Troubleshooting the Docker Daemon

--- title: Troubleshooting the Docker daemon description: Learn how to troubleshoot errors and misconfigurations in the Docker daemon keywords: | docker, daemon, configuration, troubleshooting, error, fail to start, networking, dns resolver, ip forwarding, dnsmasq, firewall, Cannot connect to the Docker daemon. Is 'docker daemon' running on this host? aliases: - /engine/install/troubleshoot/ - /storage/troubleshooting_volume_errors/ - /config/daemon/troubleshooting/ tags: [Troubleshooting] --- This page describes how to troubleshoot and debug the daemon if you run into issues. You can turn on debugging on the daemon to learn about the runtime activity of the daemon and to aid in troubleshooting. If the daemon is unresponsive, you can also [force a full stack trace](logs.md#force-a-stack-trace-to-be-logged) of all threads to be added to the daemon log by sending the `SIGUSR` signal to the Docker daemon. ## Daemon ### Unable to connect to the Docker daemon ```text Cannot connect to the Docker daemon. Is 'docker daemon' running on this host? ``` This error may indicate: - The Docker daemon isn't running on your system. Start the daemon and try running the command again. - Your Docker client is attempting to connect to a Docker daemon on a different host, and that host is unreachable. ### Check whether Docker is running The operating-system independent way to check whether Docker is running is to ask Docker, using the `docker info` command. You can also use operating system utilities, such as `sudo systemctl is-active docker` or `sudo status docker` or `sudo service docker status`, or checking the service status using Windows utilities. Finally, you can check in the process list for the `dockerd` process, using commands like `ps` or `top`. #### Check which host your client is connecting to To see which host your client is connecting to, check the value of the `DOCKER_HOST` variable in your environment. ```console $ env | grep DOCKER_HOST ``` If this command returns a value, the Docker client is set to connect to a Docker daemon running on that host. If it's unset, the Docker client is set to connect to the Docker daemon running on the local host. If it's set in error, use the following command to unset it: ```console $ unset DOCKER_HOST ``` You may need to edit your environment in files such as `~/.bashrc` or `~/.profile` to prevent the `DOCKER_HOST` variable from being set erroneously. If `DOCKER_HOST` is set as intended, verify that the Docker daemon is running on the remote host and that a firewall or network outage isn't preventing you from connecting. ### Troubleshoot conflicts between the `daemon.json` and startup scripts If you use a `daemon.json` file and also pass options to the `dockerd` command manually or using start-up scripts, and these options conflict, Docker fails to start with an error such as: ```text unable to configure the Docker daemon with file /etc/docker/daemon.json: the following directives are specified both as a flag and in the configuration file: hosts: (from flag: [unix:///var/run/docker.sock], from file: [tcp://127.0.0.1:2376]) ``` If you see an error similar to this one and you are starting the daemon manually with flags, you may need to adjust your flags or the `daemon.json` to remove the conflict. > [!NOTE] > > If you see this specific error message about `hosts`, continue to the > [next section](#configure-the-daemon-host-with-systemd) > for a workaround. If you are starting Docker using your operating system's init scripts, you may need to override the defaults in these scripts in ways that are specific to the operating system. #### Configure the daemon host with systemd One notable example of a configuration conflict that's difficult to troubleshoot is when you want to specify a different daemon address from the default. Docker listens on a socket by default. On Debian and Ubuntu systems using `systemd`, this means that a host flag `-H` is always used when starting

This document provides guidance on troubleshooting issues with the Docker daemon. It covers enabling debug mode, forcing a stack trace, resolving common errors such as inability to connect to the daemon, and resolving conflicts between `daemon.json` configuration and startup scripts. It also provides tips on checking Docker's running status, host connection, and configuring the daemon host with systemd.