add notes for needed environtment

kerberos_notes.md

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+### Required environment for Kerberos authentication ###
+1. Kerberos server
+2. DNS server
+3. Opensearch Core & Dashboards server
+4. Client browser with SPNEGO ( I testing on google-chrome )
+
+---
+### KEBEROS SERVER ###
+Required
+1. User principle to be authenticate
+2. Service principle eg. HTTP/<server_host> ( When created with kadmin should get as HTTP/<server_host>@<your_domain>)
+3. Keytab for Service principle ( Make sure to give owner to opensearch core )
+
+**NOTE** 
+1. I only test both core and dashboards on the same host.
+2. Dashboards isn't the one authenticate with Kerberos but the core did so keytab doesn't need for dashboards.
+
+---
+
+### DNS SERVER ###
+Required
+1. DNS Service if using **bind9** should have dns address to opensearch & kerberos like
+```
+_kerberos._udp.<YOUR DOMAIN>.     IN SRV 1  0 88  kdc.<your domain>.
+_kerberos._tcp.<YOUR DOMAIN>.     IN SRV 1  0 88  kdc.<your domain>.
+_kerberos-adm._tcp.<YOUR DOMAIN>. IN SRV 1  0 749 kdc.<your domain>.
+_kpasswd._udp.<YOUR DOMAIN>.      IN SRV 1  0 464 kdc.<your domain>.
+
+kdc     IN      A       <kerberos kdc server address>
+opensearch  IN      A       <opensearch server address>
+```
+
+---
+
+### Opensearch core ###
+Required
+1. set DNS point to your DNS server
+2. Add config to opensearch.yml
+```
+plugins.security.kerberos.krb5_filepath: '/etc/krb5.conf' # this is your kerberos config location
+plugins.security.kerberos.acceptor_keytab_filepath: '<path to keytab file>' # keytab file for kerberos ( don't forget to give owner to opensearch )
+plugins.security.kerberos.acceptor_principal: 'HTTP/<server_host>' # this is your service principle on your kerberos for opensearch
+```
+
+3. Configuration Opensearch-security config file ( don't forget to apply )
+```
+    authc:
+      kerberos_auth_domain:
+        http_enabled: true # enable this
+        transport_enabled: false
+        order: 6
+        http_authenticator:
+          type: kerberos
+          challenge: false
+          config:
+            krb_debug: true
+            strip_realm_from_principal: true
+        authentication_backend:
+          type: noop
+
+
+      jwt_auth_domain:
+        description: "Authenticate via Json Web Token"
+        http_enabled: true
+        transport_enabled: false
+        order: 0
+        http_authenticator:
+          type: jwt
+          challenge: false
+          config:
+            signing_key: "<encoded secret key>" # edit this to your key (encoded by base64)
+            jwt_header: "Authorization"
+            jwt_url_parameter: null
+#            jwt_clock_skew_tolerance_seconds: 30
+            roles_key: roles
+            subject_key: user
+        authentication_backend:
+          type: noop
+```
+
+---
+
+### Opensearch Dashboards ###
+Required
+Edit config file for dashboards
+
+```
+# define auth type
+opensearch_security.auth.type: kerberos
+
+# set your secret key
+opensearch_security.kerberos.jwt_siging_key: '<your secret key>' #NOTE as plain text not encoded
+
+```
+
+---
+
+### Client Browser ###
+Required ( For google chrome )
+1. Make sure to that browser have **SPNEGO**
+1. Edit policy section for ```AuthServerAllowlist```
+
+For google chrome debian package should locate at ```/etc/opt/chrome/policies/managed/```
+create your policy file eg.
+```
+{
+    "AuthServerAllowlist" : "<opensearch_hostname>"
+}
+```
+
+**NOTE**
+- When search on browser you must access hostname according to policies your defined.
+
+- Make sure that you already use **kinit** and see your tokens via **klist**
+
+- You can test with curl for checking kerberos by
+```curl <opensearch_hostname>:<port> -u ':' --negotiate -v```
+( You should see Negotiate token when request to server. If not it may problem with misconfiguration kerberos (usually principle) or DNS )
+
+- **Don't forget** to add permission to user name same as kerberos principle of that user.