UNIX(4)

NAME
unix - UNIX-domain protocol family

SYNOPSIS
#include <sys/types.h>
#include <sys/un.h>

DESCRIPTION
The UNIX-domain protocol family is a collection of protocols that provides local (on-machine) interprocess communication through the normal socket(2) mechanisms. The UNIX-domain family supports the SOCK_STREAM, SOCK_DGRAM and SOCK_SEQPACKET socket types and uses filesystem pathnames for addressing.

ADDRESSING
UNIX-domain addresses are variable-length filesystem pathnames of at most 104 characters. The include file <sys/un.h> defines this address:

    struct sockaddr_un {
    u_char		sun_len;
    sa_family_t	sun_family;
    char		sun_path[104];
    };

Binding a name to a UNIX-domain socket with bind(2) causes a socket file to be created in the filesystem. This file is not removed when the socket is closed — unlink(2) must be used to remove the file.

The UNIX-domain protocol family does not support broadcast addressing or any form of “wildcard” matching on incoming messages. All addresses are absolute- or relative-pathnames of other UNIX-domain sockets. Normal filesystem access-control mechanisms are also applied when referencing pathnames; e.g., the destination of a connect(2) or sendto(2) must be writable.

PROTOCOLS
The UNIX-domain protocol family is comprised of simple transport protocols that support the SOCK_STREAM, SOCK_DGRAM and SOCK_SEQPACKET abstractions. SOCK_STREAM, SOCK_DGRAM and SOCK_SEQPACKET 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 struct cmsghdr that is defined in the include file <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 sender's descriptor, as if it were created via dup(fd) or fcntl(fd, F_DUPFD_CLOEXEC, 0) depending on whether MSG_CMSG_CLOEXEC is passed in the recvmsg(2) call. 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 struct xucred (defined 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.

SEE ALSO
socket(2), intro(4) An Introductory 4.3 BSD Interprocess Communication Tutorial, PS1, 7. An Advanced 4.3 BSD Interprocess Communication Tutorial, PS1, 8.