User identities are always cached, as well as information about the domain services. However, user credentials are not cached by default. This means that SSSD always checks with the back end identity provider for authentication requests. If the identity provider is offline or unavailable, there is no way to process those authentication requests, so user authentication could fail.

It is possible to enable offline credentials caching, which stores credentials (after successful login) as part of the user account in the SSSD cache. Therefore, even if an identity provider is unavailable, users can still authenticate, using their stored credentials. Offline credentials caching is primarily configured in each individual domain entry, but there are some optional settings that can be set in the PAM service section, because credentials caching interacts with the local PAM service as well as the remote domain.

[domain/EXAMPLE]
cache_credentials = true

There are optional parameters that set when those credentials expire. Expiration is useful because it can prevent a user with a potentially outdated account or credentials from accessing local services indefinitely.

The credentials expiration itself is set in the PAM service, which processes authentication requests for the system.

[sssd]
services = nss,pam
...

[pam]
offline_credentials_expiration = 3
...

[domain/EXAMPLE]
cache_credentials = true
...

offline_credentials_expiration sets the number of days after a successful login that a single credentials entry for a user is preserved in cache. Setting this to zero (0) means that entries are kept forever.

While not related to the credentials cache specifically, each domain has configuration options on when individual user and service caches expire:

For example:

[sssd]
services = nss,pam
...

[pam]
offline_credentials_expiration = 3
...

[domain/EXAMPLE]
cache_credentials = true
account_cache_expiration = 7
entry_cache_timeout = 14400
...