UNIX(4) FreeBSD 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
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.
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.
Prior to binding a socket, fchmod(2) can be used to set the permissions
of the socket file. This avoids the race that would otherwise occur
between creation of the file and a subsequent call to chmod(2). Once the
socket is bound to a file name, the permissions of the file can not be
changed this way.
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
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.
SOCKET OPTIONS
UNIX domain sockets support a number of socket options for the options
level SOL_LOCAL, 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 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.
LOCAL_CREDS_PERSISTENT This option is similar to LOCAL_CREDS, except
that socket credentials are passed on every read
from a SOCK_STREAM or SOCK_SEQPACKET socket,
instead of just the first read. Additionally,
the msg_control field in the msghdr structure
points to a buffer that contains a cmsghdr
structure followed by a variable length sockcred2
structure, defined in <sys/socket.h> as follows:
struct sockcred2 {
int sc_version; /* version of this structure */
pid_t sc_pid; /* PID of sending process */
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 version is zero.
The cmsghdr fields have the following values:
cmsg_len = CMSG_LEN(SOCKCRED2SIZE(ngroups))
cmsg_level = SOL_SOCKET
cmsg_type = SCM_CREDS2
The LOCAL_CREDS and LOCAL_CREDS_PERSISTENT
options are mutually exclusive.
LOCAL_PEERCRED Requested via getsockopt(2) on a SOCK_STREAM or
SOCK_SEQPACKET 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 */
pid_t cr_pid; /* process id of the sending process */
};
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.
BUFFERING
Due to the local nature of the Unix-domain sockets, they do not implement
send buffers. The send(2) and write(2) families of system calls attempt
to write data to the receive buffer of the destination socket.
The default buffer sizes for SOCK_STREAM and SOCK_SEQPACKET Unix-domain
sockets can be configured with net.local.stream and net.local.seqpacket
branches of sysctl(3) MIB respectively. Note that setting the send
buffer size (sendspace) affects only the maximum write size.
The Unix-domain sockets of type SOCK_DGRAM are unreliable and always non-
blocking for write operations. The default receive buffer can be
configured with net.local.dgram.recvspace. The maximum allowed datagram
size is limited by net.local.dgram.maxdgram. A SOCK_DGRAM socket that
has been bound with bind(2) can have multiple peers connected at the same
time. The modern FreeBSD implementation will allocate
net.local.dgram.recvspace sized private buffers in the receive buffer of
the bound socket for every connected socket, preventing a situation when
a single writer can exhaust all of buffer space. Messages coming from
unconnected sends using sendto(2) land on the shared buffer of the
receiving socket, which has the same size limit. A side effect of the
implementation is that it doesn't guarantee that writes from different
senders will arrive at the receiver in the same chronological order they
were sent. The order is preserved for writes coming through a particular
connection.
SEE ALSO
connect(2), dup(2), fchmod(2), fcntl(2), getsockopt(2), listen(2),
recvmsg(2), sendto(2), setsockopt(2), socket(2), CMSG_DATA(3), intro(4),
sysctl(8)
"An Introductory 4.3 BSD Interprocess Communication Tutorial", PS1, 7.
"An Advanced 4.3 BSD Interprocess Communication Tutorial", PS1, 8.
FreeBSD 15.1-STABLE-HBSD October 31, 2024 UNIX(4)