The Unix Ns -domain protocol family is comprised of simple transport protocols that support the SOCK_STREAM and SOCK_DGRAM abstractions. SOCK_STREAM sockets also support the communication of Unix file descriptors through the use of the msg_control field in the msg argument to sendmsg(2) and recvmsg(2).
Any valid descriptor may be sent in a message. The file descriptor(s) to be passed are described using a
.Vt "struct cmsghdr" that is defined in the include file
.In sys/socket.h . The type of the message is SCM_RIGHTS, and the data portion of the messages is an array of integers representing the file descriptors to be passed. The number of descriptors being passed is defined by the length field of the message; the length field is the sum of the size of the header plus the size of the array of file descriptors.
The received descriptor is a duplicate of the senders descriptor, as if it were created with a call to dup(2). Per-process descriptor flags, set with fcntl(2), are not passed to a receiver. Descriptors that are awaiting delivery, or that are purposely not received, are automatically closed by the system when the destination socket is closed.
The effective credentials (i.e., the user ID and group list) of a peer on a SOCK_STREAM socket may be obtained using the LOCAL_PEERCRED socket option. This may be used by a server to obtain and verify the credentials of its client, and vice versa by the client to verify the credentials of the server. These will arrive in the form of a filled in
.Vt "struct xucred" (defined in
.In sys/ucred.h ) . The credentials presented to the server (the listen(2) caller) are those of the client when it called connect(2); the credentials presented to the client (the connect(2) caller) are those of the server when it called listen(2). This mechanism is reliable; there is no way for either party to influence the credentials presented to its peer except by calling the appropriate system call (e.g., connect(2) or listen(2)) under different effective credentials.
UNIX domain sockets support a number of socket options which can be set with setsockopt(2) and tested with getsockopt(2):