After updating Ubuntu to 21.04 from 20.10 docker seems to be running in:

/run/docker.sock

but the docker client goes to look for it on:

/var/run/docker.sock

How can I fix this?

Elaborating a bit, running a docker command I get:

$ docker ps

Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

But the docker service seems to be running with the docker socket in another location (out of var):

$ sudo systemctl status docker

● docker.service - Docker Application Container Engine

     Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled)

     Active: active (running) since Mon 2021-05-17 10:14:49 CEST; 5s ago

TriggeredBy: ● docker.socket

       Docs: https://docs.docker.com

   Main PID: 10163 (dockerd)

      Tasks: 18

     Memory: 40.5M

     CGroup: /system.slice/docker.service

             └─10163 /usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock



may 17 10:14:49 xps-laptop dockerd[10163]: time="2021-05-17T10:14:49.599580310+02:00" level=warning msg="Your kernel does not support CPU realtime scheduler"

may 17 10:14:49 xps-laptop dockerd[10163]: time="2021-05-17T10:14:49.599624934+02:00" level=warning msg="Your kernel does not support cgroup blkio weight"

may 17 10:14:49 xps-laptop dockerd[10163]: time="2021-05-17T10:14:49.599642841+02:00" level=warning msg="Your kernel does not support cgroup blkio weight_device"

may 17 10:14:49 xps-laptop dockerd[10163]: time="2021-05-17T10:14:49.599907138+02:00" level=info msg="Loading containers: start."

may 17 10:14:49 xps-laptop dockerd[10163]: time="2021-05-17T10:14:49.740160234+02:00" level=info msg="Default bridge (docker0) is assigned with an IP address 172.17.0.0/16. Daemon option --bip can be use>

may 17 10:14:49 xps-laptop dockerd[10163]: time="2021-05-17T10:14:49.838137738+02:00" level=info msg="Loading containers: done."

may 17 10:14:49 xps-laptop dockerd[10163]: time="2021-05-17T10:14:49.854381886+02:00" level=info msg="Docker daemon" commit=20.10.2-0ubuntu2 graphdriver(s)=overlay2 version=20.10.2

may 17 10:14:49 xps-laptop dockerd[10163]: time="2021-05-17T10:14:49.854462072+02:00" level=info msg="Daemon has completed initialization"

may 17 10:14:49 xps-laptop systemd[1]: Started Docker Application Container Engine.

may 17 10:14:49 xps-laptop dockerd[10163]: time="2021-05-17T10:14:49.879020578+02:00" level=info msg="API listen on /run/docker.sock"

Edit (content of /lib/systemd/system/docker.socket):

$ cat /lib/systemd/system/docker.socket

[Unit]

Description=Docker Socket for the API



[Socket]

ListenStream=/var/run/docker.sock

SocketMode=0660

SocketUser=root

SocketGroup=docker



[Install]

WantedBy=sockets.target

Edit 2 (content of /lib/systemd/system/docker.service):

$ cat /lib/systemd/system/docker.service

[Unit]

Description=Docker Application Container Engine

Documentation=https://docs.docker.com

After=network-online.target firewalld.service containerd.service

Wants=network-online.target

Requires=docker.socket

Wants=containerd.service



[Service]

Type=notify

# the default is not to use systemd for cgroups because the delegate issues still

# exists and systemd currently does not support the cgroup feature set required

# for containers run by docker

ExecStart=/usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock

ExecReload=/bin/kill -s HUP $MAINPID

TimeoutSec=0

RestartSec=2

Restart=always



# Note that StartLimit* options were moved from "Service" to "Unit" in systemd 229.

# Both the old, and new location are accepted by systemd 229 and up, so using the old location

# to make them work for either version of systemd.

StartLimitBurst=3



# Note that StartLimitInterval was renamed to StartLimitIntervalSec in systemd 230.

# Both the old, and new name are accepted by systemd 230 and up, so using the old name to make

# this option work for either version of systemd.

StartLimitInterval=60s



# Having non-zero Limit*s causes performance problems due to accounting overhead

# in the kernel. We recommend using cgroups to do container-local accounting.

LimitNOFILE=infinity

LimitNPROC=infinity

LimitCORE=infinity



# Comment TasksMax if your systemd version does not support it.

# Only systemd 226 and above support this option.

TasksMax=infinity



# set delegate yes so that systemd does not reset the cgroups of docker containers

Delegate=yes



# kill only the docker process, not all processes in the cgroup

KillMode=process

OOMScoreAdjust=-500



[Install]

WantedBy=multi-user.target