--- name: "ddp" section: "7" platform: "linux" package: "man-pages" version: "6.19-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/ddp" --- # ddp(7) Linux AppleTalk protocol implementation ## NAME ddp - Linux AppleTalk protocol implementation ## SYNOPSIS ``` #include #include ``` *ddp\_socket* **= socket(AF\_APPLETALK, SOCK\_DGRAM, 0);** *raw\_socket* **= socket(AF\_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**(7)](/socket/7.md). An AppleTalk socket is created by calling the [**socket**(2)](/socket/2.md) 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. ### 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 { 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)](/bind/2.md) 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. ### Socket options No protocol-specific socket options are supported. ### /proc interfaces 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. ### Ioctls All ioctls described in [**socket**(7)](/socket/7.md) apply to DDP. ## ERRORS - **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)](/accept/2.md). - **EHOSTUNREACH** No routing table entry matches the destination address. - **EINVAL** Invalid argument passed. - **EISCONN** [**connect**(2)](/connect/2.md) 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** **ENOBUFS** Not enough memory available. - **ENOPKG** A kernel subsystem was not configured. - **ENOPROTOOPT** **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. ## VERSIONS AppleTalk is supported by Linux 2.0 or higher. The */proc* interfaces exist since Linux 2.2. ## 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. ### 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 support the alternative CAP package and AppleTalk monitoring tools more easily. ## BUGS 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 [**recvmsg**(2)](/recvmsg/2.md), [**sendmsg**(2)](/sendmsg/2.md), [**capabilities**(7)](/capabilities/7.md), [**socket**(7)](/socket/7.md)