unix - Linux Command - Unix Command

Linux

Filed In:

Linux

Linux / Unix Command: unix

Command Library

NAME

unix, PF_UNIX, AF_UNIX, PF_LOCAL, AF_LOCAL - Sockets for local interprocess communication.

SYNOPSIS

#include <sys/socket.h>
#include <sys/un.h>

unix_socket = socket(PF_UNIX, type, 0);
error = socketpair(PF_UNIX, type, 0, int *sv);

DESCRIPTION

The PF_UNIX (also known as PF_LOCAL) socket family is used to communicate between processes on the same machine efficiently. Unix sockets can be either anonymous (created by socketpair(2)) or associated with a file of socket type. 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. Unix sockets are always reliable and don't reorder datagrams.

Unix sockets support passing file descriptors or process credentials to other processes as ancillary data to datagrams.

ADDRESS FORMAT

A unix address is defined as a filename in the filesystem or as a unique string in the abstract namespace. Sockets created by socketpair(2) are anonymous. For non-anonymous sockets the target address can be set using connect(2). The local address can be set using bind(2). When a socket is connected and it doesn't already have a local address a unique address in the abstract namespace will be generated automatically.

#define UNIX_PATH_MAX   108


struct sockaddr_un {
    sa_family_t  sun_family;              /* AF_UNIX */
    char         sun_path[UNIX_PATH_MAX]; /* pathname */
};

sun_family always contains AF_UNIX. sun_path contains the zero-terminated pathname of the socket in the file system. If sun_path starts with a zero byte it refers to the abstract namespace maintained by the Unix protocol module. The socket's address in this namespace is given by the rest of the bytes in sun_path. Note that names in the abstract namespace are not zero-terminated.

SOCKET OPTIONS

For historical reasons these socket options are specified with a SOL_SOCKET type even though they are PF_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 connected yet an unique name in the abstract namespace will be generated automatically. Expects an integer boolean flag.

ANCILLARY MESSAGES

For historical reasons these ancillary message type are specified with a SOL_SOCKET type even though they are PF_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 a integer array of the file descriptors. The passed file descriptors behave as like 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.


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 */ 
};

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 his own. The sender must specify its own process id (unless it has CAP_SYS_ADMIN), its user id, effective user id or set user id (unless it has CAP_SETUID), and its group id, effective group id or set group id (unless it has CAP_SETGID). To receive a struct ucred message the SO_PASSCRED option must be enabled on the socket.