如何在命名管道上传输文件描述符

Question

My goal is to create a concept for daemons which interact locally (ie not via network) with other processes, all with different user ids/rights.

To restrict access to the daemons to processes with the right access levels, I throught of the following concept:

• Each daemon opens a named pipe for reading in /foo/{daemon-name}
• The access to that named pipe can be easily managed via normal file access rights, so other processes not in the correct user group will have no write access to the pipe
• changing access rights to the daemon later is easily done without the need to recompile
• a process that wants to communicate with the daemon sends one end of a socketpair() via the named pipe and communication continues over that link

That way, each dameon could implement their own API or packet format to communicate via the socketpair. But there would be no need for authentication and such because the access rights to the named pipe already took care of only a specific group being able to send the socketpair to communicate over.

Now the only problem I have is that I can't get the transfer of the socketpair() file descritor to work over the named pipe. sendmsg() apparently does not work on named pipes.

How can I send one of the socketpair() fds via the named pipe so that the daemon can access the connection and start communicating?

Answer 1

A named pipe won't work for what you want to do - the pipe retains no information regarding where the data in it came from. It's a pipe for raw data bits, not a connection like a socket that retains a lot more information about where the data came from and where it's going.

Unix-domain sockets and named pipes are not the same - they have different capabilities. Unix-domain sockets can be used to send file descriptors between processes, named pipes can't. Why they have been implemented that way is another question.

Since you're trying to create a socket connection between your daemon and client processes, just use Unix-domain sockets to get such a connection directly. File system permissions work with Unix-domain sockets just as they do for named pipes.