Manual Pages  — NG_SOCKET

A new ng_socket node is created by creating a new socket of type NG_CONTROL in the protocol family PF_NETGRAPH, using the socket(2) system call. Any control messages received by the node and not having a cookie value of NGM_SOCKET_COOKIE are received by the process, using recvfrom(2); the socket address argument is a struct sockaddr_ng containing the sender's netgraph address. Conversely, control messages can be sent to any node by calling sendto(2), supplying the recipient's address in a struct sockaddr_ng. The bind(2) system call may be used to assign a global netgraph name to the node.

To transmit and receive netgraph data packets, a NG_DATA socket must also be created using socket(2) and associated with a ng_socket node. NG_DATA sockets do not automatically have nodes associated with them; they are bound to a specific node via the connect(2) system call. The address argument is the netgraph address of the ng_socket node already created. Once a data socket is associated with a node, any data packets received by the node are read using recvfrom(2) and any packets to be sent out from the node are written using sendto(2). In the case of data sockets, the struct sockaddr_ng contains the name of the hook on which the data was received or should be sent.

As a special case, to allow netgraph data sockets to be used as stdin or stdout on naive programs, a sendto(2) with a NULL sockaddr pointer, a send(2) or a write(2) will succeed in the case where there is exactly ONE hook attached to the socket node, (and thus the path is unambiguous).

There is a user library that simplifies using netgraph sockets; see netgraph(3).