Synced files deleted if proxy returns an unexpected response (e.g. 404) when Nextcloud service is offline

[NeXX451]

⚠️ Before posting ⚠️

• This is a bug, not a question or an enhancement.
• I've searched for similar issues and didn't find a duplicate.
• I've written a clear and descriptive title for this issue, not just "Bug" or "Crash".
• I agree to follow Nextcloud's Code of Conduct.

Steps to reproduce

1. set up VPN connection to nextcloud server
2. sync some files
3. kill vpn connection such that nextcloud client looses connection
4. wait a few hours until programs such as davx start complaining about lost connection
5. synced files are now deleted

Expected behaviour

The synced files should not be deleted under any circumstances unless the user explicitly deletes them. The client loosing connection to the server should not result in synced files being deleted.

Actual behaviour

I have asked this question on the help.nextcloud.com forum, so I will just copy and paste.

For whatever reason when my phone (Samsung S23, Nextcloud Android app, version 3.27.0) looses connection to my nextcloud server, it decides that all the synced files are to be deleted. This happened three times already and I have to re-download (resync) everything, so you can imagine this is supremely frustrating.

I access my nextcloud server via a vpn connection, so if the vpn server dies → nextcloud connection dies. I woke up today with a bunch of DavX notification that all read along the lines “cannot sync”, “no connection”, etc. I promptly restarted the server. If there are any app logs I can share with you, just tell me where they are and I shall share them.

It does not start syncing the files on it’s own, which is strange in its own way, the files are completely gone from the phone and I have to go into the folder and click on the three dots → sync.

Also I have noticed it also happens on my tablet as well.

Android version

14

Device brand and model

Samsung S23, Samsung Galaxy Tab S4

Stock or custom OS?

Stock

Nextcloud android app version

3.27.0

Nextcloud server version

28.0.1

Using a reverse proxy?

Yes

Android logs

No response

Server error logs

No response

Additional information

I have not found any logs on my phone unfortunately. The folder /data/ does not exist on my phone.

[joshtrichards]

I assume green check marks on the file(s) remain and it's purely the on-disk files disappearing?

Can you trigger the same condition just by putting you device in offline/airplane mode?

What path are you using? The one without tmp in it? Or this is through the documents provider maybe?

If the green checkmark is disappearing, it would be interesting to see logs from the client app to see why it is "unsyncing" mysteriously.

There are a couple ways of getting logs...

The easiest may be to install the Dev edition of the app since it can be installed alongside your regular edition. It'll use its own storage folder so as long as you don't configure an overlapping auto-upload folder, there should be no conflict. In the Dev app you can access the logs directly within the app via Settings->Logs. The Dev app can be installed via F-Droid. You can even use it with a separate test account on your Nextcloud instance if you wish to further isolate any testing.

Alternatively you can get logs from your existing installation by using logcat.

EDIT: Clarified my initial response.

[NeXX451]

No even the green check-marks disappear.

I'll see what happens in offline/airplane mode and let you know. I suspect in offline-mode nothing will happen, but in airplane-mode it will block the vpn connection and result in the ominous desync.

I haven't messed with the path at all, it's the default Internal storage/Android/media/com.nextcloud.client/nextcloud/user@domain/

I'll also look into the logs with the dev-version.

[NeXX451]

Ok I have reproduced it. It won't work with either offline or airplane mode as I suspect the app takes notice and doesn't do anything. So what I have done is actually stopped the nextcloud server altogether, which should be equivalent to VPN loosing connection. I also have the logs

What I have noticed is that, the moment the server goes offline, the client completely forgets that there ever were some files, and instead of showing me the folders (from cache or something) all it shows is "No files here", which then results in deletion.

Edit: I uploaded logs from main account.. I will upload test account logs in a bit.

[NeXX451]

The logs:
logs.txt

[joshtrichards]

Thanks for the logs. I'll take a look. What type of VPN connection is this, btw?

[NeXX451]

No problem. As I have said it's not related to the vpn connection, since the deletion occurs when I turn the nextcloud server off (connected from local network to the server, so vpn is not needed). It's OpenVPN

[NeXX451]

Ok, I can norrow it down further. Today I had to turn off the whole server and nothing was deleted. So if connection to the server exists but the nextcloud instance is turned off it desyncs, since it connects to the server but can't contact the instance.

[borisdigital]

Looks like I'm having equal issues with the android app. I'm not using a VPN, but from time to time the nextcloud server goes down (mostly because of diy layer 8 docker autoupdate problems). This always seems to delete all synced files on the android device. I normally recognize the absence of the files, when I try to play one of my m3u lists in vlc...

[joshtrichards]

Ok, I can norrow it down further. Today I had to turn off the whole server and nothing was deleted. So if connection to the server exists but the nextcloud instance is turned off it desyncs, since it connects to the server but can't contact the instance.

How are you "turning off Nextcloud" when you trigger this condition? And what reverse proxy are you using?

Based on your logs it appears the HTTPS still on the other end of the connection starts happily returning 404s (file not found) to our checks for the existence of files/etc (i.e. whatever is still responding server-side is behaving as if it's the actual server we're trying to talk to instead of timing out like I'd expect the proxy to do with a 502 Bad Gateway or similar). A proxy won't return 404s if the target service is offline typically so something weird is going on here.

Can you provide the output of curl -I https://<your_domain while Nextcloud is offline (when things are not working)?

[NeXX451]

All is running in docker. I'm using Traefik as reverse proxy. I also I have a pihole instance, where I have a custom DNS record pointing to the nextcloud server. Since I can't be bothered to actually expose anything I run everything locally, but I still decided to use self-signed certificates, so Traefik uses a self-signed certificate for the custom DNS record.

Turning off nextcloud boils down to shutting down the container, but since traefik is still up, it will return 404 (my guess). At this point I am not sure if that's intended behaviour or not... I mean if it isn't than that means I screwed up my config somewhere along the line, but would that even matter?

Since the certificate for https is self-signed I ran curl with -k

curl -Ik https://nc.vpn                                                                                      ✔ 
HTTP/2 404 
content-type: text/plain; charset=utf-8
x-content-type-options: nosniff
content-length: 19
date: Tue, 05 Mar 2024 17:36:34 GMT

[NeXX451]

Actually I can also post the config, maybe that helps..

Traefik:

version: '3'

services:
  traefik:
    image: traefik
    command:
      - "--log.level=DEBUG"
      - "--entrypoints.websecure.address=:443"
      - "--providers.docker=true"
      - "--providers.file.directory=/certs/config/"
      - "--providers.file.watch=true"
    container_name: traefik
    restart: always
    security_opt:
      - no-new-privileges:true
    networks:
      - proxy
    ports:
      - 80:80
      - 443:443
    volumes:
      - /etc/localtime:/etc/localtime:ro
      - /var/run/docker.sock:/var/run/docker.sock:ro

      - /mnt/NC/self_sign/compose/certs/:/certs/
      - /mnt/NC/self_sign/compose/certs/config/:/certs/config/

      - ./data/logs:/var/log/traefik
    labels:
      - "traefik.http.middlewares.sslheader.headers.customrequestheaders.X-Forwarded-Proto=https"

networks:
  proxy:
    external: true

and in the nextcloud docker-compose:

    labels:

      - "traefik.enable=true"
      - "traefik.http.routers.nextcloud.rule=Host(`nc.vpn`)"
      - "traefik.http.routers.nextcloud.entrypoints=websecure"
      - "traefik.http.routers.nextcloud.tls=true"
      
      - "traefik.docker.network=proxy"