Custom Database Secrets Engines

The database secrets engine allows new functionality to be added through a plugin interface without needing to modify Vault's core code. This allows you write your own code to generate credentials in any database you wish. It also allows databases that require dynamically linked libraries to be used as plugins while keeping Vault itself statically linked.

Please read the Plugins internals docs for more information about the plugin system before getting started building your Database plugin.

Database plugins can be made to implement plugin multiplexing which allows a single plugin process to be used for multiple database connections. To enable multiplexing, the plugin must be compiled with the ServeMultiplex function call from Vault's dbplugin package.

Plugin Interface

All plugins for the database secrets engine must implement the same interface. This interface is found in sdk/database/dbplugin/v5/database.go

type Database interface {
    // Initialize the database plugin. This is the equivalent of a constructor for the
    // database object itself.
    Initialize(ctx context.Context, req InitializeRequest) (InitializeResponse, error)

    // NewUser creates a new user within the database. This user is temporary in that it
    // will exist until the TTL expires.
    NewUser(ctx context.Context, req NewUserRequest) (NewUserResponse, error)

    // UpdateUser updates an existing user within the database.
    UpdateUser(ctx context.Context, req UpdateUserRequest) (UpdateUserResponse, error)

    // DeleteUser from the database. This should not error if the user didn't
    // exist prior to this call.
    DeleteUser(ctx context.Context, req DeleteUserRequest) (DeleteUserResponse, error)

    // Type returns the Name for the particular database backend implementation.
    // This type name is usually set as a constant within the database backend
    // implementation, e.g. "mysql" for the MySQL database backend. This is used
    // for things like metrics and logging. No behavior is switched on this.
    Type() (string, error)

    // Close attempts to close the underlying database connection that was
    // established by the backend.
    Close() error
}

Each of the request and response objects can also be found in sdk/database/dbplugin/v5/database.go.

In each of the requests, you will see at least 1 Statements object (in UpdateUserRequest they are in sub-fields). This object represents the set of commands to run for that particular operation. For the NewUser function, this is a set of commands to create the user (and often set permissions for that user). These statements are from the following fields in the API:

In many of the built-in plugins, they replace {{name}} (or {{username}}), {{password}}, and/or {{expiration}} with the associated values. It is up to your plugin to perform these string replacements. There is a helper function located in sdk/database/helper/dbutil called QueryHelper that assists in doing this string replacement. You are not required to use it, but it will make your plugin's behavior consistent with the built-in plugins.

The InitializeRequest object contains a map of keys to values. This data is what the user specified as the configuration for the plugin. Your plugin should use this data to make connections to the database. The response object contains a similar configuration map. The response object should contain the configuration map that should be saved within Vault. This allows the plugin to manipulate the configuration prior to saving it.

It is also passed a boolean value (InitializeRequest.VerifyConnection) indicating if your plugin should initialize a connection to the database during the Initialize call. This function is called when the configuration is written. This allows the user to know whether the configuration is valid and able to connect to the database in question. If this is set to false, no connection should be made during the Initialize call, but subsequent calls to the other functions will need to open a connection.

Serving A Plugin

Serving A Plugin with Multiplexing

The plugin runs as a separate binary outside of Vault, so the plugin itself will need a main function. Use the ServeMultiplex function within sdk/database/dbplugin/v5 to serve your multiplexed plugin. You will also need to pass some TLS configuration information that Vault uses when initializing the plugin.

Below is an example setup:

package main

import (
    "github.com/hashicorp/vault/api"
    dbplugin "github.com/hashicorp/vault/sdk/database/v5"
)

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

    err := Run()
    if err != nil {
        log.Println(err)
        os.Exit(1)
    }
}

func Run() error {
    dbplugin.ServeMultiplex(dbType.(dbplugin.New))

    return nil
}

func New() (interface{}, error) {
    db, err := newDatabase()
    if err != nil {
        return nil, err
    }

    // This middleware isn't strictly required, but highly recommended to prevent accidentally exposing
    // values such as passwords in error messages. An example of this is included below
    db = dbplugin.NewDatabaseErrorSanitizerMiddleware(db, db.secretValues)
    return db, nil
}

type MyDatabase struct {
    // Variables for the database
    password string
}

func newDatabase() (MyDatabase, error) {
    // ...
    db := &MyDatabase{
        // ...
    }
    return db, nil
}

func (db *MyDatabase) secretValues() map[string]string {
    return map[string]string{
        db.password: "[password]",
    }
}

Replacing MyDatabase with the actual implementation of your database plugin.

Serving A Plugin without Multiplexing

Serving a plugin without multiplexing requires calling the Serve function from sdk/database/dbplugin/v5 to serve your plugin. You will also need to pass some TLS configuration information that Vault uses when initializing the plugin.

The setup is exactly the same as the multiplexed case above, except for the Run function:

func Run() error {
    dbType, err := New()
    if err != nil {
        return err
    }

    dbplugin.Serve(dbType.(dbplugin.Database))

    return nil
}

Running your plugin

The above main package, once built, will supply you with a binary of your plugin. We also recommend if you are planning on distributing your plugin to build with gox for cross platform builds.

To use your plugin with the database secrets engine you need to place the binary in the plugin directory as specified in the plugin internals docs.

You should now be able to register your plugin into the vault catalog. To do this your token will need sudo permissions.

$ vault write sys/plugins/catalog/database/mydatabase-database-plugin \
    sha256="..." \
    command="mydatabase"
Success! Data written to: sys/plugins/catalog/database/mydatabase-database-plugin

Now you should be able to configure your plugin like any other:

$ vault write database/config/mydatabase \
    plugin_name=mydatabase-database-plugin \
    allowed_roles="readonly" \
    myplugins_connection_details="..."

Upgrading database plugins to leverage plugin multiplexing

Background

Scaling many external plugins can become resource intensive. To address performance problems with scaling external plugins, database plugins can be made to implement plugin multiplexing which allows a single plugin process to be used for multiple database connections. To enable multiplexing, the plugin must be compiled with the ServeMultiplex function call from Vault's dbplugin package.

Upgrading your database plugin to leverage plugin multiplexing

There is only one step required to upgrade from a non-multiplexed to a multiplexed database plugin: Change the Serve function call to ServeMultiplex.

This will run the RPC server for the plugin just as before. However, the ServeMultiplex function takes the factory function directly as its argument. This factory function is a function that returns an object that implements the dbplugin.Database interface.

When should plugin multiplexing be avoided?

Some use cases that should avoid plugin multiplexing might include:

• Plugin process level separation is required
• Avoiding restart across all mounts/database connections for a plugin type on crashes or plugin reload calls

Upgrading database plugins to the V5 interface

Background

In Vault 1.6, the database interface changed. The new version is referred to as version 5 and the previous version as version 4. This is due to prior versioning of the interface that was not explicitly exposed.

The new interface was introduced for several reasons:

1. Password policies introduced in Vault 1.5 required that Vault be responsible for generating passwords. In the prior version, the database plugin was responsible for generating passwords. This prevented integration with password policies.
2. Passwords needed to be generated by database plugins. This meant that plugin authors were responsible for generating secure passwords. This should be done with a helper function available within the Vault SDK, however there was nothing preventing an author from generating insecure passwords.
3. There were a number of inconsistencies within the version 4 interface that made it confusing for authors. For instance: passwords were handled in 3 different ways. CreateUser generated a password and returned it, SetCredentials receives a password via a configuration struct and returns it, and RotateRootCredentials generated a password and was expected to return an updated copy of its entire configuration with the new password.
4. The SetCredentials and RotateRootCredentials used for static credential rotation, and rotating the root user's credentials respectively were essentially the same operation: change a user's password. The only practical difference was which user it was referring to. This was especially evident when SetCredentials was used when rotating root credentials (unless static credential rotation wasn't supported by the plugin in question).
5. The old interface included both Init and Initialize adding to the confusion.

The new interface is roughly modeled after a gRPC interface. It has improved future compatibility by not requiring changes to the interface definition to add additional data in the requests or responses. It also simplifies the interface by merging several into a single function call.

Upgrading your custom database

Vault 1.6 supports both version 4 and version 5 database plugins. The support for version 4 plugins will be removed in a future release. Version 5 database plugins will not function with Vault prior to version 1.6. If you upgrade your database plugins, ensure that you are only using Vault 1.6 or later. To determine if a plugin is using version 4 or version 5, the following is a list of changes in no particular order that you can check against your plugin to determine the version:

1. The import path for version 4 is github.com/hashicorp/vault/sdk/database/dbplugin whereas the import path for version 5 is github.com/hashicorp/vault/sdk/database/dbplugin/v5
2. Version 4 has the following functions: Initialize, Init, CreateUser, RenewUser, RevokeUser, SetCredentials, RotateRootCredentials, Type, and Close. You can see the full function signatures in sdk/database/dbplugin/plugin.go.
3. Version 5 has the following functions: Initialize, NewUser, UpdateUser, DeleteUser, Type, and Close. You can see the full function signatures in sdk/database/dbplugin/v5/database.go.

If you are using a version 4 custom database plugin, the following are basic instructions for upgrading to version 5.

1. Change the import path from github.com/hashicorp/vault/sdk/database/dbplugin to github.com/hashicorp/vault/sdk/database/dbplugin/v5. The package name is the same, so any references to dbplugin can remain as long as those symbols exist within the new package (such as the Serve function).
2. An easy way to see what functions need to be implemented is to put the following as a global variable within your package: var _ dbplugin.Database = (*MyDatabase)(nil). This will fail to compile if the MyDatabase type does not adhere to the dbplugin.Database interface.
3. Replace Init and Initialize with the new Initialize function definition. The fields that Init was taking (config and verifyConnection) are now wrapped into InitializeRequest. The returned map[string]interface{} object is now wrapped into InitializeResponse. Only Initialize is needed to adhere to the Database interface.
4. Update CreateUser to NewUser. The NewUserRequest object contains the username and password of the user to be created. It also includes a list of statements for creating the user as well as several other fields that may or may not be applicable. Your custom plugin should use the password provided in the request, not generate one. If you generate a password instead, Vault will not know about it and will give the caller the wrong password.
5. SetCredentials, RotateRootCredentials, and RenewUser are combined into UpdateUser. The request object, UpdateUserRequest contains three parts: the username to change, a ChangePassword and a ChangeExpiration object. When one of the objects is not nil, this indicates that particular field (password or expiration) needs to change. For instance, if the ChangePassword field is not-nil, the user's password should be changed. This is equivalent to calling SetCredentials. If the ChangeExpiration field is not-nil, the user's expiration date should be changed. This is equivalent to calling RenewUser. Many databases don't need to do anything with the updated expiration.
6. Update RevokeUser to DeleteUser. This is the simplest change. The username to be deleted is enclosed in the DeleteUserRequest object.