Two Factor Authentication (2FA) is an additional layer of protection you can add to your Minecraft server. You should already be relying on SSH keys to access your server, but those keys can be lost or leaked if you use them on untrusted machines. 2FA protects you from hacks resulting from someone gaining access to your password or SSH key.

DuoSecurity is a company that provides a 2FA service that is free for personal use. A Mobile Push notification is sent via their Android / iOS app whenever a login is attempted and requires an affirmative response before login can proceed. There is also SMS and computer-generated voice calls, but those consume credits that have a cost to refill. There is also a cost for going over 10 users, which should not be an issue for adminstation of a Minecraft server.

I performed a source install to get the latest version, there are also packages for RHEL/CentOS/Debian/Ubuntu on their website.

They offer a SSH-specific installation and a PAM installation that covers all auth on the machine. PAM is likely the more comprehensive solution but is a more involved process, plus requires a SELinux policy update (and the server resulting from this series has SELinux enabled). Their website tells you how to install the relevant SELinux policy but the necessary objects are missing from their download. I’ve send them a mail and will update if I get clarification. For the record, the error I got was:

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make: Entering directory `/home/centos/duo_unix-1.9.14/pam_duo'
checkmodule -M -m -o authlogin_duo.mod authlogin_duo.te
checkmodule:  loading policy configuration from authlogin_duo.te
checkmodule:  unable to open authlogin_duo.te
make: [semodule] Error 1 (ignored)
semodule_package -o authlogin_duo.pp -m authlogin_duo.mod
semodule_package:  Could not open file No such file or directory:  authlogin_duo.mod
make: [semodule] Error 1 (ignored)
semodule -i authlogin_duo.pp
semodule:  Failed on authlogin_duo.pp!
make: [semodule] Error 1 (ignored)
make: Leaving directory `/home/centos/duo_unix-1.9.14/pam_duo'

SSH setup is straightforward. Install the DuoSecurity app on your mobile device. Create an account on their website. Navigate their portal and select Applications, then +New Application. Give it name of Minecraft SSH. This will result in a window showing a Integration key, Secret key (requiring a click to show), and an API hostname. Collect those and store them to the side (securely!)

https://www.duosecurity.com/docs/duounix#instructions describes the compile process:

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wget https://dl.duosecurity.com/duo_unix-latest.tar.gz
tar zxf duo_unix-latest.tar.gz
cd duo_unix-1.9.14
./configure --prefix=/usr && make && sudo make install

After installing the binaries, modify /etc/duo/login_duo.conf and fill in the ikey/skey/host values from above. Uncomment pushinfo as well so that more details about the login are sent to you.

https://www.duosecurity.com/docs/duounix#centos describes modifying SSH to work with the new system. Keep an alternate root-logged in session open in an alternate window while attempting this so you can back out if you make a mistake. Do not log out of the ‘backup’ window until you’ve thoroughly exercised the system. I will not be responsible if you lock yourself out.

Add ForceCommand /usr/sbin/login_duo to the end of /etc/sshd/sshd_config, then restart sshd (which does not kick out existing sessions) with systemctl restart sshd.service

Now attempt to log in to your account. You will be prompted to enroll in 2FA. Upon logging in again you will be prompted for your 2FA method, which will look like:

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$  ssh minecraft.example.com -i ~/.ssh/minecraft.pem -l centos
Duo two-factor login for centos

Enter a passcode or select one of the following options:

 1. Duo Push to XXX-XXX-1234
 2. Phone call to XXX-XXX-1234
 3. SMS passcodes to XXX-XXX-1234

I always choose #1 as I do not want to expend credits, though if push failed I could resort to the other two. When I choose #1 I get a popup on my phone asking me to accept / deny the login, and after choosing Accept I am able to log in.

Now I do not have to worry someone will get ahold of my minecraft.pem and gain access to the server. Further, if I get a login-request on my phone and haven’t attemped to log in, I know my key has been compromised.