ddp - Linux AppleTalk protocol implementation
#include <sys/socket.h>
#include <netatalk/at.h>
ddp_socket = socket(AF_APPLETALK, SOCK_DGRAM, 0);
raw_socket = socket(AF_APPLETALK, SOCK_RAW,
protocol);
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(7).
An AppleTalk socket is created by calling the
socket(2) function with a
AF_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 opened only by a process with effective user ID 0 or when the
process has the
CAP_NET_RAW capability.
An AppleTalk socket address is defined as a combination of a network number, a
node number, and a port number.
struct at_addr {
unsigned short s_net;
unsigned char s_node;
};
struct sockaddr_atalk {
sa_family_t sat_family; /* address family */
unsigned char sat_port; /* port */
struct at_addr sat_addr; /* net/node */
};
sat_family is always set to
AF_APPLETALK.
sat_port contains
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
capability may
bind(2) to these sockets.
sat_addr is the host
address. The
net member of
struct at_addr contains the host
network in network byte order. The value of
AT_ANYNET is a wildcard 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.
No protocol-specific socket options are supported.
IP supports a set of
/proc interfaces to configure some global AppleTalk
parameters. The parameters can be accessed by reading or writing files in the
directory
/proc/sys/net/atalk/.
- 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.
All ioctls described in
socket(7) apply to DDP.
- 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_SERVICE.
- EADDRINUSE
- Tried to bind to an address already in use.
- EADDRNOTAVAIL
- A nonexistent interface was requested or the requested source address was
not local.
- EAGAIN
- Operation on a nonblocking socket would block.
- EALREADY
- A connection operation on a nonblocking socket is already in
progress.
- ECONNABORTED
- A connection was closed during an accept(2).
- EHOSTUNREACH
- No routing table entry matches the destination address.
- EINVAL
- Invalid argument passed.
- EISCONN
- connect(2) was called on an already connected socket.
- EMSGSIZE
- Datagram is bigger than the DDP MTU.
- ENODEV
- Network device not available or not capable of sending IP.
- ENOENT
- SIOCGSTAMP was called on a socket where no packet arrived.
- ENOMEM and ENOBUFS
- Not enough memory available.
- ENOPKG
- A kernel subsystem was not configured.
- ENOPROTOOPT and EOPNOTSUPP
- Invalid socket option passed.
- ENOTCONN
- The operation is defined only on a connected socket, but the socket wasn't
connected.
- EPERM
- User doesn't have permission to set high priority, make a configuration
change, or send signals to the requested process or group.
- EPIPE
- The connection was unexpectedly closed or shut down by the other end.
- ESOCKTNOSUPPORT
- The socket was unconfigured, or an unknown socket type was requested.
AppleTalk is supported by Linux 2.0 or higher. The
/proc interfaces exist
since Linux 2.2.
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.
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 support the alternative CAP
package and AppleTalk monitoring tools more easily.
There are too many inconsistent error values.
The ioctls used to configure routing tables, devices, AARP tables, and other
devices are not yet described.
recvmsg(2),
sendmsg(2),
capabilities(7),
socket(7)