Can't run containers when running in LXC (`open sysctl net.ipv4.ip_unprivileged_port_start file: reopen fd 8: permission denied`)

[privatefound]

Description

i've tried a lot of distro, debian 12/13 and like 3 versions of ubuntu, but i keep getting this error running hello-world and also other containers (ps. running via root and also with other users) the users are inside docker group and i freshly installed docker from the official website guide https://docs.docker.com/engine/install/debian/ this is the error i get "docker: Error response from daemon: failed to create task for container: failed to create shim task: OCI runtime create failed: runc create failed: unable to start container process: error during container init: open sysctl net.ipv4.ip_unprivileged_port_start file: reopen fd 8: permission denied: unknown" can you guys help me out? that's not my first installation i got a lot of debian and ubuntu servers running docker containers...

Reproduce

1. Install debian 12 or 13 CT on proxmox 9
2. install docker via https://docs.docker.com/engine/install/debian/
3. run docker run hello-world via root or via another user inside docker group

Expected behavior

run the docker

docker version

root@testdc:/home/sec# docker version
Client: Docker Engine - Community
 Version:           28.5.1
 API version:       1.51
 Go version:        go1.24.8
 Git commit:        e180ab8
 Built:             Wed Oct  8 12:17:24 2025
 OS/Arch:           linux/amd64
 Context:           default

Server: Docker Engine - Community
 Engine:
  Version:          28.5.1
  API version:      1.51 (minimum version 1.24)
  Go version:       go1.24.8
  Git commit:       f8215cc
  Built:            Wed Oct  8 12:17:24 2025
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          v1.7.28
  GitCommit:        b98a3aace656320842a23f4a392a33f46af97866
 runc:
  Version:          1.3.3
  GitCommit:        v1.3.3-0-gd842d771
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

docker info

Client: Docker Engine - Community
 Version:    28.5.1
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.29.1
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.40.3
    Path:     /usr/libexec/docker/cli-plugins/docker-compose

Server:
 Containers: 14
  Running: 0
  Paused: 0
  Stopped: 14
 Images: 4
 Server Version: 28.5.1
 Storage Driver: overlay2
  Backing Filesystem: zfs
  Supports d_type: true
  Using metacopy: false
  Native Overlay Diff: true
  userxattr: true
 Logging Driver: json-file
 Cgroup Driver: systemd
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local splunk syslog
 CDI spec directories:
  /etc/cdi
  /var/run/cdi
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: b98a3aace656320842a23f4a392a33f46af97866
 runc version: v1.3.3-0-gd842d771
 init version: de40ad0
 Security Options:
  seccomp
   Profile: builtin
  cgroupns
 Kernel Version: 6.14.11-2-pve
 Operating System: Debian GNU/Linux 12 (bookworm)
 OSType: linux
 Architecture: x86_64
 CPUs: 2
 Total Memory: 7.906GiB
 Name: testdc
 ID: 34b2a960-be2e-452d-b92c-e7212b38b29d
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Experimental: false
 Insecure Registries:
  ::1/128
  127.0.0.0/8
 Live Restore Enabled: false

Additional Info

No response

[vvoland]

Possibly related to the new runc 1.3.3?

https://github.com/opencontainers/runc/releases/tag/v1.3.3

[vvoland]

EDIT: For a temporary workaround, see: containerd/containerd#12484 (comment)

Old comment

Could you try downgrading the containerd.io package to a previous one that doesn't include the new runc?

EDIT: Please note that THIS IS NOT A SOLUTION! Downgrading makes you vulnerable to 3 CVEs that were fixed by the new version.
This is only for diagnostic purposes.

sudo apt install containerd.io=1.7.28-1~debian.12~bookworm

[thaJeztah]

related discussion in the containerd repository mentions that (if you're using LXC), that setting AppArmor to unconfined can be used as a workaround;

• [Docker, Inc.'s package] net.ipv4.ip_unprivileged_port_start permission denied only on 1.7.28-2~debian.13~trixie containerd/containerd#12484

And further exploration in this LXC ticket;

• nested docker 28.5.2 unable start containers; net.ipv4.ip_unprivileged_port_start permission denied lxc/incus#2623

[cyphar]

I've figured out the cause -- see containerd/containerd#12484 (comment).

[privatefound]

Ty guys 👯