[Bug 1674330] Re: Please consider dropping /etc/network/if-up.d/openssh-server

[Andreas Hasenack]

Hm, cosmic rays, my comment wasn't saved :/

I also did some testing and the only case that won't work without either
the hook, or using the IP_FREEBIND socket option, is when ListenAddress
is set to an IP that doesn't exist yet. Meaning, sshd won't listen on
that IP when it comes up.

Maybe people caught in this corner case should use 0.0.0.0 (or the IPv6
equivalent) for ListenAddress and firewall rules to control which
traffic can reach the sshd daemon. That's the only way in artful now
anyway.

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Title:
  Please consider dropping /etc/network/if-up.d/openssh-server

Status in openssh package in Ubuntu:
  New

Bug description:
  The /etc/network/if-up.d/openssh-server hack was introduced ten years ago [1] as a response to bug 
  103436. At least from today's perspective this isn't justified:

  I can't seem to be able to actually reproduce that issue: I can start
  a VM with no network interfaces, remove the above hack, then start
  sshd, then bring up an ethernet interface, and I can connect to ssh
  via ethernet just fine. Also, e. g. Fedora has no counterpart of this
  hack, and these days a lot of people would complain if that would
  cause problems, as hotpluggable/roaming network devices are
  everywhere.

  The hack introduces a race: you run into connection errors after
  bringing up a new interface as sshd stops listening briefly while
  being reloaded. That's the reason why I looked at it, as this
  regularly happens in upstream's cockpit integration tests.

  Also, /etc/network/if-up.d/ isn't being run when using
  networkd/netplan, i. e. in more recent Ubuntnu cloud instances. So far
  this doesn't seem to have caused any issues.

  I asked the original reporter of bug 103436 for some details, and to
  check whether that hack is still necessary. There is actually a
  proposed patch upstream [2] to use IP_FREEBIND, which is the modern
  solution to listening to all "future" interfaces as well. But at least
  for the majority of cases it seems to work fine without that even.

  So I wonder if it's time to bury that hack?

  [1] https://anonscm.debian.org/cgit/pkg-ssh/openssh.git/commit/?id=ba6b55ed6
  [2] https://bugzilla.mindrot.org/show_bug.cgi?id=2512

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