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The adoption of KIP-255: OAuth Authentication via SASL/OAUTHBEARER in release 2.0.0 creates the possibility of using information in the bearer token to make authorization decisions.  Unfortunately, however, Kafka connections are long-lived, so there is no ability to change the bearer token associated with a particular connection.  Allowing SASL connections to periodically re-authenticate would resolve this.  In addition to this motivation there are two others that are security-related.  First, to eliminate access to Kafka for connected clients, the current requirement is to remove all authorizations (i.e. remove all ACLs).  This is necessary because of the long-lived nature of the connections.  It is operationally simpler to shut off access at the point of authentication, and with the release of KIP-86: Configurable SASL Callback Handlers it is going to become more and more likely that installations will authenticate users against external directories (e.g. via LDAP).  The ability to stop Kafka access by simply disabling an account in an LDAP directory (for example) is desirable.  The second motivating factor for re-authentication related to security is that the use of short-lived tokens is a common OAuth security recommendation, but issuing a short-lived token to a Kafka client (or a broker when OAUTHBEARER is the inter-broker protocol) currently has no benefit because once a client is connected to a broker the client is never challenged again and the connection may remain intact beyond the token expiration time (and may remain intact indefinitely under perfect circumstances).  This KIP proposes adding the ability for SASL clients (and brokers when a SASL mechanism is the inter-broker protocol) to re-authenticate their connections to brokers .  This KIP also proposes to add the ability for and for brokers to close connections that continue to use expired sessions.

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This KIP proposes the addition of a configuration option to enable the server-side expired-connection-kill feature (the option default results in no functionality change, of course, so there is no change to existing behavior in the absence of an explicit opt-in).  This KIP also proposes bumping the version number for the SASL_AUTHENTICATE API to 1 (with a change in wire format) so that servers brokers can indicate the session expiration time to clients via an additional value on the last round-trip response.  Clients can use the max SASL_AUTHENTICATE version number supported by the server to determine if they are connected to a broker that supports re-authentication (true if version > 0).  This KIP also adds new metrics as described below.

The configuration option this KIP proposes to add to enable server-side expired-connection-kill is 'connections.max.reauth.ms'  – it must be (not prefixed with listener prefix and or SASL mechanism name in lower-case– this is a single value for the cluster). For example, "sasl_ssl.oauthbearer.connections.max.reauth.ms=3600000".  The value represents the maximum value that could potentially be communicated as part of the new V1 SaslAuthenticateResponse.  The default value is 0, which means there is effectively no maximum communicated (0 will be sent, meaning "none"), server-side kill of expired connections is disabled, clients are not required to re-authenticate, and whether clients re-authenticate or not and at what interval is entirely up to them.  Existing SASL clients upgraded to v2.1.0 will be coded to not re-authenticate in this scenario.  The default value of 0 therefore results in no change whatsoever.

When 'connections.max.reauth.ms' is explicitly set to a positive number the server will disconnect any SASL connection that does not re-authenticate and subsequently uses the connection for any purpose other than re-authentication at any point beyond the communicated expiration point (which will not exceed the configured maximum value).  For example, if the configured value is 3600000 (1 hour) and the remaining token lifetime at lifetime of a bearer token presented at the time of authentication is 45 minutes, then 45 minutes is communicated back to the client and the server would kill the connection if it is not re-authenticated within 45 minutes and it is then actively used for anything other than re-authentication.  As a further example, if the configured value is 3600000 and the credential lifetime is either unspecified or greater than 1 hour then 1 hour would be communicated to the client and the server would kill the connection if it is not re-authenticated within 1 hour and it is then actively used for anything other than re-authentication.

Older clients will of course not have the session expiration time communicated to them since they will use a version 0 SaslAuthenticateRequest and will receive the existing version 0 SaslAuthenticateResponse.  The broker will disconnect these connections upon session expiration regardless of the fact that the client is an older one.  Such connections will be captured via a metric (described below) to help with migration.

The 'connections.max.reauth.ms' The above configuration option will not be dynamically changeable; restarts will be required if the value is to be changedrestarts will be required if the value is to be changed.  However, if a new listener is dynamically added, the value could be set for that listener at that time.  This dynamic override ability will be addressed as a separate ticket and may not be delivered with the initial KIP implementation.

From a behavior perspective on the client side (including the broker when it is acting as an inter-broker client), when a v2.1.0-or-later SASL client connects to a v2.1.0 or later broker that supports re-authentication, the broker will communicate the session expiration time as part of the final SASL_AUTHENTICATE response.  If this value is positive, then the client will automatically re-authenticate before anything else unrelated to re-authentication is sent beyond that expiration point.  If the re-authentication attempt fails then the connection will be closed by the broker; retries are not supported.  If re-authentication succeeds then any requests that queued up during re-authentication will subsequently be able to flow through, and eventually the connection will re-authenticate again, etc.

From a behavior perspective on the server (broker) side, when the expired-connection-kill feature is enabled with a positive value for a particular SASL mechanism the broker will communicate a session time via SASL_AUTHENTICATE and will close a connection when the connection is used past the expiration time and the specific API request is not directly related to re-authentication (ApiVersionsRequest, SaslHandshakeRequest, and  and SaslAuthenticateRequest).  In other words, if a connection sits idle, it will not be closed – something unrelated to re-authentication must traverse the connection before a disconnect will occur.

Metrics documenting re-authentications will be maintained.  They will mirror existing metrics that document authentications.  For example: failed-reauthentication-{rate,total} and successful-reauthentication-{rate,total}.  There will also be a separate pair of metrics tracking the subset of clients that authenticate with a V0 SaslAuthenticateRequest: failed-v0separate successful-authentication-{rate,total} and successful-v0-authentication-{rate,total}total metric tagged with "auth=v0" to indicate it tracks the subset of clients that successfully authenticate with a V0 SaslAuthenticateRequest.   This metric is helpful during migration (see Migration Plan for details) as it will identify if/when all clients are properly upgraded before server-side expired-connection-kill functionality is enabled: the metric will be zero across all brokers when it is appropriate to enable the feature.

A client-side metric will be created that documents the latency imposed by re-authentication.  It is unclear if this latency will be problematic, and the data collected via this metric may be useful as we consider this issue in the future.

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1. Upgrade all brokers to v2.1.0 or later at whatever rate is desired with 'connections.max.reauth.ms' allowed to default to 0.  If SASL is used for the inter-broker protocol then brokers will check the SASL_AUTHENTICATE API version and use a V1 request when communicating to a broker that has been upgraded to 2.1.0, but the client will see the "0" session max lifetime and will not re-authenticate.  Their connections will not be killed.
2. In parallel with (1) above, upgrade non-broker clients to v2.1.0 or later at whatever rate is desired.  SASL clients will check the SASL_AUTHENTICATE API version and use a V1 request when communicating to a broker that has been upgraded to 2.1.0, but the client will see the "0" session max lifetime and will not re-authenticate.  Their connections will not be killed.
3. After (1) and (2) are complete, perform a rolling restart of all brokers and check the broker metrics failed- v0-authentication-{rate,total} and tagged metrics successful-v0-authentication-{rate,total} to confirm that they remain at zero.  This gives confidence that (1) and (2) are indeed complete.
4. Update '[listener].[mechanism].connections.max.reauth.ms' to a positive value and perform a rolling restart of brokers again. 
5. Monitor the failed- v0-authentication-{rate,total} and tagged metrics successful-v0-authentication-{rate,total} metrics –  – they will remain at 0 unless an older client connects to the broker.

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