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====== Ubuntu - SSH - Override socket-activated SSH ======

===== Override socket-activated SSH =====

Occasionally when systemd gets into a broken state, socket activation doesn’t work, which can make a system inaccessible if ssh is the only option. This can be avoided configuring a permanently active SSH daemon that forks for each incoming connection.

To do this directly on the CoreOS machine, begin by replacing the default sshd unit file at **/etc/systemd/system/sshd.service** with the following:

<file bash /etc/systemd/system/sshd.service>
# /etc/systemd/system/sshd.service
[Unit]
Description=OpenSSH server daemon

[Service]
Type=forking
PIDFile=/var/run/sshd.pid
ExecStart=/usr/sbin/sshd
ExecReload=/bin/kill -HUP $MAINPID
KillMode=process
Restart=on-failure
RestartSec=30s

[Install]
WantedBy=multi-user.target
</file>

Next mask the systemd.socket unit:

<code bash>
systemctl mask --now sshd.socket
</code>

Finally, execute a daemon-reload, stop the sshd.socket service, and start the sshd.service unit:

<code bash>
systemctl daemon-reload
systemctl restart sshd.service
</code>

The same configuration can be achieved and an actively listening sshd started by providing user-data like:

cloud-config:

<file bash>
#cloud-config

coreos:
  units:
  - name: sshd.socket
    command: stop
    mask: true

  - name: sshd.service
    command: start
    content: |
      [Unit]
      Description=OpenSSH server daemon

      [Service]
      Type=forking
      PIDFile=/var/run/sshd.pid
      ExecStart=/usr/sbin/sshd
      ExecReload=/bin/kill -HUP $MAINPID
      KillMode=process
      Restart=on-failure
      RestartSec=30s

      [Install]
      WantedBy=multi-user.target

write_files:
  - path: "/var/run/sshd.pid"
    permissions: "0644"
    owner: "root"
</file>

    
Ignition:

<file bash>
{
  "ignition": {
    "version": "2.0.0"
  },
  "systemd": {
    "units": [
      {
        "name": "sshd.socket",
        "mask": true
      },
      {
        "name": "sshd.service",
        "enable": true,
        "contents": "[Unit]\nDescription=OpenSSH server daemon\n[Service]\nType=forking\nPIDFile=/var/run/sshd.pid\nExecStart=/usr/sbin/sshd\nExecReload=/bin/kill -HUP $MAINPID\nKillMode=process\nRestart=on-failure\nRestartSec=30s\n[Install]\nWantedBy=multi-user.target\n"
      }
    ]
  }
}
</file>