If 'rsa-sha2-512' Host Key Algorithm is the only algorithm in sshd_coonfig or sshd is forced to start using only 'rsa-sha2-512' as Host Key Algorithm no algorithm is offered to client during negotiation. Start server with: sshd -oHostKeyAlgorithms="rsa-sha2-512" When client tries to connect no HostKeyAlgorithm is offered. debug2: peer server KEXINIT proposal debug2: KEX algorithms: curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1 debug2: host key algorithms: debug2: ciphers ctos: aes256-ctr,aes192-ctr,aes128-ctr debug2: ciphers stoc: aes256-ctr,aes192-ctr,aes128-ctr and connection is not established. Same problem with "rsa-sha2-256".
There's a fix for this in the patch at https://bugzilla.mindrot.org/show_bug.cgi?id=2799
This was fixed by the following commit and will be in OpenSSH 7.8: commit 4ba0d54794814ec0de1ec87987d0c3b89379b436 Author: djm@openbsd.org <djm@openbsd.org> Date: Tue Jul 3 11:39:54 2018 +0000 upstream: Improve strictness and control over RSA-SHA2 signature In ssh, when an agent fails to return a RSA-SHA2 signature when requested and falls back to RSA-SHA1 instead, retry the signature to ensure that the public key algorithm sent in the SSH_MSG_USERAUTH matches the one in the signature itself. In sshd, strictly enforce that the public key algorithm sent in the SSH_MSG_USERAUTH message matches what appears in the signature. Make the sshd_config PubkeyAcceptedKeyTypes and HostbasedAcceptedKeyTypes options control accepted signature algorithms (previously they selected supported key types). This allows these options to ban RSA-SHA1 in favour of RSA-SHA2. Add new signature algorithms "rsa-sha2-256-cert-v01@openssh.com" and "rsa-sha2-512-cert-v01@openssh.com" to force use of RSA-SHA2 signatures with certificate keys. feedback and ok markus@ OpenBSD-Commit-ID: c6e9f6d45eed8962ad502d315d7eaef32c419dde
Close RESOLVED bugs with the release of openssh-8.0