16:52:53 [error] 18#18: *676 connect() failed (111: Connection refused) while connecting to upstream, client: 172.18.0.1, server: _, request: "GET /rest/system/connections HTTP/1.1", upstream: "http://[::1]:3000/rest/system/connections", host: "sync.ud.coop", referrer: "https://sync.ud.coop/"
Apr 15 17:52:53 2025/04/15 16:52:53 [warn] 18#18: *676 upstream server temporarily disabled while connecting to upstream, client: 172.18.0.1, server: _, request: "GET /rest/system/connections HTTP/1.1", upstream: "http://[::1]:3000/rest/system/connections", host: "sync.ud.coop", referrer: "https://sync.ud.coop/"

Chatting with ChatGTP about it:

Good catch — and fair challenge.

You're right to ask: where did I get "Nginx is configured to proxy to http://[::1]:3000"?

The clue is in the error messages from your original post:

connect() failed (111: Connection refused) while connecting to upstream, client: 172.18.0.1, server: _, request: "GET /rest/system/discovery HTTP/1.1", upstream: "http://[::1]:3000/rest/system/discovery"

That part:

upstream: "http://[::1]:3000/rest/system/discovery"

shows that Nginx is attempting to connect to Syncthing at [::1]:3000 — which is IPv6 localhost on port 3000.

That’s not coming from the config.xml you pasted, but from the Nginx reverse proxy config that Cloudron auto-generates for apps. Cloudron sets up Nginx to expect the app's internal web interface on localhost:3000, and it prefers IPv6 when resolving localhost, hence the [::1].

So unless Syncthing is bound to localhost (which includes both 127.0.0.1 and [::1]), Nginx will fail to connect — which is exactly what the log shows.

---

TL;DR

• That upstream: "http://[::1]:3000/... is from your Nginx error logs.
• It shows that Nginx is trying IPv6 localhost.
• But your Syncthing is only listening on IPv4 (127.0.0.1), so connection is refused.

Changing the Syncthing bind address from 127.0.0.1:8384 to localhost:3000 would fix it.

Want me to explain how Cloudron handles the reverse proxy internals if you're curious?

[jdaviescoates]

@jdaviescoates said in IPv6 stuff seems to have broken Syncthing:

It was working fine, but today it doesn't.

But, um, I just rebooted my laptop and now everything is working again

(I still have those warnings and errors, but if it's working I don't care)

[jdaviescoates]

@jdaviescoates said in IPv6 stuff seems to have broken Syncthing:

Changing the Syncthing bind address from 127.0.0.1:8384 to localhost:3000 would fix it.

I wasn't sure about this, but I tried it. It didn't make any difference, so I reverted the change.

But in short: syncthing is not syncing

[BrutalBirdie]

Have you tried to access the ipv4 and ipv6 version of a service manually via some tool?
e.g.

curl --ipv6 https://my.hackradt.com/api/v1/cloudron/status 

which Resolve names to IPv6 addresses.

Which I can identify in the /var/log/nginx/access.log:

-IPV6_REDACTED- - - [15/Apr/2025:17:21:43 +0000] "GET /api/v1/cloudron/status HTTP/2.0" 200 24 "-" "curl/8.5.0"

That is really was IPv6

[BrutalBirdie]

Just tried:

 curl -6 https://sync.ud.coop/

Which returns valid html

[BrutalBirdie]

I am setting up Syncthing right now to understand what is happening since I never used this app before.

[BrutalBirdie]

LGTM!
I am on IPv6 with tunneled IPv4.

I also see these erros and warnings:

Apr 15 19:41:49 2025/04/15 17:41:49 [error] 28#28: *1696 connect() failed (111: Connection refused) while connecting to upstream, client: 172.18.0.1, server: _, request: "GET /rest/system/discovery HTTP/1.1", upstream: "http://[::1]:3000/rest/system/discovery", host: "syncthing.hackradt.com", referrer: "https://syncthing.hackradt.com/"
Apr 15 19:41:49 2025/04/15 17:41:49 [warn] 28#28: *1696 upstream server temporarily disabled while connecting to upstream, client: 172.18.0.1, server: _, request: "GET /rest/system/discovery HTTP/1.1", upstream: "http://[::1]:3000/rest/system/discovery", host: "syncthing.hackradt.com", referrer: "https://syncthing.hackradt.com/"

But since it does sync I don't care Works?

But you are saying, it does not sync for you, right? @jdaviescoates

Also, this "error" is recurring, almost like a cron.

I can even validate that my client uses IPv6 from the app log:
2a02:8109:ab82:c600:1c05:d233:2fc7:122a is my computers IPv6.

331:2025-04-15T17:34:56Z [KENJT] 2025/04/15 17:34:56 INFO: Established secure connection to R2XDFAS at [fd00:c107:d509::22]:22000-[2a02:8109:ab82:c600:1c05:d233:2fc7:122a]:22000/tcp-client/TLS1.3-TLS_AES_128_GCM_SHA256/WAN-P30-61MHPLH87LIT6L1RIDSDOIF0OM
332:2025-04-15T17:34:56Z [KENJT] 2025/04/15 17:34:56 INFO: Device R2XDFAS client is "syncthing v1.29.5" named "bratan" at [fd00:c107:d509::22]:22000-[2a02:8109:ab82:c600:1c05:d233:2fc7:122a]:22000/tcp-client/TLS1.3-TLS_AES_128_GCM_SHA256/WAN-P30-61MHPLH87LIT6L1RIDSDOIF0OM

[jdaviescoates]

It was working fine, but today it doesn't.

[jdaviescoates]

@jdaviescoates said in IPv6 stuff seems to have broken Syncthing:

It was working fine, but today it doesn't.

But, um, I just rebooted my laptop and now everything is working again

(I still have those warnings and errors, but if it's working I don't care)

[jdaviescoates]

BTW, what I use this for is to auto-sync my local ~/Music file with my Hetzner Storage Box where I've also got all my music stored (and then have that set-up as a volume which is mounted into Navidrome, Syncthing, etc) - it's so much better than the rsync set-up I was using before!! (because that checkes all ~100k files it takes ages whereas syncthing just quickly syncs what has actually changed immediately)

[jdaviescoates]

@jdaviescoates said in IPv6 stuff seems to have broken Syncthing:

@jdaviescoates said in IPv6 stuff seems to have broken Syncthing:

It was working fine, but today it doesn't.

But, um, I just rebooted my laptop and now everything is working again

(I still have those warnings and errors, but if it's working I don't care)

And the reason that fixed everything is that Syncthing was already running on my laptop but then I tried to open it again and that confused it and locked the DB or something.

I've now finished setting it up properly so that it auto starts and also installed a handy little Syncthing Tray app so I can see what's up

[BrutalBirdie]

Dangit! So a reboot was the solution again.