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* plugin/pkg/proxy: add max_age for per-connection lifetime cap Introduce a max_age setting on Transport that closes connections based on creation time, independent of idle-timeout (expire). Background: PR #7790 changed the connection pool from LIFO to FIFO for source-port diversity. Under FIFO, every connection is cycled through the pool and its used timestamp is refreshed continuously. When request rate is high enough that pool_size / request_rate < expire, no connection ever becomes idle and expire never fires. This prevents CoreDNS from opening new connections to upstreams that scale out (e.g. new Kubernetes pods behind a ClusterIP service with conntrack pinning). max_age addresses this by enforcing an absolute upper bound on connection lifetime regardless of activity: - persistConn gains a created field set at dial time. - Transport gains maxAge (default 0 = unlimited, preserving existing behaviour). - Dial(): rejects cached connections whose creation age exceeds max_age. - cleanup(): when maxAge > 0, uses a linear scan over both idle-timeout and max-age predicates; when maxAge == 0, preserves the original binary-search path on used time (sorted by FIFO insertion order). - Both hot paths pre-compute the deadline outside any inner loop to avoid repeated time.Now() calls. Tests added: - TestMaxAgeExpireByCreation: connection with old created but fresh used must be rejected even though idle-timeout would pass. - TestMaxAgeFIFORotation: three FIFO-rotated connections (old created, fresh used) must all be rejected, confirming that continuous rotation cannot prevent max-age expiry. Signed-off-by: cangming <cangming@cangming.app> * plugin/forward: add max_age option Expose Transport.SetMaxAge through the forward plugin so operators can set an absolute upper bound on connection lifetime via the Corefile. Usage: forward . 1.2.3.4 { max_age 30s } Default is 0 (unlimited), which preserves existing behaviour. A positive value causes connections older than max_age to be closed and re-dialled on the next request, ensuring CoreDNS reconnects to newly scaled-out upstream pods even under sustained load where the idle timeout (expire) would never fire. If max_age is set, it must not be less than expire; the parser rejects this combination at startup with a clear error message. Signed-off-by: cangming <cangming@cangming.app> --------- Signed-off-by: cangming <cangming@cangming.app>