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     ng_socket — netgraph socket node type


     #include <sys/types.h>
     #include <netgraph/ng_socket.h>


     A socket node is both a BSD socket and a netgraph node.  The ng_socket node type allows
     user-mode processes to participate in the kernel netgraph(4) networking subsystem using the
     BSD socket interface.  The process must have root privileges to be able to create netgraph
     sockets however once created, any process that has one may use it.

     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

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


     This node type supports hooks with arbitrary names (as long as they are unique) and always
     accepts hook connection requests.


     This node type supports the generic control messages, plus the following:

          When the last hook is removed from this node, it will shut down as if it had received a
          NGM_SHUTDOWN message.  Attempts to access the sockets associated will return ENOTCONN.

          This is the default mode.  When the last hook is removed, the node will continue to
          exist, ready to accept new hooks until it is explicitly shut down.

     All other messages with neither the NGM_SOCKET_COOKIE or NGM_GENERIC_COOKIE will be passed
     unaltered up the NG_CONTROL socket.


     This node type shuts down and disappears when both the associated NG_CONTROL and NG_DATA
     sockets have been closed, or a NGM_SHUTDOWN control message is received.  In the latter
     case, attempts to write to the still-open sockets will return ENOTCONN.  If the
     NGM_SOCK_CMD_NOLINGER message has been received, closure of the last hook will also initiate
     a shutdown of the node.


     socket(2), netgraph(3), netgraph(4), ng_ksocket(4), ngctl(8)


     The ng_socket node type was implemented in FreeBSD 4.0.


     Julian Elischer <>


     It is not possible to reject the connection of a hook, though any data received on that hook
     can certainly be ignored.

     The controlling process is not notified of all events that an in-kernel node would be
     notified of, e.g. a new hook, or hook removal.  Some node-initiated messages should be
     defined for this purpose (to be sent up the control socket).