Ubuntu – How to force SSH to only allow users with a key to log in


I tried following the instructions here:

to only allow users with a public key on the server to authenticate, but I can't get SSH to disallow logging in with only a username/password.

Here is my sshd_config file – am I missing anything? I already tried restarting SSH and the computer itself.

# 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 ::
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
#Privilege Separation is turned on for security
UsePrivilegeSeparation yes

# Lifetime and size of ephemeral version 1 server key
KeyRegenerationInterval 3600
ServerKeyBits 768

# Logging
SyslogFacility AUTH
LogLevel INFO

# Authentication:
LoginGraceTime 120
PermitRootLogin yes
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 without-password".
# 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 no

Best Answer

By default PasswordAuthentication is set to yes, so explicitly commenting it in /etc/ssh/sshd_config and restart sshd has no effect.

You'll need to explicitly set PasswordAuthentication no to allow only Public Key Authentication.

# To disable tunneled clear text passwords, change to no here!
PasswordAuthentication no

PubkeyAuthentication yes

NOTE (man sshd_config): PasswordAuthentication specifies whether password authentication is allowed. The default is yes.

And restart sshd

  • for sysvinit service ssh restart
  • for systemd systemctl restart sshd.service.

Additionally, it is best practice to use the following directives (in order) DenyUsers AllowUsers DenyGroups AllowGroups for finer SSH access control granularity and flexibility. -> Reference: man 5 sshd_config ---> Ubuntu openssh man page does not include this any more as it absorbs openssh upstream docs (but FreeBSD, EL 7, 8 man page still have them).

Last but not least

NOTE: be careful with setting UsePAM no as that way password locked user accounts (this is different from disabled / expired user accounts -> man passwd and man usermod) will NOT be able to login even if they have public key authentication configured in ~/.ssh/authorized_keys.

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