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UNIX(4) FreeBSD Kernel Interfaces Manual UNIX(4)

unix
UNIX-domain protocol family

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

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_SEQPACKET, and SOCK_DGRAM socket types and uses file system pathnames for addressing.

UNIX-domain addresses are variable-length file system pathnames of at most 104 characters. The include file <sys/un.h> defines this address:
struct sockaddr_un { 
	u_char	sun_len; 
	u_char	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 file system. This file is not removed when the socket is closed — unlink(2) must be used to remove the file.
The length of UNIX-domain address, required by bind(2) and connect(2), can be calculated by the macro SUN_LEN() defined in <sys/un.h>. The sun_path field must be terminated by a NUL character to be used with SUN_LEN(), but the terminating NUL is not part of the address.
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 file system access-control mechanisms are also applied when referencing pathnames; e.g., the destination of a connect(2) or sendto(2) must be writable.

The UNIX-domain sockets support the communication of UNIX file descriptors and process credentials through the use of the msg_control field in the msg argument to sendmsg(2) and recvmsg(2). The items to be passed are described using a struct cmsghdr that is defined in the include file <sys/socket.h>.
To send file descriptors, 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.
Credentials of the sending process can be transmitted explicitly using a control message of type SCM_CREDS with a data portion of type struct cmsgcred, defined in <sys/socket.h> as follows:
struct cmsgcred { 
  pid_t	cmcred_pid;		/* PID of sending process */ 
  uid_t	cmcred_uid;		/* real UID of sending process */ 
  uid_t	cmcred_euid;		/* effective UID of sending process */ 
  gid_t	cmcred_gid;		/* real GID of sending process */ 
  short	cmcred_ngroups;		/* number of groups */ 
  gid_t	cmcred_groups[CMGROUP_MAX];	/* groups */ 
};
The sender should pass a zeroed buffer which will be filled in by the system.
The group list is truncated to at most CMGROUP_MAX GIDs.
The process ID cmcred_pid should not be looked up (such as via the KERN_PROC_PID sysctl) for making security decisions. The sending process could have exited and its process ID already been reused for a new process.

UNIX domain sockets support a number of socket options which can be set with setsockopt(2) and tested with getsockopt(2):
This option may be enabled on SOCK_DGRAM, SOCK_SEQPACKET, or a SOCK_STREAM socket. This option provides a mechanism for the receiver to receive the credentials of the process calling write(2), send(2), sendto(2) or sendmsg(2) as a recvmsg(2) control message. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by a variable length sockcred structure, defined in <sys/socket.h> as follows:
struct sockcred { 
  uid_t	sc_uid;		/* real user id */ 
  uid_t	sc_euid;	/* effective user id */ 
  gid_t	sc_gid;		/* real group id */ 
  gid_t	sc_egid;	/* effective group id */ 
  int	sc_ngroups;	/* number of supplemental groups */ 
  gid_t	sc_groups[1];	/* variable length */ 
};
    
The current implementation truncates the group list to at most CMGROUP_MAX groups.
The SOCKCREDSIZE() macro computes the size of the sockcred structure for a specified number of groups. The cmsghdr fields have the following values:
cmsg_len = CMSG_LEN(SOCKCREDSIZE(ngroups)) 
cmsg_level = SOL_SOCKET 
cmsg_type = SCM_CREDS
    
On SOCK_STREAM and SOCK_SEQPACKET sockets credentials are passed only on the first read from a socket, then the system clears the option on the socket.
This option and the above explicit struct cmsgcred both use the same value SCM_CREDS but incompatible control messages. If this option is enabled and the sender attached a SCM_CREDS control message with a struct cmsgcred, it will be discarded and a struct sockcred will be included.
Many setuid programs will write(2) data at least partially controlled by the invoker, such as error messages. Therefore, a message accompanied by a particular sc_euid value should not be trusted as being from that user.
Used with SOCK_STREAM sockets, this option causes the connect(2) function to block until accept(2) has been called on the listening socket.
Requested via getsockopt(2) on a SOCK_STREAM socket returns credentials of the remote side. These will arrive in the form of a filled in xucred structure, defined in <sys/ucred.h> as follows:
struct xucred { 
  u_int	cr_version;		/* structure layout version */ 
  uid_t	cr_uid;			/* effective user id */ 
  short	cr_ngroups;		/* number of groups */ 
  gid_t	cr_groups[XU_NGROUPS];	/* groups */ 
};
    
The cr_version fields should be checked against XUCRED_VERSION define.
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.
To reliably obtain peer credentials on a SOCK_DGRAM socket refer to the LOCAL_CREDS socket option.

connect(2), dup(2), fcntl(2), getsockopt(2), listen(2), recvmsg(2), sendto(2), setsockopt(2), socket(2), CMSG_DATA(3), intro(4)
An Introductory 4.3 BSD Interprocess Communication Tutorial, PS1, 7.
An Advanced 4.3 BSD Interprocess Communication Tutorial, PS1, 8.
August 19, 2018 FreeBSD 12.0-RELEASE

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