When it comes to secure access to a remote server, such as an AWS EC2 instance, you have couple of options. The preferred option is to have the instance (or the server in a data center or other similar environment) within a private network (such as a VPC in AWS), only accessible by SSH over a VPN (either your own OpenVPN setup, or IPsec available from AWS). However, not having a fall-back SSH connectivity is not always practical or feasible, even if it be just to access a gateway instance that likely serves as a bastion host. This is obviously not applicable to environments where nothing is strictly ever done over SSH, and everything is only ever done over configuration management, but such environments are far and few between.
The following outlines my preferred method of setting up SSH access on the gateway instance. Because the configuration parameters differ between the MFA-protected, but IP-unrestricted SSH server, and the one that servers connections from the allowed addresses/CIDR ranges, it is best to run two separate SSH daemons.
Before starting this process, make sure that the normal OpenSSH access to your server/instance has been configured, as this article mainly outlines the deltas from the standard SSH setup. So let’s get started!
- Install `libpam-google-authenticator`:
sudo apt-get install libpam-google-authenticator
- Make a copy of the existing `sshd_config`:
sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config_highport
- Modify the newly created sshd_config_highpoprt:
- Select a different port, such as 22222:
- Allow users who should be allowed to use the MFA-protected, but IP-unrestricted SSH access:
- Set Google Authenticator -compatible authentication method:
# require Google Authenticator after pubkey
- Set `ChallengeResponseAuthentication`:
- If you have configured any `Match address` or `Match user` entries in your primary SSH server’s sshd_config file (whose copy we’re editing), remove them from the copy. For example, you might have something like this configured for the primary SSH instance:
Match address 10.10.10.0/24
Match User root Address 10.10.10.0/24
Match User root Address 100.100.100.50/32
If you do, remove them from `sshd_config_highport`.
- Make a copy of `/etc/pam.d/sshd`, and modify the copy, like so:
sudo cp /etc/pam.d/sshd /etc/pam.d/sshd2
Then add on top of the `sshd2` file:
auth required pam_google_authenticator.so
.. and comment out the following line in the file:
# @include common-auth
- Now run `google-authenticator` as the user who you want to be able to log in as over the MFA-protected, but IP-unrestricted SSH-service. Do not use root; use an unprivileged user account instead! Once you run it, you will be prompted to answer: `Do you want authentication tokens to be time-based (y/n)`. Answer `yes`, and the system will display a QR code. If you don’t have a Google Authenticator -compatible app on your smart phone/tablet yet, install one. I recommend Authy (Android / iOS). Once you have installed it and created an account in it, scan the QR code off the screen, and also write down the presented five “emergency scratch codes” in a safe place. Then answer the remaining questions, all affirmative:
– Do you want me to update your “~/.google_authenticator” file (y/n) y
– Do you want to disallow multiple uses of the same authentication token? .. y
– By default, tokens are good for 30 seconds and in order to compensate .. y
– .. Do you want to enable rate-limiting (y/n) .. y
- Create a `ssh2` symlink to the existing `ssh` executable (a requirement by service autostart):
sudo ln -s /usr/sbin/sshd /usr/sbin/sshd2
- Add the following in `/etc/default/ssh`:
# same for the highport SSH daemon
- Create a new `systemd` service file for the `ssh2` daemon:
sudo cp /lib/systemd/system/ssh.service /etc/systemd/system/sshd2.service
.. then modify the `sshd2.service` file; modify the existing ExecStart and Alias lines, like so:
ExecStart=/usr/sbin/sshd2 -D $SSHD2_OPTS -f /etc/ssh/sshd_config_highport
Note that the Alias name (as set above) must be different from the file name for the service. In other words, as in the example above, the file name is `sshd2.service`, the “Alias” must be set to something else that doesn’t previously exist in the `/etc/systemd/system` directory (i.e. in the above example `sshd-highport.service`).
Then start the service, test it, and finally enable it (to persist reboots):
sudo systemctl daemon-reload
sudo systemctl start sshd2
sudo systemctl status sshd2
sudo systemctl enable sshd2
sudo systemctl status sshd2
When you enable the service with `sudo systemctl enable sshd2`, a symlink is created by the name of the alias you defined. Following the above example, it would be like so:
`/etc/systemd/system/sshd-highport.service` [symlink] -> `/etc/systemd/system/sshd2.service` [physical file]
You’re all set! Other considerations: If you’re running this in AWS, remember to configure the Security Groups to allow access to the high port SSH from any source (assuming you want it to be accessed from any source), as well as adjust the ACLs and iptables/ufw if you use them. Furthermore, since the MFA-protected SSH port will be accessible publicly, it’s a good idea to install sshguard (some install info here) to prevent port knocking (besides stressing the system some, brute force attacks aren’t much of a threat since the port is now protected by the MFA which also implements access rate limiting).
Finally, since the MFA is time-based, it’s a good idea to make sure `ntp` is running on the your server/instance. Additionally I run `ntpdate` from system crontab once a day to make sure a possible greater drift than what the maximum ntp slew rate can correct in a reasonable amount of time is corrected at least once a day:
sudo apt-get update
sudo apt-get install ntp
sudo systemctl enable ntp
.. and in `/etc/crontab:`
# Force time-sync once a day (in case of a time difference too big for stepped adjustment via ntp)
0 5 * * * root service ntp stop && ntpdate -u us.pool.ntp.org && service ntp start
(`ntpdate` won’t run if `ntp` service is active)
And there you have it! Now when you connect to the server/instance over SSH from a random external IP (such as from a hotel) using a public key to your chosen user account, you will be prompted for an MFA code (which you have to enter from the Authy app) before the connection is authorized. And because the primary SSH daemon still serves the default SSH port 22, and because it is restricted to the private networks (plus possibly some strictly limited public IPs), those “known”, or “internal” IPs are not presented with an MFA challenge in order to get connected.