====== Mar 8 2023 SSH Error no hostkey alg ======

When connecting via SSH from an older OS, there may be cases where connections cannot be made due to the following error:
From RHEL6 to RHEL8, for example,

The RHEL6 side needs to support ECDSA connections.

Although there is a need to run commands like "make," it's possible that following [[#3. Update the client's SSH]] is the simplest solution.

===== Error =====

<code>
$ ssh 10.10.0.25
no hostkey alg
</code>

===== Solution =====

==== 1. How to create ECDSA keys ====

<code>
ssh-keygen -t ecdsa -b 256
</code>

The private key and public key are these:

<code>
$ ls  .ssh/id_ecdsa*
.ssh/id_ecdsa  .ssh/id_ecdsa.pub
</code>

==== Settings for connecting using ECDSA ====

You can use password authentication with this method.
If set to connect using ECDSA, the connection will be possible.

<code|.ssh/config>
Host 10.10.0.25
    HostKeyAlgorithms ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521
</code>

==== Wildcards can also be used ====
<code|.ssh/config>
Host 10.10.10.*
    HostKeyAlgorithms ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521
</code>

===== 2. Server-side solution =====

==== Add to sshd_config ====

<code>
cat << __EOM__ >> /etc/ssh/sshd_config
HostKeyAlgorithms +ssh-rsa
PubkeyAcceptedKeyTypes +ssh-rsa
__EOM__
</code>

==== sshd restart ====

<code>
systemctl restart ssh.service
</code>


===== 3. Update the client's SSH =====

From here, download the new version of OpenSSH.
[[http://ftp.jaist.ac.jp/pub/OpenBSD/OpenSSH/portable/]]

<code>
$ tar zxvf openssh-8.0p1.tar.gz
$ cd openssh-8.0p1
$ ./configure
$ make
# cp ssh /usr/local/bin/ssh
</code>

Write it as an alias.

<code>
$ cat .bashrc
alias ssh='/usr/local/bin/ssh'

$ ssh -V
OpenSSH_9.2p1, OpenSSL 1.0.2k-fips  26 Jan 2017
</code>
 


{{tag>Diary SSH}}