Create a fake server transport plugin that writes PT protocol strings to stderr, not stdout: ``` #!/bin/sh ( echo "VERSION 1" echo "SMETHOD testpt 127.0.0.1:9999" echo "SMETHODS DONE" ) 2>&1 ``` Verify that it writes nothing to stdout: ``` $ ./testpt.stderr.sh >/dev/null | wc 0 0 0 ``` Run tor with this torrc: ``` PublishServerDescriptor 0 AssumeReachable SOCKSPort 0 ORPort auto ServerTransportPlugin testpt exec ./testpt.stderr.sh Bridge testpt 127.0.0.1:9999 ``` A log line shows that tor has interpreted PT protocol messages from stderr, when it should only be looking for them on stdout. ``` Oct 10 16:21:11.000 [notice] Registered server transport 'testpt' at '127.0.0.1:9999' ``` ---- I think this is a bug. [pt-spec says](https://gitweb.torproject.org/torspec.git/tree/pt-spec.txt?id=d890052d5525a09829c798ab0ad6dcdcede1a8ef#n368) "All Pluggable Transport Proxies communicate to the parent process via writing NL-terminated lines to stdout." It does not say that tor should inspect stderr and interpret any lines that happen to be a well-formed PT protocol message that it finds there, as if they had been sent on stdout. The only mention of stderr in the spec is [in the context of LOG and STATUS](https://gitweb.torproject.org/torspec.git/tree/pt-spec.txt?id=d890052d5525a09829c798ab0ad6dcdcede1a8ef#n599), and that is [a fairly recent addition](https://gitweb.torproject.org/torspec.git/commit/pt-spec.txt?id=d890052d5525a09829c798ab0ad6dcdcede1a8ef) with legacy/trac#28179/legacy/trac#28181. I always thought that the intention was to copy anything seen on stderr to tor's log, as if it had been part of a LOG message on stdout. The mention of stderr in relation to STATUS is something I missed during review of that part of the spec, because I don't think it makes sense there. What I expected to see was this, treating verbatim stderr lines as things to be logged: ``` [notice] Unknown line received by managed proxy (VERSION 1) [notice] Unknown line received by managed proxy (SMETHOD testpt 127.0.0.1:9999) [notice] Unknown line received by managed proxy (SMETHODS DONE) ``` Otherwise, tor is essentially holding transport plugins to a contract that is unstated in the spec: transport plugins cannot safely write anything to stderr at all, because there is always a danger that a future version of tor will recognize it as a PT message and interpret it, instead of just logging it.