Configuring Anonymous Authentication in OpenSearch blog post

I recently configured anonymous authentication in OpenSearch.  While the
documentation has some information about this, there is no guide
available that list all requirements, and I had to find information in
multiple places and had to look at the code to fully understand what was
needed.  Not great for a tasks that I assumed would be simple.

More recently, people asked on Slack how to enable anonymous auth, so
maybe a blog post is welcome to document the process?

Signed-off-by: Romain Tartière <[email protected]>

_posts/xxxx-xx-xx-configure-anonymous-authentication.md

@@ -0,0 +1,98 @@
+---
+layout: post
+title: Configuring Anonymous Authentication in OpenSearch
+authors:
+  - smortex
+date: 2023-12-14 14:20:00 -1000
+category:
+  - technical-posts
+meta_keywords: anonymous, authentication, authorization
+meta_description: Learn how to set up OpenSearch for anonymous authentication
+---
+
+The OpenSearch Security plugin has support multiple authentication backends.
+It is also possible to enable anonymous authentication to allow access to OpenSearch without prior identification.
+In this article, we configure anonymous authentication which is disabled by default.
+
+## Enable anonymous authentication in OpenSearch
+
+Anonymous access to OpenSearch is done in the Security plugin by modifying `opensearch-security/config.yml` and reloading the configuration with `securityadmin.sh`.
+
+Ensure `anonymous_auth_enabled` is set to true:
+
+```yaml
+config:
+  dynamic:
+    http:
+      anonymous_auth_enabled: true
+```
+
+Then, inject the new configuration (this will replace the configuration, exercise caution if you have changed OpenSearch configuration outside of these files):
+
+```
+OPENSEARCH_JAVA_HOME=/usr/share/opensearch/jdk /usr/share/opensearch/plugins/opensearch-security/tools/securityadmin.sh \
+  -cacert /etc/opensearch/root-ca.pem \
+  -cert /etc/opensearch/kirk.pem \
+  -key /etc/opensearch/kirk-key.pem \
+  -cd /etc/opensearch/opensearch-security
+```
+
+## Enable anonymous authentication in OpenSearch-Dashboards
+
+
+Now that OpenSearch anonymous access is enabled, we must enable it in OpenSearch Dashboards by modifing `opensearch_dashboards.yml` and adding the following line:
+
+```yaml
+opensearch_security.auth.anonymous_auth_enabled: true
+```
+
+When done, we restart the `opensearch-dashboards` service.
+
+When accessing OpenSearch Dashboards from a browser, you will automatically log in as the **opendistro_security_anonymous** user.  It is possible to authenticate as another user after logging out: the usual log in screen will be presented, with a new "Log in as anonymous" button.
+
+## Adjusting premissions of anonymous users
+
+With the default OpenSearch configuartion, the **opendistro_security_anonymous** user has the *backend role* `opendistro_security_anonymous_backendrole` and the *role* `own_index`.
+This is not sufficient to access OpenSearch data and roles must be mapped to the `opendistro_security_anonymous_backendrole` *backend role* to grant access.
+
+There is no one-size-fit-all configuration, so we will consider some simple use cases.
+
+Configuration can be done using the OpenSearch Dashboards user interface as an administrator, or by modifing the Security plugin configuration in `opensearch-security/roles_mapping.yml` and applying changes with `securityadmin.sh`.
+
+### Provide read-only access to anonymous users
+
+Map the `readall` and `kibana_user` roles to the `opendistro_security_anonymous_backendrole` backend role:
+
+```
+kibana_user:
+  reserved: false
+  backend_roles:
+  - "kibanauser"
+  - "opendistro_security_anonymous_backendrole" # <--- added
+  description: "Maps kibanauser to kibana_user"
+
+readall:
+  reserved: false
+  backend_roles:
+  - "readall"
+  - "opendistro_security_anonymous_backendrole" # <--- added
+```
+
+### Provide full access to anonymous users
+
+Map the `all_access` role to the `opendistro_security_anonymous_backendrole` backend role:
+
+```
+all_access:
+  reserved: false
+  backend_roles:
+  - "admin"
+  - "opendistro_security_anonymous_backendrole" # <--- added
+  description: "Maps admin to all_access"
+```
+
+## Conclusion
+
+In this article, we saw how to enable anonymous access to OpenSearch Dashboards.
+
+In your deployment, you will likely create custom roles to give access to a subset of your data through [document-level](https://opensearch.org/docs/latest/security/access-control/document-level-security/) or [field-level](https://opensearch.org/docs/latest/security/access-control/field-level-security/) security.