After invoking with emacs -Q, opening a file with tramp fails with an error: unix_listener: "/var/folders/l9/vwn57w053mzdv_mrtkjqqnj00000gn/T/tramp.91898jdU.david@death.porkrind.org:22.zrNS5blwPkoWXlHh" too long for Unix domain socket This error is printed in buffer called "*tramp/scp david@death.porkrind.org*". The particular file I was opening was "/david@death.porkrind.org:". This also fails in Emacs 24.4. It does not fail in Emacs 24.3. I'm going to try to binary search to narrow down the problem. Done. I tracked it to somewhere between 2014-06-13 and 2014-06-06 (I used the nightly builds at http://emacsformacosx.com/builds/all instead of building it all myself. The only patch in that area that looks like it has anything to do with it is a336b2eae3b52f6fba5a19cc98e780feca618115, but I don't understand why (maybe the regexp wasn't matching before?). When running as batch I noticed the ControlPath seems to be the difference with when it works or not. The message when it works: Tramp: Sending command `exec ssh -l david -o ControlPersist=no -e none deadth.porkrind.org' And when it doesn't: Tramp: Sending command `exec ssh -l david -o ControlPath=/var/folders/l9/vwn57w053mzdv_mrtkjqqnj00000gn/T/tramp.95670s_p.%r@%h:%p -o ControlMaster=auto -o ControlPersist=no -e none death.porkrind.org' I'm running on Mac OS X 10.10 (Yosemite). Ok. More info: I'm running a locally built version of OpenSSH. My local version is "OpenSSH_6.7p1, OpenSSL 1.0.2 22 Jan 2015". The system version is "OpenSSH_6.2p2, OSSLShim 0.9.8r 8 Dec 2011". When I run the command listed above with the long control path, the 2 versions act differently. The locally compiled 6.7p1 version prints: unix_listener: "/var/folders/l9/vwn57w053mzdv_mrtkjqqnj00000gn/T/tramp.95670s.david@death.porkrind.org:22.1NyTKIqLDRhV1uyV" too long for Unix domain socket ...and then exits. The system 6.2p2 version prints: ControlPath "/var/folders/l9/vwn57w053mzdv_mrtkjqqnj00000gn/T/tramp.95670s.david@death.porkrind.org:22.92nYwUmCrMTDpjyE" too long for Unix domain socket ...but then logs me in anyway (presumably without a ControlMaster). So it looks like Emacs is generating command line paths that are too long for ssh. New ssh versions treat this as a fatal error but old versions treat it as a warning. Shortening the path to just "/tmp/tramp.95670s_p.%r@%h:%p" makes it work with both ssh versions. -David In GNU Emacs 25.0.50.1 (x86_64-apple-darwin13.4.0, NS appkit-1265.21 Version 10.9.5 (Build 13F34)) of 2015-02-11 on builder10-9.porkrind.org Windowing system distributor `Apple', version 10.3.1344 Configured using: `configure --with-ns' Configured features: ACL LIBXML2 ZLIB Important settings: value of $LANG: en_US.UTF-8 locale-coding-system: utf-8-unix Major mode: Fundamental Minor modes in effect: shell-dirtrack-mode: t tooltip-mode: t global-eldoc-mode: t electric-indent-mode: t mouse-wheel-mode: t tool-bar-mode: t menu-bar-mode: t file-name-shadow-mode: t global-font-lock-mode: t font-lock-mode: t blink-cursor-mode: t auto-composition-mode: t auto-encryption-mode: t auto-compression-mode: t line-number-mode: t Recent messages: Tramp: Sending command `exec ssh -l david -o ControlPath=/var/folders/l9/vwn57w053mzdv_mrtkjqqnj00000gn/T/tramp.91898lKp.%r@%h:%p -o ControlMaster=auto -o ControlPersist=no -e none death.porkrind.org' Tramp: Waiting for prompts from remote shell... Tramp failed to connect. If this happens repeatedly, try `M-x tramp-cleanup-this-connection' Tramp: Waiting for prompts from remote shell...failed Tramp: Opening connection for david@death.porkrind.org using scp...failed Use M-x make-directory RET RET to create the directory and its parents Tramp: Checking `vc-registered' for /david@death.porkrind.org:src...failed Use M-x make-directory RET RET to create the directory and its parents Load-path shadows: None found. 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