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[Seyeong Kim <2111226@xxxxxxxxxxxxxxxxxx>]

Thanks for the clarification. I understand what you said.

So, I installed fresh 22.04 and 24.04 (without IP settting) then added
just two line in ssh.socket.d as you mentioned in description.
(confirmed that ssh.socket is active)

then tried to access them.
22.04 success.
24.04 failed.

Thanks.

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https://bugs.launchpad.net/bugs/2111226

Title:
  ssh@.service is still needed for hv_sock in Noble release

Status in openssh package in Ubuntu:
  Fix Released
Status in openssh source package in Jammy:
  Fix Released
Status in openssh source package in Noble:
  In Progress
Status in openssh source package in Oracular:
  Fix Released
Status in openssh source package in Plucky:
  Fix Released
Status in openssh source package in Questing:
  Fix Released

Bug description:
  [Impact]
  On Ubuntu VMs running under Microsoft Hyper-V, users commonly rely on 'hv_sock' (Hyper-V socket) to enable seamless SSH access using the 'hvc.exe' tool on the Windows host. This works correctly on Ubuntu Jammy and earlier ( and Oracular and later with different mechanism ), but fails silently in Noble due to a missing 'ssh@.service' systemd unit.

  The failure is due to a combination of systemd and OpenSSH changes:

  * In older versions (e.g., Jammy with systemd 249), the 'ssh@.service' unit was  relied upon for socket activation ('Accept=yes' mode) and the unit file exists.
  * With the Ubuntu Kinetic release, 'ssh@.service' was removed, and no template unit was shipped by default.
  * systemd introduced systemd-ssh-generator in version 256 checks sshd@.service unit and openssh provides sshd@.service unit.
  * Ubuntu Noble ships with systemd 255, which lacks this feature, resulting in the absence of sshd@.service.
  * Debian has restored a static 'sshd@.service' template in recent OpenSSH packaging. Noble’s OpenSSH package currently lacks it.

  As a result, the typical 'ssh.socket' activation workflow fails on
  Noble, breaking compatibility for 'hv_sock' SSH access.

  This issue affects all Ubuntu series between Kinetic and Noble
  (inclusive) where:

  * systemd < 256 is used (no dynamic generator)
  * 'ssh@.service' has been removed

  But I think Noble only needs SRU for now since the others are EOL.

  Basically user should setup ssh.socket correctly to use hv_sock(e.g
  changing Accept=no to Accept=yes) but creating whole service file
  (ssh@.service) might be different story since Jammy was working fine.

  [Test Case]

  1. Launch a Noble VM on Hyper-V.
  2. Ensure the 'hv_sock' kernel module is loaded:

     echo 'hv_sock' >> /etc/modules

  3. Adding the socket conf for SSH to listen on vsock:

     # cat > /etc/systemd/system/ssh.socket.d/vsock.conf << EOF
     [Socket]
     ListenStream=vsock::22
     EOF

  4. Reload and reconfigure systemd units:

     systemctl disable ssh.service
     systemctl daemon-reload
     systemctl stop ssh.service
     systemctl enable ssh.socket
     systemctl start ssh.socket

  5. Attempt to connect from the Hyper-V host:

     hvc ssh user@ubuntu-vm

  Expected Result: Connection succeeds and SSH login is presented.
  Actual Result: The connection hangs. No systemd unit is spawned due to missing 'ssh@.service'.

  [Where problems could occur]
  Adding a static 'ssh@.service' template unit, as done in Debian(although it is sshd@.service), is unlikely to interfere with traditional SSH service setups (i.e., 'ssh.service'). The '@' template only activates in conjunction with 'Accept=yes' sockets and does not conflict with existing unit files. Also it was working in Jammy.

  [Other Info]

  * Debian commit restoring 'sshd@.service'
  https://salsa.debian.org/ssh-team/openssh/-/commit/eb25ab611967996a0d57b4ee565faa7de58b41f6
  * systemd 256 adding 'systemd-ssh-generator'
  https://github.com/systemd/systemd/commit/0e3220684c6184a2f70396d991200ae207a25377
  * OpenSSH in Ubuntu removed 'ssh@.service' in
  https://launchpadlibrarian.net/619116456/openssh_1%3A9.0p1-1_1%3A9.0p1-1ubuntu1.diff.gz
  during Kinetic development.

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