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diff --git a/blog/2022-03-26-ssh-mfa.org b/blog/2022-03-26-ssh-mfa.org index 3560dcf..1d7141e 100644 --- a/blog/2022-03-26-ssh-mfa.org +++ b/blog/2022-03-26-ssh-mfa.org @@ -1,82 +1,88 @@ -+++ -date = 2022-03-26 -title = "Enable TOTP MFA for SSH" -description = "Harden your SSH target with user-enabled MFA options, such as TOTP." -draft = false -+++ - -## 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. +#+title: Enable TOTP MFA for SSH +#+date: 2022-03-26 + +** Why Do I Need MFA for SSH? +:PROPERTIES: +:CUSTOM_ID: why-do-i-need-mfa-for-ssh +:END: +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 +[[/blog/hardening-a-public-facing-home-server/][Hardening a +Public-Facing Home Server]]. + +** Install MFA PAM Module +:PROPERTIES: +:CUSTOM_ID: install-mfa-pam-module +:END: +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: + +#+begin_src sh sudo apt-get update -``` +#+end_src -```sh +#+begin_src 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 +#+end_src + +** Initialize the PAM Module +:PROPERTIES: +:CUSTOM_ID: initialize-the-pam-module +:END: +*** Interactive Method +:PROPERTIES: +:CUSTOM_ID: interactive-method +:END: +Once the package is installed, initialize it and following the +interactive prompts to generate your OTP or TOTP: + +#+begin_src sh google-authenticator -``` +#+end_src -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. +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 +#+begin_src txt OUTPUT Do you want authentication tokens to be time-based (y/n) y -``` +#+end_src -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. +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 +#+begin_src txt OUTPUT Do you want me to update your "/home/user/.google_authenticator" file? (y/n) y -``` +#+end_src -```txt +#+begin_src 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 -``` +#+end_src -```txt +#+begin_src txt OUTPUT By default, a new token is generated every 30 seconds by the mobile app. @@ -89,30 +95,32 @@ 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 -``` +#+end_src -```txt +#+begin_src 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 -``` +#+end_src -### Non-Interactive Method +*** Non-Interactive Method +:PROPERTIES: +:CUSTOM_ID: non-interactive-method +:END: +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: -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 +#+begin_src sh google-authenticator -t -d -f -r 3 -R 30 -w 3 -``` +#+end_src The options referenced above are as follows: -```txt +#+begin_src txt google-authenticator [<options>] -h, --help Print this message -c, --counter-based Set up counter-based (HOTP) verification @@ -132,65 +140,69 @@ google-authenticator [<options>] -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 +#+end_src + +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 +:PROPERTIES: +:CUSTOM_ID: pam-configuration-settings +:END: +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: + +#+begin_src sh sudo nano /etc/pam.d/sshd -``` +#+end_src -You need to do two things in this file. First, add the following lines to the -bottom of the file: +You need to do two things in this file. First, add the following lines +to the bottom of the file: -```config +#+begin_src config auth required pam_google_authenticator.so nullok auth required pam_permit.so -``` +#+end_src 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: +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 +#+begin_src config #@include common-auth -``` - -## SSH Configuration Settings +#+end_src -Finally, edit the `sshd_config` file again: +** SSH Configuration Settings +:PROPERTIES: +:CUSTOM_ID: ssh-configuration-settings +:END: +Finally, edit the =sshd_config= file again: -```sh +#+begin_src sh sudo nano /etc/ssh/sshd_config -``` +#+end_src -You'll need to change `ChallengeResponseAuthentication` to yes and add the -`AuthenticationMethods` line to the bottom of the file. +You'll need to change =ChallengeResponseAuthentication= to yes and add +the =AuthenticationMethods= line to the bottom of the file. -```config +#+begin_src config ChallengeResponseAuthentication yes AuthenticationMethods publickey,password publickey,keyboard-interactive -``` +#+end_src -Finally, restart the `ssh` service: +Finally, restart the =ssh= service: -```sh +#+begin_src sh sudo systemctl restart sshd.service -``` +#+end_src -The next time you log in, you should be greeted with a verification code +The next time you log in, you should be greeted with a verification code request! |