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unix(4)
UNIX(4) BSD Kernel Interfaces Manual 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 pro‐
vides 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 path‐
names for addressing.
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.
CONTROL MESSAGES
The UNIX-domain sockets support the communication of UNIX file descrip‐
tors 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 auto‐
matically 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.
SOCKET OPTIONS
UNIX domain sockets support a number of socket options which can be set
with setsockopt(2) and tested with getsockopt(2):
LOCAL_CREDS This option may be enabled on SOCK_DGRAM, SOCK_SEQPACKET,
or a SOCK_STREAM socket. This option provides a mecha‐
nism 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 par‐
tially 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.
LOCAL_CONNWAIT Used with SOCK_STREAM sockets, this option causes the
connect(2) function to block until accept(2) has been
called on the listening socket.
LOCAL_PEERCRED 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.
SEE ALSO
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.
BSD August 19, 2018 BSD