This extension adds an Authenticator which is used to protect HTTP Endpoints using the simple and protected GSSAPI negotiation mechanism [SPNEGO](https://en.wikipedia.org/wiki/SPNEGO).
To use the Kerberos authenticator, add an authenticator with type `kerberos` to the authenticatorChain. The example above uses the name "MyKerberosAuthenticator" for the Authenticator.
Configuration of the named authenticator is assigned through properties with the form:
|`druid.auth.authenticator.kerberos.serverPrincipal`|`HTTP/_HOST@EXAMPLE.COM`| SPNego service principal used by druid processes|empty|Yes|
|`druid.auth.authenticator.kerberos.serverKeytab`|`/etc/security/keytabs/spnego.service.keytab`|SPNego service keytab used by druid processes|empty|Yes|
|`druid.auth.authenticator.kerberos.authToLocal`|`RULE:[1:$1@$0](druid@EXAMPLE.COM)s/.*/druid DEFAULT`|It allows you to set a general rule for mapping principal names to local user names. It will be used if there is not an explicit mapping for the principal name that is being translated.|DEFAULT|No|
|`druid.auth.authenticator.kerberos.cookieSignatureSecret`|`secretString`| Secret used to sign authentication cookies. It is advisable to explicitly set it, if you have multiple druid ndoes running on same machine with different ports as the Cookie Specification does not guarantee isolation by port.|<Randomvalue>|No|
As a note, it is required that the SPNego principal in use by the druid processes must start with HTTP (This specified by [RFC-4559](https://tools.ietf.org/html/rfc4559)) and must be of the form "HTTP/_HOST@REALM".
In older releases, the Kerberos authenticator had an `excludedPaths` property that allowed the user to specify a list of paths where authentication checks should be skipped. This property has been removed from the Kerberos authenticator because the path exclusion functionality is now handled across all authenticators/authorizers by setting `druid.auth.unsecuredPaths`, as described in the [main auth documentation](../../design/auth.md).
The syntax for mapping rules is `RULE:\[n:string](regexp)s/pattern/replacement/g`. The integer n indicates how many components the target principal should have. If this matches, then a string will be formed from string, substituting the realm of the principal for $0 and the n‘th component of the principal for $n. e.g. if the principal was druid/admin then `\[2:$2$1suffix]` would result in the string `admindruidsuffix`.
If this string matches regexp, then the s//\[g] substitution command will be run over the string. The optional g will cause the substitution to be global over the string, instead of replacing only the first match in the string.
### Increasing HTTP Header size for large SPNEGO negotiate header
In Active Directory environment, SPNEGO token in the Authorization header includes PAC (Privilege Access Certificate) information,
which includes all security groups for the user. In some cases when the user belongs to many security groups the header to grow beyond what druid can handle by default.
In such cases, max request header size that druid can handle can be increased by setting `druid.server.http.maxRequestHeaderSize` (default 8Kb) and `druid.router.http.maxRequestBufferSize` (default 8Kb).
Druid internal processes communicate with each other using an escalated http Client. A Kerberos enabled escalated HTTP Client can be configured by following properties -
Note: Above command will authenticate the user first time using SPNego negotiate mechanism and store the authentication cookie in file. For subsequent requests the cookie will be used for authentication.
Many HTTP client libraries, such as Apache Commons [HttpComponents](https://hc.apache.org/), already have support for performing SPNEGO authentication. You can use any of the available HTTP client library to communicate with druid cluster.
