Smuxi

                internal static string BuildPathFromPortName (string portName)
                {
                        if (!Win32.IpcChannelHelper.IsValidPipeName (portName))
                                throw new RemotingException ("Invalid IPC port name");
                        return Path.Combine (Path.GetTempPath (), portName);
                }

            listener = new UnixListener (path);
            Mono.Unix.Native.Syscall.chmod (path,
                                     Mono.Unix.Native.FilePermissions.S_IRUSR |
                                     Mono.Unix.Native.FilePermissions.S_IWUSR |
                                     Mono.Unix.Native.FilePermissions.S_IRGRP |
                                     Mono.Unix.Native.FilePermissions.S_IWGRP |
                                     Mono.Unix.Native.FilePermissions.S_IROTH |
                                     Mono.Unix.Native.FilePermissions.S_IWOTH);

from ./mcs/class/System.Runtime.Remoting/System.Runtime.Remoting.Channels.Ipc.Unix/README:

System.Runtime.Remoting.Channels.Ipc.Unix
=========================================

*.Ipc.Unix is a wrapper for Mono.Remoting.Channels.Unix.
The Unix channels are loaded via reflection.

The wrapper is performing the following mappings:

IPC                     UNIX
--------------------------------------
portName="foo"          path=$TEMP/foo
ipc://foo/bar.rem    unix://$TEMP/foo?/bar.rem

So Mono is creating a unix socket in /tmp, chmodded 666, named after the port name of the IPC channel. According to MSDN docs [0] it is supposed to be only accessible by the same user by default, that would be chmod 600 on Unix.

https://msdn.microsoft.com/en-us/library/ms172351(v=vs.80)

Path.GetTempPath() -> [ICall] Path.get_temp_path() -> ves_icall_System_IO_get_temp_path() -> g_get_tmp_dir() of eglib [0] or glib [1]

https://github.com/mono/mono/blob/mono-3.2.8-branch/eglib/src/gmisc-win32.c#L146

https://developer.gnome.org/glib/stable/glib-Miscellaneous-Utility-Functions.html#g-get-tmp-dir

So the TMP, TMPDIR and TEMP environment variables influences the location of the chosen temp directory

Applied in changeset 709af0de6cdd439c307aeb359c7a309e2eede50e.