Secure VPS with Certificate SSH

Generate a new key on the machine that is to access the VPS

ssh-keygen -a 1000 -b 4096 -C "" -o -t rsa

ssh-keygen -a 1000 -C "" -o -t ed25519

-a specifies # of KDF rounds
-b specifies the bit size for RSA keys
-C removes the common comment at the end of the key
-o specifies that we should use the new format provided by OpenSSH
-t specifies the key type (RSA)

Copy key to the VPS

Copy the public key to the VPS

ssh-copy-id username@remote_host

Check that it worked

Look at the contents of ~/.ssh/id_rsa.pub

cat ~/.ssh/id_ed25519.pub

and make sure it is the same as the public key on the VPS

cat ~/.ssh/known_hosts

Modify the VPS configuration

Edit both these files

/etc/ssh/ssh_config
/etc/ssh/sshd_config

HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_dsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key

Should be

HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ed25519_key

PasswordAuthentication no
ChallengeResponseAuthentication no

Restart ssh on the VPS

service ssh restart

Test that SSH is actually using the certs and no passwords

This happens on the machine you are using to connect TO the VPS

move the contents of ~/.ssh to a backup location
Be careful!!! If you screw this up you won’t be able to get into the server.
Once the contents of ~/.ssh are carefully moved, try to ssh in
Receive the following message
- Permission denied (publickey).
If you can stil log in with a password, then something is wrong.

TODO

Change Default port for SSH from 22

Update firewall

Disable root logon

Create a new non-root user
Add user to ???
Can’t remember how I did this
PermitRootLogin no

Copy of my sshd_config 20171204

# Package generated configuration file
# See the sshd_config(5) manpage for details

# What ports, IPs and protocols we listen for
Port 22
# Use these options to restrict which interfaces/protocols sshd will bind to
#ListenAddress ::
#ListenAddress 0.0.0.0
Protocol 2
# HostKeys for protocol version 2
HostKey /etc/ssh/ssh_host_rsa_key
#HostKey /etc/ssh/ssh_host_dsa_key
#HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key
#Privilege Separation is turned on for security
UsePrivilegeSeparation yes

# Lifetime and size of ephemeral version 1 server key
KeyRegenerationInterval 3600
ServerKeyBits 1024

# Logging
SyslogFacility AUTH
LogLevel INFO

# Authentication:
LoginGraceTime 120
PermitRootLogin no
StrictModes yes

RSAAuthentication yes
PubkeyAuthentication yes
#AuthorizedKeysFile	%h/.ssh/authorized_keys

# Don't read the user's ~/.rhosts and ~/.shosts files
IgnoreRhosts yes
# For this to work you will also need host keys in /etc/ssh_known_hosts
RhostsRSAAuthentication no
# similar for protocol version 2
HostbasedAuthentication no
# Uncomment if you don't trust ~/.ssh/known_hosts for RhostsRSAAuthentication
#IgnoreUserKnownHosts yes

# To enable empty passwords, change to yes (NOT RECOMMENDED)
PermitEmptyPasswords no

# Change to yes to enable challenge-response passwords (beware issues with
# some PAM modules and threads)
ChallengeResponseAuthentication no

# Change to no to disable tunnelled clear text passwords
PasswordAuthentication no

# Kerberos options
#KerberosAuthentication no
#KerberosGetAFSToken no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes

# GSSAPI options
#GSSAPIAuthentication no
#GSSAPICleanupCredentials yes

X11Forwarding yes
X11DisplayOffset 10
PrintMotd no
PrintLastLog yes
TCPKeepAlive yes
#UseLogin no

#MaxStartups 10:30:60
#Banner /etc/issue.net

# Allow client to pass locale environment variables
AcceptEnv LANG LC_*

Subsystem sftp /usr/lib/openssh/sftp-server

# Set this to 'yes' to enable PAM authentication, account processing,
# and session processing. If this is enabled, PAM authentication will
# be allowed through the ChallengeResponseAuthentication and
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication via ChallengeResponseAuthentication may bypass
# the setting of "PermitRootLogin yes
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and ChallengeResponseAuthentication to 'no'.
UsePAM yes

# Added by DigitalOcean build process
ClientAliveInterval 120
ClientAliveCountMax 2

REFERENCES

https://www.digitalocean.com/community/questions/best-practices-for-hardening-new-sever-in-2017

https://blog.g3rt.nl/upgrade-your-ssh-keys.html

https://www.digitalocean.com/community/tutorials/how-to-configure-ssh-key-based-authentication-on-a-linux-server

Developer.