Configuring NATS Server

While the NATS server has many flags that allow for simple testing of features, the NATS server products provide a flexible configuration format that combines the best of traditional formats and newer styles such as JSON and YAML.

The NATS configuration file supports the following syntax:

  • Lines can be commented with # and //

  • Values can be assigned to properties with:

    • Equals sign: foo = 2

    • Colon: foo: 2

    • Whitespace: foo 2

  • Arrays are enclosed in brackets: ["a", "b", "c"]

  • Maps are enclosed in braces: {foo: 2}

  • Maps can be assigned with no key separator

  • Semicolons can be used as terminators

The NATS configuration file is parsed with UTF-8 encoding.


The NATS configuration in the file can also be rendered as a JSON object (with comments!), but to combine it with variables the variables still have to be unquoted.

Strings and Numbers

The configuration parser is very forgiving, as you have seen:

  • values can be a primitive, or a list, or a map

  • strings and numbers typically do the right thing

  • numbers support units such as, 1K for 1000, 1KB for 1024

String values that start with a digit can create issues. To force such values as strings, quote them.

BAD Config:

authorization: {
    # BAD!
    token: 3secret

Fixed Config:

authorization: {
    token: "3secret"


Server configurations can specify variables. Variables allow you to reference a value from one or more sections in the configuration


  • Are block-scoped

  • Are referenced with a $ prefix. They have to be unquoted when being referenced, for example, an assignment like foo = "$example" will result in foo being the literal string "$example".

  • Can be resolved from environment variables having the same name

If the environment variable value begins with a number you may have trouble resolving it depending on the server version you are running.

# Define a variable in the config
TOKEN: "secret"

# Reference the variable
authorization {
    token: $TOKEN

A similar configuration, but this time, the value is in the environment:

# TOKEN is defined in the environment
authorization {
    token: $TOKEN

export TOKEN="hello"; nats-server -c /config/file

Include Directive

The include directive allows you to split a server configuration into several files. This is useful for separating configuration into chunks that you can easily reuse between different servers.

Includes must use relative paths, and are relative to the main configuration (the one specified via the -c option):


include ./auth.conf

Note that include is not followed by = or :, as it is a directive.


authorization: {
    token: "f0oBar"
> nats-server -c server.conf

Configuration Properties




Host for client connections.


Port for client connections.



Listen specification <host>:<port> for client connections. Either use this or the options host and/or port.

same as host, port


Alternative client listen specification <host>:<port> or just <host> to advertise to clients and other server. Useful in cluster setups with NAT.

Advertise what host and port specify.

Configuration map for tls for client and http monitoring.

Configuration map for gateway.

Configuration map for leafnodes.

Configuration map for mqtt.

Configuration map for websocket.



Configuration map for cluster.

Connection Timeouts



Duration at which pings are sent to clients, leaf nodes and routes. In the presence of client traffic, such as messages or client side pings, the server will not send pings. Therefore it is recommended to keep this value bigger than what clients use.



After how many unanswered pings the server will allow before closing the connection.



Maximum number of seconds the server will block when writing. Once this threshold is exceeded the connection will be closed. See slow consumer on how to deal with this on the client.





Maximum number of active client connections.



Maximum length of a protocol line (including combined length of subject and queue group). Increasing this value may require client changes to be used. Applies to all traffic.



Maximum number of bytes in a message payload. Reducing this size may force you to implement chunking in your clients. Applies to client and leafnode payloads. It is not recommended to use values over 8MB but max_payload can be set up to 64MB. The max payload must be equal to or smaller than the max_pending value.



Maximum number of bytes buffered for a connection. Applies to client connections. Note that applications can also set 'PendingLimits' (number of messages and total size) for their subscriptions.



Maximum numbers of subscriptions per client and leafnode accounts connection.

0, unlimited


You can enable JetStream in the server's configuration by simply adding a jetstream {} map. By default, the JetStream subsystem will store data in the /tmp directory, but you can specify the directory to use via the store_dir, as well as the limits for JetStream storage (a value of 0 means no limit).



Directory to use for JetStream storage.




Maximum size of the 'memory' storage

75% of available memory



Maximum size of the 'file' storage

Up to 1TB if available



Set to enable storage-level encryption at rest. Choose either chachapoly or aes.

(not set)



The encryption key to use when encryption is enabled. A key length of at least 32 bytes is recommended. Note, this key is HMAC-256 hashed on startup which reduces the byte length to 64.

(not set)



Max in-flight bytes for stream catch-up




Change the default fsync interval for page cache in the filestore.



Here's an example minimal file that will store data in a local "nats" directory with some limits.

$ nats-server -c js.conf

jetstream {
  store_dir: nats

  # 1GB
  max_memory_store: 1073741824

  # 10GB
  max_file_store: 10737418240

Normally JetStream will be run in clustered mode and will replicate data, so the best place to store JetStream data would be locally on a fast SSD. One should specifically avoid NAS or NFS storage for JetStream. Note that each JetStream enabled server should use its own individual storage directory.

Authentication and Authorization

Centralized Authentication and Authorization


Configuration map for client authentication/authorization.

Configuration map for multi tenancy via accounts.

Username present in the authorization block or an account. A client connecting without any form of authentication will be associated with this user, its permissions and account.

Decentralized Authentication and Authorization

The Configuration options here refer to JWT based authentication and authorization.


Path to an operator JWT.

The built-in NATS resolver, MEMORY for static or URL(<url>) to use an external account server. (When the operator JWT contains an account URL, it will be used as default. In this case resolver is only needed to overwrite the default.)

tls configuration map for tls connections to the resolver. (This is for an outgoing connection and therefore does not use timeout, verify and map_and_verify)

Map to preload account public keys and their corresponding JWT. Keys consist of <account public nkey>, value is the <corresponding jwt>.

Runtime Configuration



If true disable subscription caches for all accounts. This is saves resources in situations where different subjects are used all the time.

false, cache enabled


In lame duck mode the server rejects new clients and slowly closes client connections. After this duration is over the server shuts down. This value cannot be set lower than 30 seconds. Start lame duck mode with: nats-server --signal ldm.



This is the duration the server waits, after entering lame duck mode, before starting to close client connections


Monitoring and Tracing



The servers name, shows up in logging. Defaults to the server's id. When JetStream is used, within a domain, all server names need to be unique.

Generated Server ID


A set of tags describing properties of the server. This will be exposed through /varz and can be used for system resource requests, such as placement of streams. It is recommended to use key:value style notation.



If true enable protocol trace log messages. Excludes the system account.

false, disabled


If true enable protocol trace log messages. Includes the system account.

false, disabled


If true enable debug log messages

false, disabled


If set to false, log without timestamps

true, include timestamp


Log file name, relative to...

No log file

Size in bytes after the log file rolls over to a new one

0, unlimited


Set a limit to the trace of the payload of a message.

0, unlimited


Log to syslog.

false, disabled


Syslog server address.

http port for server monitoring.

Listen specification <host>:<port>for server monitoring.

https port for server monitoring. This is influenced by the tls property.

base path for monitoring endpoints.


Listen specification <host>:<port>for TLS server monitoring.


Name of the system account. Users of this account can subscribe to system events. See System Accounts for more details.


File containing PID, relative to ... This can serve as input to nats-server --signal


Directory to write a file containing the servers open ports to, relative to ...


Number of attempts at which a repeated failed route, gateway or leaf node connection is reported. Connect attempts are made once every second.

3600, approx every hour


Number of failed attempts to reconnect a route, gateway or leaf node connection. Default is to report every attempt.

1, every failed attempt

Configuration Reloading

A server can reload most configuration changes without requiring a server restart or clients to disconnect by sending the nats-server a signal:

nats-server --signal reload

As of NATS v2.10.0, a reload signal can be sent on a NATS service using a system account user, where <server-id> is the unique ID of the server be targeted.

nats --user sys --password sys request '$SYS.REQ.SERVER.<server-id>.RELOAD' ""

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