SO_PEERCRED vs SCM_CREDENTIALS - why there are both of them?
credentials, linux, unix-socket
Solution
If I understand correctly, there is a subtle difference between the two. `SO_PEERCRED` retrieves the credentials of the peer process, without requiring any interaction from the peer process. In contrast, `SCM_CREDENTIALS` is a mechanism to send / receive credentials of the peer process, which are then checked by the kernel. This subtle difference may matter when a process is running as UID 0. `SCM_CREDENTIALS` allows a process running as UID 0, to declare itself less privileged (e.g., UID 50), whereas this would not be possible with `SO_PEERCRED`.
See above. I guess using `SCM_CREDENTIALS` is encouraged and `SO_PEERCRED` is only supported for compatibility.
The dbus daemon seems to use `SO_PEERCRED` and `getpeereid()`. I think it is best to copy their code in order to portably get the credentials.
http://cgit.freedesktop.org/dbus/dbus/tree/dbus/dbus-sysdeps-unix.c?id=edaa6fe253782dda959d78396b43e9fd71ea77e3
Problem
SO_PEERCRED is simple way to get pid/uid/gid of connected AF_UNIX stream socket, SCM_CREDENTIALS is more or less the same, but more complex (various ancillary messages). Links to example showing both ways. - Why there are two ways to get more or less the same information? - Why the more comfortable SO_PEERCRED is not listed in unix(7) manpage? - Which is use more in real-life applicatins? What should I use?