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On Ubuntu Server, a practical way to require two factors for SSH is to authenticate first with an SSH key and then with a time-based one-time password (TOTP) through PAM. Before enforcing it, enroll every SSH user, confirm key-only access, and verify that you can recover the server through your VPS provider’s console. The procedure below protects SSH logins; it does not automatically protect provider accounts, web apps, databases, or every user account on the VPS.
What SSH two-factor authentication protects
With the PAM-backed Ubuntu configuration, SSH requires a public-key authentication step followed by a keyboard-interactive prompt for an OTP. The private key stays with the client; the OTP comes from an authenticator using a secret configured for that user. Ubuntu’s documented configuration disables SSH password authentication while requiring both methods.
This is separate from MFA on your VPS provider account. Provider-console access is another administrative route, and its MFA and recovery controls must be checked separately. The instructions here concern SSH; sudo requires its own deliberate configuration if you also want a second factor for privilege elevation.
This guide centers on Ubuntu Server’s current TOTP/HOTP instructions, last updated June 26, 2026. PAM layout, package names, OpenSSH versions, and configuration directives can differ on other distributions. Do not copy Ubuntu PAM changes to a different system without following that distribution’s current documentation.
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Prepare access and recovery before changing SSH
- Identify the distribution and release:
cat /etc/os-release. Ubuntu 20.04 LTS and earlier use a legacy SSH directive noted below. - Confirm you can log in over SSH now, have a separate sudo-capable administrator account, and can access your VPS provider’s out-of-band console or rescue route. Recovery procedures vary by provider; verify the route before relying on it.
- Keep an existing privileged SSH session open while making changes. Use a second terminal for all new-login tests, and do not close the first session until the full key-plus-code flow succeeds.
- Set up and test SSH public-key access for every person or automation account that needs SSH access. Do not assume password login is a safe fallback once MFA is enforced.
- Enroll each intended SSH user with an OTP secret before requiring the second factor. A user without a configured key and OTP secret may be unable to complete SSH setup after enforcement.
- Keep recovery material somewhere protected and separate from the VPS where practical. A copy of the OTP secret or emergency codes can let someone bypass the intended protection if exposed.
Vultr’s April 1, 2025 guide also lists updating the system, configuring a firewall, and using SSH keys among its prerequisites. Its specific console and configuration guidance applies to Vultr; check your own provider’s recovery process.
Choose TOTP/HOTP or a hardware security key
| Method | Credential and requirements | Important failure mode | Recovery consideration |
|---|---|---|---|
| PAM TOTP/HOTP | A per-user shared secret generates a code. The SSH path uses PAM and keyboard-interactive prompts. | TOTP depends on aligned clocks. HOTP can desynchronize if codes are generated without the server advancing in step. | Securely protect emergency codes, authenticator backups, or alternate administration access; exposure can defeat the extra factor. |
| OpenSSH U2F/FIDO security key | Uses an OpenSSH security-key credential, such as ecdsa-sk or ed25519-sk, with compatible client, server, and hardware. |
The hardware device must be present and available to authenticate. | Plan a suitable backup device or alternate access route for your deployment; requirements depend on the hardware and SSH setup. |
Ubuntu recommends U2F/FIDO hardware authentication devices for the best two-factor security where practical. TOTP is a practical alternative when a hardware route is not suitable. Ubuntu’s TOTP guide says its presented U2F/FIDO and TOTP/HOTP configurations are not recommended together because that combination has not been tested there; choose a documented path rather than mixing them casually.
Configure PAM-backed OTP on Ubuntu
Install the PAM module
- On Ubuntu, update package metadata and install the documented module:
sudo apt update && sudo apt install libpam-google-authenticator. - As each SSH user, run
google-authenticatorand follow the prompts for that installed version. Import the displayed QR code into a compatible authenticator app, or enter the secret manually. Keep the secret and any generated emergency passcodes confidential. - Repeat enrollment for every user who needs SSH access. Keep your working privileged session open while you configure and test.
The user’s authenticator configuration file contains the shared secret, emergency passcodes, and settings, so protect it like a credential. Use the current module prompts and version-specific guidance rather than assuming an older tutorial’s defaults or prompts still apply. Ubuntu’s older tutorial describes rate limiting and preventing multiple uses of a token as options; treat those as version-specific setup choices, not timeless defaults.
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Configure the SSH daemon and PAM
Follow the current Ubuntu Server TOTP/HOTP procedure for the relevant Ubuntu release. Its SSH configuration excerpt is:
KbdInteractiveAuthentication yes
PasswordAuthentication no
AuthenticationMethods publickey,keyboard-interactiveUbuntu 20.04 LTS and earlier use ChallengeResponseAuthentication yes instead of KbdInteractiveAuthentication yes in this configuration. Check the active SSH configuration and included files for existing or conflicting directives; adding duplicate lines without understanding which value takes effect can leave authentication in an unintended state.
Ensure /etc/pam.d/sshd invokes the OTP module in the way specified by Ubuntu’s current instructions. An older Ubuntu tutorial gives the PAM line auth required pam_google_authenticator.so, but its examples use legacy configuration names; prefer the current Ubuntu Server procedure over that older variant.
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Audit the PAM path before enforcing it
KeyboardInteractiveAuthentication is a way to present interactive prompts; it is not inherently an OTP-only channel. PAM can also offer password authentication through that path. Mozilla’s OpenSSH guidance warns that PasswordAuthentication no alone does not prove SSH password authentication is impossible if PAM still enables it.
- Inspect
/etc/pam.d/sshdand the PAM files it includes. Understand which authentication modules can answer the keyboard-interactive prompt. - Verify that the effective SSH settings require
publickey,keyboard-interactive, and that the PAM path implements the intended OTP step without an unintended password fallback. - Test with a fresh client session and confirm that a valid key alone does not complete login and that the expected OTP prompt appears. Do not infer security from a configuration line in isolation.
PAM stacks differ across distributions and releases, so there is no safe universal replacement file for every Linux VPS. Use the vendor’s current instructions for non-Ubuntu systems and understand their PAM includes before changing authentication.
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- Review the SSH and PAM edits against the instructions for your exact release. Resolve conflicting directives rather than leaving multiple competing values.
- Validate the SSH daemon configuration using the command and reload or restart procedure documented for that release. A syntax check can catch some errors, but it does not prove that the authentication flow works.
- From a second terminal, start a new SSH connection. Confirm that the intended key is accepted and that SSH then requests the OTP. Complete login successfully before ending the existing session.
- If the test fails, use the still-open session to correct the configuration. If you have lost SSH access, use the provider’s verified console or rescue route; do not assume the recovery path is identical across providers.
OTP timing, backups, and lockout recovery
TOTP clock mismatch
TOTP derives the expected code from time, so the authenticator and server clocks must be sufficiently aligned. If valid-looking codes are rejected, check time synchronization on both devices before changing PAM settings. Ubuntu generally prefers TOTP when the authenticator supports it.
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HOTP desynchronization
HOTP advances through a sequence as codes are requested. If the authenticator generates codes but the server does not advance in step, the two can get out of sync. Use your planned out-of-band administration route to recover rather than repeatedly generating codes and making the mismatch worse.
Lost or unavailable authenticator
Ubuntu lists authenticator backup or sync, written backup codes, multiple enrolled TOTP devices, and another authentication path to rerun setup as possible mitigations. These backups also weaken the extra factor if an attacker gets them. Protect them accordingly; avoid keeping a raw shared secret in an unencrypted notes-sync service.
Vultr’s April 1, 2025 guide gives an example of generating 10 emergency codes and using a 30-second TOTP period. Those are values in that guide’s example, not universal defaults. Follow the prompts and documentation for your actual module and release.
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Troubleshoot common SSH MFA failures
| Symptom | Likely cause | What to check |
|---|---|---|
| SSH accepts the key but never prompts for a code | Keyboard-interactive is disabled, the required authentication methods are not effective, or PAM does not invoke the OTP module. | Check the effective SSH configuration, release-specific directive, and /etc/pam.d/sshd path and includes. |
| A password prompt appears instead of an OTP prompt | The PAM keyboard-interactive path may still allow password authentication. | Inspect the PAM stack and its includes; PasswordAuthentication no alone may not disable password authentication through PAM. |
| The OTP is rejected | The server and authenticator clocks may be misaligned, or the user may be entering a code for a different secret. | Check clock synchronization and verify that the enrolled secret belongs to the SSH account being tested. |
| HOTP codes stop working after generating several | The client and server counters may have become desynchronized. | Use the prepared recovery route to resynchronize or reconfigure securely. |
| A user cannot log in after enforcement | The user may not have enrolled an OTP secret, configured a key, or used the expected authentication path. | Use the existing admin session or provider console to restore access, then enroll and test that user before re-enforcing requirements. |
| SSH fails to reload after edits | A syntax error or conflicting setting may be present. | Keep the original session open, validate configuration using release-appropriate tools, and correct the error before trying another login. |
Keep the rest of the VPS secure
SSH MFA is one control, not a complete VPS security plan. Keep the operating system and exposed services maintained, restrict network access with a firewall appropriate to your services, protect provider-account access, and review which users and keys retain administrative access. MFA on SSH does not automatically extend to applications or databases running on the server.
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