For Ubuntu Server, the documented PAM-based SSH setup requires a public key first and a one-time code second. Before enabling it, verify key-based access, enroll every SSH user, and confirm you can recover through your VPS provider’s console. SSH two-factor authentication protects SSH logins; it does not secure every account or service on the VPS.
What SSH two-factor authentication protects
This guide configures a second factor for SSH access to the Linux guest operating system. In Ubuntu’s documented TOTP/HOTP flow, OpenSSH first checks a public key, then prompts for a one-time password through PAM and keyboard-interactive authentication. The documented configuration disables SSH password authentication.
This is separate from multi-factor authentication on your VPS provider account or access through its web console. It also does not automatically require a second factor for web applications, databases, or sudo. Configure those separately if you need them protected.
Prepare before changing SSH authentication
- Identify your Linux distribution and release. The commands and PAM arrangement below target Ubuntu Server; other distributions may package the module or structure PAM differently.
- Confirm you can log in over SSH now and have a separate sudo-capable administrator account. Vultr’s prerequisites also recommend an updated system, a firewall, and SSH-key access; see Vultr’s guide.
- Verify how to reach your provider’s out-of-band console or rescue environment. The recovery method varies by provider; do not assume console access has the same authentication requirements as SSH.
- Keep your existing privileged SSH session open while changing settings. Use a second terminal for the test login, and do not close the first session until the full key-plus-code flow works.
- List every user who needs SSH access and enroll each one before making the second factor mandatory. Ubuntu warns that users need their public-key access and 2FA secret configured before enforcement, or they may be unable to log in to finish setup.
Choose a second-factor method
PAM-backed TOTP or HOTP
Ubuntu documents the libpam-google-authenticator PAM module and a per-user setup command. Each user enrolls a secret in a compatible authenticator app. The resulting per-user file contains the shared secret, emergency passcodes, and configuration, so treat it as sensitive authentication material.
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Ubuntu generally prefers TOTP when the authenticator supports it. TOTP codes depend on both the server and authenticator having sufficiently aligned clocks. HOTP codes advance through a sequence; if codes are generated but the server does not advance in step, the two sides can get out of sync.
Hardware-backed U2F/FIDO
Ubuntu recommends hardware authentication devices that support U2F/FIDO for the strongest 2FA security. Its separate OpenSSH guide covers security-key types such as ecdsa-sk and ed25519-sk. This is a different setup path: it requires compatible OpenSSH support and a supported hardware device to be present for authentication. Consult Ubuntu’s U2F/FIDO documentation. Ubuntu’s TOTP guide says combining that method with its PAM TOTP/HOTP setup is not recommended because the combination has not been tested there.
| Method | Credential and requirements | Important failure or recovery consideration |
|---|---|---|
| PAM TOTP/HOTP | Per-user secret and one-time code; PAM module and SSH keyboard-interactive configuration. | TOTP depends on clock alignment; HOTP can desynchronize. Protect backup codes and secrets. |
| OpenSSH U2F/FIDO | Hardware security device and compatible OpenSSH security-key support. | The device must be available. Plan an alternate access or recovery route appropriate to your setup. |
Set up PAM-backed TOTP on Ubuntu Server
Use the current Ubuntu Server procedure for your release as the authority for exact PAM edits and service handling. The Ubuntu documentation is titled “Two factor authentication with TOTP/HOTP” and was last updated June 26, 2026.
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1. Install the PAM module
On Ubuntu, update package metadata and install the module:
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For each user, run the module’s setup command as that user, following its prompts and the current Ubuntu instructions. Scan the displayed QR code or enter the secret into a compatible authenticator. Store emergency passcodes securely and keep the generated secret file private. Do not store an unencrypted copy of the shared secret in a notes-sync service.
Do not enable mandatory OTP for an account until that user has a working SSH key and has completed enrollment. Review the current prompts rather than assuming an older guide’s defaults are unchanged. Ubuntu’s older tutorial advises rate limiting and preventing multiple uses of a token; treat those as version-sensitive setup choices, not universal defaults.
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3. Configure PAM for SSH
Follow Ubuntu’s current procedure to make the SSH PAM stack invoke the OTP module in /etc/pam.d/sshd. Ubuntu’s older tutorial shows the line auth required pam_google_authenticator.so, but its example uses legacy configuration names; do not substitute it blindly for the current release’s documented PAM setup.
4. Configure the SSH daemon
Ubuntu’s documented SSH configuration excerpt is:
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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 daemon’s main file and included configuration files for existing or conflicting directives before editing; duplicate settings can produce unexpected effective values.
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5. Check the PAM authentication path
Keyboard-interactive provides text prompts and can invoke PAM. That does not mean it is limited to OTP: PAM may also offer password authentication. Mozilla’s OpenSSH guidance warns that PasswordAuthentication no alone does not prove password authentication is impossible if PAM still enables it. Inspect /etc/pam.d/sshd and any included stacks to confirm the actual path requires the intended factors and has no unintended password fallback.
PAM stacks are distribution-specific. Do not replace a PAM file with a generic snippet from another distribution. If you are not on the Ubuntu release covered by the instructions, use your vendor’s current documentation and understand the included PAM rules before changing authentication.
6. Apply and test the change
Apply the SSH configuration using the restart or reload procedure documented for your Ubuntu release. Keep your current session open, then connect from a second terminal. Confirm the server accepts the intended key and prompts for the OTP, and verify that the login completes only with both. If it fails, use the open session or provider recovery console rather than closing your last working administrative path.
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Plan recovery before relying on MFA
Decide what happens if a phone is lost, damaged, replaced, or unavailable. Ubuntu lists authenticator backup or sync, securely stored backup codes, multiple enrolled TOTP devices, and another authentication path for rerunning setup as possible mitigations. Each backup can weaken the additional factor if an attacker obtains it, so protect it appropriately.
Separately verify your provider’s console or rescue procedure before enforcement. Vultr documents its web console as a way to recover from SSH lockout; your provider may work differently. Keep recovery material somewhere other than the VPS where practical, and ensure authorized administrators can reach it without relying on the device that may be lost.
Troubleshoot common login failures
- The SSH client never asks for a code: Confirm the effective daemon configuration enables keyboard-interactive authentication and requires
publickey,keyboard-interactive. Check included SSH configuration files and the PAM SSH stack. - The key works but the OTP is rejected: For TOTP, check that the server and authenticator clocks are aligned. For HOTP, codes generated without a matching server advance can desynchronize the sequence. Use your documented recovery path if correcting the cause does not restore access.
- A user cannot log in after enforcement: Verify that the user enrolled a secret and has a working public key before enforcement. Use an already-open administrator session or provider console to correct the account configuration.
- A password prompt still appears or a password works: Inspect PAM and its included stacks. Disabling
PasswordAuthenticationdoes not by itself rule out password authentication through keyboard-interactive and PAM. - Configuration changes lock out new sessions: Do not close the known-good session before testing. If all SSH access is lost, use the provider’s verified console or rescue process.
Keep the rest of the VPS secure
SSH MFA is one layer, not a complete VPS security plan. Maintain key-based access, keep the system updated, limit network exposure with a firewall, protect provider-account access, and review which users have administrative privileges. If you intentionally want MFA for sudo, configure and test that separately; this SSH procedure does not enable it.
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