ClickHouse/docs/en/operations/external-authenticators/kerberos.md

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# Kerberos {#external-authenticators-kerberos}
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Existing and properly configured ClickHouse users can be authenticated via Kerberos authentication protocol.
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Currently, Kerberos can only be used as an external authenticator for existing users, which are defined in `users.xml` or in local access control paths. Those users may only use HTTP requests and must be able to authenticate using GSS-SPNEGO mechanism.
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For this approach, Kerberos must be configured in the system and must be enabled in ClickHouse config.
## Enabling Kerberos in ClickHouse {#enabling-kerberos-in-clickhouse}
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To enable Kerberos, one should include `kerberos` section in `config.xml`. This section may contain additional parameters.
#### Parameters:
- `principal` - canonical service principal name that will be acquired and used when accepting security contexts.
- This parameter is optional, if omitted, the default principal will be used.
- `realm` - a realm, that will be used to restrict authentication to only those requests whose initiator's realm matches it.
- This parameter is optional, if omitted, no additional filtering by realm will be applied.
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Example (goes into `config.xml`):
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```xml
<yandex>
<!- ... -->
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<kerberos />
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</yandex>
```
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With principal specification:
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```xml
<yandex>
<!- ... -->
<kerberos>
<principal>HTTP/clickhouse.example.com@EXAMPLE.COM</principal>
</kerberos>
</yandex>
```
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With filtering by realm:
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```xml
<yandex>
<!- ... -->
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<kerberos>
<realm>EXAMPLE.COM</realm>
</kerberos>
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</yandex>
```
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!!! warning "Note"
You can define only one `kerberos` section. The presence of multiple `kerberos` sections will force ClickHouse to disable Kerberos authentication.
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!!! warning "Note"
`principal` and `realm` sections cannot be specified at the same time. The presence of both `principal` and `realm` sections will force ClickHouse to disable Kerberos authentication.
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## Kerberos as an external authenticator for existing users {#kerberos-as-an-external-authenticator-for-existing-users}
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Kerberos can be used as a method for verifying the identity of locally defined users (users defined in `users.xml` or in local access control paths). Currently, **only** requests over the HTTP interface can be *kerberized* (via GSS-SPNEGO mechanism).
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Kerberos principal name format usually follows this pattern:
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- *primary/instance@REALM*
The */instance* part may occur zero or more times. **The *primary* part of the canonical principal name of the initiator is expected to match the kerberized user name for authentication to succeed**.
### Enabling Kerberos in `users.xml` {#enabling-kerberos-in-users-xml}
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In order to enable Kerberos authentication for the user, specify `kerberos` section instead of `password` or similar sections in the user definition.
Parameters:
- `realm` - a realm that will be used to restrict authentication to only those requests whose initiator's realm matches it.
- This parameter is optional, if omitted, no additional filtering by realm will be applied.
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Example (goes into `users.xml`):
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```xml
<yandex>
<!- ... -->
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<users>
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<!- ... -->
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<my_user>
<!- ... -->
<kerberos>
<realm>EXAMPLE.COM</realm>
</kerberos>
</my_user>
</users>
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</yandex>
```
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!!! warning "Warning"
Note that Kerberos authentication cannot be used alongside with any other authentication mechanism. The presence of any other sections like `password` alongside `kerberos` will force ClickHouse to shutdown.
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!!! info "Reminder"
Note, that now, once user `my_user` uses `kerberos`, Kerberos must be enabled in the main `config.xml` file as described previously.
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### Enabling Kerberos using SQL {#enabling-kerberos-using-sql}
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When [SQL-driven Access Control and Account Management](../access-rights.md#access-control) is enabled in ClickHouse, users identified by Kerberos can also be created using SQL statements.
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```sql
CREATE USER my_user IDENTIFIED WITH kerberos REALM 'EXAMPLE.COM'
```
...or, without filtering by realm:
```sql
CREATE USER my_user IDENTIFIED WITH kerberos
```