Address format¶

A UNIX domain socket address is represented in the following structure:

struct sockaddr_un {


    sa_family_t sun_family;               /* AF_UNIX */


    char        sun_path[108];            /* Pathname */
};

The sun_family field always contains AF_UNIX. On Linux, sun_path is 108 bytes in size; see also BUGS, below.

Various systems calls (for example, bind(2), connect(2), and sendto(2)) take a sockaddr_un argument as input. Some other system calls (for example, getsockname(2), getpeername(2), recvfrom(2), and accept(2)) return an argument of this type.

Three types of address are distinguished in the sockaddr_un structure:

pathname

a UNIX domain socket can be bound to a null-terminated filesystem pathname using bind(2). When the address of a pathname socket is returned (by one of the system calls noted above), its length is

offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1
    

and sun_path contains the null-terminated pathname. (On Linux, the above offsetof() expression equates to the same value as sizeof(sa_family_t), but some other implementations include other fields before sun_path, so the offsetof() expression more portably describes the size of the address structure.)

For further details of pathname sockets, see below.

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, its length is sizeof(sa_family_t), and sun_path should not be inspected.

abstract

an abstract socket address is distinguished (from a pathname socket) by the fact that sun_path[0] is a null byte ('\0'). The socket's address in this namespace is given by the additional bytes in sun_path that are covered by the specified length of the address structure. (Null bytes in the name have no special significance.) The name has no connection with filesystem pathnames. When the address of an abstract socket is returned, the returned addrlen is greater than sizeof(sa_family_t) (i.e., greater than 2), and the name of the socket is contained in the first (addrlen - sizeof(sa_family_t)) bytes of sun_path.

Pathname sockets¶

When binding a socket to a pathname, a few rules should be observed for maximum portability and ease of coding:

•

The pathname in sun_path should be null-terminated.

•

The length of the pathname, including the terminating null byte, should not exceed the size of sun_path.

•

The addrlen argument that describes the enclosing sockaddr_un structure should have a value of at least:

offsetof(struct sockaddr_un, sun_path)+strlen(addr.sun_path)+1
    

or, more simply, addrlen can be specified as sizeof(struct sockaddr_un).

There is some variation in how implementations handle UNIX domain socket addresses that do not follow the above rules. For example, some (but not all) implementations append a null terminator if none is present in the supplied sun_path.

When coding portable applications, keep in mind that some implementations have sun_path as short as 92 bytes.

Various system calls (accept(2), recvfrom(2), getsockname(2), getpeername(2)) return socket address structures. When applied to UNIX domain sockets, the value-result addrlen argument supplied to the call should be initialized as above. Upon return, the argument is set to indicate the actual size of the address structure. The caller should check the value returned in this argument: if the output value exceeds the input value, then there is no guarantee that a null terminator is present in sun_path. (See BUGS.)

Pathname socket ownership and permissions¶

In the Linux implementation, pathname sockets honor the permissions of the directory they are in. Creation of a new socket fails if the process does not have write and search (execute) permission on the directory in which the socket is created.

On Linux, connecting to a stream socket object requires write permission on that socket; sending a datagram to a datagram socket likewise requires write permission on that socket. POSIX does not make any statement about the effect of the permissions on a socket file, and on some systems (e.g., older BSDs), the socket permissions are ignored. Portable programs should not rely on this feature for security.

When creating a new socket, the owner and group of the socket file are set according to the usual rules. The socket file has all permissions enabled, other than those that are turned off by the process umask(2).

The owner, group, and permissions of a pathname socket can be changed (using chown(2) and chmod(2)).

Abstract sockets¶

Socket permissions have no meaning for abstract sockets: the process umask(2) has no effect when binding an abstract socket, and changing the ownership and permissions of the object (via fchown(2) and fchmod(2)) has no effect on the accessibility of the socket.

Abstract sockets automatically disappear when all open references to the socket are closed.

The abstract socket namespace is a nonportable 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

Enabling this socket option causes receipt of the credentials of the sending process in an SCM_CREDENTIALS ancillary message in each subsequently received message. The returned credentials are those specified by the sender using SCM_CREDENTIALS, or a default that includes the sender's PID, real user ID, and real group ID, if the sender did not specify SCM_CREDENTIALS ancillary data.

When this option is set and the socket is not yet connected, a unique name in the abstract namespace will be generated automatically.

The value given as an argument to setsockopt(2) and returned as the result of getsockopt(2) is an integer boolean flag.

SO_PASSSEC

Enables receiving of the SELinux security label of the peer socket in an ancillary message of type SCM_SECURITY (see below).

The value given as an argument to setsockopt(2) and returned as the result of getsockopt(2) is an integer boolean flag.

The SO_PASSSEC option is supported for UNIX domain datagram sockets since Linux 2.6.18; support for UNIX domain stream sockets was added in Linux 4.2.

SO_PEEK_OFF

See socket(7).

SO_PEERCRED

This read-only socket option returns the credentials of the peer process connected to this socket. The returned credentials are those that were in effect at the time of the call to connect(2) or socketpair(2).

The argument to getsockopt(2) is a pointer to a ucred structure; define the _GNU_SOURCE feature test macro to obtain the definition of that structure from <sys/socket.h>.

The use of this option is possible only for connected AF_UNIX stream sockets and for AF_UNIX stream and datagram socket pairs created using socketpair(2).

SO_PEERSEC

This read-only socket option returns the security context of the peer socket connected to this socket. By default, this will be the same as the security context of the process that created the peer socket unless overridden by the policy or by a process with the required permissions.

The argument to getsockopt(2) is a pointer to a buffer of the specified length in bytes into which the security context string will be copied. If the buffer length is less than the length of the security context string, then getsockopt(2) returns -1, sets errno to ERANGE, and returns the required length via optlen. The caller should allocate at least NAME_MAX bytes for the buffer initially, although this is not guaranteed to be sufficient. Resizing the buffer to the returned length and retrying may be necessary.

The security context string may include a terminating null character in the returned length, but is not guaranteed to do so: a security context "foo" might be represented as either {'f','o','o'} of length 3 or {'f','o','o','\0'} of length 4, which are considered to be interchangeable. The string is printable, does not contain non-terminating null characters, and is in an unspecified encoding (in particular, it is not guaranteed to be ASCII or UTF-8).

The use of this option for sockets in the AF_UNIX address family is supported since Linux 2.6.2 for connected stream sockets, and since Linux 4.18 also for stream and datagram socket pairs created using socketpair(2).

Autobind feature¶

If a bind(2) call specifies addrlen as sizeof(sa_family_t), or the SO_PASSCRED socket option was specified for a socket that was not explicitly bound to an address, then the socket is autobound to an abstract address. The address consists of a null byte followed by 5 bytes in the character set [0-9a-f]. Thus, there is a limit of 2^20 autobind addresses. (From Linux 2.1.15, when the autobind feature was added, 8 bytes were used, and the limit was thus 2^32 autobind addresses. The change to 5 bytes came in Linux 2.3.15.)

Sockets API¶

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.

Before Linux 3.4, the use of MSG_TRUNC in the flags argument of recv(2) was 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.

Commonly, this operation is referred to as "passing a file descriptor" to another process. However, more accurately, what is being passed is a reference to an open file description (see open(2)), and in the receiving process it is likely that a different file descriptor number will be used. Semantically, this operation is equivalent to duplicating (dup(2)) a file descriptor into the file descriptor table of another process.

If the buffer used to receive the ancillary data containing file descriptors is too small (or is absent), then the ancillary data is truncated (or discarded) and the excess file descriptors are automatically closed in the receiving process.

If the number of file descriptors received in the ancillary data would cause the process to exceed its RLIMIT_NOFILE resource limit (see getrlimit(2)), the excess file descriptors are automatically closed in the receiving process.

The kernel constant SCM_MAX_FD defines a limit on the number of file descriptors in the array. Attempting to send an array larger than this limit causes sendmsg(2) to fail with the error EINVAL. SCM_MAX_FD has the value 253 (or 255 before Linux 2.6.38).

SCM_CREDENTIALS

Send or receive UNIX credentials. This can be used for authentication. The credentials are passed as a struct ucred ancillary message. This 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 (before including any header files) in order to obtain the definition of this structure.

The credentials which the sender specifies are checked by the kernel. A privileged process 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, in which case the PID of any existing process may be specified), its real user ID, effective user ID, or saved set-user-ID (unless it has CAP_SETUID), and its real 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.

SCM_SECURITY

Receive the SELinux security context (the security label) of the peer socket. The received ancillary data is a null-terminated string containing the security context. The receiver should allocate at least NAME_MAX bytes in the data portion of the ancillary message for this data.

To receive the security context, the SO_PASSSEC option must be enabled on the socket (see above).

When sending ancillary data with sendmsg(2), only one item of each of the above types may be included in the sent message.

At least one byte of real data should be sent when sending ancillary data. On Linux, this is required to successfully send ancillary data over a UNIX domain stream socket. When sending ancillary data over a UNIX domain datagram socket, it is not necessary on Linux to send any accompanying real data. However, portable applications should also include at least one byte of real data when sending ancillary data over a datagram socket.

When receiving from a stream socket, ancillary data forms a kind of barrier for the received data. For example, suppose that the sender transmits as follows:

Suppose that the receiver now performs recvmsg(2) calls each with a buffer size of 20 bytes. The first call will receive five bytes of data, along with the ancillary data sent by the second sendmsg(2) call. The next call will receive the remaining four bytes of data.

If the space allocated for receiving incoming ancillary data is too small then the ancillary data is truncated to the number of headers that will fit in the supplied buffer (or, in the case of an SCM_RIGHTS file descriptor list, the list of file descriptors may be truncated). If no buffer is provided for incoming ancillary data (i.e., the msg_control field of the msghdr structure supplied to recvmsg(2) is NULL), then the incoming ancillary data is discarded. In both of these cases, the MSG_CTRUNC flag will be set in the msg.msg_flags value returned by recvmsg(2).

Ioctls¶

The following ioctl(2) calls return information in value. The correct syntax is:

int value;
error = ioctl(unix_socket, ioctl_type, &value);

ioctl_type can be:

SIOCINQ

For SOCK_STREAM sockets, this call returns the number of unread bytes in the receive buffer. The socket must not be in LISTEN state, otherwise an error (EINVAL) is returned. SIOCINQ is defined in <linux/sockios.h>. Alternatively, you can use the synonymous FIONREAD, defined in <sys/ioctl.h>. For SOCK_DGRAM sockets, the returned value is the same as for Internet domain datagram sockets; see udp(7).

Example output¶

$ ./server &
[1] 25887
$ ./client 3 4
Result = 7
$ ./client 11 -5
Result = 6
$ ./client DOWN
Result = 0
[1]+  Done                    ./server
$

Program source¶

/*


 * File connection.h


 */
#define SOCKET_NAME "/tmp/9Lq7BNBnBycd6nxy.socket"
#define BUFFER_SIZE 12
/*


 * File server.c


 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#include "connection.h"
int
main(int argc, char *argv[])
{


    struct sockaddr_un name;


    int down_flag = 0;


    int ret;


    int connection_socket;


    int data_socket;


    int result;


    char buffer[BUFFER_SIZE];


    /* Create local socket. */


    connection_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0);


    if (connection_socket == -1) {


        perror("socket");


        exit(EXIT_FAILURE);


    }


    /*


     * For portability clear the whole structure, since some


     * implementations have additional (nonstandard) fields in


     * the structure.


     */


    memset(&name, 0, sizeof(name));


    /* Bind socket to socket name. */


    name.sun_family = AF_UNIX;


    strncpy(name.sun_path, SOCKET_NAME, sizeof(name.sun_path) - 1);


    ret = bind(connection_socket, (const struct sockaddr *) &name,


               sizeof(name));


    if (ret == -1) {


        perror("bind");


        exit(EXIT_FAILURE);


    }


    /*


     * Prepare for accepting connections. The backlog size is set


     * to 20. So while one request is being processed other requests


     * can be waiting.


     */


    ret = listen(connection_socket, 20);


    if (ret == -1) {


        perror("listen");


        exit(EXIT_FAILURE);


    }


    /* This is the main loop for handling connections. */


    for (;;) {


        /* Wait for incoming connection. */


        data_socket = accept(connection_socket, NULL, NULL);


        if (data_socket == -1) {


            perror("accept");


            exit(EXIT_FAILURE);


        }


        result = 0;


        for (;;) {


            /* Wait for next data packet. */


            ret = read(data_socket, buffer, sizeof(buffer));


            if (ret == -1) {


                perror("read");


                exit(EXIT_FAILURE);


            }


            /* Ensure buffer is 0-terminated. */


            buffer[sizeof(buffer) - 1] = 0;


            /* Handle commands. */


            if (!strncmp(buffer, "DOWN", sizeof(buffer))) {


                down_flag = 1;


                break;


            }


            if (!strncmp(buffer, "END", sizeof(buffer))) {


                break;


            }


            /* Add received summand. */


            result += atoi(buffer);


        }


        /* Send result. */


        sprintf(buffer, "%d", result);


        ret = write(data_socket, buffer, sizeof(buffer));


        if (ret == -1) {


            perror("write");


            exit(EXIT_FAILURE);


        }


        /* Close socket. */


        close(data_socket);


        /* Quit on DOWN command. */


        if (down_flag) {


            break;


        }


    }


    close(connection_socket);


    /* Unlink the socket. */


    unlink(SOCKET_NAME);


    exit(EXIT_SUCCESS);
}
/*


 * File client.c


 */
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#include "connection.h"
int
main(int argc, char *argv[])
{


    struct sockaddr_un addr;


    int ret;


    int data_socket;


    char buffer[BUFFER_SIZE];


    /* Create local socket. */


    data_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0);


    if (data_socket == -1) {


        perror("socket");


        exit(EXIT_FAILURE);


    }


    /*


     * For portability clear the whole structure, since some


     * implementations have additional (nonstandard) fields in


     * the structure.


     */


    memset(&addr, 0, sizeof(addr));


    /* Connect socket to socket address. */


    addr.sun_family = AF_UNIX;


    strncpy(addr.sun_path, SOCKET_NAME, sizeof(addr.sun_path) - 1);


    ret = connect(data_socket, (const struct sockaddr *) &addr,


                   sizeof(addr));


    if (ret == -1) {


        fprintf(stderr, "The server is down.\n");


        exit(EXIT_FAILURE);


    }


    /* Send arguments. */


    for (size_t i = 1; i < argc; ++i) {


        ret = write(data_socket, argv[i], strlen(argv[i]) + 1);


        if (ret == -1) {


            perror("write");


            break;


        }


    }


    /* Request result. */


    strcpy(buffer, "END");


    ret = write(data_socket, buffer, strlen(buffer) + 1);


    if (ret == -1) {


        perror("write");


        exit(EXIT_FAILURE);


    }


    /* Receive result. */


    ret = read(data_socket, buffer, sizeof(buffer));


    if (ret == -1) {


        perror("read");


        exit(EXIT_FAILURE);


    }


    /* Ensure buffer is 0-terminated. */


    buffer[sizeof(buffer) - 1] = 0;


    printf("Result = %s\n", buffer);


    /* Close socket. */


    close(data_socket);


    exit(EXIT_SUCCESS);
}

For examples of the use of SCM_RIGHTS, see cmsg(3) and seccomp_unotify(2).