CentOS, users, groups, sudo and SSH keys

Create sudo users and groups in CentOS

I will show you how to organise groups and users in CentOS.

As an example we will create account for user named developer. The purpose of this account is:

  • logging via SSH instead of using root account
  • access to sudo command for management tasks
  • write access to website files
  • read access to logs

Create list with available groups

If you are on fresh system it's handy to create a list with all available groups. It's also handy to check which users belongs to that groups. It might be helpful after some time when you need to decide if given group was at the beginning or can it be safely removed. getent command can help you with that.

getent group > /etc/initial-group-list
cat /etc/initial-group-list

Remove group from the system

If you need to remove group from CentOS simply use following command:

sudo groupdel NAME_OF_THE_GROUP_TO_DELETE

Create new group

In order to create group you need to use groupadd command:

sudo groupadd NAME_OF_THE_NEW_GROUP

I usually add group named www (or www-data, whatever works for you). To this group I add php daemons, nginx workers etc. It makes life easier with writing to files. In order to create such group execute following command:

sudo groupadd www

Create list with available users

Same like with group, I like to have list of initial users. In order to create such list you can use getent too:

getent passwd > /etc/initial-users-list
cat /etc/initial-users-list

Delete user from CentOS

In case you would like to remove any user from the system, use following command:

sudo userdel -r USERNAME_TO_REMOVE

-r flag will remove also his home directory. If you wish to delete the user, but to keep his files, omit this flag.

Create new user in CentOS

Let's create new user developer that we mentioned at the beginning:

sudo adduser developer

and create the password for his account:

sudo passwd developer

If you want to add developer user to www group created before use usermod command:

sudo usermod -g www developer

If you want to add this user sudo powers (and you should if you want to use this user instead of root), add it to wheel group. wheel group is special group in CentOS configured in sudoers file. Whoever belongs to this group can have sudo powers.

sudo usermod -g wheel developer

Optional parameters to useradd command

There are lot of additional parameters for useradd command but there are two especially useful.

First one is helpful when you don't want to create user home directory. It means that user will not have it's own place under /home directory to store it's files. This option is helpful when you are creating user for system service like Apache httpd for instance. So in order to create user with no home directory use --no-create-home:

sudo useradd httpd --no-create-home

Another useful feature is to specify shell of given user. It's nice if you want to cut of possibility to login to the system via SSH for instance. Add --shell /sbin/nologin to disable login for given user, like so:

sudo useradd httpd --shell /sbin/nologin --no-create-home

You can use --shell and --no-create-home parameters separately:)

 

How to setup SSH keys for new created user?

Each user should have RSA key-pair. It makes life easier and you should use it if you want to login to different servers, use GIT etc. In order to create such user key-pair you first need to login to user you created. Most probably you are using root account to execute all commands, but you should never ever login via SSH as a root.

It's much better to create separate user for system management and use only this account. Login via SSH to your server to account your created. In my case it's developer user so my command looks like this:

ssh developer@IP_OF_THE_SERVER_HERE

Once you'll be logged in (after providing the password), you can create RSA key pair. Execute following command:

ssh-keygen -t rsa -b 4096

-t rsa means that it will be RSA key, but this is standard for creating SSH keys. Fun part is with strength of the key -b 4096. By default it's 1024 bits, but to make it harder to break I usually provides 4096. It's not necessary, but you should do that. Some services requires key length to be minimum 2048, but it's better to create even longer one.

Generator will ask you some questions, but you should generally confirm them with enter and leave the defaults. When it comes to SSH on the server, I usually don't set the password. It makes life easier in automated scripts etc.

After that private and public key should be generated as expected. You can find them in ~/.ssh directory.

Add authorized key to user

In order to login with SSH keys to the server, instead of using password you need to add authorized key to developer user. In my opinion it's must have feature as using password login is super risky. Again, been there, done that, I was hacked, even when my password was strong. With SSH logging even strongest bruteforce attack will fail:)

You need to add your key to ~/.ssh/authorized_keys on the server. If You have ssh-copy-id command available just execute:

ssh-copy-id developer@IP_OF_YOUR_SERVER

Make sure that you are executing this command from your computer, not from the server. If you don't have SSH key created locally, you can generate it in the same way as on the server, by using ssh-keygen command.

If uou don't have ssh-copy-id available (for instance from Windows), you can do it manually.

ssh developer@IP_OF_YOUR_SERVER
cd ~/.ssh
vi authorized_keys
//Press "i" to enter in input mode, paste there your code (usually it's right click of the mouse) and :wq (colon, w, q) it will save and quit from vi 
chmod 600 authorized_keys

So here how it goes:

  1. ssh to the server as usual with password.
  2. Change location to .ssh directory.
  3. Create authorized_keys file with vi
  4. Paste there your local public key, save the file and quit
  5. Set permissions on authorized_keys.

Test ssh login with keys

Now You can try to log in with Your key.

ssh developer@IP_OF_YOUR_SERVER -i path/to/your/PRIVATE/key/file

You shouldn't be prompted for your account password!

Easier way?

If you don't want to spend your precious time executing each of these commands by hand, you can use Ansible and our LAMP on steroids project to speed things up!

If you don't know what Ansible is - you can read our tutorial about it here.

LAMP on steroids project is available on GitHub here.