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Connecting to your database server is usually one of the first tasks you need to accomplish when designing and configuring database-backed applications. While there are many methods of providing the address, listening port, credentials, and other details to applications, connection URIs, sometimes called connection strings or connection URLs, are one of the most powerful and flexible ways of specifying complex configuration in a compact format.

In this guide, we'll talk about how to format a connection URI with your MongoDB database information and authentication details. Connection URIs are divided into sections, so we'll cover each part as we go along.

Percent encoding values

Before we begin, we should mention that MongoDB connection URIs expect percent-encoded values. This means that any characters that have a special meaning within the URL must be converted to their percent-encoded counterparts to ensure that libraries and applications can interpret them correctly.

Characters you should percent-encode include:

  • :: %3A
  • /: %2F
  • ?: %3F
  • #: %23
  • [: %5B
  • ]: %5D
  • @: %40

These have special meaning within the connection URI.

So if your password is...:

pe@ce&lo\/3'll want to specify it within the connection URI as:


If you are unsure about whether a character should be percent-encoded, it's usually best to encode it anyways. For example, if you are unsure if the \ character is reserved, you can use it's percent-encoded equivalent, %5C, to be safe:


Keep this in mind as you build your connection URIs.

A quick overview

Before we get into the details, we can look at the spec for a MongoDB connection URI:

| | | | |
|- schema |- auth credentials | | |- parameter list
| |
| |- default authentication database
|- host specifier

The parts in square brackets indicate optional parts. You may have noticed that most parts of the URI are optional. It might also be apparent that there are many pieces of information you can encode in the URI.

A quick description of each of the individual components:

  • mongodb://: The schema identifier used to identify the string as a MongoDB connection URI.
  • auth credentials: An optional component of the URI that can be used to specify the user and password to connect as.
    • username: An optional username. If included, it should start after the second slash (/) and continue until a colon (:). Must be accompanied by a password if included.
    • password: An optional password. If included, it begins after a colon (:) and continues until the at sign (@). Must be accompanied by a username if included.
  • host specifier: A required component used to specify one or more hostnames and ports to connect to.
    • host: An IP address, DNS name, or locally resolvable name of the server to connect to. The host continues until a colon (:) (if a port is included), until a comma (,) if more than one host is specified, or else until a slash (/). At least one host must be provided.
    • port: An optional port specification to indicate which port MongoDB is listening to on the host. The port begins with a colon (:) and continues until a comma (,) if another host is provided or until the slash (/) if not.
  • default authentication database: The name of the database to authenticate to if a more specific authSource is not provided in the parameter list. If no database is specified here or with authSource, MongoDB will attempt to authenticate to the standard admin database.
  • parameter list: An optional list of additional parameters that can affect the connection behavior. The parameter list begins with a question mark (?). If no default authentication database is provided, you must begin the parameter list with both the slash and question mark (/?) after the last host definition.
    • parameter pairs: The parameter list is composed of key-value pairs. The key and value within each pair are separated by an equal sign (=) and each pair is separated from the next by an ampersand (&).

Here is an example of a MongoDB connection URI that incorporates all of these components:

^ ^ ^ ^ ^ ^ ^
|- schema | |- password |- host | | |- parameter list
| | |
|- username | |- default authentication database
|- port

Specifying the URI type

The item in a connection URI is usually the protocol specification or application type. Since the URI will be used to connect and authenticate to a MongoDB database, we need to use a signifier that signifies that to the applications and libraries we're using.

The MongoDB project only accepts mongodb as valid URI schema designators. Therefore, you should always start your connection URI like this:


The schema designator will ensure that the information that follows is interpreted in the correct context.

Specifying a username and password

The next part of the URI is the user credentials. User credentials are optional, but typically required if you don't want to rely on defaults configured by either your application or the database.

To include user credentials, provide the username after the schema identifier, followed by a colon (:), the password, and finally an at sign (@):


User credentials are optional, but if included, you must provide both the username and password components.

Specifying where the server is listening

After the user credentials comes the host specifier which defines where the server is listening. One or more hosts can be defined within the host specifier, but since the host specifier is required, at least one host must be provided.

Each host definition consists of a host and an optional port. The host can either be a locally resolvable host name, a name resolved by an external name system like DNS, or an IP address or other direct address. The port signifies the port number on the host where MongoDB is listening.

To specify that the application should attempt to connect to the default MongoDB port (27017) on the local computer, you can use:


If you needed to include a username and password, that information would come first and be separated by the at sign:


To specify a remote server running on a non-standard port, separate those details with a colon. For example, to connect to port 3333 on a host at, you could use:


To define more than one host and port pair, separate the sets by commas (,) to tell the application to try the latter servers if the first cannot be reached. For example, to extend the previous example to include a fallback server listening on port 5555 on, you could use:


Conforming clients and applications will try to first connect to the server listening at If that fails, they will try to reach a MongoDB database listening on

Providing the default authentication database

After the host specifier, the next piece of data is the default authentication database. While not true for all database management systems, with MongoDB, you must authenticate against a specific database when establishing a connection.

The database name begins with a forward slash (/) and proceeds until either the end of the line or a question mark (?). The default authentication database will be used if an authSource option is not provided within the parameter list. If neither are provided, the client will authenticate against the admin database.

To connect to a database called sales hosted on a MongoDB server listening on, you could type:


Specifying additional parameters

The last part of the connection URI is used to provide additional parameters for the connection. The list of parameters is introduced by a leading question mark (?) and continues until the end of the line. If no default authentication database is provided, the trailing slash indicating the end of the host specification must directly precede the question mark (/?).

Each parameter listed is defined as a key and value pair joined with an equals sign (=). After the first parameter pair, each additional key-value pair is separated by an ampersand (&).

For example, to specify that the client should apply a 10 second timeout for the connection we were previously defining, you could use:


If you wanted to provide additional parameters, you'd add them afterwards with an ampersand (&) between each pair. For instance, we could additionally specify that we require SSL and that the specified hosts are members of a replica set we want to connect to:


The MongoDB documentation has a full list of parameters that you can read to learn more.


In this guide, we discussed what a MongoDB connection URI is, how to interpret the various components, and how to construct your own URIs given a set of connection information. Connection URIs encode all of the information required to connect to a given database within a single string. Because of this flexibility and due to their wide adoption, understanding how to parse and construct those strings can be pretty helpful.

About the Author(s)
Justin Ellingwood

Justin Ellingwood

Justin has been writing about databases, Linux, infrastructure, and developer tools since 2013. He currently lives in Berlin with his wife and two rabbits. He doesn't usually have to write in the third person, which is a relief for all parties involved.