The host identification of my borg host has changed and I cannot backup anymore

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The Basics

  • Nextcloud Server version (e.g., 29.x.x):
    • Nextcloud AIO
  • Operating system and version (e.g., Ubuntu 24.04):
    • Debian Trixie
  • Web server and version (e.g, Apache 2.4.25):
    • Nextcloud AIO
  • Reverse proxy and version _(e.g. nginx 1.27.2)
    • nginx
  • PHP version (e.g, 8.3):
    • Nextcloud AIO
  • Is this the first time you’ve seen this error? (Yes / No):
    • Yes
  • When did this problem seem to first start?
    • Today
  • Installation method (e.g. AlO, NCP, Bare Metal/Archive, etc.)
    • AIO
  • Are you using CloudfIare, mod_security, or similar? (Yes / No)
    • No

Summary of the issue you are facing:

I have a borg repository accessible through SSH and I needed to move it from one server to another one. Now backup does not work anymore. I see the following error:

2025-12-17T10:47:29.616871293Z SSH command line: [‘ssh’, ‘-o’, ‘StrictHostKeyChecking=accept-new’, ‘-i’, ‘/nextcloud_aio_volumes/nextcloud_aio_mastercontainer/data/id_borg’, ‘-p’, ‘8022’, ‘backup@b68.krzm.de’, ‘borg’, ‘serve’, ‘–debug’]
2025-12-17T10:47:29.832280829Z Remote: @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
2025-12-17T10:47:29.832353736Z Remote: @ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
2025-12-17T10:47:29.832359497Z Remote: @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
2025-12-17T10:47:29.832362943Z Remote: IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
2025-12-17T10:47:29.832366059Z Remote: Someone could be eavesdropping on you right now (man-in-the-middle attack)!
2025-12-17T10:47:29.832369244Z Remote: It is also possible that a host key has just been changed.
2025-12-17T10:47:29.832372530Z Remote: The fingerprint for the ED25519 key sent by the remote host is
2025-12-17T10:47:29.832376237Z Remote: SHA256:RRRc0lx+hKtgDK2Rw8rvtC1OGKPnrNSlkabCqqSIh0o.
2025-12-17T10:47:29.832379704Z Remote: Please contact your system administrator.
2025-12-17T10:47:29.832382950Z Remote: Add correct host key in /root/.ssh/known_hosts to get rid of this message.
2025-12-17T10:47:29.832447191Z Remote: Offending ED25519 key in /root/.ssh/known_hosts:3
2025-12-17T10:47:29.832451028Z Remote: Host key for [XXX]:8022 has changed and you have requested strict checking.
2025-12-17T10:47:29.832454184Z Remote: Host key verification failed.

My question: How do I fix that in the borgbackup container of AIO? I have no way of running the container such that I could exec into it and change the known_hosts file.

Steps to replicate it (hint: details matter!):

  1. Move an existing borg repository from one host to another

  2. Create a new backup

Log entries

Nextcloud

See above

1 Like
2

Hi, you could reset the backup location via the AIO interface (there is now a button to do this) and then try to re-do the initial bacup setup

3

Thanks for the suggestion, but wouldn’t I lose the old backups then?

4

I’ve tried that - did not change the outcome. But thanks anyway!

5

No, it should be able to catch up on the existing backups if you dont delete the target backup repository.

6

Okay, I’ve changed the host id of the new server of the one the old server had. Now SSH access works again, but I’ve run into a different problem now:

2025-12-17T11:35:44.384288776Z Did not find borg.config file in the mastercontainer volume.
2025-12-17T11:35:44.385797641Z Cannot create a backup as this is wrong.

I have no idea where this is coming from.

Any idea what this means?

7

Hm… simply run touch /var/lib/docker/volumes/nextcloud_aio_mastercontainer/_data/data/borg.config and try again?

1 Like
8

Wow. That seems to have done the trick. Thanks very much!

A backup seems to be running now. Let’s hope for the best…

9

I have the same issue and cannot set the remote locations key to the one it had before.

Any idea how to update the borgbackup stored key?

I tried to SSH from the nextcloud server into the remote location.

I said there was a new key and asked me to store it. It is stored ok as I could verify.

Yet Borg seems to have it stored somewhere unknown. There is no borg.config file in /var/lib/docker/volumes/nextcloud_aio_mastercontainer/_data/data/ as I never made a successful backup before

I keep getting the same error:

2026-01-04T17:26:48.736377078Z Your public ssh key for borgbackup is: ssh-ed25519 AAAAC... root@nextcloud-aio-borgbackup
2026-01-04T17:26:49.593769708Z Remote: @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
2026-01-04T17:26:49.593803356Z Remote: @    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
2026-01-04T17:26:49.593806948Z Remote: @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
2026-01-04T17:26:49.593809319Z Remote: IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
2026-01-04T17:26:49.593811708Z Remote: Someone could be eavesdropping on you right now (man-in-the-middle attack)!
2026-01-04T17:26:49.593813930Z Remote: It is also possible that a host key has just been changed.
2026-01-04T17:26:49.593816467Z Remote: The fingerprint for the ED25519 key sent by the remote host is
2026-01-04T17:26:49.593818615Z Remote: SHA256:bzCAy2....
2026-01-04T17:26:49.593820930Z Remote: Please contact your system administrator.
2026-01-04T17:26:49.593823041Z Remote: Add correct host key in /root/.ssh/known_hosts to get rid of this message.
2026-01-04T17:26:49.593825337Z Remote: Offending ECDSA key in /root/.ssh/known_hosts:4
2026-01-04T17:26:49.593827393Z Remote: Host key for 192.168.178.31 has changed and you have requested strict checking.
2026-01-04T17:26:49.593829633Z Remote: Host key verification failed.
2026-01-04T17:26:49.597627994Z Connection closed by remote host. Is borg working on the server?
10

The only way i could manage to get around this is to assign a new IP adress and use the IP adress in the remote location rather than the host name.