Controlling the User Cache

User credentials are cached in memory on each node to avoid connecting to a remote authentication server or hitting the disk for every incoming request. You can configure characteristics of the user cache with the cache.ttl, cache.max_users, and cache.hash_algo realm settings.

PKI realms do not use the user cache.

The cached user credentials are hashed in memory. By default, Shield uses a salted sha-256 hash algorigthm. You can use a different algorithm by setting the cache-hash_algo setting to any of the supported cache hash algorithms:

Table 6. Cache hash algorithms

Algorithm	Description
`ssha256`	Uses a salted `sha-256` algorithm (default).
`md5`	Uses `MD5` algorithm.
`sha1`	Uses `SHA1` algorithm.
`bcrypt`	Uses `bcrypt` algorithm with salt generated in 10 rounds.
`bcrypt4`	Uses `bcrypt` algorithm with salt generated in 4 rounds.
`bcrypt5`	Uses `bcrypt` algorithm with salt generated in 5 rounds.
`bcrypt6`	Uses `bcrypt` algorithm with salt generated in 6 rounds.
`bcrypt7`	Uses `bcrypt` algorithm with salt generated in 7 rounds.
`bcrypt8`	Uses `bcrypt` algorithm with salt generated in 8 rounds.
`bcrypt9`	Uses `bcrypt` algorithm with salt generated in 9 rounds.
`noop`,`clear_text`	Doesn’t hash the credentials and keeps it in clear text in memory. CAUTION: keeping clear text is considered insecure and can be compromised at the OS level (for example through memory dumps and using `ptrace`).

Evicting Users from the Cache

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Shield exposes a Clear Cache API you can use to force the eviction of cached users. For example, the following request evicts all users from the ad1 realm:

$ curl -XPOST 'http://localhost:9200/_shield/realm/ad1/_clear_cache'

To clear the cache for multiple realms, specify the realms as a comma-separated list:

$ curl -XPOST 'http://localhost:9200/_shield/realm/ad1,ad2/_clear_cache'

You can also evict specific users:

$ curl -XPOST 'http://localhost:9200/_shield/realm/ad1/_clear_cache?usernames=rdeniro,alpacino'

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