All Vault auth and secret backends are considered plugins. This simple concept allows both built-in and external plugins to be treated like Legos. Any plugin can exist at multiple different locations. Different versions of a plugin may be at each one, with each version differing from Vault's version.

Built-In Plugins

Built-in plugins are shipped with Vault, often for commonly used implementations, and require no additional operator intervention to run. Built-in plugins are just like any other backend code inside Vault.

To use a different or edited version of a built-in plugin, you would first edit the plugin's code or navigate to the Vault version holding the version of the plugin you desire. Then, you'd $ cd into the cmd/:plugin-name directory contained alongside that plugin's code. For instance, for AppRole, you would: $ cd vault/builtin/credential/approle/cmd/approle. Once in that directory, you would run $ go build to obtain a new binary for the AppRole plugin. Then you would add it to the plugin catalog as per normal, and enable it.

Plugin Architecture

Vault's plugins are completely separate, standalone applications that Vault executes and communicates with over RPC. This means the plugin process does not share the same memory space as Vault and therefore can only access the interfaces and arguments given to it. This also means a crash in a plugin can not crash the entirety of Vault.

It is possible to enable a custom plugin with a name that's identical to a built-in plugin. In such a situation, Vault will always choose the custom plugin when enabling it.

Plugin Communication

Vault creates a mutually authenticated TLS connection for communication with the plugin's RPC server. While invoking the plugin process, Vault passes a wrapping token to the plugin process' environment. This token is single use and has a short TTL. Once unwrapped, it provides the plugin with a uniquely generated TLS certificate and private key for it to use to talk to the original Vault process.

The api_addr must be set in order for the plugin process establish communication with the Vault server during mount time. If the storage backend has HA enabled and supports automatic host address detection (e.g. Consul), Vault will automatically attempt to determine the api_addr as well.

Plugin Registration

An important consideration of Vault's plugin system is to ensure the plugin invoked by Vault is authentic and maintains integrity. There are two components that a Vault operator needs to configure before external plugins can be run, the plugin directory and the plugin catalog entry.

Plugin Directory

The plugin directory is a configuration option of Vault, and can be specified in the configuration file. This setting specifies a directory in which all plugin binaries must live; this value cannot be a symbolic link. A plugin can not be added to Vault unless it exists in the plugin directory. There is no default for this configuration option, and if it is not set plugins can not be added to Vault.

Plugin Catalog

The plugin catalog is Vault's list of approved plugins. The catalog is stored in Vault's barrier and can only be updated by a Vault user with sudo permissions. Upon adding a new plugin, the plugin name, SHA256 sum of the executable, and the command that should be used to run the plugin must be provided. The catalog will make sure the executable referenced in the command exists in the plugin directory. When added to the catalog the plugin is not automatically executed, it instead becomes visible to backends and can be executed by them. For more information on the plugin catalog please see the Plugin Catalog API docs.

An example of plugin registration in current versions of Vault:

$ vault plugin register -sha256=<SHA256 Hex value of the plugin binary> \
    secret \                  # type
    myplugin-database-plugin

Success! Registered plugin: myplugin-database-plugin

Vault versions prior to v0.10.4 lacked the vault plugin operator and the registration step for them is:

$ vault write sys/plugins/catalog/database/myplugin-database-plugin \
     sha256=<SHA256 Hex value of the plugin binary> \
     command="myplugin"

Success! Data written to: sys/plugins/catalog/database/myplugin-database-plugin

Plugin Execution

When a backend wants to run a plugin, it first looks up the plugin, by name, in the catalog. It then checks the executable's SHA256 sum against the one configured in the plugin catalog. Finally Vault runs the command configured in the catalog, sending along the JWT formatted response wrapping token and mlock settings. Like Vault, plugins support the use of mlock when available.

Troubleshooting

Unrecognized remote plugin message

If the following error is encountered when enabling a plugin secret engine or auth method:

Unrecognized remote plugin message:

This usually means that the plugin is either invalid or simply
needs to be recompiled to support the latest protocol.

Verify whether the Vault process has mlock enabled, and if so run the following command against the plugin binary:

sudo setcap cap_ipc_lock=+ep <plugin-binary>

Plugin Development

Because Vault communicates to plugins over a RPC interface, you can build and distribute a plugin for Vault without having to rebuild Vault itself. This makes it easy for you to build a Vault plugin for your organization's internal use, for a proprietary API that you don't want to open source, or to prototype something before contributing it back to the main project.

In theory, because the plugin interface is HTTP, you could even develop a plugin using a completely different programming language! (Disclaimer, you would also have to re-implement the plugin API which is not a trivial amount of work.)

Developing a plugin is simple. The only knowledge necessary to write a plugin is basic command-line skills and basic knowledge of the Go programming language.

Your plugin implementation needs to satisfy the interface for the plugin type you want to build. You can find these definitions in the docs for the backend running the plugin.

package main

import (
    "os"

    myPlugin "your/plugin/import/path"
    "github.com/hashicorp/vault/api"
    "github.com/hashicorp/vault/sdk/plugin"
)

func main() {
    apiClientMeta := &api.PluginAPIClientMeta{}
    flags := apiClientMeta.FlagSet()
    flags.Parse(os.Args[1:])

    tlsConfig := apiClientMeta.GetTLSConfig()
    tlsProviderFunc := api.VaultPluginTLSProvider(tlsConfig)

    err := plugin.Serve(&plugin.ServeOpts{
        BackendFactoryFunc: myPlugin.Factory,
        TLSProviderFunc:    tlsProviderFunc,
    })
    if err != nil {
        logger := hclog.New(&hclog.LoggerOptions{})

        logger.Error("plugin shutting down", "error", err)
        os.Exit(1)
    }
}

And that's basically it! You would just need to change myPlugin to your actual plugin. For more information on how to register and enable your plugin, check out the Building Plugin Backends tutorial.