User credentials on a unix datagram socket

[John Kelly <jak@isp2dial.com>]

The socket(7) man page seems to imply that user credentials cannot be
sent on a unix datagram socket, unless socketpair() created it.

> SO_PEERCRED
>   Return the credentials of the foreign process connected to this socket.
>   This is only possible for connected AF_UNIX stream sockets and AF_UNIX
>   stream and datagram socket pairs created using socketpair(2);


But through trial and error, without reading any kernel source, I
learned that you can send user credentials on a regular unix datagram
socket which was not created with socketpair().

I'm unsure what SO_PEERCRED is intended for; I used SO_PASSCRED in my
server code, and it works.  I'm glad it does.  Stream sockets would
require a file descriptor for each client.  Ugh.  What I want to do is
so much simpler with datagram sockets.

I have sample code at ftp://ftp.isp2dial.com/users/jak/src/test/uxdg/

I hope someone will revise the man page and explain that you can pass
user credentials on a unix datagram socket, even one not created with
socketpair().

This is a very useful feature.  Please keep it in the kernel!


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