Keys generated with 8.0p1 server is rebooted and made up with 5.3p1 server. But sshd is not able to read the keys generated with new version 8.0p1. How to fix this problem? LOGS: ===== ======================== debug3: checking syntax for 'Match (null)' debug1: sshd version OpenSSH_5.3p1 debug3: Not a RSA1 key file /etc/ssh/ssh_host_rsa_key. debug1: PEM_read_PrivateKey failed debug1: read PEM private key done: type <unknown> Could not load host key: /etc/ssh/ssh_host_rsa_key debug3: Not a RSA1 key file /etc/ssh/ssh_host_dsa_key. debug1: PEM_read_PrivateKey failed debug1: read PEM private key done: type <unknown> Could not load host key: /etc/ssh/ssh_host_dsa_key Disabling protocol version 2. Could not load host key sshd: no hostkeys available -- exiting.
The OpenSSH since 7.8 generate new keys in the new format that is not understood by OpenSSH 5.3. The release notes [1] explain it in a whole: * ssh-keygen(1): write OpenSSH format private keys by default instead of using OpenSSL's PEM format. The OpenSSH format, supported in OpenSSH releases since 2014 and described in the PROTOCOL.key file in the source distribution, offers substantially better protection against offline password guessing and supports key comments in private keys. If necessary, it is possible to write old PEM-style keys by adding "-m PEM" to ssh-keygen's arguments when generating or updating a key. [1] http://www.openssh.com/txt/release-7.8
Thanks Jakub.
closing resolved bugs as of 8.6p1 release