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Externally Sourced Configuration File Values

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New in version 4.2.

MongoDB supports using expansion directives in configuration files to load externally sourced values. Expansion directives can load values for specific configuration file options or load the entire configuration file. Expansion directives help obscure confidential information like security certificates and passwords.

storage:
dbPath: "/var/lib/mongo"
systemLog:
destination: file
path: "/var/log/mongodb/mongod.log"
net:
bindIp:
__exec: "python /home/user/getIPAddresses.py"
type: "string"
trim: "whitespace"
digest: 85fed8997aac3f558e779625f2e51b4d142dff11184308dc6aca06cff26ee9ad
digest_key: 68656c6c30303030307365637265746d796f6c64667269656e64
tls:
mode: requireTLS
certificateKeyFile: "/etc/tls/mongod.pem"
certificateKeyFilePassword:
__rest: "https://myrestserver.example.net/api/config/myCertKeyFilePassword"
type: "string"
digest: b08519162ba332985ac18204851949611ef73835ec99067b85723e10113f5c26
digest_key: 6d795365637265744b65795374756666

To use expansion directives, you must specify the --configExpand command-line option with the complete list of expansion directives used:

mongod --config "/path/to/config/mongod.conf" --configExpand "rest,exec"

If you omit the --configExpand option or if you do not specify the complete list of expansion directives used in the configuration file, the mongod/mongos returns an error and terminates. You can only specify the --configExpand option on the command line.

The __rest expansion directive loads configuration file values from a REST endpoint. __rest supports loading specific values in the configuration file or loading the entire configuration file.

Important

The value returned by the specified REST endpoint cannot include any additional expansion directives. The mongod/mongos does not perform additional processing on the returned data and will terminate with an error code if the returned data includes additional expansion directives.

The __exec expansion directive loads configuration file values from a shell or terminal command. __exec supports loading specific values in the configuration file or loading the entire configuration file.

Important

The data returned by executing the specified __exec string cannot include any additional expansion directives. The mongod/mongos does not perform additional processing on the returned data and will terminate with an error code if the returned data includes additional expansion directives.

__rest

The __rest expansion directive loads configuration file values from a REST endpoint. __rest supports loading specific values in the configuration file or loading the entire configuration file. The mongod/mongos then starts using the externally sourced values as part of its configuration.

The __rest expansion directive has the following syntax:

  • To specify a REST endpoint for a specific configuration file setting or settings:

    <some configuration file setting>:
    __rest: "<string>"
    type: "string"
    trim: "none|whitespace"
    digest: "<string>"
    digest_key: "<string>"
  • To specify a REST endpoint for the entire configuration file:

    __rest: "<string>"
    type: "yaml"
    trim: "none|whitespace"

    If specifying the entire configuration file via REST endpoint, the expansion directive and its options must be the only values specified in the configuration file.

__rest takes the following fields:

Field
Type
Description
string

Required The URL against which the mongod/mongos issues a GET request to retrieve the externally sourced value.

For non-localhost REST endpoints (e.g. a REST endpoint hosted on a remote server), __rest requires encrypted (https://) URLs where both the host machine and the remote server support TLS 1.1 or later.

If the REST endpoint specified in the URL requires authentication, encode credentials into the URL with the standard RFC 3986 User Information format.

For localhost REST endpoints (e.g. a REST endpoint listening on the host machine), __rest allows unencrypted (http://) URLs.

Important

The value returned by the specified REST endpoint cannot include any additional expansion directives. The mongod/mongos does not perform additional processing on the returned data and will terminate with an error code if the returned data includes additional expansion directives.

type
string

Optional Controls how __rest parses the returned value from the specified URL.

Possible values are:

trim
string
Optional Specify whitespace to direct __rest to trim any leading or trailing whitespace, specifically occurrences of " ", "\r", "\n", "\t", "\v", and "\f". Defaults to none, or no trimming.
string

Optional. The SHA-256 digest of the expansion result.

If specified, you must also specify the digest_key.

string

Optional. The hexadecimal string representation of the secret used to calculate the SHA-256 digest.

If specified, you must also specify the digest.

Note

For examples, see Use the __rest Expansion Directive.

__exec

The __exec expansion directive loads configuration file values from the output of a shell or terminal command. __exec supports loading specific values in the configuration file or loading the entire configuration file. The mongod/mongos then starts using the externally sourced values as part of its configuration.

The __exec expansion directive has the following syntax:

  • To specify a shell or terminal command for a specific configuration file setting or settings:

    <some configuration file setting>:
    __exec: "<string>"
    type: "string"
    trim: "none|whitespace"
  • To specify a a shell or terminal command for the entire configuration file:

    __exec: "<string>"
    type: "yaml"
    trim: "none|whitespace"

    If specifying the entire configuration file via a terminal or shell command, the expansion directive and its options must be the only values specified in the configuration file.

__exec takes the following fields:

Field
Type
Description
__exec
string

Required The string which the mongod/mongos executes on the terminal or shell to retrieve the externally sourced value.

On Linux and OSX hosts, execution is handled via POSIX popen(). On Windows hosts, execution is handled via the process control API. __exec opens a read-only pipe as the same user that started the mongod or mongos.

Important

The data returned by executing the specified command cannot include any additional expansion directives. The mongod/mongos does not perform additional processing on the returned data and will terminate with an error code if the returned data includes additional expansion directives.

type
string

Optional Controls how __exec parses the value returned by the executed command.

Possible values are:

trim
string
Optional Specify whitespace to direct __exec to trim any leading or trailing whitespace, specifically occurrences of " ", "\r", "\n", "\t", "\v", and "\f". Defaults to none, or no trimming.
string

Optional. The SHA-256 digest of the expansion result.

If specified, you must also specify the digest_key

string

Optional. The hexadecimal string representation of the secret used to calculate the SHA-256 digest.

If specified, you must also specify the digest

Note

For examples, see Use the __exec Expansion Directive.

You can test the final output of a configuration file that specifies one or more expansion directives by starting the mongod/mongos with the --outputConfig option. A mongod/mongos started with --outputConfig outputs the resolved YAML configuration document to stdout and halts. If any expansion directive specified in the configuration file returns additional expansion directives, the mongod/mongos throws an error and terminates.

Warning

The --outputConfig option returns the resolved values for any field using an expansion directive. This includes any private or sensitive information previously obscured by using an external source for the configuration option.

For example, the following configuration file mongod.conf contains a __rest expansion directive:

storage:
dbPath: "/var/lib/mongo"
systemLog:
destination: file
path: "/var/log/mongodb/mongod.log"
net:
port:
__rest: "https://mongoconf.example.net:8080/record/1"
type: string

The string recorded at the specified URL is 20128

If the configuration file includes the __rest expansion, on Linux/macOS, the read access to the configuration file must be limited to the user running the mongod/mongos process only.

Start the mongod with the --configExpand "rest" and --outputConfig options:

mongod -f mongod.conf --configExpand rest --outputConfig

The mongod outputs the following to stdout before terminating:

config: mongod.conf
storage:
dbPath: "/var/lib/mongo"
systemLog:
destination: file
path: "/var/log/mongodb/mongod.log"
net:
port: 20128
outputConfig: true
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