sshd

We will be primarily working with one configuration file in this article:

• OpenSSH – /etc/ssh/sshd_config

OpenSSH

For locking down which users may or may not access the server you will want to look into one, or more, of the following directives:

User/Group Based Access

AllowGroups

This keyword can be followed by a list of group name patterns, separated by spaces.If specified, login is allowed only for users whose primary group or supplementary group list matches one of the patterns.`*' and `?' can be used as wildcards in the patterns.Only group names are valid; a numerical group ID is not recognized.By default, login is allowed for all groups.

AllowUsers

This keyword can be followed by a list of user name patterns, separated by spaces.If specified, login is allowed only for user names that match one of the patterns.`*' and `?' can be used as wildcards in the patterns.Only user names are valid; a numerical user ID is not recognized.By default, login is allowed for all users.If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts.

DenyGroups

This keyword can be followed by a list of group name patterns, separated by spaces.Login is disallowed for users whose primary group or supplementary group list matches one of the patterns.
`*' and `?' can be used as wildcards in the patterns.Only group names are valid; a numerical group ID is not recognized. By default, login is allowed for all groups.

DenyUsers

This keyword can be followed by a list of user name patterns, separated by spaces.Login is disallowed for user names that match one of the patterns.`*' and `?' can be used as wildcards in the patterns.Only user names are valid; a numerical user ID is not recognized.By default, login is allowed for all users.
If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts.

The first thing to do is backup the original configuration file:

cp /etc/ssh/sshd_config /etc/ssh/sshd_config{,.`date +%s`}

We will now need to edit the configuration file with your favorite editor (vi/vim/ed/joe/nano/pico/emacs.)

An example of only allowing two specific users, admin and bob, to login to the server will be:

/etc/ssh/sshd_config:

AllowUsers admin bob

Ifyou would like to more easily control this for the future then you can create a Group on the server that will be allowed to login to the server, adding individual users as needed (replace username with the actual user):

shell:

groupadd –r sshusers

usermod –a –G sshusers username

With this we will no longer be using AllowUsers but AllowGroups

/etc/ssh/sshd_config:

AllowGroups sshusers

The alternatives to these directives are DenyGroups and DenyUsers which perform the exact opposite of the aforementioned AllowGroups and AllowUsers.
When complete you will want to make sure that sshd will read in the new configuration without breaking.

/usr/sbin/sshd –t

echo $?

We will want to see a 0 following the “echo $?’’ command.Otherwise we should also see an error stating what the erroneous data is:

sshd_config: line 112: Bad configuration option: allowuser
sshd_config: terminating, 1 bad configuration options

After verification we will simply need to restart sshd.This can be performed via many different methods, for which we will assume a sysv-compatible system:

/etc/init.d/sshd restart

Make sure to not disconnect your ssh session but create a new one as a ‘just incase’.
Verify that you can perform any required actions with this user(eg: su into root if you are not allowing root logins.)