Openssh 7.0, released 2015-08-11, deprecated the use of DSA (ssh-dss)
keys and RSA keys smaller than 1024 bits [0]. We have been applying
some backwards compatibility configuration changes to ssh bastion
servers in both Cloud VPS and Toolforge for some time to continue to
support old keys using these deprecated algorithms. I was supposed to
announce this to the community about 1.5 years ago, but apparently I
did not [1].
We have noticed with the introduction of Debian Stretch ssh bastion
servers running Openssh 7.4 that users with DSA keys (and possibly
short RSA keys) are being denied access by the newer software. The
easiest fix for this is for users to generate new keys and upload
their new public key using the form at
<https://toolsadmin.wikimedia.org/profile/settings/ssh-keys> or
<https://wikitech.wikimedia.org/wiki/Special:Preferences#mw-prefsection-open…>.
We currently recommend using either ed25519 or 4096-bit RSA keys. See
<https://wikitech.wikimedia.org/wiki/Production_shell_access#Generating_your…>
for more information.
[0]: https://www.openssh.com/txt/release-7.0
[1]: https://phabricator.wikimedia.org/T168433
Bryan, on behalf of the Wikimedia Cloud Services team
--
Bryan Davis Wikimedia Foundation <bd808(a)wikimedia.org>
[[m:User:BDavis_(WMF)]] Manager, Technical Engagement Boise, ID USA
irc: bd808 v:415.839.6885 x6855