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keeping ssh host keys up-to-date with monkeysphere
Posted by dkg on Thu 23 Jul 2009 at 23:04
Enrico posted a neat trick to track the SSH host keys of debian machines, thanks to Zobel. I wanted to mention monkeysphere, a project i'm involved with which provides a more generalized structure for doing this kind of update by taking advantage of the OpenPGP Web of Trust to distribute and authenticate SSH keys.

Enrico's known_hosts update strategy is nice, but:

  • it's centralized,
  • it's useful only for debian developers (and only when connecting to debian machines),
  • it relies on users regularly running an update they wouldn't need to do otherwise (re-fetching the file from master.debian.org via scp) and
  • it relies on the maintainers of master.debian.org:
    • to avoid compromise (there are a ton of other problems if master was compromised, of course),
    • to keep /etc/ssh/ssh_known_hosts up-to-date, and
    • not to change the host key for master itself (or the user's regular scp updates would fail).

These are relatively small flaws, and as a project debian is able to work around them because we have infrastructure in place like the machines database (though checking the machines db manually is tedious and therefore error-prone). But most other projects don't have that level of organization, and the process doesn't scale to other projects we (or our users) might be involved in. And other projects (including debian, i'd think) might prefer to have a less centralized process, to minimize bottlenecks and single points of failure.

Check out Monkeysphere's documentation for a server administrator for a quick rundown about how to easily publish your SSH host keys via the Web of Trust (it's not mutually-exclusive with the technique Enrico describes).

And this is just part of what the monkeysphere can do: using the same web of trust, monkeysphere is capable of helping a host authenticate ssh users based on their OpenPGP identities, which gives full re-keying and revocation functionality for these accounts. But that's a separate discussion!