# resolver

The `resolver` configuration option is used in conjunction with [NATS JWT Authentication](securing_nats/auth_intro/jwt_auth.md) and [nsc](../../nats-tools/nsc/nsc). The `resolver` option specifies a URL where the nats-server can retrieve an account JWT. There are two built-in resolver implementations:

 - `URL`
 - `MEMORY`
 
## URL Resolver

The `URL` resolver specifies a URL where the server can append an account public key to retrieve that account's JWT. Convention for [NATS Account JWT Servers](../../nats-tools/nas) is to serve JWTs at: `http://localhost:9090/jwt/v1/accounts/`. For such a configuration you would specify the resolver as follows:

```yaml
resolver: URL(http://localhost:9090/jwt/v1/accounts/)
```

Note that if you are not using a nats-account-server, the URL can be anything as long as by appending the public key for an account, the requested JWT is returned.

For more information on how to configure an account server, see [NATS Account JWT Server](../../nats-tools/nas).

## MEMORY

The `MEMORY` resolver is statically configured in the server's configuration file. The memory resolver makes use of the `resolver_preload` directive, which specifies a map of a public key to an account JWT:

```yaml
resolver: MEMORY
resolver_preload: {
ACSU3Q6LTLBVLGAQUONAGXJHVNWGSKKAUA7IY5TB4Z7PLEKSR5O6JTGR: eyJ0eXAiOiJqd3QiLCJhbGciOiJlZDI1NTE5In0.eyJqdGkiOiJPRFhJSVI2Wlg1Q1AzMlFJTFczWFBENEtTSDYzUFNNSEZHUkpaT05DR1RLVVBISlRLQ0JBIiwiaWF0IjoxNTU2NjU1Njk0LCJpc3MiOiJPRFdaSjJLQVBGNzZXT1dNUENKRjZCWTRRSVBMVFVJWTRKSUJMVTRLM1lERzNHSElXQlZXQkhVWiIsIm5hbWUiOiJBIiwic3ViIjoiQUNTVTNRNkxUTEJWTEdBUVVPTkFHWEpIVk5XR1NLS0FVQTdJWTVUQjRaN1BMRUtTUjVPNkpUR1IiLCJ0eXBlIjoiYWNjb3VudCIsIm5hdHMiOnsibGltaXRzIjp7InN1YnMiOi0xLCJjb25uIjotMSwibGVhZiI6LTEsImltcG9ydHMiOi0xLCJleHBvcnRzIjotMSwiZGF0YSI6LTEsInBheWxvYWQiOi0xLCJ3aWxkY2FyZHMiOnRydWV9fX0._WW5C1triCh8a4jhyBxEZZP8RJ17pINS8qLzz-01o6zbz1uZfTOJGvwSTS6Yv2_849B9iUXSd-8kp1iMXHdoBA
}
```

The `MEMORY` resolver is recommended when the server has a small number of accounts that don't change very often.

For more information on how to configure a memory resolver, see [this tutorial](../../nats-tools/nas/mem_resolver.md).