mjhjmtu

Today, after doing updates in Debian Stretch, it started displaying these warnings when restarting the ssh service with my current config:

/etc/ssh/sshd_config line 17: Deprecated option KeyRegenerationInterval
/etc/ssh/sshd_config line 18: Deprecated option ServerKeyBits
/etc/ssh/sshd_config line 29: Deprecated option RSAAuthentication
/etc/ssh/sshd_config line 36: Deprecated option RhostsRSAAuthentication
[....] Restarting OpenBSD Secure Shell server: sshd
/etc/ssh/sshd_config line 17: Deprecated option KeyRegenerationInterval
/etc/ssh/sshd_config line 18: Deprecated option ServerKeyBits
/etc/ssh/sshd_config line 29: Deprecated option RSAAuthentication
/etc/ssh/sshd_config line 36: Deprecated option RhostsRSAAuthentication

What is happening here?

Using Debian 9 with OpenSSH 7.4

In the current Stretch update, openssh version changed from 7.3 to 7.4, released on 2016-Dec-19.

As it can be inferred from the Release notes, and from @Jakuje comments, OpenSSH maintainers have removed the corresponding configuration options for good, as they are obsolete.

So the lines can be safely removed.

Also, take head of:

Future deprecation notice

We plan on retiring more legacy cryptography in future releases,
specifically:

• In approximately August 2017, removing remaining support for the
  
  SSH v.1 protocol (client-only and currently compile-time disabled).




• In the same release, removing support for Blowfish and RC4 ciphers
  and the RIPE-MD160 HMAC. (These are currently run-time disabled).




• Refusing all RSA keys smaller than 1024 bits (the current minimum
  
  is 768 bits)




• The next release of OpenSSH will remove support for running sshd(8)
  with privilege separation disabled.




• The next release of portable OpenSSH will remove support for
  
  OpenSSL version prior to 1.0.1.

You can remove deprecated configuration lines with this:

sed -i '/KeyRegenerationInterval/d' /etc/ssh/sshd_config
sed -i '/ServerKeyBits/d' /etc/ssh/sshd_config
sed -i '/RSAAuthentication/d' /etc/ssh/sshd_config
sed -i '/RhostsRSAAuthentication/d' /etc/ssh/sshd_config
sed -i '/UsePrivilegeSeparation/d' /etc/ssh/sshd_config

And restart SSH daemon: systemctl restart sshd