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SSH key-based authentication

Why?

  • No chance to brute-force SSH service if password-based login is disabled
  • No need to type-in password on every login via SSH

Disadvantages:

  • No way to login if key-file is lost
  • No way to login if passphrase from key-file is forgotten

Create SSH keys

On local linux-based (mac or linux) machine:

ssh-keygen

# "Enter file in which to save the key"
# it's a good idea to have separate
# keys for every host.
# Create a key with the name related to host
# ~/.ssh/my-app-proj

# "Enter passphrase"
# Skip "empty for no passphrase"
# or set passphrase to encrypt key
# so, even if it's leaked or stolen
# it will be useless without passphrase

Add to keychain (mac os)

# To add keys permanently, use -K flag

ssh-add -K ~/.ssh/my-app-proj

# If the key file is created with the passphrase
# You will be asked to enter it

Add key to server

On local machine copy public key:

# Copy contents of ~/.ssh/my-app-proj.pub
cat ~/.ssh/my-app-proj.pub

On a Server, to login as root (less secure):

# Paste copied public key to new line at:
nano ~/.ssh/authorized_keys
# Create file/folder if not exists
# Save file with: "ctrl + o" combination
# Close file with: "ctrl + x" combination

Skip this step if root user will be used for login. On a Server, to login as any other user (secure, read how to create a user)

Note: Do not forget to change username to actual users' name below:

# Go to users' home directory
cd /home/username

# Create .ssh directory (if not yet exists)
mkdir -p /home/username/.ssh

# Paste copied public key to the new line at:
nano /home/username/.ssh/authorized_keys
# Save file with: "ctrl + o" combination
# Close file with: "ctrl + x" combination

# Set right permissions
chmod -R 700 /home/username/.ssh
chmod -R 640 /home/username/.ssh/authorized_keys
chown -R username:username /home/username/.ssh

Disable password auth (extra security)

If SSH keys are 100% secure and won't be lost. Disabling password authentication would add an extra layer of security:

# nano /etc/ssh/sshd_config
# find the line with PasswordAuthentication
# remove "#" if presented at the beginning of the line

PasswordAuthentication no

Restart SSH service:

service sshd restart

Issue with multiple keys (mac os)

Having multiple keys in the system keychain would result SSH command to try login with each of added "identity file" keys in alphabet order. By default, Linux server will accept only 5 login attempts per connection, so having more than 5 SSH keys in the keychain will result in rejected connection. To solve this issue use -i flag to explicitly set "identity file":

ssh -i ~/.ssh/my-app-proj username@host -o PubkeyAuthentication=no

Login with password (mac os)

To force login with password, use -o PubkeyAuthentication=no and/or -o PreferredAuthentications="password" flags:

ssh username@host -o PubkeyAuthentication=no
# or
ssh username@host -o PreferredAuthentications="password"
# or use both flags:
ssh username@host -o PreferredAuthentications="password" -o PubkeyAuthentication=no

"Too many authentication failures" error can get fixed with -o IdentitiesOnly=yes flag:

ssh username@host -o IdentitiesOnly=yes

Further reading: