ddp - Linux AppleTalk protocol implementation
SYNOPSIS
#include <sys/socket.h>
#include <netatalk/at.h>
ddp_socket = socket(PF_APPLETALK, SOCK_DGRAM, 0);
raw_socket = socket(PF_APPLETALK, SOCK_RAW, protocol);
DESCRIPTION
Linux implements the Appletalk protocols described in
Inside Appletalk. Only the DDP layer and AARP are present
in the kernel. They are designed to be used via the
netatalk protocol libraries. This page documents the
interface for those who wish or need to use the DDP layer
directly.
The communication between Appletalk and the user program
works using a BSD-compatible socket interface. For more
information on sockets, see socket(4).
An AppleTalk socket is created by calling the socket(2)
function with a PF_APPLETALK socket family argument. Valid
socket types are SOCK_DGRAM to open a ddp socket or
SOCK_RAW to open a raw socket. protocol is the Appletalk
protocol to be received or sent. For SOCK_RAW you must
specify ATPROTO_DDP.
Raw sockets may be only opened by a process with effective
user id 0 or when the process has the CAP_NET_RAW capabil
ity.
ADDRESS FORMAT
An Appletalk socket address is defined as a combination of
a network number, a node number, and a port number.
struct at_addr {
u_short s_net;
u_char s_node;
};
struct sockaddr_atalk {
sa_family_t sat_family; /* address family */
u_char sat_port; /* port */
struct at_addr sat_addr; /* net/node */
};
sat_family is always set to AF_APPLETALK. sat_port con
tains the port. The port numbers below 129 are known as
reserved ports. Only processes with the effective user id
0 or the CAP_NET_BIND_SERVICE attribute set may bind(2) to
these sockets. sat_addr is the host address. The net
and also implies "this network." The node member of
struct at_addr contains the host node number. The value
of AT_ANYNODE is a wildcard and also implies "this node."
The value of ATADDR_BCAST is a link local broadcast
address.
SOCKET OPTIONS
No protocol-specific socket options are supported.
SYSCTLS
IP supports a sysctl interface to configure some global
AppleTalk parameters. The sysctls can be accessed by
reading or writing the /proc/sys/net/atalk/* files or with
the sysctl(2) interface.
aarp-expiry-time
The time interval (in seconds) before an AARP cache
entry expires.
aarp-resolve-time
The time interval (in seconds) before an AARP cache
entry is resolved.
aarp-retransmit-limit
The number of retransmissions of an AARP query
before the node is declared dead.
aarp-tick-time
The timer rate (in seconds) for the timer driving
AARP.
The default values match the specification and should
never need to be changed.
IOCTLS
These ioctls can be accessed using ioctl(2). The correct
syntax is:
error = ioctl(atalk_socket, ioctl_type, value_ptr);
SIOCGSTAMP
Return a struct timeval with the receive timestamp
of the last packet passed to the user. This is use
ful for accurate round trip time measurements. See
setitimer(2) for a description of struct timeval.
FIOCSETOWN and SIOCSPGRP
Set the process or process group (negative value
passed with a process group id of the absolute
value) to send SIGIO signal to when an asynchronous
IO operation has finished. Argument is a pid_t.
Only processes of effective user id 0 may set this
value to arbitrary pids; all others are only for
FIOASYNC
Set a flag to enable or disable asynchronous mode
of the socket. Asynchronous mode means that SIGIO
is raised when a new I/O event occurs.
Valid I/O events are: new data arrives; the socket
send buffer has enough room to queue new data; a
new connection arrives (for connection-oriented
protocols); or the connection is broken. SIGIO is
not sent when the connection is broken from the
local end using shutdown(2) or close(2). In some
situations (multiple processes or the kernel send
ing data to a single socket) the condition that
caused the SIGIO might already have disappeared
when the SIGIO is processed by the user process.
When this happens the user process should just wait
again because Linux guarantees to resend a new
SIGIO later.
FIOCGETOWN and SIOCGPGRP
Get the current process or process group that
receive SIGIO or SIGURG signals, or 0 when none is
set. Argument is a pid_t.
NOTES
Be very careful with the SO_BROADCAST option - it is not
privileged in Linux. It is easy to overload the network
with careless sending to broadcast addresses.
VERSIONS
Appletalk is supported by Linux 2.0 or higher. The sysctl
interface is new in Linux 2.2.
ERRORS
ENOTCONN
The operation is only defined on a connected
socket, but the socket wasn't connected.
EINVAL Invalid argument passed.
EMSGSIZE
Datagram is bigger than the DDP MTU.
EACCES The user tried to execute an operation without the
necessary permissions. These include sending to a
broadcast address without having the broadcast
flag set, and trying to bind to a reserved port
without effective user id 0 or CAP_NET_BIND_SER
VICE.
EADDRINUSE
Not enough memory available.
ENOPROTOOPT and EOPNOTSUPP
Invalid socket option passed.
EPERM User doesn't have permission to set high priority,
make a configuration change, or send signals to
the requested process or group,
EADDRNOTAVAIL
A non-existent interface was requested or the
requested source address was not local.
EAGAIN Operation on a nonblocking socket would block.
ESOCKTNOSUPPORT
The socket was unconfigured, or an unknown socket
type was requested.
EISCONN connect(2) was called on an already connected
socket.
EALREADY
A connection operation on a non-blocking socket is
already in progress.
ECONNABORTED
A connection was closed during an accept(2).
EPIPE The connection was unexpectedly closed or shut
down by the other end.
ENOENT SIOCGSTAMP was called on a socket where no packet
arrived.
EHOSTUNREACH
No routing table entry matches the destination
address.
ENODEV Network device not available or not capable of
sending IP.
ENOPKG A kernel subsystem was not configured.
COMPATIBILITY
The basic AppleTalk socket interface is compatible with
netatalk on BSD-derived systems. Many BSD systems fail to
check SO_BROADCAST when sending broadcast frames; this can
lead to compatibility problems.
The raw socket mode is unique to Linux and exists to sup
port the alternative CAP package and AppleTalk monitoring
There are too many inconsistent error values.
The ioctls used to configure routing tables, devices, AARP
tables and other devices are not yet described.
SEE ALSO
sendmsg(2), recvmsg(2), socket(4)
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