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diff --git a/content/blog/2022-03-26-ssh-mfa.md b/content/blog/2022-03-26-ssh-mfa.md deleted file mode 100644 index 30b6a84..0000000 --- a/content/blog/2022-03-26-ssh-mfa.md +++ /dev/null @@ -1,198 +0,0 @@ -+++ -date = 2022-03-26 -title = "Enable TOTP MFA for SSH" -description = "Learn how to enable timed one-time passcodes for SSH." -+++ - -## Why Do I Need MFA for SSH? - -If you are a sysadmin of a server anywhere (that includes at home!), you -may want an added layer of protection against intruders. This is not a -replacement for other security measures, such as: - -- Disable root SSH -- Disable SSH password authentication -- Allow only certain users to login via SSH -- Allow SSH only from certain IPs - -However, MFA can be added as an additional security measure to ensure -that your server is protected. This is especially important if you need -to allow password authentication for SSH. - -For more guidance on server security measures, see my other post: -[Hardening a Public-Facing Home -Server](/blog/hardening-a-public-facing-home-server/). - -## Install MFA PAM Module - -PAM, which stands for Pluggable Authentication Module, is an -authentication infrastructure used on Linux systems to authenticate a -user. In order to use this technology, let's install the -`libpam-google-authenticator` package: - -```sh -sudo apt-get update -``` - -```sh -sudo apt-get install libpam-google-authenticator -``` - -## Initialize the PAM Module - -### Interactive Method - -Once the package is installed, initialize it and following the -interactive prompts to generate your OTP or TOTP: - -```sh -google-authenticator -``` - -If you are not sure how to answer, read the prompts carefully and think -about having to how each situation would affect your normal login -attempts. If you are still not sure, use my default responses below. - -```txt -OUTPUT - -Do you want authentication tokens to be time-based (y/n) y -``` - -At this point, use an authenticator app somewhere one of your devices to -scan the QR code. Any future login attempts after our upcoming -configuration changes will require that TOTP. - -```txt -OUTPUT - -Do you want me to update your "/home/user/.google_authenticator" file? (y/n) y -``` - -```txt -OUTPUT - -Do you want to disallow multiple uses of the same authentication -token? This restricts you to one login about every 30s, but it increases -your chances to notice or even prevent man-in-the-middle attacks (y/n) y -``` - -```txt -OUTPUT - -By default, a new token is generated every 30 seconds by the mobile app. -In order to compensate for possible time-skew between the client and the server, -we allow an extra token before and after the current time. This allows for a -time skew of up to 30 seconds between authentication server and client. If you -experience problems with poor time synchronization, you can increase the window -from its default size of 3 permitted codes (one previous code, the current -code, the next code) to 17 permitted codes (the 8 previous codes, the current -code, and the 8 next codes). This will permit for a time skew of up to 4 minutes -between client and server. -Do you want to do so? (y/n) n -``` - -```txt -OUTPUT - -If the computer that you are logging into isn't hardened against brute-force -login attempts, you can enable rate-limiting for the authentication module. -By default, this limits attackers to no more than 3 login attempts every 30s. -Do you want to enable rate-limiting? (y/n) y -``` - -### Non-Interactive Method - -If you need to do this quickly, know your responses to the prompts, or -are setting this up for numerous users, the non-interactive method can -be much faster: - -```sh -google-authenticator -t -d -f -r 3 -R 30 -w 3 -``` - -The options referenced above are as follows: - -```txt -google-authenticator [<options>] - -h, --help Print this message - -c, --counter-based Set up counter-based (HOTP) verification - -t, --time-based Set up time-based (TOTP) verification - -d, --disallow-reuse Disallow reuse of previously used TOTP tokens - -D, --allow-reuse Allow reuse of previously used TOTP tokens - -f, --force Write file without first confirming with user - -l, --label=<label> Override the default label in "otpauth://" URL - -i, --issuer=<issuer> Override the default issuer in "otpauth://" URL - -q, --quiet Quiet mode - -Q, --qr-mode={NONE,ANSI,UTF8} QRCode output mode - -r, --rate-limit=N Limit logins to N per every M seconds - -R, --rate-time=M Limit logins to N per every M seconds - -u, --no-rate-limit Disable rate-limiting - -s, --secret=<file> Specify a non-standard file location - -S, --step-size=S Set interval between token refreshes - -w, --window-size=W Set window of concurrently valid codes - -W, --minimal-window Disable window of concurrently valid codes - -e, --emergency-codes=N Number of emergency codes to generate -``` - -This fully configures the authenticator, saves it to a file, and then -outputs the secret key, QR code, and recovery codes. (If you add the -flag `-q`, then there won't be any output). If you use this -command in an automated fashion, make sure your script captures the -secret key and/or recovery codes and makes them available to the user. - -## PAM Configuration Settings - -Once you've enabled the T/OTP and have it saved to an MFA app on your -phone or other device, open the PAM `sshd` file: - -```sh -sudo nano /etc/pam.d/sshd -``` - -You need to do two things in this file. First, add the following lines -to the bottom of the file: - -```config -auth required pam_google_authenticator.so nullok -auth required pam_permit.so -``` - -Second, comment-out the following line near the top of the file. - -If you leave this line uncommented, every SSH login attempt will ask for -the following three authentication factors: - -1. Publickey -2. Password -3. T/OTP code - -```config -#@include common-auth -``` - -## SSH Configuration Settings - -Finally, edit the `sshd_config` file again: - -```sh -sudo nano /etc/ssh/sshd_config -``` - -You'll need to change `ChallengeResponseAuthentication` to -yes and add the `AuthenticationMethods` line to the bottom of -the file. - -```config -ChallengeResponseAuthentication yes -AuthenticationMethods publickey,password publickey,keyboard-interactive -``` - -Finally, restart the `ssh` service: - -```sh -sudo systemctl restart sshd.service -``` - -The next time you log in, you should be greeted with a verification code -request! |