c2e3c52e0d
This change allows creating file descriptors with close-on-exec set in some situations. SOCK_CLOEXEC and SOCK_NONBLOCK can be OR'ed in socket() and socketpair()'s type parameter, and MSG_CMSG_CLOEXEC to recvmsg() makes file descriptors (SCM_RIGHTS) atomically close-on-exec. The numerical values for SOCK_CLOEXEC and SOCK_NONBLOCK are as in NetBSD. MSG_CMSG_CLOEXEC is the first free bit for MSG_*. The SOCK_* flags are not passed to MAC because this may cause incorrect failures and can be done later via fcntl() anyway. On the other hand, audit is expected to cope with the new flags. For MSG_CMSG_CLOEXEC, unp_externalize() is extended to take a flags argument. Reviewed by: kib
305 lines
7.5 KiB
Groff
305 lines
7.5 KiB
Groff
.\" Copyright (c) 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)unix.4 8.1 (Berkeley) 6/9/93
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.\" $FreeBSD$
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.\"
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.Dd March 19, 2013
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.Dt UNIX 4
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.Os
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.Sh NAME
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.Nm unix
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.Nd UNIX-domain protocol family
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/un.h
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.Sh DESCRIPTION
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The
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.Ux Ns -domain
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protocol family is a collection of protocols
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that provides local (on-machine) interprocess
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communication through the normal
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.Xr socket 2
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mechanisms.
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The
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.Ux Ns -domain
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family supports the
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.Dv SOCK_STREAM ,
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.Dv SOCK_SEQPACKET ,
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and
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.Dv SOCK_DGRAM
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socket types and uses
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file system pathnames for addressing.
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.Sh ADDRESSING
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.Ux Ns -domain
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addresses are variable-length file system pathnames of
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at most 104 characters.
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The include file
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.In sys/un.h
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defines this address:
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.Bd -literal -offset indent
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struct sockaddr_un {
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u_char sun_len;
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u_char sun_family;
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char sun_path[104];
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};
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.Ed
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.Pp
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Binding a name to a
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.Ux Ns -domain
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socket with
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.Xr bind 2
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causes a socket file to be created in the file system.
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This file is
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.Em not
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removed when the socket is closed \(em
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.Xr unlink 2
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must be used to remove the file.
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.Pp
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The length of
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.Ux Ns -domain
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address, required by
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.Xr bind 2
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and
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.Xr connect 2 ,
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can be calculated by the macro
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.Fn SUN_LEN
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defined in
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.In sys/un.h .
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The
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.Va sun_path
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field must be terminated by a
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.Dv NUL
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character to be used with
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.Fn SUN_LEN ,
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but the terminating
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.Dv NUL
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is
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.Em not
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part of the address.
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.Pp
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The
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.Ux Ns -domain
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protocol family does not support broadcast addressing or any form
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of
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.Dq wildcard
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matching on incoming messages.
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All addresses are absolute- or relative-pathnames
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of other
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.Ux Ns -domain
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sockets.
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Normal file system access-control mechanisms are also
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applied when referencing pathnames; e.g., the destination
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of a
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.Xr connect 2
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or
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.Xr sendto 2
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must be writable.
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.Sh PASSING FILE DESCRIPTORS
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The
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.Ux Ns -domain
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sockets support the communication of
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.Ux
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file descriptors through the use of the
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.Va msg_control
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field in the
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.Fa msg
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argument to
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.Xr sendmsg 2
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and
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.Xr recvmsg 2 .
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.Pp
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Any valid descriptor may be sent in a message.
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The file descriptor(s) to be passed are described using a
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.Vt "struct cmsghdr"
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that is defined in the include file
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.In sys/socket.h .
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The type of the message is
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.Dv SCM_RIGHTS ,
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and the data portion of the messages is an array of integers
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representing the file descriptors to be passed.
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The number of descriptors being passed is defined
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by the length field of the message;
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the length field is the sum of the size of the header
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plus the size of the array of file descriptors.
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.Pp
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The received descriptor is a
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.Em duplicate
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of the sender's descriptor, as if it were created via
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.Li dup(fd)
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or
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.Li fcntl(fd, F_DUPFD_CLOEXEC, 0)
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depending on whether
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.Dv MSG_CMSG_CLOEXEC
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is passed in the
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.Xr recvmsg 2
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call.
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Descriptors that are awaiting delivery, or that are
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purposely not received, are automatically closed by the system
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when the destination socket is closed.
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.Sh SOCKET OPTIONS
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.Tn UNIX
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domain sockets support a number of socket options which can be set with
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.Xr setsockopt 2
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and tested with
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.Xr getsockopt 2 :
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.Bl -tag -width ".Dv LOCAL_CONNWAIT"
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.It Dv LOCAL_CREDS
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This option may be enabled on
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.Dv SOCK_DGRAM ,
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.Dv SOCK_SEQPACKET ,
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or a
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.Dv SOCK_STREAM
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socket.
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This option provides a mechanism for the receiver to
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receive the credentials of the process as a
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.Xr recvmsg 2
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control message.
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The
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.Va msg_control
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field in the
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.Vt msghdr
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structure points to a buffer that contains a
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.Vt cmsghdr
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structure followed by a variable length
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.Vt sockcred
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structure, defined in
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.In sys/socket.h
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as follows:
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.Bd -literal
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struct sockcred {
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uid_t sc_uid; /* real user id */
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uid_t sc_euid; /* effective user id */
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gid_t sc_gid; /* real group id */
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gid_t sc_egid; /* effective group id */
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int sc_ngroups; /* number of supplemental groups */
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gid_t sc_groups[1]; /* variable length */
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};
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.Ed
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.Pp
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The
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.Fn SOCKCREDSIZE
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macro computes the size of the
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.Vt sockcred
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structure for a specified number
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of groups.
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The
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.Vt cmsghdr
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fields have the following values:
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.Bd -literal
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cmsg_len = CMSG_LEN(SOCKCREDSIZE(ngroups))
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cmsg_level = SOL_SOCKET
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cmsg_type = SCM_CREDS
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.Ed
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.Pp
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On
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.Dv SOCK_STREAM
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and
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.Dv SOCK_SEQPACKET
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sockets credentials are passed only on the first read from a socket,
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then system clears the option on socket.
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.It Dv LOCAL_CONNWAIT
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Used with
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.Dv SOCK_STREAM
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sockets, this option causes the
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.Xr connect 2
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function to block until
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.Xr accept 2
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has been called on the listening socket.
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.It Dv LOCAL_PEERCRED
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Requested via
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.Xr getsockopt 2
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on a
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.Dv SOCK_STREAM
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socket returns credentials of the remote side.
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These will arrive in the form of a filled in
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.Vt xucred
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structure, defined in
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.In sys/ucred.h
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as follows:
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.Bd -literal
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struct xucred {
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u_int cr_version; /* structure layout version */
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uid_t cr_uid; /* effective user id */
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short cr_ngroups; /* number of groups */
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gid_t cr_groups[XU_NGROUPS]; /* groups */
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};
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.Ed
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The
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.Vt cr_version
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fields should be checked against
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.Dv XUCRED_VERSION
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define.
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.Pp
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The credentials presented to the server (the
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.Xr listen 2
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caller) are those of the client when it called
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.Xr connect 2 ;
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the credentials presented to the client (the
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.Xr connect 2
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caller) are those of the server when it called
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.Xr listen 2 .
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This mechanism is reliable; there is no way for either party to influence
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the credentials presented to its peer except by calling the appropriate
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system call (e.g.,
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.Xr connect 2
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or
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.Xr listen 2 )
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under different effective credentials.
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.Pp
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To reliably obtain peer credentials on a
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.Dv SOCK_DGRAM
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socket refer to the
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.Dv LOCAL_CREDS
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socket option.
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.El
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.Sh SEE ALSO
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.Xr connect 2 ,
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.Xr dup 2 ,
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.Xr fcntl 2 ,
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.Xr getsockopt 2 ,
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.Xr listen 2 ,
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.Xr recvmsg 2 ,
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.Xr sendto 2 ,
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.Xr setsockopt 2 ,
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.Xr socket 2 ,
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.Xr intro 4
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.Rs
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.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial"
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.%B PS1
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.%N 7
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.Re
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.Rs
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.%T "An Advanced 4.3 BSD Interprocess Communication Tutorial"
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.%B PS1
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.%N 8
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.Re
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