Failed to connect to RCON serverrcon: authentication failed

[ktpeng2]

Tried to add minecraft backups to mc service that is running a modpack. after service starts up, logs will say

[02:32:37] [Server thread/INFO] [minecraft/DedicatedServer]: Starting remote control listener
[02:32:37] [Server thread/INFO] [minecraft/GenericThread]: Thread RCON Listener started
[02:32:37] [Server thread/INFO] [minecraft/RconThread]: RCON running on 0.0.0.0:25575
...
[02:32:49] [RCON Listener #1/INFO] [minecraft/GenericThread]: Thread RCON Client /xxx.xx.0.3 started
[02:32:49] [RCON Client /172.18.0.3 #2/INFO] [minecraft/RconClient]: Thread RCON Client /xxx.xx.0.3 shutting down
[02:32:59] [RCON Listener #1/INFO] [minecraft/GenericThread]: Thread RCON Client /xxx.xx.0.3 started
[02:32:59] [RCON Client /172.18.0.3 #3/INFO] [minecraft/RconClient]: Thread RCON Client /xxx.xx.0.3 shutting down
[02:33:09] [RCON Listener #1/INFO] [minecraft/GenericThread]: Thread RCON Client /xxx.xx.0.3 started
[02:33:09] [RCON Client /172.18.0.3 #4/INFO] [minecraft/RconClient]: Thread RCON Client /xxx.xx.0.3 shutting down
[02:33:19] [RCON Listener #1/INFO] [minecraft/GenericThread]: Thread RCON Client /xxx.xx.0.3 started
[02:33:19] [RCON Client /172.18.0.3 #5/INFO] [minecraft/RconClient]: Thread RCON Client /xxx.xx.0.3 shutting down
[02:33:29] [RCON Listener #1/INFO] [minecraft/GenericThread]: Thread RCON Client /xxx.xx.0.3 started
[02:33:29] [RCON Client /172.18.0.3 #6/INFO] [minecraft/RconClient]: Thread RCON Client /xxx.xx.0.3 shutting down
[02:33:39] [RCON Listener #1/INFO] [minecraft/GenericThread]: Thread RCON Client /xxx.xx.0.3 started
[02:33:39] [RCON Client /172.18.0.3 #7/INFO] [minecraft/RconClient]: Thread RCON Client /xxx.xx.0.3 shutting down

Checking the logs for the minecraft-backups container has

2025-04-09T02:32:49+0000 ERROR Unable to execute rcon-cli save-on - try 0/5. Retrying in 10s
2025-04-09T02:32:49+0000 ERROR Failure reason: 2025/04/09 02:32:49 Failed to connect to RCON serverrcon: authentication failed
...
2025-04-09T02:33:39+0000 ERROR Unable to execute rcon-cli save-on - try 5/5. Retrying in 10s
2025-04-09T02:33:39+0000 ERROR Failure reason: 2025/04/09 02:33:39 Failed to connect to RCON serverrcon: authentication failed

compose file:

  mc:
    image: itzg/minecraft-server
    tty: true
    stdin_open: true
    ports:
      - "25565:25565"
    environment:
      EULA: true
      MOD_PLATFORM: AUTO_CURSEFORGE
      CF_API_KEY: ${CF_API_KEY}
      CF_SLUG: all-the-mods-10
      VERSION: 1.21.1
      MEMORY: 16G
      CF_OVERRIDES_EXCLUSIONS: |
        shaderpacks/**
      ENABLE_RCON: "true"
    volumes:
      - ./data:/data
  backups:
    image: itzg/mc-backup
    user: "1000"
    depends_on:
      mc:
        condition: service_healthy
    environment:
      BACKUP_INTERVAL: "4h"
      RCON_HOST: mc
      INITIAL_DELAY: 0
      PRUNE_BACKUPS_DAYS: 3
    volumes:
      - ./mc-data:/data:ro
      - ./mc-backup1s:/backups

Any ideas?

[shoodidagen]

###IGNORE THIS POST, SEE REPLY BELOW FROM @itzg

Logs show that the credentials you're attempting to use for RCON connection are incorrect.

Do you know what the RCON password is? (dont post it here)

My first port of call would be to try manually connecting to RCON with the credentials that are generated when the MC container runs to double check if that's working.

Its been a few months since i've messed with itzg images, so i can only recommend general best practice - but perhaps you need to specify the RCON password as a environment variable for both containers.

AFAIK - mc will create a random generated RCON password, but but your backup container has no way of knowing what this is.
Below is how i would tackle it - but perhaps itzg has already provided a solution to this.

It might be a good idea to jump on the discord server and ask there.

mc:
  image: itzg/minecraft-server
  ports:
    - "25565:25565"
  environment:
    EULA: "TRUE"
    ENABLE_RCON: "true"
    RCON_PASSWORD: "mySuperSecret"
    # ...other envs...
  volumes:
    - ./data:/data
  # ...

put the same RCON password in the backup container

backups:
  image: itzg/mc-backup
  depends_on:
    mc:
      condition: service_healthy
  environment:
    RCON_HOST: mc
    RCON_PASSWORD: "mySuperSecret"
    # ...other envs...
  volumes:
    - ./mc-data:/data:ro
    - ./mc-backups:/backups
  # ...

HIGHLY recommend using variables rather than keeping credentials in plain text though.
Instead of using 'environment:' i use 'env-file:' and point to a .env file where all my secrets exist.

[itzg]

@ktpeng2 your /data volumes are not matching between the services -- they need to bind from the same host directory. It first locates the shared password file there. Besides, you have backed up an empty directory with those mismatched.

[shoodidagen]

@itzg Well Spotted!!!!!

ignore my previous post in that case @ktpeng2

[shoodidagen]

@itzg it makes so much sense to get the credential from the file!!!

[itzg]

Designed out of necessity after enhancing the password to be randomly generated by default 😁

...and accidentally turns out to be a way to confirm the backup source directory is the intended one

[shoodidagen]

It makes perfect sense. Very smart!