unix(4) manual page

The Unix Ns -domain protocol family is a collection of protocols that provides local (on-machine) interprocess communication through the normal socket(2) mechanisms. The Unix Ns -domain family supports the SOCK_STREAM, SOCK_SEQPACKET, and SOCK_DGRAM socket types and uses file system pathnames for addressing.

Unix Ns -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 Ns -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 Ns -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 Ns -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 Ns -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 Ns -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 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_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` 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.

Due to the local nature of the Unix Ns -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 Ns -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 Ns -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.

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)

PS1, 7, An Introductory 4.3 BSD Interprocess Communication Tutorial,

PS1, 8, An Advanced 4.3 BSD Interprocess Communication Tutorial,

Manual Pages — UNIX

NAME

CONTENTS

SYNOPSIS

DESCRIPTION

ADDRESSING

CONTROL MESSAGES

SOCKET OPTIONS

BUFFERING

SEE ALSO