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A Plink invocation of the form 'plink -shareexists <session>' tests for a currently live connection-sharing upstream for the session in question. <session> can be any syntax you'd use with Plink to make the actual connection (a host/port number, a bare saved session name, -load, whatever). I envisage this being useful for things like adaptive proxying - e.g. if you want to connect to host A which you can't route to directly, and you might already have a connection to either of hosts B or C which are viable proxies, then you could write a proxy shell script which checks whether you already have an upstream for B or C and goes via whichever one is currently active. Testing for the upstream's existence has to be done by actually connecting to its socket, because on Unix the mere existence of a Unix-domain socket file doesn't guarantee that there's a process listening to it. So we make a test connection, and then immediately disconnect; hence, that shows up in the upstream's event log.