The
AF_UNIX
(also known as
AF_LOCAL)
socket family is used to communicate between processes on the same machine
efficiently.
Traditionally, Unix sockets can be either unnamed,
or bound to a file system pathname (marked as being of type socket).
Linux also supports an abstract namespace which is independent of the
file system.
Valid types are:
SOCK_STREAM,
for a stream-oriented socket and
SOCK_DGRAM,
for a datagram-oriented socket that preserves message boundaries
(as on most Unix implementations, Unix domain datagram
sockets are always reliable and don't reorder datagrams);
and (since Linux 2.6.4)
SOCK_SEQPACKET,
for a connection-oriented socket that preserves message boundaries
and delivers messages in the order that they were sent.
Unix sockets support passing file descriptors or process credentials
to other processes using ancillary data.
Address Format
A Unix domain socket address is represented in the following structure:
Three types of address are distinguished in this structure:
*
pathname:
a Unix domain socket can be bound to a null-terminated file
system pathname using
bind(2).
When the address of the socket is returned by
getsockname(2),
getpeername(2),
and
accept(2),
its length is
sizeof(sa_family_t) + strlen(sun_path) + 1,
and
sun_path
contains the null-terminated pathname.
*
unnamed:
A stream socket that has not been bound to a pathname using
bind(2)
has no name.
Likewise, the two sockets created by
socketpair(2)
are unnamed.
When the address of an unnamed socket is returned by
getsockname(2),
getpeername(2),
and
accept(2),
its length is
sizeof(sa_family_t),
and
sun_path
should not be inspected.
*
abstract:
an abstract socket address is distinguished by the fact that
sun_path[0]
is a null byte ('\0').
All of the remaining bytes in
sun_path
define the "name" of the socket.
(Null bytes in the name have no special significance.)
The name has no connection with file system pathnames.
The socket's address in this namespace is given by the rest of the
bytes in
sun_path.
When the address of an abstract socket is returned by
getsockname(2),
getpeername(2),
and
accept(2),
its length is
sizeof(struct sockaddr_un),
and
sun_path
contains the abstract name.
The abstract socket namespace is a non-portable Linux extension.
Socket Options
For historical reasons these socket options are specified with a
SOL_SOCKET
type even though they are
AF_UNIX
specific.
They can be set with
setsockopt(2)
and read with
getsockopt(2)
by specifying
SOL_SOCKET
as the socket family.
SO_PASSCRED
Enables the receiving of the credentials of the sending process
ancillary message.
When this option is set and the socket is not yet connected
a unique name in the abstract namespace will be generated automatically.
Expects an integer boolean flag.
(Un)supported Features
The following paragraphs describe domain-specific details and
unsupported features of the sockets API for Unix domain sockets on Linux.
Unix domain sockets do not support the transmission of
out-of-band data (the
MSG_OOB
flag for
send(2)
and
recv(2)).
The
send(2)
MSG_MORE
flag is not supported by Unix domain sockets.
The
SO_SNDBUF
socket option does have an effect for Unix domain sockets, but the
SO_RCVBUF
option does not.
For datagram sockets, the
SO_SNDBUF
value imposes an upper limit on the size of outgoing datagrams.
This limit is calculated as the doubled (see
socket(7))
option value less 32 bytes used for overhead.
Ancillary Messages
Ancillary data is sent and received using
sendmsg(2)
and
recvmsg(2).
For historical reasons the ancillary message types listed below
are specified with a
SOL_SOCKET
type even though they are
AF_UNIX
specific.
To send them set the
cmsg_level
field of the struct
cmsghdr
to
SOL_SOCKET
and the
cmsg_type
field to the type.
For more information see
cmsg(3).
SCM_RIGHTS
Send or receive a set of open file descriptors from another process.
The data portion contains an integer array of the file descriptors.
The passed file descriptors behave as though they have been created with
dup(2).
SCM_CREDENTIALS
Send or receive Unix credentials.
This can be used for authentication.
The credentials are passed as a
struct ucred
ancillary message.
Thus structure is defined in
<sys/socket.h>
as follows:
struct ucred {
pid_t pid; /* process ID of the sending process */
uid_t uid; /* user ID of the sending process */
gid_t gid; /* group ID of the sending process */
};
Since glibc 2.8, the
__GNU_SOURCE
feature test macro must be defined in order to obtain the definition
of this structure.
The credentials which the sender specifies are checked by the kernel.
A process with effective user ID 0 is allowed to specify values that do
not match its own.
The sender must specify its own process ID (unless it has the capability
CAP_SYS_ADMIN),
its user ID, effective user ID, or saved set-user-ID (unless it has
CAP_SETUID),
and its group ID, effective group ID, or saved set-group-ID
(unless it has
CAP_SETGID).
To receive a
struct ucred
message the
SO_PASSCRED
option must be enabled on the socket.
ERRORS
EADDRINUSE
Selected local address is already taken or file system socket
object already exists.
ECONNREFUSED
connect(2)
called with a socket object that isn't listening.
This can happen when
the remote socket does not exist or the filename is not a socket.
ECONNRESET
Remote socket was unexpectedly closed.
EFAULT
User memory address was not valid.
EINVAL
Invalid argument passed.
A common cause is the missing setting of AF_UNIX
in the
sun_type
field of passed addresses or the socket being in an
invalid state for the applied operation.
EISCONN
connect(2)
called on an already connected socket or a target address was
specified on a connected socket.
ENOMEM
Out of memory.
ENOTCONN
Socket operation needs a target address, but the socket is not connected.
EOPNOTSUPP
Stream operation called on non-stream oriented socket or tried to
use the out-of-band data option.
EPERM
The sender passed invalid credentials in the
struct ucred.
EPIPE
Remote socket was closed on a stream socket.
If enabled, a
SIGPIPE
is sent as well.
This can be avoided by passing the
MSG_NOSIGNAL
flag to
sendmsg(2)
or
recvmsg(2).
EPROTONOSUPPORT
Passed protocol is not AF_UNIX.
EPROTOTYPE
Remote socket does not match the local socket type
(SOCK_DGRAM
vs.
SOCK_STREAM)
ESOCKTNOSUPPORT
Unknown socket type.
Other errors can be generated by the generic socket layer or
by the file system while generating a file system socket object.
See the appropriate manual pages for more information.
VERSIONS
SCM_CREDENTIALS
and the abstract namespace were introduced with Linux 2.2 and should not
be used in portable programs.
(Some BSD-derived systems also support credential passing,
but the implementation details differ.)
NOTES
In the Linux implementation, sockets which are visible in the
file system honor the permissions of the directory they are in.
Their owner, group and their permissions can be changed.
Creation of a new socket will fail if the process does not have write and
search (execute) permission on the directory the socket is created in.
Connecting to the socket object requires read/write permission.
This behavior differs from many BSD-derived systems which
ignore permissions for Unix sockets.
Portable programs should not rely on
this feature for security.
Binding to a socket with a filename creates a socket
in the file system that must be deleted by the caller when it is no
longer needed (using
unlink(2)).
The usual Unix close-behind semantics apply; the socket can be unlinked
at any time and will be finally removed from the file system when the last
reference to it is closed.
To pass file descriptors or credentials over a
SOCK_STREAM,
you need
to send or receive at least one byte of non-ancillary data in the same
sendmsg(2)
or
recvmsg(2)
call.
Unix domain stream sockets do not support the notion of out-of-band data.
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
