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NETGRAPH(4)		 BSD Kernel Interfaces Manual		   NETGRAPH(4)

     netgraph -- graph based kernel networking subsystem

     The netgraph system provides a uniform and	modular	system for the imple-
     mentation of kernel objects which perform various networking functions.
     The objects, known	as nodes, can be arranged into arbitrarily complicated
     graphs. Nodes have	hooks which are	used to	connect	two nodes together,
     forming the edges in the graph.  Nodes communicate	along the edges	to
     process data, implement protocols,	etc.

     The aim of	netgraph is to supplement rather than replace the existing
     kernel networking infrastructure.	It provides:

       +o   A flexible way of combining protocol	and link level drivers
       +o   A modular way to implement new protocols
       +o   A common framework for kernel entities to inter-communicate
       +o   A reasonably	fast, kernel-based implementation

Nodes and Types
     The most fundamental concept in netgraph is that of a node.  All nodes
     implement a number	of predefined methods which allow them to interact
     with other	nodes in a well	defined	manner.

     Each node has a type, which is a static property of the node determined
     at	node creation time.  A node's type is described	by a unique ASCII type
     name.  The	type implies what the node does	and how	it may be connected to
     other nodes.

     In	object-oriented	language, types	are classes and	nodes are instances of
     their respective class. All node types are	subclasses of the generic node
     type, and hence inherit certain common functionality and capabilities
     (e.g., the	ability	to have	an ASCII name).

     Nodes may be assigned a globally unique ASCII name	which can be used to
     refer to the node.	 The name must not contain the characters "." or ":"
     and is limited to NG_NODELEN + 1 characters (including NUL	byte).

     Each node instance	has a unique ID	number which is	expressed as a 32-bit
     hex value.	This value may be used to refer	to a node when there is	no
     ASCII name	assigned to it.

     Nodes are connected to other nodes	by connecting a	pair of	hooks, one
     from each node. Data flows	bidirectionally	between	nodes along connected
     pairs of hooks.  A	node may have as many hooks as it needs, and may as-
     sign whatever meaning it wants to a hook.

     Hooks have	these properties:

       +o   A hook has an ASCII name which is unique among all hooks on that
	   node	(other hooks on	other nodes may	have the same name).  The name
	   must	not contain a "." or a ":" and is limited to NG_HOOKLEN	+ 1
	   characters (including NUL byte).
       +o   A hook is always connected to another hook. That is,	hooks are cre-
	   ated	at the time they are connected,	and breaking an	edge by	remov-
	   ing either hook destroys both hooks.

     A node may	decide to assign special meaning to some hooks.	 For example,
     connecting	to the hook named "debug" might	trigger	the node to start
     sending debugging information to that hook.

Data Flow
     Two types of information flow between nodes: data messages	and control
     messages. Data messages are passed	in mbuf	chains along the edges in the
     graph, one	edge at	a time.	The first mbuf in a chain must have the
     M_PKTHDR flag set.	Each node decides how to handle	data coming in on its

     Control messages are type-specific	C structures sent from one node	di-
     rectly to some arbitrary other node.  Control messages have a common
     header format, followed by	type-specific data, and	are binary structures
     for efficiency.  However, node types also may support conversion of the
     type specific data	between	binary and ASCII for debugging and human in-
     terface purposes (see the NGM_ASCII2BINARY	and NGM_BINARY2ASCII generic
     control messages below).  Nodes are not required to support these conver-

     There are two ways	to address a control message. If there is a sequence
     of	edges connecting the two nodes,	the message may	be "source routed" by
     specifying	the corresponding sequence of hooks as the destination address
     for the message (relative addressing).  Otherwise,	the recipient node
     global ASCII name (or equivalent ID based name) is	used as	the destina-
     tion address for the message (absolute addressing).  The two types	of ad-
     dressing may be combined, by specifying an	absolute start node and	a se-
     quence of hooks.

     Messages often represent commands that are	followed by a reply message in
     the reverse direction. To facilitate this,	the recipient of a control
     message is	supplied with a	"return	address" that is suitable for address-
     ing a reply.

     Each control message contains a 32	bit value called a typecookie indicat-
     ing the type of the message, i.e.,	how to interpret it.  Typically	each
     type defines a unique typecookie for the messages that it understands.
     However, a	node may choose	to recognize and implement more	than one type
     of	message.

Netgraph is Functional
     In	order to minimize latency, most	netgraph operations are	functional.
     That is, data and control messages	are delivered by making	function calls
     rather than by using queues and mailboxes.	 For example, if node A	wishes
     to	send a data mbuf to neighboring	node B,	it calls the generic netgraph
     data delivery function. This function in turn locates node	B and calls
     B's "receive data"	method.	While this mode	of operation results in	good
     performance, it has a few implications for	node developers:

       +o   Whenever a node delivers a data or control message, the node	may
	   need	to allow for the possibility of	receiving a returning message
	   before the original delivery	function call returns.
       +o   Netgraph nodes and support routines generally run at	splnet().
	   However, some nodes may want	to send	data and control messages from
	   a different priority	level. Netgraph	supplies queueing routines
	   which utilize the NETISR system to move message delivery to
	   splnet().  Note that	messages are always received at	splnet().
       +o   It's	possible for an	infinite loop to occur if the graph contains

     So	far, these issues have not proven problematical	in practice.

Interaction With Other Parts of	the Kernel
     A node may	have a hidden interaction with other components	of the kernel
     outside of	the netgraph subsystem,	such as	device hardware, kernel	proto-
     col stacks, etc.  In fact,	one of the benefits of netgraph	is the ability
     to	join disparate kernel networking entities together in a	consistent
     communication framework.

     An	example	is the node type socket	which is both a	netgraph node and a
     socket(2) BSD socket in the protocol family PF_NETGRAPH.  Socket nodes
     allow user	processes to participate in netgraph.  Other nodes communicate
     with socket nodes using the usual methods,	and the	node hides the fact
     that it is	also passing information to and	from a cooperating user

     Another example is	a device driver	that presents a	node interface to the

Node Methods
     Nodes are notified	of the following actions via function calls to the
     following node methods (all at splnet()) and may accept or	reject that
     action (by	returning the appropriate error	code):

     Creation of a new node
	  The constructor for the type is called. If creation of a new node is
	  allowed, the constructor must	call the generic node creation func-
	  tion (in object-oriented terms, the superclass constructor) and then
	  allocate any special resources it needs. For nodes that correspond
	  to hardware, this is typically done during the device	attach rou-
	  tine.	Often a	global ASCII name corresponding	to the device name is
	  assigned here	as well.

     Creation of a new hook
	  The hook is created and tentatively linked to	the node, and the node
	  is told about	the name that will be used to describe this hook. The
	  node sets up any special data	structures it needs, or	may reject the
	  connection, based on the name	of the hook.

     Successful	connection of two hooks
	  After	both ends have accepted	their hooks, and the links have	been
	  made,	the nodes get a	chance to find out who their peer is across
	  the link and can then	decide to reject the connection. Tear-down is

     Destruction of a hook
	  The node is notified of a broken connection. The node	may consider
	  some hooks to	be critical to operation and others to be expendable:
	  the disconnection of one hook	may be an acceptable event while for
	  another it may affect	a total	shutdown for the node.

     Shutdown of a node
	  This method allows a node to clean up	and to ensure that any actions
	  that need to be performed at this time are taken. The	method must
	  call the generic (i.e., superclass) node destructor to get rid of
	  the generic components of the	node.  Some nodes (usually associated
	  with a piece of hardware) may	be persistent in that a	shutdown
	  breaks all edges and resets the node,	but doesn't remove it, in
	  which	case the generic destructor is not called.

Sending	and Receiving Data
     Three other methods are also supported by all nodes:

     Receive data message
	  An mbuf chain	is passed to the node.	The node is notified on	which
	  hook the data	arrived, and can use this information in its process-
	  ing decision.	 The node must must always m_freem() the mbuf chain on
	  completion or	error, or pass it on to	another	node (or kernel	mod-
	  ule) which will then be responsible for freeing it.

	  In addition to the mbuf chain	itself there is	also a pointer to a
	  structure describing meta-data about the message (e.g. priority in-
	  formation). This pointer may be NULL if there	is no additional in-
	  formation. The format	for this information is	described in
	  sys/netgraph/netgraph.h.  The	memory for meta-data must allocated
	  via malloc() with type M_NETGRAPH.  As with the data itself, it is
	  the receiver's responsibility	to free() the meta-data. If the	mbuf
	  chain	is freed the meta-data must be freed at	the same time. If the
	  meta-data is freed but the real data on is passed on,	then a NULL
	  pointer must be substituted.

	  The receiving	node may decide	to defer the data by queueing it in
	  the netgraph NETISR system (see below).

	  The structure	and use	of meta-data is	still experimental, but	is
	  presently used in frame-relay	to indicate that management packets
	  should be queued for transmission at a higher	priority than data
	  packets. This	is required for	conformance with Frame Relay stan-

     Receive queued data message
	  Usually this will be the same	function as Receive data message. This
	  is the entry point called when a data	message	is being handed	to the
	  node after having been queued	in the NETISR system.  This allows a
	  node to decide in the	Receive	data message method that a message
	  should be deferred and queued, and be	sure that when it is processed
	  from the queue, it will not be queued	again.

     Receive control message
	  This method is called	when a control message is addressed to the
	  node.	 A return address is always supplied, giving the address of
	  the node that	originated the message so a reply message can be sent
	  anytime later.

	  It is	possible for a synchronous reply to be made, and in fact this
	  is more common in practice.  This is done by setting a pointer (sup-
	  plied	as an extra function parameter)	to point to the	reply.	Then
	  when the control message delivery function returns, the caller can
	  check	if this	pointer	has been made non-NULL,	and if so then it
	  points to the	reply message allocated	via malloc() and containing
	  the synchronous response. In both directions,	(request and response)
	  it is	up to the receiver of that message to free() the control mes-
	  sage buffer. All control messages and	replies	are allocated with
	  malloc() type	M_NETGRAPH.

     Much use has been made of reference counts, so that nodes being free'd of
     all references are	automatically freed, and this behaviour	has been
     tested and	debugged to present a consistent and trustworthy framework for
     the "type module" writer to use.

     The netgraph framework provides an	unambiguous and	simple to use method
     of	specifically addressing	any single node	in the graph. The naming of a
     node is independent of its	type, in that another node, or external	compo-
     nent need not know	anything about the node's type in order	to address it
     so	as to send it a	generic	message	type. Node and hook names should be
     chosen so as to make addresses meaningful.

     Addresses are either absolute or relative.	An absolute address begins
     with a node name, (or ID),	followed by a colon, followed by a sequence of
     hook names	separated by periods. This addresses the node reached by
     starting at the named node	and following the specified sequence of	hooks.
     A relative	address	includes only the sequence of hook names, implicitly
     starting hook traversal at	the local node.

     There are a couple	of special possibilities for the node name.  The name
     "." (referred to as ".:") always refers to	the local node.	 Also, nodes
     that have no global name may be addressed by their	ID numbers, by enclos-
     ing the hex representation	of the ID number within	square brackets.  Here
     are some examples of valid	netgraph addresses:


     Consider the following set	of nodes might be created for a	site with a
     single physical frame relay line having two active	logical	DLCI channels,
     with RFC-1490 frames on DLCI 16 and PPP frames over DLCI 20:

     [type SYNC	]		   [type FRAME]			[type RFC1490]
     [ "Frame1"	](uplink)<-->(data)[<un-named>](dlci16)<-->(mux)[<un-named>  ]
     [	  A	]		   [	B     ](dlci20)<---+	[     C	     ]
							   |	  [ type PPP ]
								  [    D     ]

     One could always send a control message to	node C from anywhere by	using
     the name Frame1:uplink.dlci16.  Similarly,	Frame1:uplink.dlci20 could re-
     liably be used to reach node D, and node A	could refer to node B as
     .:uplink, or simply uplink.  Conversely, B	can refer to A as data.	 The
     address could be used by both nodes C and	D to address a message
     to	node A.

     Note that this is only for	control	messages.  Data	messages are routed
     one hop at	a time,	by specifying the departing hook, with each node mak-
     ing the next routing decision. So when B receives a frame on hook data it
     decodes the frame relay header to determine the DLCI, and then forwards
     the unwrapped frame to either C or	D.

     A similar graph might be used to represent	multi-link PPP running over an
     ISDN line:

     [ type BRI	](B1)<--->(link1)[ type	MPP  ]
     [	"ISDN1"	](B2)<--->(link2)[ (no name) ]
     [		](D) <-+
      +->(switch)[ type	Q.921 ](term1)<---->(datalink)[	type Q.931 ]
		 [ (no name)  ]			      [	(no name)  ]

Netgraph Structures
     Interesting members of the	node and hook structures are shown below:

     struct  ng_node {
       char    *name;		     /*	Optional globally unique name */
       void    *private;	     /*	Node implementation private info */
       struct  ng_type *type;	     /*	The type of this node */
       int     refs;		     /*	Number of references to	this struct */
       int     numhooks;	     /*	Number of connected hooks */
       hook_p  hooks;		     /*	Linked list of (connected) hooks */
     typedef struct ng_node *node_p;

     struct  ng_hook {
       char	      *name;	     /*	This node's name for this hook */
       void	      *private;	     /*	Node implementation private info */
       int	      refs;	     /*	Number of references to	this struct */
       struct ng_node *node;	     /*	The node this hook is attached to */
       struct ng_hook *peer;	     /*	The other hook in this connected pair */
       struct ng_hook *next;	     /*	Next in	list of	hooks for this node */
     typedef struct ng_hook *hook_p;

     The maintenance of	the name pointers, reference counts, and linked	list
     of	hooks for each node is handled automatically by	the netgraph subsys-
     tem.  Typically a node's private info contains a back-pointer to the node
     or	hook structure,	which counts as	a new reference	that must be regis-
     tered by incrementing node->refs.

     From a hook you can obtain	the corresponding node,	and from a node	the
     list of all active	hooks.

     Node types	are described by these structures:

     /** How to	convert	a control message from binary <-> ASCII	*/
     struct ng_cmdlist {
       u_int32_t		  cookie;     /* typecookie */
       int			  cmd;	      /* command number	*/
       const char		  *name;      /* command name */
       const struct ng_parse_type *mesgType;  /* args if !NGF_RESP */
       const struct ng_parse_type *respType;  /* args if NGF_RESP */

     struct ng_type {
       u_int32_t version;		     /*	Must equal NG_VERSION */
       const  char *name;		     /*	Unique type name */

       /* Module event handler */
       modeventhand_t  mod_event;	     /*	Handle load/unload (optional) */

       /* Constructor */
       int    (*constructor)(node_p *node);  /*	Create a new node */

       /** Methods using the node **/
       int    (*rcvmsg)(node_p node,	     /*	Receive	control	message	*/
		 struct	ng_mesg	*msg,		     /*	The message */
		 const char *retaddr,		     /*	Return address */
		 struct	ng_mesg	**resp);	     /*	Synchronous response */
       int    (*shutdown)(node_p node);	     /*	Shutdown this node */
       int    (*newhook)(node_p	node,	     /*	create a new hook */
		 hook_p	hook,			     /*	Pre-allocated struct */
		 const char *name);		     /*	Name for new hook */

       /** Methods using the hook **/
       int    (*connect)(hook_p	hook);	     /*	Confirm	new hook attachment */
       int    (*rcvdata)(hook_p	hook,	     /*	Receive	data on	a hook */
		 struct	mbuf *m,		     /*	The data in an mbuf */
		 meta_p	meta);			     /*	Meta-data, if any */
       int    (*disconnect)(hook_p hook);    /*	Notify disconnection of	hook */

       /** How to convert control messages binary <-> ASCII */
       const struct ng_cmdlist *cmdlist;     /*	Optional; may be NULL */

     Control messages have the following structure:

     #define NG_CMDSTRLEN    15	     /*	Max command string (16 with null) */

     struct ng_mesg {
       struct ng_msghdr	{
	 u_char	     version;	     /*	Must equal NG_VERSION */
	 u_char	     spare;	     /*	Pad to 2 bytes */
	 u_short     arglen;	     /*	Length of cmd/resp data	*/
	 u_long	     flags;	     /*	Message	status flags */
	 u_long	     token;	     /*	Reply should have the same token */
	 u_long	     typecookie;     /*	Node type understanding	this message */
	 u_long	     cmd;	     /*	Command	identifier */
	 u_char	     cmdstr[NG_CMDSTRLEN+1]; /*	Cmd string (for	debug) */
       } header;
       char  data[0];		     /*	Start of cmd/resp data */

     #define NG_VERSION	     1		     /*	Netgraph version */
     #define NGF_ORIG	     0x0000	     /*	Command	*/
     #define NGF_RESP	     0x0001	     /*	Response */

     Control messages have the fixed header shown above, followed by a vari-
     able length data section which depends on the type	cookie and the com-
     mand. Each	field is explained below:

	  Indicates the	version	of netgraph itself. The	current	version	is

	  This is the length of	any extra arguments, which begin at data.

	  Indicates whether this is a command or a response control message.

	  The token is a means by which	a sender can match a reply message to
	  the corresponding command message; the reply always has the same to-

	  The corresponding node type's	unique 32-bit value.  If a node
	  doesn't recognize the	type cookie it must reject the message by re-
	  turning EINVAL.

	  Each type should have	an include file	that defines the commands, ar-
	  gument format, and cookie for	its own	messages.  The typecookie in-
	  sures	that the same header file was included by both sender and re-
	  ceiver; when an incompatible change in the header file is made, the
	  typecookie must be changed.  The de facto method for generating
	  unique type cookies is to take the seconds from the epoch at the
	  time the header file is written (i.e., the output of date -u +'%s').

	  There	is a predefined	typecookie NGM_GENERIC_COOKIE for the
	  "generic" node type, and a corresponding set of generic messages
	  which	all nodes understand.  The handling of these messages is auto-

	  The identifier for the message command. This is type specific, and
	  is defined in	the same header	file as	the typecookie.

	  Room for a short human readable version of "command" (for debugging
	  purposes only).

     Some modules may choose to	implement messages from	more than one of the
     header files and thus recognize more than one type	cookie.

Control	Message	ASCII Form
     Control messages are in binary format for efficiency.  However, for de-
     bugging and human interface purposes, and if the node type	supports it,
     control messages may be converted to and from an equivalent ASCII form.
     The ASCII form is similar to the binary form, with	two exceptions:

     o	  The cmdstr header field must contain the ASCII name of the command,
	  corresponding	to the cmd header field.
     o	  The args field contains a NUL-terminated ASCII string	version	of the
	  message arguments.

     In	general, the arguments field of	a control messgage can be any arbi-
     trary C data type.	 Netgraph includes parsing routines to support some
     pre-defined datatypes in ASCII with this simple syntax:

     o	  Integer types	are represented	by base	8, 10, or 16 numbers.
     o	  Strings are enclosed in double quotes	and respect the	normal C lan-
	  guage	backslash escapes.
     o	  IP addresses have the	obvious	form.
     o	  Arrays are enclosed in square	brackets, with the elements listed
	  consecutively	starting at index zero.	 An element may	have an	op-
	  tional index and equals sign preceding it.  Whenever an element does
	  not have an explicit index, the index	is implicitly the previous el-
	  ement's index	plus one.
     o	  Structures are enclosed in curly braces, and each field is specified
	  in the form "fieldname=value".
     o	  Any array element or structure field whose value is equal to its
	  "default value" may be omitted. For integer types, the default value
	  is usually zero; for string types, the empty string.
     o	  Array	elements and structure fields may be specified in any order.

     Each node type may	define its own arbitrary types by providing the	neces-
     sary routines to parse and	unparse.  ASCII	forms defined for a specific
     node type are documented in the documentation for that node type.

Generic	Control	Messages
     There are a number	of standard predefined messages	that will work for any
     node, as they are supported directly by the framework itself.  These are
     defined in	ng_message.h along with	the basic layout of messages and other
     similar information.

	  Connect to another node, using the supplied hook names on either

	  Construct a node of the given	type and then connect to it using the
	  supplied hook	names.

	  The target node should disconnect from all its neighbours and	shut
	  down.	 Persistent nodes such as those	representing physical hardware
	  might	not disappear from the node namespace, but only	reset them-
	  selves.  The node must disconnect all	of its hooks.  This may	result
	  in neighbors shutting	themselves down, and possibly a	cascading
	  shutdown of the entire connected graph.

	  Assign a name	to a node. Nodes can exist without having a name, and
	  this is the default for nodes	created	using the NGM_MKPEER method.
	  Such nodes can only be addressed relatively or by their ID number.

	  Ask the node to break	a hook connection to one of its	neighbours.
	  Both nodes will have their "disconnect" method invoked.  Either node
	  may elect to totally shut down as a result.

	  Asks the target node to describe itself. The four returned fields
	  are the node name (if	named),	the node type, the node	ID and the
	  number of hooks attached. The	ID is an internal number unique	to
	  that node.

	  This returns the information given by	NGM_NODEINFO, but in addition
	  includes an array of fields describing each link, and	the descrip-
	  tion for the node at the far end of that link.

	  This returns an array	of node	descriptions (as for NGM_NODEINFO)
	  where	each entry of the array	describes a named node.	 All named
	  nodes	will be	described.

	  This is the same as NGM_LISTNAMES except that	all nodes are listed
	  regardless of	whether	they have a name or not.

	  This returns a list of all currently installed netgraph types.

	  The node may return a	text formatted status message.	The status in-
	  formation is determined entirely by the node type.  It is the	only
	  "generic" message that requires any support within the node itself
	  and as such the node may elect to not	support	this message. The text
	  response must	be less	than NG_TEXTRESPONSE bytes in length (pres-
	  ently	1024). This can	be used	to return general status information
	  in human readable form.

	  This message converts	a binary control message to its	ASCII form.
	  The entire control message to	be converted is	contained within the
	  arguments field of the NGM_BINARY2ASCII message itself.  If success-
	  ful, the reply will contain the same control message in ASCII	form.
	  A node will typically	only know how to translate messages that it
	  itself understands, so the target node of the	NGM_BINARY2ASCII is
	  often	the same node that would actually receive that message.

	  The opposite of NGM_BINARY2ASCII.  The entire	control	message	to be
	  converted, in	ASCII form, is contained in the	arguments section of
	  the NGM_ASCII2BINARY and need	only have the flags, cmdstr, and
	  arglen header	fields filled in, plus the NUL-terminated string ver-
	  sion of the arguments	in the arguments field.	 If successful,	the
	  reply	contains the binary version of the control message.

     Data moving through the netgraph system can be accompanied	by meta-data
     that describes some aspect	of that	data. The form of the meta-data	is a
     fixed header, which contains enough information for most uses, and	can
     optionally	be supplemented	by trailing option structures, which contain a
     cookie (see the section on	control	messages), an identifier, a length and
     optional data. If a node does not recognize the cookie associated with an
     option, it	should ignore that option.

     Meta data might include such things as priority, discard eligibility, or
     special processing	requirements. It might also mark a packet for debug
     status, etc. The use of meta-data is still	experimental.

     The base netgraph code may	either be statically compiled into the kernel
     or	else loaded dynamically	as a KLD via kldload(8).  In the former	case,

	   options NETGRAPH

     in	your kernel configuration file.	You may	also include selected node
     types in the kernel compilation, for example:

	   options NETGRAPH
	   options NETGRAPH_ECHO

     Once the netgraph subsystem is loaded, individual node types may be
     loaded at any time	as KLD modules via kldload(8).	Moreover, netgraph
     knows how to automatically	do this; when a	request	to create a new	node
     of	unknown	type type is made, netgraph will attempt to load the KLD mod-
     ule ng_type.ko.

     Types can also be installed at boot time, as certain device drivers may
     want to export each instance of the device	as a netgraph node.

     In	general, new types can be installed at any time	from within the	kernel
     by	calling	ng_newtype(), supplying	a pointer to the type's	struct ng_type

     The NETGRAPH_INIT() macro automates this process by using a linker	set.

     Several node types	currently exist. Each is fully documented in its own
     man page:

	  The socket type implements two new sockets in	the new	protocol do-
	  main PF_NETGRAPH.  The new sockets protocols are NG_DATA and
	  NG_CONTROL, both of type SOCK_DGRAM.	Typically one of each is asso-
	  ciated with a	socket node.  When both	sockets	have closed, the node
	  will shut down. The NG_DATA socket is	used for sending and receiving
	  data,	while the NG_CONTROL socket is used for	sending	and receiving
	  control messages.  Data and control messages are passed using	the
	  sendto(2) and	recvfrom(2) calls, using a struct sockaddr_ng socket

	  Responds only	to generic messages and	is a "black hole" for data,
	  Useful for testing. Always accepts new hooks.

	  Responds only	to generic messages and	always echoes data back
	  through the hook from	which it arrived. Returns any non generic mes-
	  sages	as their own response. Useful for testing.  Always accepts new

     TEE  This node is useful for "snooping".  It has 4	hooks: left, right,
	  left2right, and right2left.  Data entering from the right is passed
	  to the left and duplicated on	right2left, and	data entering from the
	  left is passed to the	right and duplicated on	left2right.  Data en-
	  tering from left2right is sent to the	right and data from right2left
	  to left.

     RFC1490 MUX
	  Encapsulates/de-encapsulates frames encoded according	to RFC 1490.
	  Has a	hook for the encapsulated packets ("downstream") and one hook
	  for each protocol (i.e., IP, PPP, etc.).

	  Encapsulates/de-encapsulates Frame Relay frames.  Has	a hook for the
	  encapsulated packets ("downstream") and one hook for each DLCI.

	  Automatically	handles	frame relay "LMI" (link	management interface)
	  operations and packets.  Automatically probes	and detects which of
	  several LMI standards	is in use at the exchange.

     TTY  This node is also a line discipline. It simply converts between mbuf
	  frames and sequential	serial data, allowing a	tty to appear as a
	  netgraph node. It has	a programmable "hotkey"	character.

	  This node encapsulates and de-encapsulates asynchronous frames ac-
	  cording to RFC 1662. This is used in conjunction with	the TTY	node
	  type for supporting PPP links	over asynchronous serial lines.

	  This node is also a system networking	interface. It has hooks	repre-
	  senting each protocol	family (IP, AppleTalk, IPX, etc.) and appears
	  in the output	of ifconfig(8).	 The interfaces	are named ng0, ng1,

     Whether a named node exists can be	checked	by trying to send a control
     message to	it (e.g., NGM_NODEINFO).  If it	does not exist,	ENOENT will be

     All data messages are mbuf	chains with the	M_PKTHDR flag set.

     Nodes are responsible for freeing what they allocate.  There are three

     1	   Mbufs sent across a data link are never to be freed by the sender.

     2	   Any meta-data information traveling with the	data has the same re-
	   striction.  It might	be freed by any	node the data passes through,
	   and a NULL passed onwards, but the caller will never	free it.  Two
	   macros NG_FREE_META(meta) and NG_FREE_DATA(m, meta) should be used
	   if possible to free data and	meta data (see netgraph.h).

     3	   Messages sent using ng_send_message() are freed by the callee. As
	   in the case above, the addresses associated with the	message	are
	   freed by whatever allocated them so the recipient should copy them
	   if it wants to keep that information.

	    Definitions	for use	solely within the kernel by netgraph nodes.
	    Definitions	needed by any file that	needs to deal with netgraph
	    Definitions	needed to use netgraph socket type nodes.
	    Definitions	needed to use netgraph {type} nodes, including the
	    type cookie	definition.
	    Netgraph subsystem loadable	KLD module.
	    Loadable KLD module	for node type {type}.

     There is a	library	for supporting user-mode programs that wish to inter-
     act with the netgraph system. See netgraph(3) for details.

     Two user-mode support programs, ngctl(8) and nghook(8), are available to
     assist manual configuration and debugging.

     There are a few useful techniques for debugging new node types.  First,
     implementing new node types in user-mode first makes debugging easier.
     The tee node type is also useful for debugging, especially	in conjunction
     with ngctl(8) and nghook(8).

     socket(2),	netgraph(3), ng_async(4), ng_bpf(4), ng_cisco(4), ng_echo(4),
     ng_ether(4), ng_frame_relay(4), ng_hole(4), ng_iface(4), ng_ksocket(4),
     ng_lmi(4),	ng_mppc(4), ng_ppp(4), ng_pppoe(4), ng_rfc1490(4),
     ng_socket(4), ng_tee(4), ng_tty(4), ng_UI(4), ng_vjc(4), ngctl(8),

     The netgraph system was designed and first	implemented at Whistle Commu-
     nications,	Inc. in	a version of FreeBSD 2.2 customized for	the Whistle
     InterJet.	It first made its debut	in the main tree in FreeBSD 3.4.

     Julian Elischer <>, with	contributions by Archie	Cobbs

BSD			       January 19, 1999				   BSD

NAME | DESCRIPTION | Nodes and Types | Hooks | Data Flow | Netgraph is Functional | Interaction With Other Parts of the Kernel | Node Methods | Sending and Receiving Data | Addressing | Netgraph Structures | Control Message ASCII Form | Generic Control Messages | Metadata | INITIALIZATION | EXISTING NODE TYPES | NOTES | FILES | USER MODE SUPPORT | SEE ALSO | HISTORY | AUTHORS

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