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diff --git a/src/yubikey.thrust b/src/yubikey.thrust new file mode 100644 index 0000000..e91143d --- /dev/null +++ b/src/yubikey.thrust @@ -0,0 +1,135 @@ +--- +title: YubiKey +date: 2020-05-14 +--- +{% extends 'templates/base.html' %} +{% block body %} + <nav> + <a href='/projects'>> projects</a> + </nav> + + <header> + <h1>{{ title }}</h1> + </header> + + <article> + {% markdown %} + This article contains tips & tricks for using [YubiKeys](https://www.yubico.com) for authentication. + + [TOC] + + ## `sudo` over SSH using YubiKey + + Remembering root passwords for machines is annoying and error-prone. + You can reuse the same passwords and be insecure. + You can use different passwords and forget them. + You can type them into IRC and then have to reset them. + But there is a better way. + + In modern Linux & Unix systems, [Pluggable Authentication Modules (PAM)](https://en.wikipedia.org/wiki/Pluggable_authentication_module) can provide multiple ways of authenticating a user. + The most common one is "asking for a password", but they can also query external network services or require a second factor. + The module `pam_ssh_agent_auth` authenticates users by checking against SSH keys from a connected SSH agent, which means that we can have passwordless `sudo` when logged in remotely. + + Using `pam_ssh_agent_auth` with a YubiKey is both more convenient and more secure than passwords: you only need to tap the YubiKey, and because you _need_ to tap the YubiKey, becoming root requires a physical action on your part. + + NB: Technically this allows all SSH keys loaded into the SSH Agent to be used to become root. + If you have a mixture of YubiKey keys and on-disk SSH keys, I would recommend using `ssh-askpass` / `SSH_ASKPASS` to require confirmation. + + ### Debian & Ubuntu + + TL;DR: + + - Install `pam_ssh_agent_auth`. + - Enable `pam_ssh_agent_auth` for use with `sudo`. + - Allow `sudo` to maintain access to `SSH_AUTH_SOCK`. + - Connect to our remote machine with _SSH agent forwarding_ and never have to remember a server password again. + + On your remote machine, first install the SSH agent authentication PAM: + + ```sh + $ apt install libpam-ssh-agent-auth + ``` + + Then, enable and configure the PAM module for use with `sudo`: + + ```sh + $ cat /etc/pam.d/sudo + #%PAM-1.0 + + # Allow users to use their regular authorized SSH keys for sudo, + # and allow them to manage the keys themselves. + auth sufficient pam_ssh_agent_auth.so file=~/.ssh/authorized_keys allow_user_owned_authorized_keys_file + + # # Alternatively, have a single central key file, owned by root. + # # This is useful if you only want a subset of SSH keys to grant root permissions. + # auth sufficient pam_ssh_agent_auth.so file=/etc/ssh/sudo_authorized_keys + + @include common-auth + @include common-account + @include common-session-noninteractive + ``` + + Allow `sudo` to maintain access to the SSH agent by keeping `SSH_AUTH_SOCK`: + + ```sh + $ cat /etc/sudoers + Defaults env_keep += SSH_AUTH_SOCK + Defaults env_reset + Defaults mail_badpass + Defaults secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin" + + # User privilege specification + root ALL=(ALL:ALL) ALL + + # Allow members of group sudo to execute any command + %sudo ALL=(ALL:ALL) ALL + ``` + + To test it, connect to the machine with `ssh -A server.example.com`, and try using `sudo`: it should _not_ ask for a password, and the YubiKey should flash for a tap. + + For convenience, add `ForwardAgent yes` to the relevant hosts your SSH config to set `-A` by default: + + ```sh + $ cat ~/.ssh/config + Host server.example.com + ForwardAgent yes + + Host 192.168.16.* + ForwardAgent yes + ``` + + ### NixOS + + On [NixOS](https://nixos.org), the configuration for the server is much simpler: + + ```sh + $ cat /etc/nixos/configuration.nix + { config, pkgs, ... }: + { + . . . + security.pam.enableSSHAgentAuth = true; + security.pam.services.sudo.sshAgentAuth = true; + + users.users.eth.openssh.authorizedKeys.keys = [ + "ssh-rsa . . .", + "ssh-rsa . . .", + ]; + } + ``` + + To test it, connect to the machine with `ssh -A server.example.com`, and try using `sudo`: it should _not_ ask for a password, and the YubiKey should flash for a tap. + + For convenience, add `ForwardAgent yes` to the relevant hosts your SSH config to set `-A` by default: + + ```sh + $ cat ~/.ssh/config + Host server.example.com + ForwardAgent yes + + Host 192.168.16.* + ForwardAgent yes + ``` + + {% endmarkdown %} + </article> +{% endblock %} |