Intermittent connection pool exhaustion in PuTTY sessions due to disabled keepalive

Direct answers to your three questions:

1. PuTTY does not automatically enable keepalive for sessions idle beyond a configurable threshold; the default interval is 0, and this setting must be changed manually in the Connections → Keepalives panel. Automatic threshold-based enabling is not a native PuTTY feature but could be implemented via SSH server configuration or client scripting.
2. Adjusting the keepalive interval to a non-zero value (e.g., 60 seconds) can prevent intermediate firewalls or NAT devices from closing idle connections, which may reduce failed handshakes if middlebox interference is contributing to the exhaustion pattern. However, if the exhaustion stems from OS socket descriptor limits or TIME_WAIT accumulation, keepalive adjustment alone will likely be insufficient.
3. TCP socket resources can be verified after a session ends by inspecting socket states on the client or server. Commands such as ss -twn or netstat -an display established, time-wait, and closed sockets. Counting descriptors before and after a rapid-open/close cycle reveals whether the OS is recycling resources or hitting file-descriptor limits.