vault-ssh-helper

vault-ssh-helper is a counterpart to HashiCorp Vault's SSH backend. It allows a machine to consume One-Time-Passwords (OTP) created by Vault servers by allowing them to be used as client authentication credentials at SSH connection time.

All of the remote hosts that belong to the SSH backend's OTP-type roles will need this helper installed. In addition, each host must have its SSH configuration changed to enable keyboard-interactive authentication and redirect its client authentication responsibility to vault-ssh-helper.

Vault-authenticated users contact the Vault server and retrieve an OTP issued for a specific username and IP address. While establishing an SSH connection to the host, the vault-ssh-helper binary reads the OTP from the password prompt and sends it to the Vault server for verification. Client authentication is successful (and the SSH connection allowed) only if the Vault server verifies the OTP. True to its name, once the OTP has been used a single time for authentication, it is removed from Vault and cannot be used again.

vault-ssh-helper is not a PAM module, but it does the job of one. vault-ssh-helper's binary is run as an external command using pam_exec.so with access to the entered password (in this case, the issued OTP). Successful execution and exit of this command is a PAM 'requisite' for authentication to be successful. If the OTP is not validated, the binary exits with a non-zero status and authentication fails.

PAM modules are generally shared object files; rather than writing and maintaining a PAM module in C, vault-ssh-helper is written in Go and invoked as an external binary. This allows vault-ssh-helper to be contained within one code base with known, testable behavior. It also allows other authentication systems that are not PAM-based to invoke vault-ssh-helper and take advantage of its capabilities.

Usage

vault-ssh-helper [options]

Options

Option	Description
`verify-only`	Verifies that `vault-ssh-helper` is installed correctly and is able to communicate with Vault.
`config`	The path to the configuration file. Configuration options are detailed below.
`dev`	`vault-ssh-helper` communicates with Vault with TLS disabled. This is NOT recommended for production use. Use with caution.

Download vault-ssh-helper

Download the latest version of vault-ssh-helper at releases.hashicorp.com.

Installation

Install Go on your machine and set GOPATH accordingly. Clone this repository into $GOPATH/src/github.com/hashicorp/vault-ssh-helper. Install all of the dependant binaries like godep, gox, vet, etc. by bootstrapping the environment:

$ make updatedeps

Build and install vault-ssh-helper:

$ make
$ make install

Follow the instructions below to modify your SSH server configuration, PAM configuration and vault-ssh-helper configuration. Check if vault-ssh-helper is installed and configured correctly and also is able to communicate with Vault server properly. Before verifying vault-ssh-helper, make sure that the Vault server is up and running and it has mounted the SSH backend. Also, make sure that the mount path of the SSH backend is properly updated in vault-ssh-helper's config file:

$ vault-ssh-helper -verify-only -config=<path-to-config-file>
Using SSH Mount point: ssh
vault-ssh-helper verification successful!

If you intend to contribute to this project, compile a development version of vault-ssh-helper using make dev. This will put the binary in the bin and $GOPATH/bin folders.

$ make dev

If you're developing a specific package, you can run tests for just that package by specifying the TEST variable. For example below, only helper package tests will be run.

$ make test TEST=./helper
...

If you intend to cross compile the binary, run make bin.

`vault-ssh-helper` Configuration

[Note]: This configuration is applicable for Ubuntu 14.04. SSH/PAM configurations differ with each platform and distribution.

vault-ssh-helper's configuration is written in HashiCorp Configuration Language (HCL). By proxy, this means that vault-ssh-helper's configuration is JSON-compatible. For more information, please see the HCL Specification.

Properties

Property	Description
`vault_addr`	[Required] Address of the Vault server.
`ssh_mount_point`	[Required] Mount point of SSH backend in Vault server.
`ca_cert`	Path of a PEM-encoded CA certificate file used to verify the Vault server's TLS certificate. `-dev` mode ignores this value.
`ca_path`	Path to directory of PEM-encoded CA certificate files used to verify the Vault server's TLS certiciate. `-dev` mode ignores this value.
`tls_skip_verify`	Skip TLS certificate verification. Use with caution.
`allowed_cidr_list`	List of comma-separated CIDR blocks. If the IP used by the user to connect to the host is different than the address(es) of the host's network interface(s) (for instance, if the address is NAT-ed), then `vault-ssh-helper` cannot authenticate the IP. In these cases, the IP returned by Vault will be matched with the CIDR blocks in this list. If it matches, the authentication succeeds. (Use with caution)

Sample config.hcl:

vault_addr = "https://vault.example.com:8200"
ssh_mount_point = "ssh"
ca_cert = "/etc/vault-ssh-helper.d/vault.crt"
tls_skip_verify=false

PAM Configuration

Modify the /etc/pam.d/sshd file as follows; each option will be explained below.

#@include common-auth
auth requisite pam_exec.so quiet expose_authtok log=/tmp/vaultssh.log /usr/local/bin/vault-ssh-helper -config=/etc/vault-ssh-helper.d/config.hcl
auth optional pam_unix.so not_set_pass use_first_pass nodelay

First, the previous authentication mechanism common-auth, which is the standard Linux authentication module, is commented out, in favor of using our custom configuration.

Next the authentication configuration for vault-ssh-helper is set.

Keyword	Description
`auth`	PAM type that the configuration applies to.
`requisite`	If the external command fails, the authentication should fail.
`pam_exec.so`	PAM module that runs an external command (`vault-ssh-helper`).
`quiet`	Suppress the exit status of `vault-ssh-helper` from being displayed.
`expose_authtok`	Binary can read the password from stdin.
`log`	Path to `vault-ssh-helper`'s log file.
`vault-ssh-helper`	Absolute path to `vault-ssh-helper`'s binary.
`config`	The path to `vault-ssh-helper`'s config file.

The third line works around a bug between some versions of pam_exec.so and vault-ssh-helper that causes a successful authentication from vault-ssh-helper to fail due to some resources not being properly released. Because it is marked as optional, it is essentially a no-op that ensures that PAM cleans up successfully, avoiding the bug.

Option	Description
`auth`	PAM type that the configuration applies to.
`optional`	If the module fails, authentication does not fail (but if the OTP was invalid, we will have already failed previously).
`pam_unix.so`	Linux's standard authentication module.
`not_set_pass`	Module should not be allowed to set or modify passwords.
`use_first_pass`	Do not display password prompt again. Use the password from the previous module.
`nodelay`	Avoids the induced delay after entering a wrong password.

SSHD Configuration

Modify the /etc/ssh/sshd_config file. Note that for many distributions these are the default options; you may not need to set them explicitly but should verify their values if not.

ChallengeResponseAuthentication yes
UsePAM yes
PasswordAuthentication no

Option	Description
`ChallengeResponseAuthentication yes`	[Required] Enable challenge response (keyboard-interactive) authentication.
`UsePAM yes`	[Required] Enable PAM authentication modules.
`PasswordAuthentication no`	Disable password authentication.

devopstiger/vault-ssh-helper