128

I tried following the instructions here: http://lani78.wordpress.com/2008/08/08/generate-a-ssh-key-and-disable-password-authentication-on-ubuntu-server/

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 ::
#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
#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
Zanna
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user193805
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    FYI : Actually the sshd restart is not really necessary. The command ***/etc/inid.d/ssh reload*** should be enough. – Oliv Feb 24 '16 at 22:23
  • Don't forget to uncomment #AuthorizedKeysFile and to copy the public key to ~/.ssh/authorized_keys (and restart). Without this, it won't work. – ivanleoncz Jan 25 '19 at 21:04
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    If it wasn't true in 2016, it is definitely the case in 2019 that a restart is required; reload is not sufficient. – KDN May 18 '19 at 21:50

4 Answers4

165

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.

Terry Wang
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    Also we should have `UsePAM no` – Konstantinos Apr 07 '15 at 21:22
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    @YAK I like to keep things simple and I prefer not to use PAM. But somebody can use a properly configured PAM authentication. I think this link is enlightening: http://arlimus.github.io/articles/usepam/ – Konstantinos Dec 15 '15 at 14:00
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    Consider also disabling ChallengeResponseAuthentication, see http://superuser.com/a/374234/2879. – cic Jul 06 '16 at 17:36
  • Just go for the most `no` and the least possible `yes`. `sudo nano /etc/ssh/sshd_config.d/50-redhat.conf; sudo systemctl restart sshd.service` `ChallengeResponseAuthentication no` `PasswordAuthentication no` `PermitEmptyPasswords no` `PubkeyAuthentication yes` `UsePAM no` ...enables you to login with your private key only and even though the user has a password, the password login is always denied. – Alex Jul 05 '21 at 00:25
18

According to this wiki page about SSH keys and this answer, you need to change these two lines in your sshd_config:

PasswordAuthentication no
ChallengeResponseAuthentication no
McSonk
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Andy J
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    What difference does the second line about challenge response make? – Ryan Burnette Jul 21 '16 at 19:17
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    "It doesn't provide "additional security," per se. The term "ChallengeResponseAuthentication" is just an OpenSSH configuration keyword; it refers to the "keyboard-interactive" userauth method in the SSH protocol —Richard Silverman (fixunix.com) – pzkpfw May 22 '17 at 09:23
  • For some reason, adding `ChallengeResponseAuthentication no` worked even after applying `PasswordAuthentication no` and `PubkeyAuthentication yes` – Dominic Motuka Mar 28 '21 at 18:37
5

In /etc/ssh/sshd_config, below settings worked for me:

PasswordAuthentication no
UsePAM no

Finally, restart sshd daemon.

BTR Naidu
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4

The line you want is abnormally commented out by default in the sshd_config file.

# Change to no to disable tunnelled clear text passwords
--->#PasswordAuthentication yes

To disable passwords, change the yes to no and remove the comment:

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