Problem: couldn’t connect to Ubuntu 12.04 SSH Server. Everytime client wants to connect through ssh, it gives error:

Read from socket failed: Connection reset by peer

Then on the server side, checking the log using tail -f /var/log/auth.log while trying to connect from client gives:

ubuntu sshd[3207]: Server listening on 0.0.0.0 port 22.
ubuntu sshd[3207]: Server listening on :: port 22.
ubuntu sshd[3208]: error: Could not load host key: /etc/ssh/ssh_host_rsa_key
ubuntu sshd[3208]: error: Could not load host key: /etc/ssh/ssh_host_dsa_key
ubuntu sshd[3208]: error: Could not load host key: /etc/ssh/ssh_host_ecdsa_key

According to many sources, this is possibly because the host key files are not exist. To generate the host key files they do this:

sudo ssh-keygen -t rsa1 -f /etc/ssh/ssh_host_rsa_key
sudo ssh-keygen -t dsa -f /etc/ssh/ssh_host_dsa_key
sudo ssh-keygen -t ecdsa -f /etc/ssh/ssh_host_ecdsa_key

Then restart SSH server:

sudo service ssh restart

But this doesn’t solve the problem.

To solve this, we need to reinstall the openssh server and wipe all the files:

cd /etc
sudo mv ssh/ ssh.old/
sudo apt-get remove openssh-server openssh-client ssh-import-id

Watch the output and make sure it is uninstalling. Then install it again:

sudo apt-get install openssh-server openssh-client ssh-import-id

The installation process should generate new host key files, if not, check if you have removed/renamed the /etc/ssh/ folder properly.

This is the sign when your problem is about to be solved:

Unpacking openssh-server (from .../openssh-server_1%3a5.9p1-5ubuntu1_amd64.deb) ...
Selecting previously unselected package ssh-import-id.
Unpacking ssh-import-id (from .../ssh-import-id_2.10-0ubuntu1_all.deb) ...
Processing triggers for ureadahead ...
ureadahead will be reprofiled on next reboot
Processing triggers for ufw ...
Processing triggers for man-db ...
Setting up openssh-server (1:5.9p1-5ubuntu1) ...
Creating SSH2 RSA key; this may take some time ...
Creating SSH2 DSA key; this may take some time ...
Creating SSH2 ECDSA key; this may take some time ...
ssh start/running, process 21733
Setting up ssh-import-id (2.10-0ubuntu1) ...

Notice the part where the installation process generating the host keys. Now the problem is solved.

Don’t forget to secure your server by disabling root login in /etc/ssh/sshd_config:

#PermitRootLogin yes
PermitRootLogin no