├── .gitignore ├── LICENSE ├── README.md ├── ansible.cfg ├── files ├── ssh-ca ├── ssh-ca.motd └── sudoers-sshca ├── install.yml ├── ssh-csr ├── templates ├── ssh-ca.conf.j2 └── sshd_config.j2 ├── test.yml ├── users.yml └── vars.template.yml /.gitignore: -------------------------------------------------------------------------------- 1 | .ansible-vault-password.txt 2 | inventory 3 | -------------------------------------------------------------------------------- /LICENSE: -------------------------------------------------------------------------------- 1 | GNU AFFERO GENERAL PUBLIC LICENSE 2 | Version 3, 19 November 2007 3 | 4 | Copyright (C) 2007 Free Software Foundation, Inc. 5 | Everyone is permitted to copy and distribute verbatim copies 6 | of this license document, but changing it is not allowed. 7 | 8 | Preamble 9 | 10 | The GNU Affero General Public License is a free, copyleft license for 11 | software and other kinds of works, specifically designed to ensure 12 | cooperation with the community in the case of network server software. 13 | 14 | The licenses for most software and other practical works are designed 15 | to take away your freedom to share and change the works. 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It is safest 628 | to attach them to the start of each source file to most effectively 629 | state the exclusion of warranty; and each file should have at least 630 | the "copyright" line and a pointer to where the full notice is found. 631 | 632 | 633 | Copyright (C) 634 | 635 | This program is free software: you can redistribute it and/or modify 636 | it under the terms of the GNU Affero General Public License as published 637 | by the Free Software Foundation, either version 3 of the License, or 638 | (at your option) any later version. 639 | 640 | This program is distributed in the hope that it will be useful, 641 | but WITHOUT ANY WARRANTY; without even the implied warranty of 642 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 643 | GNU Affero General Public License for more details. 644 | 645 | You should have received a copy of the GNU Affero General Public License 646 | along with this program. If not, see . 647 | 648 | Also add information on how to contact you by electronic and paper mail. 649 | 650 | If your software can interact with users remotely through a computer 651 | network, you should also make sure that it provides a way for users to 652 | get its source. For example, if your program is a web application, its 653 | interface could display a "Source" link that leads users to an archive 654 | of the code. There are many ways you could offer source, and different 655 | solutions will be better for different programs; see section 13 for the 656 | specific requirements. 657 | 658 | You should also get your employer (if you work as a programmer) or school, 659 | if any, to sign a "copyright disclaimer" for the program, if necessary. 660 | For more information on this, and how to apply and follow the GNU AGPL, see 661 | . 662 | -------------------------------------------------------------------------------- /README.md: -------------------------------------------------------------------------------- 1 | Forced Command SSH CA 2 | ===================== 3 | 4 | A self-serve automated SSH CA implemented as an SSH forced command. 5 | 6 | > Copyright © 2018 Alexandre de Verteuil 7 | > 8 | > This program is free software: you can redistribute it and/or modify 9 | > it under the terms of the **GNU Affero General Public License** as published 10 | > by the Free Software Foundation, either version 3 of the License, or 11 | > (at your option) any later version. 12 | > 13 | > This program is distributed in the hope that it will be useful, 14 | > but WITHOUT ANY WARRANTY; without even the implied warranty of 15 | > MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 16 | > GNU Affero General Public License for more details. 17 | > 18 | > You should have received a copy of the GNU Affero General Public License 19 | > along with this program. If not, see [http://www.gnu.org/licenses/](http://www.gnu.org/licenses/). 20 | 21 | 22 | Design 23 | ====== 24 | 25 | This CA is made by a Linux system administrator for Linux system 26 | administrators. It leverages facilities of a mostly vanilla Debian/Ubuntu 27 | operating system to reduce on programming and maintenance effort. 28 | 29 | The user doesn't invoke `ssh-ca` directly. When an ssh connection 30 | is established to the server and the client authenticates as a user 31 | other than root, the SSH server loads `/usr/sbin/sudo -u sshca 32 | /usr/local/bin/ssh-ca` as a forced command. The `ssh-ca` process can 33 | read the `SSH_ORIGINAL_COMMAND` environment variable and take different 34 | actions according to the command parameters passed by the ssh client. 35 | 36 | When the command parameters contain an SSH certificate signature 37 | subcommand, a public key is expected on standard input then an SSH 38 | certificate is written to standard output and the connection is closed. 39 | 40 | Here's an overview of the call chain: 41 | 42 | `ssh-csr` → `ssh` → `sshd` → `sudo` → `ssh-ca` → `ssh-keygen` 43 | 44 | 45 | Functions of the Ansible playbook 46 | --------------------------------- 47 | 48 | * Create user accounts (regular Unix users) with their `authorized_keys` 49 | All users except root have a `ForceCommand` 50 | * Create the `sshca` user 51 | * Install `/usr/local/bin/ssh-ca` 52 | * Configure the CA 53 | * Configure the `ForceCommand` and `sudoers` 54 | * Install the SSH CA keypair(s) 55 | * Activate NTP 56 | * Create a host certificate signed by the CA 57 | 58 | 59 | Functions of `ssh-ca` 60 | --------------------- 61 | 62 | * Read its configuration 63 | * Path to the CA keys 64 | * Certificate options 65 | * Certificate validity (time window before and after "now") 66 | * Read a public key on standard input, sign it, write the certificate on standard output 67 | * Log the requests 68 | * Log the emitted certificates 69 | * Subcommands: 70 | * `user`: Sign a user certificate 71 | * `host`: Sign a host certificate 72 | * `report`: Report all certificates ever produced in CSV format 73 | 74 | Optionnal, future development: 75 | 76 | * Restrict certain principals to certain users 77 | 78 | 79 | Functions of the CA system administrator 80 | ---------------------------------------- 81 | 82 | * Define CA variables in the Ansible inventory 83 | * Run the CA installation playbook `install.yml` 84 | * Rotate CA keys on a recurring basis and on a security alert basis 85 | * Revoke certificates 86 | * Audit CA and CSR logs 87 | 88 | 89 | Functions of sshd 90 | ----------------- 91 | 92 | * Authenticate users authorized to obtain SSH certificates 93 | * Force the ssh-ca command on such users 94 | 95 | 96 | Functions of ssh-csr 97 | -------------------- 98 | 99 | * Read the principal list from the command parameter 100 | * Establish an SSH connection to the CA, specifying the ssh-ca subcommand and parameters 101 | * Read ~/.ssh/id\_rsa.pub (or another configured identity) and write the key to ssh-ca's standard input 102 | * Close the write stream 103 | * Read the signed certificate from ssh-ca's standard output and write it to ~/ssh.id\_rsa-cert.pub (or another configured itentity) 104 | 105 | 106 | Functions of `sudo` 107 | ------------------- 108 | 109 | Permit users of the `sshca` group to sign keys while preventing direct access to the CA keys. 110 | 111 | 112 | CA administrator set up 113 | ======================= 114 | 115 | Before you can manage the SSH CA, you must perform a few preparation steps. 116 | 117 | 118 | Inventory 119 | --------- 120 | 121 | The inventory directory should be organized in a different repository because 122 | this repository is open sourced and will be version controlled separately. 123 | 124 | In this repository's root directory, create a symbolic link named "inventory" 125 | pointing to your inventory file or directory. 126 | 127 | ln -s ../ssh-ca-inventory inventory 128 | 129 | The CA will be installed on `ssh-ca` host group. The "inventory" 130 | file/link is not tracked by Git. 131 | 132 | 133 | Variables 134 | --------- 135 | 136 | The `vars.template.yml` file is provided as documentation-by-example for 137 | the Ansible variables that must be defined in your inventory. 138 | 139 | 140 | Ansible Vault password 141 | ---------------------- 142 | 143 | If some of your variables are in an Ansible Vault, write the Vault 144 | password in the `.ansible-vault-password.txt` file at the root of this 145 | repository. This file is not tracked by Git and is referred to from the 146 | supplied `ansible.cfg` configuration file. 147 | 148 | The CA system administrators are those that have the Vault password and 149 | root access to the SSH servers of the `ssh-ca` host group. They are 150 | masters of the CA and must be trusted. 151 | 152 | 153 | SSH CA administration tasks 154 | =========================== 155 | 156 | Remove a user's key 157 | ------------------- 158 | 159 | Do not remove a user's SSH key by deleting its item from the list. This 160 | will simply stop Ansible from managing this key and would leave it 161 | present on the CA server. Rather, add the "state:" key with the value 162 | "absent". Then run the next install.yml playbook run will actually 163 | remove the SSH key from the user's authorized\_keys file. 164 | 165 | 166 | Create CA keypairs 167 | ------------------ 168 | 169 | On your workstation, use `ssh-keygen` to generate a keypair. 170 | 171 | ssh-keygen -C my-ca-01 -f my-ca-01 172 | 173 |
174 |
-C my-ca-01
175 |
Set the key's comment to "my-ca-01". By convention, 01 is the key's sequence number. Thus you can provision multiple keys in advance to facilitate key rotation.
176 |
-f my-ca-01
177 |
Provide the output filename. The public part will have `.pub` appended to the name.
178 |
179 | 180 | Then copy the content of the secret and public key files in a new `ssh_ca_keypairs` list item. 181 | 182 | 183 | Publish the CA public key for user's known hosts 184 | ------------------------------------------------ 185 | 186 | Instruct your users to add the certificate authority key to their 187 | `known_hosts` file. The format of the line is: 188 | 189 | @cert-authority * keytype base64-encoded-key comment 190 | 191 | For example: 192 | 193 | @cert-authority * ssh-rsa AAAAB3NzaC1y ..... WcS60D my-ca-01 194 | 195 | 196 | Rotate host certificates 197 | ------------------------ 198 | 199 | Host certificates have a 1 year validity. To renew them, 200 | `rm /etc/ssh/ssh_host_*_key-cert.pub` and run the playbook 201 | again. 202 | 203 | 204 | FreeIPA Integration 205 | =================== 206 | 207 | Forced Command SSH CA can be configured with FreeIPA integration for 208 | user authentication with SSH public key authentication. In this case, 209 | centralized sudo rules and Host Based Access Control must be carefully 210 | planned to limit access to the public key for regular users. 211 | 212 | I suggest joining the CA server to the ipa domain and setting 213 | `ssh_ca_configuration.freeipa_integration` to `yes` in host variables 214 | **before** running the `install.yml` playbook. 215 | 216 | I tested this on a Ubuntu 14.04 ipa client with two CentOS 7 ipa servers. 217 | 218 | 219 | Joining the FreeIPA domain 220 | -------------------------- 221 | 222 | Install FreeIPA client 223 | 224 | aptitude install freeipa-client 225 | 226 | Update the nameservers configured in /etc/network/interfaces to point 227 | to the IPA nameservers. Assuming that DNS service is enabled on the ipa 228 | servers, this will enable service discovery. 229 | 230 | ipa-client-install --mkhomedir --no-sshd 231 | 232 | Note: The `--mkhomedir` option can be set but it doesn't seem to enable 233 | PAM's mkhomedir module. There is a task in `install.yml` that takes care 234 | of that. 235 | 236 | add sudo to the list of sssd services in `/etc/sssd/sssd.conf`: 237 | 238 | services = nss, pam, ssh, sudo 239 | 240 | Remove any unneeded keys from `/root/.ssh/authorized_keys`, especially 241 | if your provisioning infrastructure automatically inserts the CA key. It 242 | is nonsensical to authorize `ssh-ca-users` to login as root on any host 243 | in the FreeIPA domain! 244 | 245 | 246 | Administration credentials 247 | -------------------------- 248 | 249 | When FreeIPA integration is used, CA administrators no longer connect 250 | to the server with the `root` user. Rather, they should have a separate 251 | user in the `ssh-ca-admins` group. For example, I have an `adeverteuil` 252 | user in the `ssh-ca-users` group and an `adeverteuil-admin` user in the 253 | `ssh-ca-admins` group. 254 | 255 | Then I run the `install.yml` playbook with the following options: 256 | 257 | ap install.yml -u adeverteuil-admin -b --become-user root 258 | 259 | 260 | FreeIPA sudo rules 261 | ------------------ 262 | 263 | -sh-4.2$ ipa sudocmd-show '/usr/local/bin/ssh-ca ""' 264 | Sudo Command: /usr/local/bin/ssh-ca "" 265 | Description: forcedcommand-ssh-ca 266 | 267 | -sh-4.2$ ipa sudorule-show forcedcommand-ssh-ca 268 | Rule name: forcedcommand-ssh-ca 269 | Enabled: TRUE 270 | User Groups: ssh-ca-users 271 | Host Groups: ssh-ca 272 | Sudo Allow Commands: /usr/local/bin/ssh-ca "" 273 | RunAs External User: sshca 274 | Sudo Option: !authenticate, env_keep+=SSH_ORIGINAL_COMMAND 275 | 276 | 277 | FreeIPA users 278 | ------------- 279 | 280 | This is my regular user. It is a indirect member of the `ssh-ca-users` 281 | group. For demonstration purposes I removed some irrelevant details. 282 | 283 | -sh-4.2$ ipa user-show adeverteuil 284 | User login: adeverteuil 285 | First name: Alexandre 286 | Last name: de Verteuil 287 | Home directory: /home/adeverteuil 288 | Login shell: /bin/sh 289 | Principal name: adeverteuil@SC.IWEB.COM 290 | Principal alias: adeverteuil@SC.IWEB.COM 291 | Email address: adeverteuil@inap.com 292 | UID: 1440900000 293 | GID: 1440900000 294 | SSH public key fingerprint: SHA256:hAJ0lT9FR85x+3IUyv6hLtv0bJSOc/1mWPXEGyAxPy0 adeverteuil@home-pc (ssh-rsa), 295 | SHA256:IpS5f34ur5ETLATde6kcX0SXSeOl3u4+e9XS8w2oiJo adeverteuil@home-laptop (ssh-rsa), 296 | SHA256:bh/6eKMulLfWwdrfR+S9OegfNjHiHeEoa3tFzpRq4bQ adeverteuil@work-laptop (ssh-rsa) 297 | Account disabled: False 298 | Password: True 299 | Member of groups: ipausers, ssh-ca-users 300 | Indirect Member of Sudo rule: forcedcommand-ssh-ca 301 | Indirect Member of HBAC rule: ssh certificate authority 302 | Kerberos keys available: True 303 | 304 | This is my admin user. It's a good practice to have a privileged 305 | user account for administrative work and a regular user account for 306 | day-to-day work. In this case, it is mandatory because any user in the 307 | `ssh-ca-users` group will never be able to get a shell via SSH due to 308 | the ForceCommand. 309 | 310 | In this case, the user is member of the `ssh-ca-admins` group. 311 | 312 | -sh-4.2$ ipa user-show adeverteuil-admin 313 | User login: adeverteuil-admin 314 | First name: Alexandre 315 | Last name: de Verteuil 316 | Home directory: /home/adeverteuil-admin 317 | Login shell: /bin/sh 318 | Principal name: adeverteuil-admin@SC.IWEB.COM 319 | Principal alias: adeverteuil-admin@SC.IWEB.COM 320 | Email address: adeverteuil-admin@sc.iweb.com 321 | UID: 1440900008 322 | GID: 1440900008 323 | SSH public key fingerprint: SHA256:bh/6eKMulLfWwdrfR+S9OegfNjHiHeEoa3tFzpRq4bQ adeverteuil@work-laptop (ssh-rsa) 324 | Account disabled: False 325 | Password: True 326 | Member of groups: ipausers, ssh-ca-admins, admins 327 | Indirect Member of Sudo rule: sc-infra-admin sudo on SC infrastructure 328 | Indirect Member of HBAC rule: ssh-ca-administration, sc-infra-admins 329 | Kerberos keys available: True 330 | 331 | 332 | FreeIPA HBAC 333 | ------------ 334 | 335 | It is important to disable the default `allow_all` HBAC rule. 336 | 337 | This new rule allows members of the `ssh-ca-admins` group to ssh into the CA server. 338 | 339 | -sh-4.2$ ipa hbacrule-show ssh-ca-administration 340 | Rule name: ssh-ca-administration 341 | Description: Members of ssh-ca-admins get shell access on the CA servers. Users of this group should not be members of ssh-ca-users, otherwise the ssh ForceCommand will be in effect and prevent ssh-ca-admins 342 | from getting a ssh terminal. 343 | Enabled: TRUE 344 | User Groups: ssh-ca-admins 345 | Host Groups: ssh-ca 346 | Services: login, sshd 347 | Service Groups: Sudo 348 | 349 | This also grants ssh access to the `ssh-ca-users` group, and the ForceCommand in `sshd_config` is enforced on this group name. 350 | 351 | -sh-4.2$ ipa hbacrule-show "ssh certificate authority" 352 | Rule name: ssh certificate authority 353 | Enabled: TRUE 354 | User Groups: ssh-ca-users 355 | Host Groups: ssh-ca 356 | Services: sshd 357 | -------------------------------------------------------------------------------- /ansible.cfg: -------------------------------------------------------------------------------- 1 | [defaults] 2 | remote_user = root 3 | retry_files_enabled = False 4 | ansible_managed = This file is managed by Ansible. Do not edit manually! 5 | vault_password_file = .ansible-vault-password.txt 6 | inventory = inventory 7 | -------------------------------------------------------------------------------- /files/ssh-ca: -------------------------------------------------------------------------------- 1 | #!/bin/bash 2 | # 3 | # ssh-ca 4 | # 5 | # This file is part of forced-command-ssh-ca, a self-serve automated SSH 6 | # certificate authority implemented as an SSH forced command 7 | # Copyright (C) 2018 Alexandre de Verteuil 8 | # 9 | # This program is free software: you can redistribute it and/or modify 10 | # it under the terms of the GNU Affero General Public License as published 11 | # by the Free Software Foundation, either version 3 of the License, or 12 | # (at your option) any later version. 13 | # 14 | # This program is distributed in the hope that it will be useful, 15 | # but WITHOUT ANY WARRANTY; without even the implied warranty of 16 | # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 17 | # GNU Affero General Public License for more details. 18 | # 19 | # You should have received a copy of the GNU Affero General Public License 20 | # along with this program. If not, see . 21 | 22 | . /etc/ssh-ca.conf 23 | 24 | run_id="$(uuid)" 25 | user_id="${SUDO_USER:-$USER}" 26 | 27 | function log_debug() { 28 | flock /var/log/ssh-ca/debug.log \ 29 | echo "$(date "+%F %T") ${run_id} ${*}" \ 30 | >> /var/log/ssh-ca/debug.log 31 | } 32 | 33 | function log_output() { 34 | log_debug "CALL ${*}" 35 | output="$("${@}" 2>&1)" 36 | rc=$? 37 | log_debug "RETURN ${rc}" 38 | log_debug "OUTPUT ${output}" 39 | return ${rc} 40 | } 41 | 42 | function log_certificate() { 43 | flock /var/log/ssh-ca/certificates.log \ 44 | dd oflag=append conv=notrunc status=none \ 45 | if="${cert}" \ 46 | of=/var/log/ssh-ca/certificates.log 47 | } 48 | 49 | function sign_key() { 50 | # Files and IO related variables 51 | mkdir -p /tmp/ssh-ca 52 | pubkey="/tmp/ssh-ca/${run_id}.pub" 53 | cert="/tmp/ssh-ca/${run_id}-cert.pub" 54 | 55 | # Expect a public key on stdin 56 | dd bs=1K count=10 of="${pubkey}" status=none 57 | log_output ssh-keygen -l -f "${pubkey}" 58 | rc=$? 59 | if [ ${rc} -gt 0 ]; then 60 | echo ERROR: input is not a well-formed SSH public key 61 | rm "${pubkey}" 62 | exit ${rc} 63 | fi 64 | 65 | # Certificate signature parameters 66 | ca_key="/usr/local/lib/ssh-ca/${SSH_CA_KEY}" 67 | serial="$(date +%s)" 68 | principals="${arguments[1]}" 69 | if [ "${arguments[0]}" = "user" ]; then 70 | validity="${SSH_CA_CERTIFICATE_VALIDITY}" 71 | identity="${user_id}" 72 | options="${SSH_CA_CERTIFICATE_OPTIONS}" 73 | host_cert="" 74 | else 75 | validity="-10m:+365d" 76 | identity="CA=${SSH_CA_KEY} REQUESTOR=${user_id}" 77 | options="" 78 | host_cert="-h" 79 | fi 80 | 81 | # Validation 82 | if [ -z "${principals}" ]; then 83 | log_debug "ERROR: Empty PRINCIPALS parameter" 84 | echo "ERROR: The PRINCIPALS parameter cannot be empty" 85 | exit 1 86 | fi 87 | 88 | # Actual signature happens here 89 | log_output ssh-keygen \ 90 | -P "" \ 91 | -s "${ca_key}" \ 92 | ${host_cert} \ 93 | -I "${identity}" \ 94 | -V "${validity}"${options} \ 95 | -n "${principals}" \ 96 | -z "${serial}" \ 97 | "${pubkey}" 98 | rc=$? 99 | 100 | # Cleanup 101 | rm "${pubkey}" 102 | if [ ${rc} -eq 0 ]; then 103 | log_certificate 104 | cat "${cert}" 105 | rm "${cert}" 106 | else 107 | echo "ERROR: no certificate was produced" 108 | echo "See the debug log on the CA server for more details" 109 | exit ${rc} 110 | fi 111 | } 112 | 113 | function print_licence() { 114 | echo "This is forced-command-ssh-ca, a self-serve automated SSH" 115 | echo "certificate authority implemented as an SSH forced command" 116 | echo "Copyright (C) 2018 Alexandre de Verteuil" 117 | echo "" 118 | echo "This program is free software: you can redistribute it and/or modify" 119 | echo "it under the terms of the GNU Affero General Public License as published" 120 | echo "by the Free Software Foundation, either version 3 of the License, or" 121 | echo "(at your option) any later version." 122 | echo "" 123 | echo "This program is distributed in the hope that it will be useful," 124 | echo "but WITHOUT ANY WARRANTY; without even the implied warranty of" 125 | echo "MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the" 126 | echo "GNU Affero General Public License for more details." 127 | echo "" 128 | echo "You should have received a copy of the GNU Affero General Public License" 129 | echo "along with this program. If not, see ." 130 | echo "" 131 | echo "The source code is available at" 132 | echo "https://github.com/adeverteuil/forced-command-ssh-ca" 133 | } 134 | 135 | function print_usage() { 136 | echo "Forced Command SSH CA" 137 | echo "Copyright (C) 2018 Alexandre de Verteuil" 138 | echo "" 139 | echo "Usage:" 140 | echo " host PRINCIPALS" 141 | echo " Sign a host certificate" 142 | echo " user PRINCIPALS" 143 | echo " Sign a user certificate" 144 | echo " PRINCIPALS is a mandatory comma-separated list of principals" 145 | echo " to include in the signed certificate" 146 | echo " report" 147 | echo " Print the list of all certificates ever signed" 148 | echo " Example report details on the last 10 certificates:" 149 | echo " $ ssh ssh-ca report | ssh-keygen -L -f <(tail)" 150 | echo " licence" 151 | echo " Print licencing information about this free software" 152 | echo "" 153 | echo "COMMAND=$SSH_ORIGINAL_COMMAND" 154 | echo "USER=$user_id" 155 | echo "SSH_CA_KEY=$SSH_CA_KEY" 156 | echo "SSH_CA_CERTIFICATE_VALIDITY=$SSH_CA_CERTIFICATE_VALIDITY" 157 | echo "SSH_CA_CERTIFICATE_OPTIONS=$SSH_CA_CERTIFICATE_OPTIONS" 158 | } 159 | 160 | log_debug "SSH_ORIGINAL_COMMAND \"${SSH_ORIGINAL_COMMAND}\"" 161 | read -a arguments <<< "${SSH_ORIGINAL_COMMAND}" 162 | case "${arguments[0]}" in 163 | user|host) 164 | sign_key "${subcommand}" 165 | ;; 166 | report) 167 | cat "/var/log/ssh-ca/certificates.log" 168 | ;; 169 | licence) 170 | print_licence 171 | ;; 172 | *) 173 | print_usage 174 | ;; 175 | esac 176 | -------------------------------------------------------------------------------- /files/ssh-ca.motd: -------------------------------------------------------------------------------- 1 | #!/bin/bash 2 | # Generated with figlet -t "Forced Command SSH CA" 3 | cat <<"EOF" 4 | _____ _ ____ _ ____ ____ _ _ ____ _ 5 | | ___|__ _ __ ___ ___ __| | / ___|___ _ __ ___ _ __ ___ __ _ _ __ __| | / ___/ ___|| | | | / ___| / \ 6 | | |_ / _ \| '__/ __/ _ \/ _` | | | / _ \| '_ ` _ \| '_ ` _ \ / _` | '_ \ / _` | \___ \___ \| |_| | | | / _ \ 7 | | _| (_) | | | (_| __/ (_| | | |__| (_) | | | | | | | | | | | (_| | | | | (_| | ___) |__) | _ | | |___ / ___ \ 8 | |_| \___/|_| \___\___|\__,_| \____\___/|_| |_| |_|_| |_| |_|\__,_|_| |_|\__,_| |____/____/|_| |_| \____/_/ \_\ 9 | 10 | * Documentation, licence and source code: https://github.com/adeverteuil/forced-command-ssh-ca 11 | EOF 12 | -------------------------------------------------------------------------------- /files/sudoers-sshca: -------------------------------------------------------------------------------- 1 | # Allow members of the sshca group to run /usr/local/bin/ssh-ca as the sshca user. 2 | # Preserve the SSH_ORIGINAL_COMMAND environment variable. 3 | Defaults!/usr/local/bin/ssh-ca env_keep += SSH_ORIGINAL_COMMAND 4 | %ssh-ca-users ALL = (sshca) NOPASSWD: /usr/local/bin/ssh-ca "" 5 | -------------------------------------------------------------------------------- /install.yml: -------------------------------------------------------------------------------- 1 | --- 2 | - hosts: ssh-ca 3 | 4 | tasks: 5 | 6 | - name: install required packages 7 | apt: 8 | name: "{{ item }}" 9 | state: present 10 | with_items: 11 | - ntp 12 | - uuid 13 | 14 | - name: create the sshca user 15 | user: 16 | name: sshca 17 | system: yes 18 | createhome: no 19 | 20 | - name: install ssh-ca 21 | copy: 22 | src: ssh-ca 23 | dest: /usr/local/bin/ssh-ca 24 | owner: sshca 25 | group: sshca 26 | mode: 0750 27 | 28 | - name: configure ssh-ca 29 | template: 30 | src: ssh-ca.conf.j2 31 | dest: /etc/ssh-ca.conf 32 | 33 | - name: configure sshd 34 | template: 35 | src: sshd_config.j2 36 | dest: /etc/ssh/sshd_config 37 | notify: restart ssh 38 | 39 | - name: create the CA directory 40 | file: 41 | dest: /usr/local/lib/ssh-ca 42 | state: directory 43 | owner: sshca 44 | group: sshca 45 | mode: 0700 46 | 47 | - name: install CA secret keys 48 | copy: 49 | content: "{{ item.secret }}" 50 | dest: /usr/local/lib/ssh-ca/{{ item.name }} 51 | owner: sshca 52 | group: sshca 53 | mode: 0600 54 | with_items: "{{ ssh_ca_keypairs }}" 55 | no_log: True 56 | 57 | - name: install CA public keys 58 | copy: 59 | content: "{{ item.public }}" 60 | dest: /usr/local/lib/ssh-ca/{{ item.name }}.pub 61 | owner: sshca 62 | group: sshca 63 | mode: 0640 64 | with_items: "{{ ssh_ca_keypairs }}" 65 | no_log: True 66 | 67 | - name: stat the key specified in ssh_ca_configuration.use_ca_key 68 | stat: 69 | path: /usr/local/lib/ssh-ca/{{ ssh_ca_configuration.use_ca_key }} 70 | register: stat_key 71 | 72 | - name: assert that use_ca_key is set to the name of an ssh_ca_keypairs item 73 | assert: 74 | that: stat_key.stat.exists 75 | msg: "{{ ssh_ca_configuration.use_ca_key }} is not a key defined in ssh_ca_keypairs" 76 | 77 | - name: create the log directory 78 | file: 79 | dest: /var/log/ssh-ca 80 | state: directory 81 | owner: sshca 82 | group: sshca 83 | mode: 0750 84 | 85 | - name: find all the host keys 86 | command: sed -n '/^HostKey/s/^HostKey *//p' /etc/ssh/sshd_config 87 | register: host_keys 88 | check_mode: no 89 | changed_when: no 90 | 91 | - name: sign the host keys 92 | command: ssh-keygen -s "/usr/local/lib/ssh-ca/{{ ssh_ca_configuration.use_ca_key }}" 93 | -h 94 | -P "" 95 | -I "{{ ssh_ca_configuration.use_ca_key }}" 96 | -V "-1h:+365d" 97 | -n "{{ ansible_eth0.ipv4.address }},{{ ansible_fqdn }}{% for principal in ssh_ca_configuration.additional_host_principals|default([]) %},{{ principal }}{% endfor %}" 98 | -z "{{ ansible_date_time.epoch }}" 99 | "{{ item }}.pub" 100 | args: 101 | creates: "{{ item }}-cert.pub" 102 | with_items: "{{ host_keys.stdout_lines }}" 103 | notify: restart ssh 104 | 105 | # https://help.ubuntu.com/lts/serverguide/sssd-ad.html#sssd-ad-mkhomedir 106 | - name: Add pam mkhomedir module for external (ipa) users 107 | lineinfile: 108 | dest: /etc/pam.d/common-session 109 | line: session required pam_mkhomedir.so skel=/etc/skel/ umask=0022 110 | insertafter: 'session required pam_unix\.so' 111 | 112 | - name: put ASCII art in the motd 113 | copy: 114 | src: ssh-ca.motd 115 | dest: /etc/update-motd.d/99-forced-command-ssh-ca 116 | mode: 0755 117 | 118 | handlers: 119 | 120 | - name: restart ssh 121 | service: 122 | name: ssh 123 | state: restarted 124 | -------------------------------------------------------------------------------- /ssh-csr: -------------------------------------------------------------------------------- 1 | #!/bin/bash 2 | # 3 | # ssh-csr 1.1 4 | # 5 | # Licence and copyright 6 | # 7 | # This file is part of forced-command-ssh-ca, a self-serve automated SSH 8 | # certificate authority implemented as an SSH forced command 9 | # Copyright (C) 2018 Alexandre de Verteuil 10 | # 11 | # This program is free software: you can redistribute it and/or modify 12 | # it under the terms of the GNU Affero General Public License as published 13 | # by the Free Software Foundation, either version 3 of the License, or 14 | # (at your option) any later version. 15 | # 16 | # This program is distributed in the hope that it will be useful, 17 | # but WITHOUT ANY WARRANTY; without even the implied warranty of 18 | # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 19 | # GNU Affero General Public License for more details. 20 | # 21 | # You should have received a copy of the GNU Affero General Public License 22 | # along with this program. If not, see . 23 | # 24 | # ssh-csr establishes an SSH session with ssh-ca, specifies the 25 | # request is for a user certificate and what principals the 26 | # request is for, sends the public key from the file specified in 27 | # the SSH_CSR_IDENTITY environment variable, receives a signed 28 | # certificate valid for the requested principals, and writes it in a 29 | # file with the -cert.pub suffix. 30 | # 31 | # How to use 32 | # 33 | # To tell ssh-csr what public key to send, define the 34 | # SSH_CSR_IDENTITY environment variable in your .bashrc. If you 35 | # don't define it, $HOME/.ssh/id_rsa.pub will be used by default. 36 | # 37 | # Add the certificate authority to your known_hosts file. You can 38 | # do so by adding a line in your ~/.ssh/known_hosts: 39 | # 40 | # @cert-authority * ssh-rsa AAAAB5W... 41 | # 42 | # Ask your CA administrator what's your @cert-authority. 43 | # 44 | # Remain attentive to your CA administrator for @revoked lines to 45 | # add and @cert-authority lines to remove or add. 46 | # 47 | # In your ~/.ssh/config file, define a Host ssh-ca and specify the 48 | # HostName, User, Identity, etc. of your CA. Request help from your 49 | # CA administrator for the correct values. 50 | # 51 | # Test your config and known_hosts with "ssh ssh-ca config" 52 | # In the debug output, you should see: 53 | # 54 | # debug1: Server host certificate: ecdsa-sha2… 55 | # debug1: Host 'ssh-ca.example.com' is known and matches the ECDSA-CERT host certificate. 56 | # debug1: Found CA key in /home/adeverteuil/.ssh/known_hosts:76 57 | # 58 | # Copy ssh-csr to your $PATH, such as ~/bin or /usr/local/bin. 59 | # 60 | # Request a certificate by calling ssh-csr with the comma-separated 61 | # list of principals to include in your certificate. 62 | # 63 | # For example: 64 | # $ ssh-csr principal1,principal2 65 | # /home/adeverteuil/.ssh/id_rsa-cert.pub: 66 | # Type: ssh-rsa-cert-v01@openssh.com user certificate 67 | # Public key: RSA-CERT SHA256:bh/6eKMulLfWwdrfR+S9OegfNjHiHeEoa3tFzpRq4bQ 68 | # Signing CA: RSA SHA256:vrAgOA20VjUdXkfQYcEdJV1qlKYqp6A67wQ5IK+0pkk 69 | # Key ID: "adeverteuil" 70 | # Serial: 1514748458 71 | # Valid: from 2017-12-31T14:17:38 to 2017-12-31T14:37:38 72 | # Principals: 73 | # principal1 74 | # principal2 75 | # Critical Options: (none) 76 | # Extensions: 77 | # permit-X11-forwarding 78 | # permit-agent-forwarding 79 | # permit-port-forwarding 80 | # permit-pty 81 | # permit-user-rc 82 | 83 | identity="${SSH_CSR_IDENTITY:-$HOME/.ssh/id_rsa.pub}" 84 | cert="${identity%.pub}-cert.pub" 85 | 86 | # Running ssh-csr without parameters prints the current certificate details 87 | if [ -z "${1}" ]; then 88 | [ -r "${cert}" ] && ssh-keygen -L -f "${cert}" 89 | exit 90 | fi 91 | 92 | output="$(ssh ssh-ca user "${1}" <"${identity}" 2>&1)" 93 | rc=$? 94 | 95 | if [ ${rc} -eq 0 ]; then 96 | echo "${output}" > "${cert}" 97 | ssh-keygen -L -f "${cert}" 98 | else 99 | echo "${output}" 100 | fi 101 | -------------------------------------------------------------------------------- /templates/ssh-ca.conf.j2: -------------------------------------------------------------------------------- 1 | # {{ ansible_managed }} 2 | {% set config = ssh_ca_configuration %} 3 | SSH_CA_KEY="{{ config.use_ca_key }}" 4 | SSH_CA_CERTIFICATE_VALIDITY="{{ config.certificate_validity }}" 5 | SSH_CA_CERTIFICATE_OPTIONS="{% for option in config.certificate_options|default([]) %} -O {{ option }}{% endfor %}" 6 | -------------------------------------------------------------------------------- /templates/sshd_config.j2: -------------------------------------------------------------------------------- 1 | # {{ ansible_managed }} 2 | 3 | Port 22 4 | Protocol 2 5 | # HostKeys for protocol version 2 6 | HostKey /etc/ssh/ssh_host_rsa_key 7 | HostKey /etc/ssh/ssh_host_dsa_key 8 | HostKey /etc/ssh/ssh_host_ecdsa_key 9 | HostKey /etc/ssh/ssh_host_ed25519_key 10 | HostCertificate /etc/ssh/ssh_host_rsa_key-cert.pub 11 | HostCertificate /etc/ssh/ssh_host_dsa_key-cert.pub 12 | HostCertificate /etc/ssh/ssh_host_ecdsa_key-cert.pub 13 | HostCertificate /etc/ssh/ssh_host_ed25519_key-cert.pub 14 | #Privilege Separation is turned on for security 15 | UsePrivilegeSeparation yes 16 | 17 | # Lifetime and size of ephemeral version 1 server key 18 | KeyRegenerationInterval 3600 19 | ServerKeyBits 1024 20 | 21 | # Logging 22 | SyslogFacility AUTH 23 | LogLevel INFO 24 | 25 | # Authentication: 26 | LoginGraceTime 120 27 | PermitRootLogin without-password 28 | StrictModes yes 29 | 30 | RSAAuthentication yes 31 | PubkeyAuthentication yes 32 | #AuthorizedKeysFile %h/.ssh/authorized_keys 33 | {% if ssh_ca_configuration.freeipa_integration|default(False) %} 34 | AuthorizedKeysCommand /usr/bin/sss_ssh_authorizedkeys 35 | AuthorizedKeysCommandUser nobody 36 | {% endif %} 37 | 38 | # Don't read the user's ~/.rhosts and ~/.shosts files 39 | IgnoreRhosts yes 40 | # For this to work you will also need host keys in /etc/ssh_known_hosts 41 | RhostsRSAAuthentication no 42 | # similar for protocol version 2 43 | HostbasedAuthentication no 44 | # Uncomment if you don't trust ~/.ssh/known_hosts for RhostsRSAAuthentication 45 | #IgnoreUserKnownHosts yes 46 | 47 | # To enable empty passwords, change to yes (NOT RECOMMENDED) 48 | PermitEmptyPasswords no 49 | 50 | # Change to yes to enable challenge-response passwords (beware issues with 51 | # some PAM modules and threads) 52 | ChallengeResponseAuthentication no 53 | 54 | # Change to no to disable tunnelled clear text passwords 55 | PasswordAuthentication no 56 | 57 | X11Forwarding no 58 | PrintMotd no 59 | PrintLastLog yes 60 | TCPKeepAlive yes 61 | 62 | #MaxStartups 10:30:60 63 | MaxStartups 50:20:100 64 | #MaxSessions 10 65 | MaxSessions 50 66 | #Banner /etc/issue.net 67 | 68 | # Allow client to pass locale environment variables 69 | AcceptEnv LANG LC_* 70 | 71 | Subsystem sftp /usr/lib/openssh/sftp-server 72 | 73 | # Set this to 'yes' to enable PAM authentication, account processing, 74 | # and session processing. If this is enabled, PAM authentication will 75 | # be allowed through the ChallengeResponseAuthentication and 76 | # PasswordAuthentication. Depending on your PAM configuration, 77 | # PAM authentication via ChallengeResponseAuthentication may bypass 78 | # the setting of "PermitRootLogin without-password". 79 | # If you just want the PAM account and session checks to run without 80 | # PAM authentication, then enable this but set PasswordAuthentication 81 | # and ChallengeResponseAuthentication to 'no'. 82 | UsePAM yes 83 | 84 | # Any non-root users are CA users which can only use the SSH server to sign SSH certificates. 85 | Match Group ssh-ca-users 86 | ForceCommand /usr/bin/sudo -u sshca /usr/local/bin/ssh-ca 87 | AllowAgentForwarding no 88 | AllowTcpForwarding no 89 | -------------------------------------------------------------------------------- /test.yml: -------------------------------------------------------------------------------- 1 | # Do various integration tests on the SSH CA 2 | -------------------------------------------------------------------------------- /users.yml: -------------------------------------------------------------------------------- 1 | --- 2 | - hosts: ssh-ca 3 | 4 | tasks: 5 | 6 | - name: add the ssh-ca-users group to the sudoers for the forced command 7 | copy: 8 | src: sudoers-sshca 9 | dest: /etc/sudoers.d/sshca 10 | mode: 0440 11 | when: ssh_ca_configuration.freeipa_integration 12 | 13 | - name: remove the local sudoers config if freeipa_integration is false 14 | file: 15 | dest: /etc/sudoers.d/sshca 16 | state: absent 17 | when: not ssh_ca_configuration.freeipa_integration 18 | 19 | - name: create the ssh-ca-users group 20 | group: 21 | name: ssh-ca-users 22 | state: present 23 | 24 | - name: create users 25 | user: 26 | name: "{{ item.name }}" 27 | comment: "{{ item.comment|default(omit) }}" 28 | groups: ssh-ca-users 29 | state: "{{ item.state|default(omit) }}" 30 | with_items: "{{ ssh_ca_users }}" 31 | 32 | - name: provision authorized SSH keys for users 33 | authorized_key: 34 | user: "{{ item.0.name }}" 35 | key: "{{ item.1.key }}" 36 | state: "{{ item.1.state|default('present') }}" 37 | with_subelements: 38 | - "{{ ssh_ca_users }}" 39 | - ssh_keys 40 | when: item.0.state|default("present") == "present" 41 | 42 | - name: provision superuser SSH keys 43 | authorized_key: 44 | user: "{{ ansible_user }}" 45 | key: "{{ item.1.key }}" 46 | state: "{% if item.0.state|default('present') == 'present' and item.0.superuser|default(False) and item.1.state|default('present') == 'present' %}present{% else %}absent{% endif %}" 47 | with_subelements: 48 | - "{{ ssh_ca_users }}" 49 | - ssh_keys 50 | -------------------------------------------------------------------------------- /vars.template.yml: -------------------------------------------------------------------------------- 1 | # vi: nowrap 2 | --- 3 | ssh_ca_users: 4 | - name: adeverteuil # Valid POSIX username 5 | comment: Alexandre de Verteuil 6 | ssh_keys: 7 | - key: ecdsa-sha2-nistp256 AAAAE2VjZHNh......2zt4HI= adeverteuil@work-laptop 8 | - key: ssh-rsa AAAAB3NzaC1yc2E......N30aU+jIn adeverteuil@work-laptop 9 | state: absent # Remove this old SSH key 10 | - name: jsmith 11 | comment: John Smith 12 | ssh_keys: 13 | - ... 14 | state: absent # Disabling the user 15 | 16 | # You better define this variable in an Ansible Vault 17 | ssh_ca_keypairs: 18 | - name: iweb-sc-01 19 | secret: | 20 | -----BEGIN RSA PRIVATE KEY----- 21 | MIIEowIBAAKCAQEAwpQh5AdhGSK5/yHjkjjBAT6tdMjeOapLUVUymiEeIxIc2GaI 22 | 3ivZ8ZaXX7WzbvSX8vXLw5DzpOHz+oa4gfUyKPePKv/WBKVP5qJrVY2R6AWINowL 23 | [...] 24 | dUBJmynMHRW1DicDedfkjBBQN2y/EOYYjX1nhLyNxPYCaL4fphc+kKHcY3bYubUq 25 | UNB7VFkSAv6FEwFyWJKUlxfPX64btbIVXC71xdZrFkWX9Pwv06gc 26 | -----END RSA PRIVATE KEY----- 27 | public: ssh-rsa AAAAB3NzaC1yc.....TUHN65v iweb-sc-01 28 | 29 | ssh_ca_configuration: 30 | use_ca_key: iweb-sc-01 # The name of an ssh_ca_keypairs item 31 | certificate_validity: "-10m:+10m" # -V option for key signing 32 | certificate_options: # Options to add to the signed certificates, if any 33 | - no-port-forwarding 34 | - source-address=1.2.3.4/24,5.6.7.8/24 35 | additional_host_principals: [] 36 | # Optional list of additional principal to include in the host 37 | # certificates in case the hostname is different from the DNS name 38 | # used to connect to the ssh-ca. 39 | --------------------------------------------------------------------------------