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X25(7)                                                                  Linux Programmer's Manual                                                                 X25(7)

NAME
       x25 - ITU-T X.25 / ISO-8208 protocol interface

SYNOPSIS
       #include <sys/socket.h>
       #include <linux/x25.h>

       x25_socket = socket(AF_X25, SOCK_SEQPACKET, 0);

DESCRIPTION
       X25  sockets  provide an interface to the X.25 packet layer protocol.  This allows applications to communicate over a public X.25 data network as standardized by
       International Telecommunication Union's recommendation X.25 (X.25 DTE-DCE mode).  X25 sockets can also be used for communication  without  an  intermediate  X.25
       network (X.25 DTE-DTE mode) as described in ISO-8208.

       Message  boundaries  are  preserved  — a read(2) from a socket will retrieve the same chunk of data as output with the corresponding write(2) to the peer socket.
       When necessary, the kernel takes care of segmenting and reassembling long messages by means of the X.25 M-bit.  There is no hard-coded upper limit for  the  mes‐
       sage  size.  However, reassembling of a long message might fail if there is a temporary lack of system resources or when other constraints (such as socket memory
       or buffer size limits) become effective.  If that occurs, the X.25 connection will be reset.

   Socket addresses
       The AF_X25 socket address family uses the struct sockaddr_x25 for representing network addresses as defined in ITU-T recommendation X.121.

           struct sockaddr_x25 {
               sa_family_t sx25_family;    /* must be AF_X25 */
               x25_address sx25_addr;      /* X.121 Address */
           };

       sx25_addr contains a char array x25_addr[] to be interpreted as a null-terminated string.  sx25_addr.x25_addr[] consists of up to 15 (not counting the  terminat‐
       ing null byte) ASCII characters forming the X.121 address.  Only the decimal digit characters from '0' to '9' are allowed.

   Socket options
       The following X.25-specific socket options can be set by using setsockopt(2) and read with getsockopt(2) with the level argument set to SOL_X25.

       X25_QBITINCL
              Controls  whether  the  X.25  Q-bit  (Qualified Data Bit) is accessible by the user.  It expects an integer argument.  If set to 0 (default), the Q-bit is
              never set for outgoing packets and the Q-bit of incoming packets is ignored.  If set to 1, an additional first byte is prepended to each message read from
              or  written  to  the  socket.  For data read from the socket, a 0 first byte indicates that the Q-bits of the corresponding incoming data packets were not
              set.  A first byte with value 1 indicates that the Q-bit of the corresponding incoming data packets was set.  If the first byte of the data written to the
              socket is 1, the Q-bit of the corresponding outgoing data packets will be set.  If the first byte is 0, the Q-bit will not be set.

VERSIONS
       The AF_X25 protocol family is a new feature of Linux 2.2.

BUGS
       Plenty, as the X.25 PLP implementation is CONFIG_EXPERIMENTAL.

       This man page is incomplete.

       There  is  no dedicated application programmer's header file yet; you need to include the kernel header file <linux/x25.h>.  CONFIG_EXPERIMENTAL might also imply
       that future versions of the interface are not binary compatible.

       X.25 N-Reset events are not propagated to the user process yet.  Thus, if a reset occurred, data might be lost without notice.

SEE ALSO
       socket(2), socket(7)

       Jonathan Simon Naylor: “The Re-Analysis and Re-Implementation of X.25.”  The URL is ⟨ftp://ftp.pspt.fi/pub/ham/linux/ax25/x25doc.tgz⟩.

Linux                                                                          2021-03-22                                                                         X25(7)