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Docker usually fails to start on Linux for one of four reasons: the daemon is stopped or crashing, the client is pointed at the wrong host, Docker’s configuration is invalid, or the host lacks a required resource such as disk space. Start with the service state and the exact daemon log; do not guess from the client error alone.
Run the checks below in order. They distinguish a Docker Engine startup failure from a shell, permission, rootless-mode, or remote-host problem, then give you a safe recovery path.
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Open a terminal and run:
docker info
sudo systemctl is-active docker
sudo systemctl status docker --no-pagerdocker info asks the Docker client for server information. If it reports Cannot connect to the Docker daemon, the daemon may be stopped, or the client may be targeting a socket or host that is not reachable. The two systemctl commands show whether the local system service is active and why systemd considers it failed.
Check for an unintended remote host
Docker honors the DOCKER_HOST environment variable. A stale value can make a healthy local daemon appear to be down:
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env | grep DOCKER_HOSTIf the command prints a host you did not intend to use, clear it for the current shell and retry:
unset DOCKER_HOST
docker infoAlso inspect shell startup files, scripts, CI variables, and service definitions if the variable returns in a new terminal. If you intentionally use a remote daemon, test that host and its network path instead of restarting the local service.
2. Read the exact startup error before changing anything
Systemd’s status summary is often too short to identify the cause. Read the Docker unit’s journal:
sudo journalctl -xu docker.serviceFor the current boot, a less verbose view is useful when you need to isolate the latest failure:
sudo journalctl -u docker.service -b --no-pagerTo watch a new start attempt live, use two terminals. In the first, follow the journal:
sudo journalctl -u docker.service -fIn the second, start Docker:
sudo systemctl start dockerLook for the first meaningful error, not only the final message that systemd prints after several retries. Depending on the distribution and how Docker was installed, older records may also be in /var/log/syslog, /var/log/messages, /var/log/daemon.log, or /var/log/docker.log. Search those files for the same boot time and error text.
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3. Start the service and check whether it stays running
If the service is inactive but the journal shows no configuration or resource error, start it explicitly:
sudo systemctl start docker
sudo systemctl status docker --no-pagerIf it starts, make it start automatically at boot:
sudo systemctl enable dockerDo not treat a successful start command as proof that the problem is solved. A daemon can start and exit seconds later. Run sudo systemctl status docker --no-pager again and test with docker info. If the unit changes to failed, return to the journal and use the first specific error as the branch point.
When systemd says the unit is missing
If systemd reports that docker.service does not exist, Docker Engine may not be installed as a systemd service, the installation is incomplete, or the distribution uses a differently named package. Check what systemd knows:
systemctl list-unit-files | grep -i docker
command -v docker
docker versionA client binary alone does not provide a running daemon. Use the installation method appropriate for your Linux distribution and then repeat the service-state checks.
4. Check containerd and other runtime dependencies
Docker Engine normally relies on a container runtime. If the journal mentions containerd, a runtime socket, or a failed dependency, inspect both units:
sudo systemctl status containerd --no-pager
sudo journalctl -u containerd.service -b --no-pagerIf containerd is stopped and its own log shows a recoverable failure, restart it before Docker:
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sudo systemctl restart containerd
sudo systemctl restart docker
sudo systemctl status docker --no-pagerDo not repeatedly restart both services while an underlying configuration or storage error remains. Fix the first containerd error, then make one clean Docker start attempt so the resulting log is easy to interpret.
5. Validate daemon.json and startup flags
A malformed or incompatible daemon configuration is one of the most common reasons for an immediate exit. The usual file is /etc/docker/daemon.json, although a custom service unit can specify another path. Inspect the file and the unit’s effective command line:
sudo sed -n '1,240p' /etc/docker/daemon.json
systemctl cat docker.serviceCheck the JSON syntax without starting Docker:
sudo python3 -m json.tool /etc/docker/daemon.jsonValid JSON requires double-quoted keys and strings, commas between properties, and no trailing comma after the final property. If the file contains a setting that duplicates a command-line flag in the systemd unit, the daemon can reject the combination. The journal will normally identify the conflicting option.
Recover from a bad configuration safely
- Make a backup before editing:
sudo cp /etc/docker/daemon.json /etc/docker/daemon.json.backup. - Correct only the invalid or conflicting property identified in the journal. Avoid deleting unrelated registry, storage, or proxy settings without recording them.
- Run the JSON check again.
- Restart Docker and immediately inspect its status and journal.
If Docker started only after you moved the file aside, restore it and reintroduce settings one at a time. That identifies the option that is incompatible with your installed Engine or host.
6. Check disk space, inodes, and Docker’s data directory
Docker can fail before it creates a container when the filesystem containing its data directory is full or out of inodes. Check both ordinary capacity and inode capacity:
df -h
df -i
sudo du -sh /var/lib/docker 2>/dev/nullRemove the accidental leading space before df -i if your shell treats it specially; the intended command is simply df -i. Pay particular attention to the filesystem containing /var/lib/docker, and check separate mounts used for logs or temporary files.
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Do not delete files directly from /var/lib/docker. Docker’s metadata and layer references can become inconsistent. Once the daemon is running, use Docker’s own cleanup commands and review their output before confirming removal:
docker system df
docker image prune
docker container pruneThese commands remove different classes of unused objects; pruning containers or images can break a workflow that depends on them. If Docker cannot start at all, free space elsewhere on the affected filesystem first, then start Docker and perform cleanup through the CLI.
7. Check permissions, the Unix socket, and rootless mode
The default client socket is normally accessible to root and to users in the Docker group. Test whether the failure is only a permission problem:
sudo docker info
ls -l /var/run/docker.sockIf sudo docker info works but unprivileged docker info does not, the daemon is running; your user lacks access to its socket or is using a different socket. Add the user to the Docker group only if that matches your security policy:
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sudo usermod -aG docker "$USER"Sign out and back in, or start a new group-aware shell, before testing again. Membership in the Docker group is effectively powerful host access, so do not grant it casually on a multi-user machine.
Rootless Docker
Rootless installations use a per-user systemd service and a user-specific socket rather than the system-wide docker.service. Check the user unit:
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systemctl --user status docker --no-pager
systemctl --user start dockerCheck DOCKER_HOST again if the client is connecting to the wrong socket. Do not alternate between rootful commands such as sudo systemctl restart docker and rootless commands while diagnosing one installation; they control different daemons.
8. Investigate network, bridge, and port errors
If the journal mentions a bridge, network controller, address pool, iptables, nftables, or a port already in use, capture the exact line and inspect the host state:
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sudo ss -ltnupA stale bridge interface, an address range that overlaps with the host or VPN, or a daemon option requiring a firewall backend unavailable on the system can prevent startup. Correct the specific setting named in the journal rather than replacing the entire networking configuration. If you changed firewall rules recently, verify that the firewall service and Docker’s selected backend agree with the distribution’s current setup.
9. Check cgroups, kernels, and security controls
On newer Linux systems, Docker needs a functioning cgroup hierarchy and a kernel configuration compatible with the selected runtime. Errors mentioning cgroups, namespaces, overlay storage, AppArmor, or SELinux point to the host rather than a stopped service.
- Record the kernel and distribution:
uname -aand your distribution’s release information. - Check whether a security policy denied an operation. SELinux and AppArmor messages may appear in their own audit logs as well as the Docker journal.
- Confirm that the filesystem used for Docker’s storage supports the configured storage driver and required extended attributes.
- If this is a virtual machine or nested container, verify that the outer host exposes the kernel features Docker needs.
Do not disable mandatory access controls or switch storage drivers as a first response. Use the denial or unsupported-feature message to choose a targeted change, then restart Docker and verify that the same error is gone.
10. Use the error text to choose the shortest fix
| Observed message or symptom | Most useful next check | Typical corrective action |
|---|---|---|
Cannot connect to the Docker daemon |
systemctl is-active docker and env | grep DOCKER_HOST |
Start the local service or unset an unintended remote-host variable. |
Unit docker.service could not be found |
systemctl list-unit-files | grep -i docker |
Complete the Engine installation or identify the service name used by that installation. |
Service starts, then immediately becomes failed |
sudo journalctl -xu docker.service |
Fix the first configuration, dependency, storage, or permission error in the journal. |
| JSON parse, unknown option, or conflicting directive | sudo python3 -m json.tool /etc/docker/daemon.json |
Correct JSON or remove the duplicate/incompatible daemon option after backing up the file. |
No space left on device |
df -h and df -i |
Free capacity or inodes on the filesystem containing Docker’s data, then clean unused objects through Docker. |
| Permission denied on the Docker socket | sudo docker info and ls -l /var/run/docker.sock |
Use the correct socket, refresh group membership, or follow the rootless service path. |
| Bridge, firewall, or address-pool failure | Journal plus ip link and sudo ss -ltnup |
Resolve the named interface, overlapping range, firewall backend, or occupied port. |
| cgroup, security-policy, or storage-driver denial | Docker journal and the host’s security audit log | Correct the kernel, policy, virtualization, or filesystem prerequisite identified by the denial. |
11. A clean recovery sequence after the cause is fixed
Once you have corrected the specific fault, use one controlled restart and verify each layer:
- Reload systemd only if you changed a unit file or drop-in:
sudo systemctl daemon-reload. - Restart a dependency first if its log showed the failure:
sudo systemctl restart containerd. - Restart Docker:
sudo systemctl restart docker. - Confirm the unit remains active:
sudo systemctl is-active docker. - Read the latest startup entries:
sudo journalctl -u docker.service -b -n 100 --no-pager. - Confirm the client can reach the server:
docker info. - Run a harmless container test appropriate for your environment, then check
docker ps.
If the restart fails again, do not keep repeating it. Preserve the journal output, the effective service configuration, the daemon configuration, and the host resource checks. Those records reveal whether the failure is deterministic and prevent a successful but temporary start from hiding the original cause.
12. Prevent the next startup failure
- Monitor free disk space and inode usage on the Docker data filesystem.
- Keep a dated backup of
/etc/docker/daemon.jsonand record why each non-default option exists. - After changing firewall, VPN, kernel, security-policy, or virtualization settings, test a Docker restart before deploying workloads.
- Use one clearly documented mode—rootful system service or rootless user service—and keep its socket selection explicit.
- When an upgrade changes Docker, containerd, the kernel, or the distribution, review the startup journal immediately rather than waiting for an application deployment to expose a problem.
The decisive diagnostic is always the daemon’s own startup log. Service state tells you whether Docker is running; the journal tells you why it is not.
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